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 ORE'S CIRCLE OF THE INDUSTRIAL ARTS. 
 
 A SERIES OP VOLUMES ON GENERAL TECHNOLOGY, AND 
 THE INDUSTRIAL AND DECORATIVE ARTS. 
 
 THE CHICLE OF THE SCIENCES, now on the eve of completion, was undertaken to supply 
 a Series of Treatises on Elementary Science. In the preparation of these Treatises, the 
 object kept in view was to produce an exposition of the more useful branches of Science, 
 written in the fullness of knowledge, and with a perfect mastery of the subject under 
 consideration, but devoid of technicalities as far as was attainable without the sacrifice 
 of scientific accuracy. In furtherance of this intention, assistance was sought in the 
 highest Scientific Circles, and obtained from the eminent men whose names are attached 
 to the respective volumes. 
 
 THE CIRCLE OF INDUSTRIAL ARTS now announced, is intended to form a Com- 
 panion Series to THE CIRCLE OF THE SCIENCES ; embodying, as it were, the Principles 
 of Science in their practical application to the various Industrial Arts ; and as assistance 
 was sought for the former Series in the highest quarters, so, in like manner, has the 
 most efficient co-operation been obtained for the present "Work, which may be viewed 
 as an extension of the former scheme. The Conductor trusts that a support, equal to 
 that so liberally accorded to THE CIRCLE OF THE SCIENCES, will be accorded to its suc- 
 cessor, which may be considered even a greater desideratum in our Literature. 
 
 The subjects contained in the Series will be comprised in Ten Volumes, arranged 
 
 as follows : 
 
 I. The Useful Metals and their Alloys. 
 II. The Precious Metals and their 
 
 Alloys. 
 
 III. Architecture find Building Con- 
 trivances. 
 
 IV. Civil Engineering in all its Branches. 
 V. The Military Art. 
 
 VI. Decorative Art, in Principle and 
 
 Practice. 
 VII. Bleaching, Dyeing, and Printing, 
 
 and the Textile Fabrics. 
 VIII. Glass, Pottery, and Porcelain. 
 IX. Painting, Sculpture, and Plastic Art. 
 X. Practical Agriculture. 
 
 The Volume now Publishing contains 
 
 THE USEFUL IftETALS AND THEIR ALLOYS. 
 
 Including Iron, Copper, Tin, Lead, Zinc, Antimony, and their several Alloys. 
 By the following "Writers : 
 
 I. (a,} Chemical Metallurgy. By JOHN 
 SCOFFERN, M.B. Lond. 
 
 II. (#.) Mining; with a Chapter on Ven- 
 tilation and Mining Jurisprudence. By a 
 Government Inspector of Mines. 
 
 II. British Iron Manufacture; including 
 the Mining, Roasting, Smelting, and Pud- 
 dling Processes. By WM. TRURAN, Esq., 
 C.E., late Engineer at the Dowlais, Hir- 
 wain, and Forest "Works. 
 
 III. Steel Manufacture ; including the 
 various processes for making Shear, Blis- 
 tered, Cast, and Indian Steel. 
 
 IV. Applications of Iron to Bridges, 
 Houses, and Ship-building. By "W. FAIR- 
 IS AIRN, Esq., Manchester. 
 
 V. On Wrought Iron in Large Masses. 
 
 By "WM. CLAY, Esq., of the Mersey Iron 
 and Steel Works. 
 
 VI. Copper, Tin, Zinc, and Antimony ; 
 their Mining, Crushing, and Smelting 
 Processes. By ROBERT XL AND, Esq., 
 Mineral Chemist, Plymouth. 
 
 VII. Lead and its Uses ; including the 
 Mining, Crushing, Smelting, and Refining 
 Processes : The Use of Lead in the Arts, 
 and its Application to Ornamental Metal- 
 lurgy. By DR. RICHARDSON, of Newcastle. 
 
 VIII. Bronze, Brass, and Ornamental 
 Iron Work, their Composition, Ancient 
 and Modern ; the Art of Moulding and 
 Casting; with their Applications to De- 
 corative Art. By W. C. AITK.EN, Cam- 
 bridge Works, Birmingham. 
 
 LONDON : WM, S. ORR AND CO., AMEN CORNER, PATERNOSTER ROW. 
 
NOTICES OF THE PEESS. 
 
 Arts.' The series is commenced with a 
 volume on ' The Useful Metals and their 
 Alloys/ and the main subject of the present 
 number is that most valuable metal, iron." 
 Portsmouth Times. 
 
 " The part before us contains ' the Useful 
 Metals and their Alloys,' including iron, 
 copper, tin, lead, zinc, antimony, and their 
 alloys. That part of the first chapter 
 which relates to British iron manufacture, 
 is by Mr. Win. Truran, late engineer at the 
 Dowlais, Hinvain, and Forest Works, and 
 includes the mining, roasting, smelting, and 
 puddling processes." Cardiff Guardian. 
 
 " In the present number, which treats of 
 the ' Useful Metals and their Alloys,' we 
 observe the very latest discoveries are in- 
 corporated, and descanted upon as, for 
 instance, Mr. Bessemer' s process of refining 
 iron, which has been attracting so much 
 attention of late. This, of itself, is sufficient 
 to show that the work is of no superficial 
 and antiquated kind, but that the materials 
 are got up fresh for the time and purpose 
 intended. When we look to the list of 
 authors on the various branches in metal- 
 lurgic art, we have a further confirmation 
 of the originality of this publication. It is 
 practical in the best sense of the word and 
 by that we mean, it is a work such ms prac- 
 tical men will read, understand, and appre- 
 ciate. We must not forget to add that each 
 art and process of art is illustrated by ap- 
 propriate drawings and diagrams, which 
 greatly tend to make the work valuable and 
 practical." Border Advertiser. 
 
 " Those who know and value, as we do, 
 the ' Circle of the Sciences,' published by 
 Messrs. Orr and Co., will be glad to see the 
 commencement of a companion series of 
 publications under the title of the ' Circle of 
 the Industrial Arts.' The former publica- 
 tion, now nearly completed, comprises 
 treatises on elementary science ; the latter 
 is designed to embody the principles of 
 science in their practical application to the 
 industrial arts. Thus, in the first part now 
 before us, we have the subject of the ' Useful 
 Metals and their Alloys ;' and all the opera- 
 tions connected with the chemistry, manu- 
 facture, &c., of iron, tin, lead, copper, &c., 
 are described and illustrated with great care 
 and minuteness. The subject is not needless- 
 ly debased ; nor is it so technically treated 
 as to be unintelligible to the many. Every- 
 thing about the publication is remarkable 
 for neatness." Cambridge Chronicle. 
 
 " The subjects for elucidation are very 
 various and very important. They will 
 comprise, in ten volumes, ' The Useful 
 Metals and their Alloys,' ' The Precious 
 Metals and their Alloys,' 'Architecture and 
 Building Contrivances,' ' Civil Engineering 
 in all its Branches,' 'The Military Art,' 
 ' Decorative Art, in Principle and Practice,' 
 ' Bleaching, Dyeing, and Printing, and the 
 Textile Fabrics,' ' Glass, Pottery, and Por- 
 celain,' ' Painting, Sculpture, and Plastic 
 Art,' and ' Practical Agriculture.' The 
 treatises arc to be written, as were the 
 treatises in the ' Circle of the Sciences,' by 
 men who have a thorough practical, as well 
 as theoretical knowledge of the subjects. The 
 first volume, of which the first part is be- 
 fore us, is devoted to the consideration of 
 the ' Useful Metals and their Alloys.' The 
 whole work will be profusely illustrated by 
 wood engravings. The first part, on ' Che 
 mical Metallurgy,' is extremely well writ- 
 ten, and very interesting." Bradford Ob- 
 server. 
 
 " The first volume treats of the Useful 
 Metals, and the information is accurate, 
 and conveyed in language as much divested 
 of technical terms as the nature of the siib- 
 ject would admit of. In fact, it is quite 
 unnecessary for us to say more in intro- 
 ducing it to our readers, when we mention 
 that among the contributors are Mr. Fair- 
 bairn, of Manchester ; Dr. Richardson, of 
 Newcastle; Mr. Clay, of the Mersey Iron 
 Works; and Mr. Wm. C. Aitken, of the 
 Cambridge Works, Birmingham." Elgin 
 and Mor at/shire Courier. 
 
 "We would strongly recommend the 
 book as being highly instructive to such 
 as engineers, machinists, &c., whose pro- 
 fessional principles are based upon that 
 mineral to which this treatise chiefly de- 
 votes itself Iron, it being here practi- 
 cally, theoretically, and technically con- 
 sidered by the mast eminent professors of 
 the day." Stockport Advertiser 
 
 " If the present work continue as it has 
 begun, we have no hesitation to state that 
 it will make for itself an equally high and 
 distinguished name." Gahvay Mercury. 
 
 " The work has the advantage of being 
 brought out under auspices which have em- 
 ployed writers of scientific eminence to 
 treat on the several subjects, thus render- 
 ing it invaluable as a text-book in connec- 
 tion with industrial and decorative arts." 
 Taunton Courier. 
 
PRACTICAL METEOROLOGY: 
 
 Jl (grate t0 tjre f !]BU0nveim jof % 
 
 PRACTICAL USE OF INSTRUMENTS FOR REGISTERING AND 
 RECORDING ATMOSPHERIC CHANGES 
 
 BY 
 
 JTBr. SCOFFERN, M.B., 
 
 )| 
 
 AUTHOR OF "ELEMENTARY CHEMISTBY," 
 AND 
 
 J, E. LOWE, ESQ., F.E.A.S., F.G.S., M.B.M.S., 
 
 OF TRX HIOHFIKLU HOUSK AND BKKSTON O)SKRTATORY, 
 
 LONDON : 
 
 HOULSTON AND STONEMAN, 65 PATERNOSTER ROW ; 
 W M . S. ORR AND CO., AMEN CORNER. 
 
 MDCCCLVI. 
 
 
O 
 
 CONTENTS. 
 
 PAGE. 
 DEFINITIONS AND LIMITATIONS. Chemical Constitution of the Atmosphere, 1 ; 
 
 Methods of Ozonizing Oxygen, 2 ; Physical Properties of Gaseous Bodies, 
 Law of Marriotte, 4; Atmosphere as a Ponderable Agent, 5; Weight of 
 the Atmospheric Column, ih. ; Influence of Weight on the Barometer, 7 ; 
 Improvements of the Barometer, 9 ; Contrivances for Measuring small 
 Divisions, 11; Thermal Expansion, 13; Atmosphere Practically Con- 
 sidered 16 
 
 THEKMOMETEES. Differential Thermometers, 22; Ordinary Thermometers, 24; 
 Systems of Graduation, 65; Correspondence between Thermometers, 29; 
 Thermal Influences in producing Winds, 30 ; the Trade Winds, 31 ; Land 
 and Sea Breezes, 32 ; Upper and Lower Currents, 33 ; Storms, 35 ; Hot 
 Winds, ib. ; Whirlwinds, 36 ; Waterspouts, 38 ; Origin of Atmospheric 
 Moisture, 39; Theory and Formation of Dew, 41; Theory of Fogs, 44; 
 Clouds, 45; Theory of Kain, 49; Snow, 51; Hail, 53; Construction of 
 Hygrometers, 55; Theory of Light, 58; Luminous Kefraction, 61; Chro- 
 matics, 62 ; Decomposition of White Light . . . . . .77 
 
 MAGNETISM AND ELECTRICITY. Terrestrial Magnetism, 79; Voltaic Conducting 
 Wires, 81 ; Deflecting power of Coils, 83 ; Dia-Magnetism, 64; Electricity, 
 86; Electric Terms, 90; Atmospheric Refraction, 91; Fata Morgana, 92; 
 Kefraction, Aberration, 93 ; Kainbows, 95 ; Halos, Parhelia, 97 ; Aurora 
 Borealis, ib. ; Thunder and Lightning, 100; Lightning Conductors, 101; Elec- 
 tric Deductions, 103; Aerolites, Shooting-stars, and Meteors, 104; Occult 
 Emanations of Meteorology 106 
 
 CLIMATOLOGY. The Gulf-Stream and its Currents, 110; Other Oceanic Currents, 
 113; Cold and its Effects, 114; Ice Pictures, 115; Climate and Organic De- 
 velopment 116 
 
 PEACTICAL METEOKOLOGY. 
 
 Methods of Observing, 117; Meteorological Observations, 118;. Newman's Ther- 
 mometer, 119; the Thermometer, 120; Glaisher's Thermometer-stand, 121; 
 Maximum and Minimum Thermometers, 122 ; Mercurial Minimum Thermo- 
 meter, 123 ; Evaporators, ib. ; Kain Gauges, 124 ; Electrometers, 125 ; Ozono- 
 
CONTENTS. 
 
 PAGE. 
 
 meters and Wind Vanes, 126 ; The Actinometer, ib. ; Anemometers and Hygro- 
 meters, 127; Thunder-storms and Aurora Borealis, 128; Hygrometrical 
 Tables, 129 ; Examples of Reading off Temperatures, 130 ; Correction of 
 Barometrical Readings, 13 1 ; Barometrical Reductions, ib. ; Corrections for 
 Temperature, 132; Dr. Drew's Theorem, 133 ; Thermometrical Reductions, 
 134 ; Mean Temperature of Dew-point, 135 ; Water contained in a Column of 
 Air, 136; Weight of a cubic foot of Air, 137; Monthly and Daily Range of 
 Temperature, 138 ; the mean amount of Clouds, 239 ; Classes of Clouds, ib. ; 
 the Daily amount of Rain, 140; of Evaporation, ib. ; the Force of Wind, ib ; 
 Table for the Deduction of the Dew-point, 141 ; Recommendation and Precau- 
 tions, ib. ; Newman's Standard Barometer, 142 ; Apparatus for a Meteorolgical 
 Observatory, 143 ; Snow Gauge, ib, ; Calendar of Nature .. , , , 144 
 
METE0110LOGY. 
 
 Definition and limitations. The term Meteorology, from juerewpos elevated, 
 was applied by Aristotle to signify phenomena occurring in elevated regions. It 
 may be considered synonymous with the study of atmospheric phenomena, though all 
 which concerns meteors proper is very nearly allied with astronomy. 
 
 In many respects Aristotle's opinions on meteorologic subjects display the usual 
 acumen of that deep thinker ; his remarks on the subject of dew are particularly 
 interesting. Deprived, however, of the barometer and thermometer deprived of 
 optical instruments the nature of electricity yet undeveloped, and the composition and 
 functions of the atmosphere unknown, ancient speculations on nieteorologic subjects 
 were necessarily unsatisfactory and vague. 
 
 The meteorological writings of Thcophrastus, Aristotle's pupil, were more diffuse 
 than those of that great philosopher, and were long recognized as constituting a text- 
 book. They constituted the groundwork of the Atoo-^eja, or prognostics of Aratus, 
 and were embodied in a versified rendering by Cicero in his youthful days. Portions 
 of these attempts at versification are still in existence, and do but little credit to the 
 great Roman orator in a poetical capacity. 
 
10 CHEMICAL CONSTITUTION OF THE ATMOSPHERE. 
 
 Meteorology, in its most common and restricted sense, may be considered synony- 
 mous with knowledge of the weather. It therefore involves a full acquaintance with 
 the nature and composition of the atmosphere ; with the laws of gaseous and vapoufous 
 elasticity ; with the conditions determining the production of fogs, dew, snow, and 
 hail ; also with the laws of atmospheric, optical, and electrical phenomena. It is the 
 province of meteorology also to study the phenomena of aerolites, and the relations 
 which subsist between atmospheric conditions and the development of organic 
 species. 
 
 The above is a general outline of the scope and limits of meteorology. Its suc- 
 cessful study will be seen to involve a pre-acquaintance with many sciences, more 
 especially those of chemistry and electricity. There does not, indeed, exist any science 
 having limits so undefined as meteorology. From a consideration of the theory of shoot- 
 ing stars to a contemplation of the mutual alliance subsisting between certain forms of 
 disease and atmospheric conditions, or the relation between certain animal and vege- 
 table tribes and given atmospheric conditions, the divergency is wide. Nevertheless, 
 all these branches of study are intimately allied with meteorology ; and perhaps the 
 most delightful part of botanical science is that which seeks to establish connections 
 between the localization of certain vegetable families in districts characterized by 
 some peculiarity of meteorologic condition. Horticulturists have been too ready to 
 overlook the influence of remote atmospheric conditions on certain vegetable families. 
 Too frequently it has been considered that a vegetable surrounded by an atmosphere of 
 temperature similar to that of its native region, and planted in a soil of similar 
 chemical composition, must necessarily thrive. There are, nevertheless, meteoric con- 
 ditions beyond these. Why is it that many species of the palm tribe refuse to grow 
 very far away from their native regions, although transplanted to localities seemingly 
 identical in all respects ? "Why is it that the cocoa-palm' refuses to grow in regions 
 very far distant from the sea ? These questions involve meteorologic considerations of 
 great interest ; and not less interesting to a meteorologist is the partiality evinced to a 
 restricted region by the cinchona tribe. An atmosphere very much rarefied and per- 
 petually moist are so essential to their existence, that they cannot live without it. 
 
 The natural approach to meteorology is the study of the atmosphere, which admits 
 of being contemplated under many aspects. It may be contemplated either as the 
 atmosphere proper or theoretical, composed of two gases, oxygen and nitrogen ; or as 
 the practical atmosphere or mixture of the gaseous theoretical atmosphere with 
 numerous vapours, extraneous gases, and fleeting undetermined miasmata. The atmo- 
 sphere, too, admits of being regarded statically, i.e. at rest, and dynamically, i.e. in 
 motion, the latter involving a study of the causes of winds. The atrrosphere, lastly, 
 may be considered in relation to the imponderable agents, to heat, light, electricity, 
 and magnetism. I shall begin by investigating the nature of our atmosphere regarded 
 chemically. 
 
 Chemical Constitution of the Atmosphere. By the ancients air was con- 
 sidered to be an elementary substance. Chemistry at length demonstrated it to consist 
 of two gases, oxygen and nitrogen combined, or rather mixed in the proportions of about 
 eighty parts, by measure, of nitrogen, and twenty of oxygen ; or, in other words, one 
 volume of oxygen to four of nitrogen. 
 
 Considerable difference of opinion once existed on the question, whether the 
 atmosphere be a chemical or a mechanical compound. To adduce evidence bearing on 
 this discussion, would be foreign to the subject of meteorology. The generally received 
 
GASEOUS DIFFUSION. 
 
 11 
 
 opinion is in favour of the mechanical constitution of the atmosphere. The law of the 
 diffusion of gases perfectly accounts for the intimate mixture with oxygen and nitrogen 
 in the atmosphere, without having recourse to the assumption of chemical union. An 
 outline of the law in question it will be proper to give. 
 
 Gaseous Diffusion. If two glass vessels betaken, as represented in the accompanying 
 diagram (Fig. 1), the upper one being filled with hydrogen gas, the lower one with 
 oxygen gas, placed in communication with each other by 
 a capillary tube passing through the cork stopper of 
 both, and allowed to remain at rest for about half an 
 hour, perfect mixture of the oxygen and hydrogen gases 
 will have ensued. Inasmuch as no chemical union takes 
 place between the two gases thus circumstanced, and 
 inasmuch as hydrogen gas filling the upper vesselis_^' ^ 
 about sixteen times kea^eiythan oxygen "gas" Ming the * 
 lower vessel, some new cause of admixture has to be 
 sought ; it depends upon the mutual tendency of the two 
 gases to become diffused through each other. 
 
 The above is an individual case exemplifying a 
 general principle ; any two gases might have been se- 
 lected and mutual diffusion would have ensued. Oxygen 
 and hydrogen gases have been here chosen because of the 
 facility wherewith the circumstance of these having become diffused may be deter- 
 mined. It is well known that neither oxygen nor hydrogen gas, taken separately, will 
 explode on the application of flame, whereas a mixture of the two readily explodes ; 
 hence the propriety of selecting these two gases for illustration of the principle in 
 question will be obvious. 
 
 Faraday appears to have been the first to direct attention to the mutual diffusibility 
 of gases. He noticed that bottles filled with gases, and corked or stoppered, or vessels 
 filled with gases and inverted over mercury, in either case occluded to all appearance 
 accurately, nevertheless almost always permitted mutual admixture of the air without 
 and the gas within. Dobereiner, Mitchell, and Graham, more especially the latter, 
 have since investigated this class of phenomena more narrowly, and Graham has suc- 
 ceeded in determining the law which regulates this diffusion. He finds that the relative 
 diffusiveness of any two gases is expressed by the reciprocals of the square root of their 
 densities. Thus, the density of air being one, its diffusiveness is one also. The density 
 
 1 1 
 
 Fig. 1. 
 
 of hydrogen being 0-693, its diffusiveness is 
 
 0-063 
 
 = 4-56 ; the density of 
 
 0-2633 
 
 ammonia being 0-5S98, its diffusiveness is 1 / . , = 77-^777 = 1'30; and generally 
 
 v O'ohyo O'/ool 
 
 representing the density of a gas by d, its diffusiveness is rr \/ - . Applying this rule 
 
 to practice, it appears that, supposing hydrogen and ammonia placed under circum- 
 stances promoting their mutual diffusion, 456 volumes of hydrogen will become mingled 
 with 1-30 of ammonia. 
 
 It will be remarked that the atmosphere has hitherto been treated of, in a theoretical 
 sense, as a mere mixture of nitrogen and oxygen gases. Practically, however, the 
 atmosphere is far more complex. It invariably contains portions of carbonic acid (about 
 one part in a thousand), also extraneous gases, besides aqueous and other vapours. A 
 
12 METHODS OF OZONISING OXYGEN GAS. 
 
 mixture of all these things may be termed, distinctively, the actual or practical 
 atmosphere. They will hereafter come under our notice seriatim ; hut even a theoretical 
 mixture of oxygen and nitrogen gases is subject to remarkable variations of property, 
 its chemical composition remaining unchanged ; portions of its oxygen are subject to be 
 converted into ozone. 
 
 The change of common oxygen into ozone, furnishes an illustration of one of the 
 most remarkable discoveries of modern science. It displays what is, perhaps, the most 
 extraordinary example of the condition of allotropism, or the existence of one body under 
 two different aspects ; it promises to render evident some of these occult atmospheric 
 causes which determine the progress of epidemics, and promote the existence of endemic 
 diseases. 
 
 A summary of our knowledge relative to ozone may be briefly stated as follows : Oxy- 
 gen gas is susceptible of undergoing a change, the nature of which is altogether veiled in 
 mystery. It is susceptible of becoming odorous, corrosive, and irritating when breathed ; 
 its chemical action is susceptible of being exalted and modified, so that whilst ordinary 
 oxygen gas neither bleaches nor corrodes silver, nor decomposes iodide of potassium, 
 the allotropic, or second form of oxygen gas, will accompliah all these results, and 
 many more too numerous for mention here. The general conclusion to which it is de- 
 sired to bring the reader is this : if causes can be proved to exist capable of changing 
 atmospheric oxygen gas in its ordinary state to oxygen gas in its extraordinary 
 state, how vast, how complicated must be the meteoric results determined thereby ! 
 That such natural causes do exist will be readily inferred from a consideration of the 
 artificial methods to which the chemist has recourse for changing ordinary oxygen into 
 ozone. 
 
 Methods of Ozonising Oxygen Gas. The most ready method of ozonising 
 oxygen gas is as follows : Take a few sticks of phosphorus, scrape them free from all 
 superficial contamination, place them in a wide-mouthed bottle containing alittle water but 
 not enough to cover the phosphorus. Let the whole remain at rest for about ten or fifteen 
 minutes, and a considerable portion of the atmospheric oxygen will have been converted 
 into ozone. The ozonized air thus generated will be at present mixed with vapours of 
 phosphorous acid ; washing will free the air from these, however, without removing the 
 ozone. That the air thus treated has become considerably modified in some way, will, 
 in the first place, be rendered evident by the smell ; atmospheric air, when pure and in 
 its ordinary state, is devoid of smell. But the atmospheric air, the product of the 
 experiment just detailed, will be found to have a very peculiar odour. It will be found, 
 moreover, to be capable of removing the colour cf sulphate of indigo, and other vege- 
 table and animal colouring bodies. All this is due to the modification which ordinary 
 oxygen gas assumed due to its assumption of the allotropic state to its conversion 
 into ozone. 
 
 Another ready method of generating ozone is this : Moisten the interior of a bell 
 glass receiver, or a large-mouthed bottle, with ether ; then take a glass rod, heat it in 
 the flame of a spirit-lamp, and plunge it into the bottle or bell-glass ; under these cir- 
 cumstances ozone will be formed, provided the glass rod has not been heated to an 
 inordinate temperature, for the circumstance has to be mentioned that ozone is recon- 
 verted into ordinary oxygen gas by contact with any body heated above a certain, but 
 not very well-determined, point. 
 
 "We have seen that tbe ordinaiy method of generating ozone consists in promoting 
 the contact of phosphorus with atmospheric air (or oxygen) under certain conditions 
 
NATURAL CAUSES WHICH GENERATE OZONE. 13 
 
 Many other substances besides phosphorus are capable of generating ozone by contact. 
 Oil of turpentine, and many other essential oils, will accomplish this ; and the fact in 
 question cannot be too forcibly remembered by the painter, who may discover in the 
 philosophy of ozone the reason why certain pigments fade, or are bleached, thus de- 
 stroying the general effect which he desired to produce. Meteorologically considered, 
 however, the most important source of ozone remains to be described ; I refer to the 
 production of ozone by means of electricity. Every person who has been much accus- 
 tomed to work with the electrical machine must have noticed the generation of some- 
 thing powerfully odorous during the friction of the cylinder, or plate, against the 
 rubber. This odour has, in point of fact, been called the electric smell. Now, if this 
 electric smell be compared with the smell of the atmospheric air which has been treated 
 with phosphorus, as just described, and washed, the two odours will be found to be 
 identical, which is a presumptive evidence that electricity lias in some way been con- 
 cerned in the formation of ozone an idea which extended experiment fully comfirms. 
 
 It has already been stated that the most prominent quality of ozone is its highly 
 developed oxidising power. By taking advantage of this property, we are 
 supplied with an easy means of recognizing it, by means of a test-paper, imbued 
 with a mixture of iodide of potassium and starch. The chemical reader need not 
 be informed that iodine colours starch blue, whereas oxide of potassium does not ; 
 therefore test-paper, imbued with iodide of potassium and starch, may occasionally be 
 resorted to for indicating the presence of certain bodies which have the faculty of 
 decomposing iodide of potassium, and liberating free iodine. Ozone is one of these? 
 which fact remembered will render the following experiment intelligible : If a piece 
 of paper, imbued with iodide of potassium and starch, be held between the prime con- 
 ductor of an electrical machine and the knuckle or a metallic ball, and electrical 
 sparks transmitted through it, spots of blue discolouration will be seen on the paper, 
 corresponding to each electrical spark. 
 
 The method of detecting the presence of atmospheric ozone is now readily in- 
 dicated. If a strip of paper, imbued with solution of iodide of potassium and starch, 
 turns blue, the existence of ozone is demonstrated. The experiment is very striking 
 when performed near the sea. During the prevalence of a land wind, the test-paper 
 will generally afford slight indications, or none at all ; during the prevalence of a sea 
 wind, however, ozone can generally be detected. 
 
 The reader will now be prepared to form some idea of the natural causes which 
 may generate ozone, "We need assume no other agency than that of electricity, 
 to be assured that the production of ozone must be universal, and, looking on. the 
 world as an aggregate, continuous ; and when we consider the potent nature of ozone, 
 the irritation it produces when breathed, the facility with which it bleaches, corrodes, 
 and destroys, we shall not be at a loss to understand that the consequences to living 
 beings of its excess or diminution must be all-important. A most important function 
 of ozone has yet to be indicated, it removes almost more rapidly than chlorine itself 
 the bad odours resulting from the decomposition of animal or vegetable bodies. If a 
 piece of putrid flesh be immersed for a few minutes in a bottle of ozonized air, the 
 odour of decomposition is totally destroyed. With these facts before us, we may 
 form some idea to ourselves of the important functions which ozone is designed to 
 accomplish. Lessen the amount of atmospheric ozone, lower it below given limits, 
 and increase the atmospheric temperature to the degree most congenial to organic 
 decomposition, and the air will soon be charged with disease-bearing putrid odours. 
 
14 PHYSICAL PROPERTIES OF GASEOUS BODIES. 
 
 It is in accordance with all that philosophy has been able to teach us in relation to 
 the laws of epidemic and endemic maladies, that the presence of such gaseous odours of 
 organic decomposition as are here assumed, must be the fruitful source of disease ; and 
 it is not possible, after having studied the qualities of ozone, to refuse assent to the pro- 
 position that the existence of this agent in competent amount must be followed by the 
 destruction of the pestiferous odours of organic decomposition. If, however, ozone be 
 naturally formed at any time in excessive amount, it is not difficult to foresee that 
 other serious consequences must result to animal life. The inhalation of an irritating 
 gas cannot but produce injurious effects on organs so delicate as the lungs, and per- 
 haps many of the now anomalous and inexplicable effects of change of air to patients 
 suffering from chest diseases may hereafter receive their solution in a more intimate 
 acquaintance with the laws of ozone. 
 
 Physical Properties of Craseous Bodies. The word gas is of German 
 origin, and was first employed by Van Helmont to signify the vapour which escaped 
 from liquids undergoing vinous fermentation. At later periods the term was applied 
 to designate every invisible substance disengaged from bodies by the application of fire. 
 Macquer, a celebrated French chemist of the eighteenth century, extended the meaning 
 of the term gas to signify every kind of air besides atmospheric air ; and modern che- 
 mists have extended the meaning of the term still further to indicate clastic fluids, 
 whatever their colour may be, which are not readily condensible. At one time it was 
 erroneously imagined that gases did not admit of condensation ; in accordance with 
 this belief a gas was defined as being a permanently elastic fluid, thus distinguishing 
 this class of bodies from mere vapours, which, so far from being permanently elastic, 
 are very readily condensible. Modern discovery has proved this distinction to be 
 untenable. All the known gases, except oxygen, nitrogen, hydrogen, nitric oxide, car- 
 bonic oxide, and coal gas have been liquefied, and a great number of them solidified, by 
 subjecting them to extreme cold and pressure. 
 
 Law of Marriotte The Volumes of Gases are inversely to the Pressure applied. This 
 is a very celebrated law, and one that intimately concerns the meteorologist ; it may 
 be otherwise termed the law of compressibility of elastic fluids. 
 
 It will be recognized that the law in question, according to the exposition of it just 
 given, is a general law applying to a vast number of gaseous and vaporous bodies. For 
 a long time its absolute truth remained unquestioned, but more recently M. Eegnault 
 and others have demonstrated it to be not of such universality. 
 
 Even atmospheric 4 air and nitrogen do not rigorously conform to the law ; and car- 
 bonic acid, and liquefiable gases generally, are so little amenable to the law, that, as 
 applied to them, it cannot be regarded as approximately correct. Even the rate of com- 
 pressibility of hydrogen is not strictly accordant with the law, although the deviation 
 in this case is in the opposite direction to the deviation when atmospheric air is con- 
 cerned ; for it suffers less compression than, according to^the law, should take place. 
 Carbonic acid and nitrogen, when compressed by a force of forty-five atmospheres, only 
 fill seven-tenths of the space they ought to occupy according to the law. 
 
 Now, inasmuch as the philosophy of estimating the height of mountains barometri- 
 cally, is intimately associated with the law of Marriott^, it is well to indicate that this 
 law is not quite correct ; nevertheless, as regards atmospheric air, it is so nearly correct 
 that we may accept it without demur. 
 
 The experiment by which the law of Marriotte was deduced is as follows : Into a 
 strong glass tube, of equal diameter throughout, bent on itself (as represented in Fig. 
 
THE ATMOSPHERE AS A PONDERABLE AGENT. 
 
 2), open at the long arid closed at the short extremity, a little mercury is poured 
 
 in such a manner that it shall be perfectly level in the two legs, as represented by the 
 
 line A B. Under these circumstances it will be evident that the air 
 
 inclosed between B C and A D will be equally compressed. Let us 
 
 assume the amount of compression to be represented by the weight x. 
 
 Let us furthermore assume that the weight of a column of mercury 
 
 between and D to be x, and let us call it m. If, then, on filling 
 
 the long arm of the syphon with mercury, the column of air originally 
 
 extending from B to C be diminished to the column extending from 
 
 B' to C, that is to say, to one-half, 
 
 then we have x -\- in = i B C, or 2 x \ B C ; 
 proving that the compression of air within the limits of the experi- 
 ment is inversely to the pressure applied. In like manner, when the 
 pressure is triple, the volume of gas is reduced to one-third ; when 
 quadruple, to one-fourth, etc. M. Arago has experimentally deter- 
 mined that the law rigorously applies to atmospheric air up to a 
 pressure of twenty-seven atmospheres. 
 
 The Atmosphere as a Ponderable Agent. Owing to the equality of 
 pressure which the atmosphere exercises on every side, we are not 
 ordinarily conscious of its possessing weight. Nevertheless, its 
 weight is no less definite, under proper limitations of demonstration, 
 than the weight of any solid or liquid. The weight of the atmos- 
 phere may be contemplated^ under two conditions : firstly, as the ] 
 equivalent weight of an atmospheric column of known sectional area 
 but unknown height ; secondly, as the equivalent weight of a known 
 atmospheric volume under proper limitations, presently to be indicated, 
 investigations will now have to be considered. 
 
 Determination of the Weight of an Atmospheric Column of "known Sectional Area biit 
 
 unknown Elevation. The Barometer, 
 Experiment 1. If a piece of glass tube 
 be taken equal in bore throughout, hav- 
 ing a length of some thirty-three or 
 thirty-four inches, closed at one ex- 
 tremity and open at the other ; if it be 
 filled with mei'cury, then closed tempo- 
 rarily by the thumb, and inverted in a 
 basin of mercury, as represented in the 
 accompanying sketch (Fig. 3), we 
 shall obtain a barometer of perhaps the 
 most simple form this useful instrument 
 can assume. "We will proceed to study 
 its philosophy. Firstly, let it be observed 
 that though the tube was quite full of 
 mercury, it does not remain quite full. 
 No sooner is the restraining thumb re- 
 moved, than a portion of the mercury 
 sinks into the basin. If, instead of hold- 
 ing the inverted tube in the hand, as represented, some permanent support be devised 
 
 Fig. 2. 
 Both these 
 
16 
 
 ATMOSPHERIC PRESSURE DEMONSTRATED. 
 
 for it; if the point corresponding with the present elevation or level of the mercury be 
 marked on the tube ; and if the tuhe be examined from day to day, the observer will 
 
 soon find the level to fluctuate ; some times it will 
 rise, at other times fall. He will find, more- 
 over, that the mercury will become depressed pre- 
 vious to stormy weather. 
 
 The instrument thus roughly extemporized is, 
 in point of fact, a measurer of the weight of an 
 atmospheric column of known sectional area (i.e., 
 the area of the interior diameter of the tube), but 
 of unknown elevation. It is a barometer ; and 
 collaterally inasmuch as depression of the mer- 
 curial column usually precedes stormy weather 
 the roughly-extemporized instrument is a weather- 
 glass. 
 
 Demonstration. It is desirable occasionally, 
 when treating of natural phenomena, to violate 
 the mathematical rule of accepting no fact until it 
 has been demonstrated. 
 
 Thus, in the present instance, I have taken the 
 fact for granted that the cause operating to maintain 
 a mercurial column in the closed tube, is atmospheric 
 pressure ; and the cause to which variations of the 
 height of that column is referable, is fluctuation of atmospheric pressure. The 
 first proposition shall now be demon- 
 strated, when the second will be ac- 
 cepted by inferential reasoning. 
 
 Experiment 2. If, instead of a tube 
 closed at one extremity, an open tube be 
 taken and one end be occluded by tj'ing 
 securely over it two or three strong 
 pieces of moistened bladder : if the tube 
 thus prepared be filled with mercury, as 
 before, and inverted in a basin of mer- 
 cury, we shall be in a position to de- 
 monstrate the proposition that it is owing 
 to atmospheric pressure and that alone 
 that the mercurial column is supported. 
 The operator has simply to prick the 
 bladder, and let in air, when the whole 
 column suddenly descends to the level of 
 the mercuiy in the basin (Fig. 4), 
 
 There is another form of demonstra- 
 tion, as follows ; but it is not so simple 
 as the last, inasmuch as it requires the 
 aid of the air-pump : Fig. 5. 
 
 A is a glass air-pump receiver (Fig. 5), 
 through the neck of which the tube B passes, inclosed in an exterior tube, which may 
 
INFLUENCE OF ELEVATION. 17 
 
 be regarded as a prolongation of the receiver. C is the plate of an air-pump, on which 
 the receiving jar is laid. 
 
 Let us assume that the barometer tube has been filled with mercury as before, 
 plunged in the basin containing mercury underneath, the receiver A slipped over it, 
 and finally extension made by the long glass sheath. Let us assume now that the air- 
 pump is worked, and exhaustion gradually effected. Under these circumstances, the 
 mercurial columns will be seen to fall, and it will fall by jerks, each jerk corresponding 
 with a stroke of the pump-handle. Hence, by the two preceding experiments it is de- 
 monstrated that to atmospheric pressure, and atmospheric pressure alone, is the varia- 
 tion mercurial column due. It is also proved , inferentially, that the fluctuations of height 
 witnessed from day to day in a barometric column are due to variations of atmospheric 
 pressure. It appears, then, that by the barometer we actually weigh a column of 
 atmospheric air equal in length to the whole elevation of the atmosphere, whatever that 
 may be, and equal in area to the internal sectional area of the barometer tube. The 
 barometer, however, gives us no information in terms of cubic dimensions of the baro- 
 metric mercury. Thus, supposing the barometric tube employed to have a sectional 
 area of one square inch, and supposing the mercurial column to be thirty inches high, 
 then we should be correct in averring the pressure of an atmospheric column a square 
 inch in sectional area, and extending the whole height of the atmosphere, to be equal 
 to the weight of thirty cubic inches of mercury. Now the weight of thirty cubic 
 inches of mercuiy will be about fifteen pounds ; hence the atmosphere is said to exert 
 a mean pressure of fifteen pounds on every square inch at the level of the sea. 
 
 Influence of Elevation on the Barometric Column. It will be evident, on 
 reflection, that the atmospheric pressure must decrease with every increment of elevation 
 above the level of the sea. Founded on this principle, the barometer is frequently em- 
 ployed to determine the height of mountains ; and, reversing the application, itmight also 
 be employed to determine the depth of mines and wells. At the elevation of about thirty- 
 six miles, the pressure of the atmosphere cannot amount to more than O'OOl of an inch 
 of the barometric column ; and conversely, at a depth of about sixty-six miles, the 
 density of the atmosphere would be about 100,000 times greater than at the surface of 
 the earth, being six times more than the density of gold and platinum ; so that, sup- 
 posing either of these metals to be plunged into such an atmosphere, they would 
 actually float. 
 
 If the atmospheric density were uniform, a barometric fall of one inch would 
 correspond to 11,065 inches, or 992 feet of air. But it is not uniform, as we have 
 already seen ; therefore such deviation from the standard of uniformity, and indeed 
 many other circumstances, must be taken cognizance of before we are enabled to 
 employ the barometer as an indicator of atmospheric elevations. 
 
 The law of Marriotte teaches that the dilation of a, volume of air is propor- 
 tional to its density, so long as the temperature to which it is exposed is constant ; 
 whence it follows, that the density of the atmosphere diminishes from below upwards 
 in geometrical progression. Reasoning on this basis, it appears that, assuming any 
 particular elevation above the point of observation, its numeral exponent may be 
 regarded as the logarithm of the density of the lowest atmospheric layer, or, in other 
 words, of the barometric column. A consideration of the mutual relation subsisting 
 between numbers and their logarithms will render this evident, for logarithms are 
 nothing more than numbers increasing by arithmetical progression, corresponding to 
 other numbers, the increase of which is also in geometrical progression. Being possessed 
 
18 
 
 ELEVATION AS INDICATED BY THE BAROMETER. 
 
 of a table of logarithms expressing the densities of atmospheric layers, one might 
 calculate the height of a mountain by two observations made at two stations ; but the 
 same result may be arrived at by using a common table of logarithms, and multiplying 
 them by a constant factor. According to Deluc, the factor is 10,000. 
 
 Many other considerations have yet to be taken cognizance of before the barometer 
 can be accurately applied as a measure of mountain elevations they are temperature, 
 
 latitude, relation subsisting between the specific gravities of air, and mercury = r^., 
 
 1046 
 
 and dilation of mercury for each degree of the thermometer =0 -0001 for each degree of 
 Fahrenheit's scale. All these general considerations have been embraced in tables. 
 
 Although the circumstances necessary to be taken cognizance of when employing 
 the barometer as an indicator of mountain elevations are nunterous and complicated, 
 nevertheless the results obtained are susceptible of a considerable degree of accuracy. 
 Subjoined is a table of comparative results between trigonometric and barometric 
 observations. The difference, it will be seen, is only trifling. 
 
 COMPARISON OF TRIGONOMETRIC AND BAROMETRIC MEASUREMENTS OF 
 MOUNTAIN HEIGHTS. 
 
 Observers. 
 
 Place of Observation. 
 
 Latitude. 
 
 j Trigonome- ! Barome- 
 Longitude. | trie height i trie height 
 in feet. I in feet. 
 
 Webb . . Gunna Nath, Stockdalej 29 45' 56' 
 Borda . . Bagha Ling, Temple . ! 29 47' 30' 
 You Buch. Teneriffe . . .* . . 28 30' 0' 
 
 Ditto 
 
 Buckle .|Sugar-loaf,SierraLeone: 8 29' 40' 
 
 Sabine .1 Ditto ditto .| 
 
 Sabine JSpitzbergen . . . .j 73 0' 0' 
 
 6,828 
 7,646 
 
 79 30' 29" 
 80 2' 27" 
 1C' 13' 0" ! 12,188 
 
 13 D 15'' 0" j 2,493 
 10 6' 0" I i,*644 
 
 6,831 
 7,635 
 
 12,131 
 
 2,521 
 1,640 
 
 "When an approximate result is alone required, and the height is inconsiderable, a 
 fall of T \jth of an inch may be allowed for every ninety feet of elevation, or T ^o^th of 
 an inch for every foot. This rule suffices for the small differences of elevation at which 
 barometers are hung, and enables the observer to institute a comparison between them. 
 Correction for temperature must, however, not be omitted. The ratio of expansion for 
 mercury, glass, and brass the materials employed in the manufacture of barometers 
 will be pointed out hereafter ; meantime I may as well indicate that perhaps, after all, 
 it is well in practice to ignore these complex elements, and to consider y^^ths of an 
 inch as the allowance for mercurial expansion for every degree above 32, and vice versa. 
 Applying the above corrections for temperature and pressure to practice, let it be 
 required to know the altitude at the level of the sea and at 32 Fah. of 29 '565 inches 
 of mercurial column at a place 150 feet above the level of the sea, and at temperature 
 55 Fah. Inches. 
 
 Actual height of mercurial column . . . . ... 29-565 
 
 3 23 69 
 
 Deduct for 23 of temperature above 32 
 
 1000 
 
 1000 
 
 369 
 
 Altitude of mercury 
 Add for elevation -001 X 
 
 150 
 
 Altitude at level of sea, at temperature 32 Fah. 
 
 29-496 
 150 
 
 29-649 
 
IMPROVEMENTS OF THE BAROMETER. 
 
 19 
 
 The reader will ren ember that the previous remarks have reference to the barometer 
 as affected by air at rest, this being the simplest atmospheric condition which, theory can 
 assume. Hereafter we shall* discover that the barometer, when influenced by the 
 atmosphere in motion by winds, in other words is subject to influences from that 
 cause. 
 
 Further Improvements of t/<e Barometer. The barometer in its simplest form, as 
 already described, is a more perfect instrument than many in which 
 simplicity of form is departed from, in deference to portability ; never- 
 theless it is not quite correct. To be absolutely correct it is indis- 
 pensable that the mercurial level in the basin should bear a constant 
 ratio to the mercury remaining in the tube, a condition which evidently 
 cannot be obtained in the instrument just described. In proportion as 
 mercury descends out of the inverted tube, the level of the mercury in 
 the basin will be elevated, and to the extent of such elevation the 
 indications of the instrument will be prejudiced. Various means 
 are had recourse to for lessening, or absolutely removing, this evil. 
 It may be lessened by increasing the width of the basin to such extent 
 that the ratio of elevation of the mercurial surface may be so greatly 
 diminished that it will practically cease to impart errors. It may be 
 absolutely removed by one of two devices. One consists in mounting 
 the receiving basin on a screw, which, by elevation and depression, 
 regulates the quicksilver to any desired level. Such is the contrivance 
 of M. Fortin, whose construction of a barometer is here subjoined 
 (Fig. 6) ; but more usual is it to attach to the barometer a long scale 
 having a slide motion, so that the lower end or commencement of the scale may be 
 
 made to coincide with the level of the mercury in 
 the basin. Practically, however, the basin is 
 usually dispensed with, the reservoir for mercury 
 being a mere extension of the barometric tube, in 
 some cases bulbed, and in others quite plain. Both 
 these forms of construction are annexed (Figs. 7, 8). 
 The Weather-glass. The primary and only 
 direct function of the barometer is as I have 
 described. Its mercurial column indicates, by rising 
 and falling, the varying weight of the superin- 
 cumbent atmosphere. Very frequently this direct 
 function is taken no account of; variation in the 
 state of the weather being all which the observer 
 desires to make himself acquainted with. Sub- 
 serviently to this intention, all direct rise and fall 
 of the barometric column is lost sight of, and 
 the indications of a dial-plate with moveable hand 
 
 Fig. C. 
 
 Fig. 8. 
 
 Fig. 7. 
 
 substituted. Such an instrument is termed the dial weather-glass, the construction 
 of which is as follows : T (Fig. 9) is a barometer tube, W is a small float attached 
 to one end of a cord, the other extremity of which is attached to a small weight, X. 
 From this arrangement it will be seen that every rise and fall of the real barometric 
 column in the long arm of the tube, will correspond with a parallel fall and rise 
 of the mercurial column in, the short arm. It will bo seen, moreover, how the 
 
20 
 
 THE WHEELED BAROMETER. 
 
 small float, w, is raised and lowered, how the pulley M will be caused to revolve, 
 and the index-hand [to traverse the dial plate of the instrument. The exterior of 
 the wheel barometer is represented in Fig. 10. 
 
 I need scarcely indicate that 
 the wheel barometer is consi- 
 dered, barometrically, a very im- 
 perfect instrument. Not only 
 is the varying] ratio between 
 the mercurial column and "the 
 level of the mercury in the re- 
 servoir here ajnecessity, the very 
 index motion depending upon 
 it ; but the presence of the float, 
 w, tends also to embarrass the 
 free ascent and descent of the 
 column of barometric mercury. 
 
 Manufacture of a Correct 
 Barometer. Not to render the 
 
 principles concerned in the barometer complex, I have 
 hitherto assumed that the act of charging a tube with 
 mercury is simple and free from difficulties. Practically 
 this is not so ; many precautions have to be taken, other- 
 wise the resulting barometer will be anything but correct. 
 Firstly, the tube selected must not be too small; many 
 instruments are rendered incorrect owing to neglect of this 
 precaution. The internal diameter of the barometer tube 
 should scarcely be less than a quarter of an inch; it 
 may be even more with advantage. A small barometer tube 
 prejudices the correctness of [the instrument in two re- 
 spects. Firstly, the variations of expansion and contrac- 
 tion of the mercury, due to variations of temperature, are 
 more considerable ; secondly, the motion of the quicksilver 
 up and down is impeded by friction against the glass. 
 The Mercury must be Pure and deprived of Atmospheric Air. Mercury, as commonly 
 existing, is generally impure. It contains uncertain quantities of tin, lead, and some- 
 times zinc. Of course these admixtures damage the mercury for barometric purposes. 
 The observer desires to read off his atmospheric pressures in terms of inches of mercury, 
 not in terms of inches of a mercurial compound. It is indispensable, therefore, that 
 the impurities be discharged or extracted. Various processes are used to this end, but 
 the process usually followed by makers of barometers consists in agitating the mercury 
 to be purified with dilute nitric acid, which gradually dissolves out the extraneous 
 metal and leaves the mercury pure. 
 
 Far greater difficulties are encountered in discharging atmospheric air from the mer- 
 cury employed. This is accomplished by boiling the mercury after it has been poured 
 into the tube. The operation requires great delicacy, and the instrument is frequently 
 broken in the operation. 
 
 Method of Heading off Barometric Indications correctly. It is not possible to read off 
 by referring to a common scale of inches and parts of inches the various small eleva- 
 
 Fig. 10. 
 
CONTRIVANCES FOR MEASURING SMALL DIVISIONS. 
 
 21 
 
 32 
 
 31 
 
 30 
 
 29 
 
 23 
 
 27 
 
 tions and depressions of a barometric column. Two methods are had recourse to for 
 obviating this difficulty : one is the diagonal tube, a contrivance altogether peculiar to 
 the barometer ; the other is the vernier or no- 
 nius scale, employed for the general purposes of 
 facilitating the reading of minute scale divi- 
 sions. 
 
 The diagonal- tubed barometer is represented 
 by the accompanying diagram (Fig. 11). 
 
 Tlie Nonius or Vernier Scale. This is an in- 
 genious contrivance for measuring small linear 
 divisions by means of larger divisions, and con- 
 sequently more easily recognizable by the eye 
 than larger divisions would be. Thus, for ex- 
 ample, by means of a vernier graduated in divi- 
 sions of an inch and one-ninth we can read 
 off tenths of an inch, as will be seen by re- 
 ference to Fig. 12. 
 
 Let A be a scale sliding in proximity to B. 
 Let each of the divisions on B be = one inch) 
 and each of the divisions on A m one inch and 
 one-ninth. From these considerations it follows 
 that nine divisions on A are equal to ten divisions on B. Directing 
 the eye to the upper limit of A, it will be seen that its edge corres- 
 ponds to thirty inches, and something more on B. In this case the 
 observer would have no difficulty in recognizing the amount over 
 and above thirty inches on B to be equal to four-tenths of an 
 inch. Our scale divisions are so large that a vernier scale is not 
 required for conveying that information. But assume the tenths 
 division between 30 and 31 to be obliterated, still we should be able 
 to discover the overplus beyond 30 inches to be four-tenths by means 
 of the vernier scale A, inasmuch as the number of tenths will be 
 equal to the number of whole parts on the 
 vernier scale A above the first line of coin- 
 cidence between it and the scale B. Now the 
 line of coincidence in question is at 26, count- 
 ing upwards, from which, to the extremity, 
 we have four divisions, which indicate a 
 fraction of four-tenths of an inch over and 
 above 30 inches. 
 
 Fig. 11. 
 
 A _ 
 
 _l 
 
 Fig. 12. 
 
 Correction of the Barometric Column for Capillarity. If mer- 
 cury be poured into a glass vessel, it will not furnish a perfectly 
 level surface, but will be elevated, as in the accompanying 
 diagram (Fig. 13) ; or, in the language of philosophy, it will 
 constitute a meniscus ; and if the line V be dropped perpen- 
 dicular to the line B, joining the two corners of the meniscus, 
 the line V will constitute, in the language of trigonometry, the 
 versed sine of the meniscus. The convexity of the meniscoid 
 surface of mercury will vary in proportion as the diameter of the tube or other vessel 
 
 Fig. 13. 
 
22 
 
 IMPERFECTION OF BAROMETERS. 
 
 containing the mercury yaries. Hence the allowance to be made for diminution oi 
 the height of a mercurial column, owing to capillarity, is determined by two con- 
 siderations the diameter of the tube, and the length of the versed sine of the cor- 
 responding meniscus. 
 
 The necessity for such allowance does not apply, as will hereafter be seen, to 
 barometers of every form ; neither is it imperative, so long as the indications of 
 one barometer are to be compared amongst themselves ; but it is indispensable if 
 
 we would compare the indi- 
 cations of barometers with 
 
 each other. 
 
 First, let us consider the 
 
 cases to which the correction 
 
 for capillarity does not apply. 
 
 It does not apply to any baro- 
 meter, the reservoir of which 
 
 constitutes part of the tube 
 
 itself. The accompanying 
 
 diagram (Fig. 14) represents 
 
 a barometer of great tubular 
 
 diameter. Two meniscoid 
 
 surfaces of mercury are there 
 
 apparent one at A, another 
 
 at B. Now it is evident that 
 Fig. 14. t k e columnar interference at 
 
 Fig. 15. 
 
 A will be exactly equal to the columnar interference at B, provided the tubular diameter be 
 equal throughout. Hence, whether the degree of elevation be counted from the summit 
 of the lower to the summit of the upper meniscus, or from the base line joining the two 
 corners of the lower meniscus to the base line joining the two corners of the upper one, 
 the two resulting columnar estimations will be strictly equal and comparable. But it 
 is different when the observer has to do with barometers constructed on the original, or 
 Toricellian, plan of a separate reservoir ; in this case the meniscoid elevation of the 
 mercury in the reservoir is practically ignored, being so inconsiderable that it amounts 
 practically to nothing, as will be seen by reference to the accompanying diagram 
 (Fig. 15). 
 
 Annexed is a small table of depressions due to capillarity. Larger and more elabo- 
 rate tables have been calculated, but the one given will suffice for, perhaps, every 
 occasion. The meteorological student cannot have the fact too strongly impressed upon 
 him, however, that the barometer is confessedly a very imperfect instrument ; therefore 
 correct results from barometric observations are to be looked for as the mean resultant 
 of a number of accumulated observations, rather than from any elaborate mathematical 
 tabulations of principles, correct enough in themselves theoretically, but which do not 
 admit of being realized in practice. 
 
 In the record of some barometrical observations, we find cognizance taken of 
 thousandths of inches. Until the errors which attach to the principle of the barometer 
 greatly diminish, a record of thousandth of inches cannot be otherwise regarded than as 
 a kind of philosophic affectation. 
 
THE11MAL EXPANSION. 
 
 23 
 
 DEPKESSTON DUE TO CAPILLARY ACTION. 
 
 Diameter. 
 Hundredth? 
 of inches. 
 
 Dcpr 
 Ivory. 
 
 ession in Decimals of 
 Young. 
 
 Inch. 
 Laplace. 
 
 5 
 
 2949 
 
 2964 
 
 
 10 
 
 1404 
 
 1424 
 
 1394 
 
 15 
 
 0865 
 
 0880 
 
 0854 
 
 20 
 
 0583 
 
 0589 
 
 0580 
 
 25 
 
 0409 
 
 0404 
 
 0412 
 
 30 
 
 ;-0293 
 
 0280 
 
 0296 
 
 35 
 
 [0212 
 
 0196 
 
 0216 
 
 40 
 
 0154 
 
 0139 
 
 0159 
 
 45 
 
 0112 
 
 0100 
 
 0117 
 
 50 
 
 0082 
 
 0074 
 
 0087 
 
 60 
 
 0043 
 
 0045 
 
 0046 
 
 70 
 
 0023 
 
 .... 
 
 0024 
 
 80 
 
 0012 
 
 
 
 0013 
 
 Thermal Expansion. I have already adverted casually to the condition of 
 thermal expansion as an interfering cause in all estimations of true barometric columnar 
 heights; and, by anticipation, I have already furnished an approximative means of 
 making allowance for it. We will now proceed to examine more narrowly the function 
 of thermal expansion, which not only intimately concerns the barometer, but is the 
 fundamental basis of the thermometer, and besides it enters as an element into so many 
 meteorologic calculations, that a thorough investigation of its laws cannot be omitted. 
 I shall, therefore, embody the laws of thermal expansion in a few propositions for suc- 
 cessive demonstration. 
 
 Heat may be regarded in the two senses of signifying temperature, or that sort of 
 heat which is recognizable to the sense of touch, and which affects the thermometer ; 
 and heat which is devoid of these manifestations, which neither creates the sensation 
 of warmth nor is amenable to thermometric demonstration. The former we may express 
 by the term sensible heat, and the second by the term latent or insensible heat. 
 
 On the supposition that all the functions of heat, sensible as well as insensible, are 
 referable to a real physical agent, the term caloric has commonly been applied as the 
 representative of such agent ; but though the term be in general use, it is perhaps ob- 
 jectionable modern science leading us to infer that the functions of heat are due to 
 a condition of matter, rather than to a separate agency. It is to evident heat, recog 
 nizable to the touch, that I shall now direct the reader's attention. 
 
 I. Heat affects the Volume of all Bodies. The general effect of heat on bodies is to 
 cause their expansion. So general is this rule, tl at we shall do well to consider it as 
 universal ; treating all deviations from it hereafter as so many exceptions. 
 
 II. The Volume of all Solids is increased by increase of Seat, This proposition is 
 demonstrated by so many instances commonly occurring, that specific experiments are 
 hardly required. The wheel-wright takes advantage of this property to bind tightly 
 together the wood-work of his carriage-wheels. He heats the annular tire, by which 
 he expands it ; he then slips the tire over and around the wood-work, and, allowing the 
 tire to cool, the wood- work is tightly braced together by an indomitable force. 
 
 Some years since the walls of the Conservatoire des Arts et Metiers, at Paris, were 
 
24 EXPANSION OF IRON BY HEAT. 
 
 found to be diverging from the perpendicular. They were restored to their original lines 
 by the following beautiful expedient : They were perforated transversely, copper bars 
 were thrust through the perforations, each bar at either extremity being supplied with 
 a nut and screw. Every alternate bar was now heated by means of a spirit-lamp flame ; 
 being heated the bars expanded, and the screw-nuts being now turned close up to the 
 wall on either side, the bars were allowed to cool. By cooling they contracted, pulled 
 the walls to some extent together, leaving the ends of the unheated bars protruding ; 
 tlicir screw-nuts were now turned close up to the wall on either side, and the heating 
 process repeated. Thus little by little the walls were restored to their original position. 
 An exemplification of the expansion of iron by heat sometimes occurs to the laun- 
 dress. Occasionally she is surprised to find that the heater of her Italian iron will not 
 
 ^frv enter its correspond- 
 
 |j '^^MiiHiiir" 11 " GL H^^^t ing sheath when red 
 
 hot, though it enters 
 readily enough when 
 cold. This is attri- 
 butable, as the stu- 
 dent will perceive, 
 to the effect of ther- 
 mal expansion on the 
 Fi ' 16 ' iron (Fig. 16). 
 
 Thousands of familiar instances might be cited, all illustrative of the same pro- 
 perty. I shall leave their consideration to the reader, concluding my remarks on this 
 part of the subject by bringing before his notice a common lecture experiment, illustra- 
 tive of solid thermal expansion. Let G (Fig. 17) be a 
 guage, into which the metallic bar, A, accurately fits, 
 whilst cold; it will be found that when the bar A is 
 heated moderate heating will suffice, such as may be 
 accomplished by means of the flame of a spirit-lamp, Fig. 17. 
 
 or a basin of hot water the heated bar will no longer fit into the guage. In this 
 way the student may demonstrate the fact that each different solid possesses its own 
 definite rate of expansion for equal degrees of temperature. 
 
 Investigations of the law regulating the thermal expansion of bodies under every 
 condition are attended with extreme difficulty. They havs been conducted by 
 Regnault, Rudberg, and others, with great industry and much success ; but, as in most 
 cases where the investigation of natural phenomena through long ranges are concerned, 
 the results are merely approximate. To give the reader a general notion of 
 one of these difficulties, let it be assumed that A B C D stand for successive 
 equal intervals of thermometric graduation. Let it be assumed that the rate 
 of expansion of a substance from A to D be known to be equal to a quantity 
 expressed by Y. It by no means, however, follows nor do philosophers believe that 
 because the rate of expansion of the body between A and D is equal to Y, therefore 
 
 Y 
 the rate of expansion of the same body between A and B is equal to y 
 
 Further consideration of this matter may, however, be omitted in a treatise on 
 meteorology. We merely want to be acquainted with approximate results of the law, 
 in order to allow for practical discrepancies between the apparent and the actual in- 
 dications of the instruments employed in the course of our researches. 
 
EXPANSIVE INFLUENCE OF HEAT. 
 
 25 
 
 To convey a general notion of this kind of knowledge to the meteorological 
 observer, the student's attention may be directed to the fact that, supposing a barometer- 
 scale to be made of brass, and supposing the tube to be filled as usual with mercury, then 
 the amount of expansion of mercury by heat, and for which allowance has to be made, 
 
 will be determined by the ratio , if M stands far the co-efficient ef expansion of 
 
 mercury, and B with the co-efficient of expansion of brass. By the term co-efficient of 
 expansion is meant the number indicating the amount of expansion peculiar to any 
 body for given ranges of temperature. 
 
 III. The Volume of all Liquids is increased by increase of Heat. Investigations prose- 
 cuted for demonstrating this law are attended with a difficulty which does not apply to the 
 previous case. Liquids require vessels to hold them, and these vessels are themselves 
 amenable to expansion. This difficulty has been very ingeniously avoided by MM. 
 Petit and Dulong, who determined the expansion of liquids by a method founded upon 
 the well-known hydrostatic principle, that the vertical heights of two fluids communi- 
 cating by a horizontal tube are in inverse ratio to their densities. The accompanying 
 apparatus (Fig. 18) was employed in their experiments. A, B, C, D is a tube bent 
 twice at right angles, and 
 
 enlarged at either extremity ; A D 
 
 the two vertical tubes are 
 connected inferiorly by a 
 horizontal tube of exceed- 
 ingly fine bore. By virtue 
 of the hydrostatic law just 
 mentioned, it follows that if 
 any liquid of homogeneous 
 density be poured into the 
 vertical leg of one side, it 
 will rise to a corresponding 
 elevation in the vertical leg of the other ; and if the fluid in one vertical leg be now 
 heated, and consequently expanded, its height will be in excess of the columnar height 
 of the other by a definite quantity. By an easy train of mathematical reasoning, the 
 expansion due to heat can be deduced from a consideration of the different levels and 
 the different temperatures of the two vertical tubes. Our diagram represents each 
 vertical tube surrounded with a cylindrical vessel. These vessels are for the purpose 
 of commanding variations of temperature, one tube being filled with ice, whilst the 
 other is filled with hot water. 
 
 By an experiment of this kind, the co-efficient of thermal expansion of mercury 
 from to 100 of the centigrade scale is -$-5 ; whence, assuming its rate of expansion 
 equal throughout r the expansion for every centigrade degree will be 3^5^, or for every 
 degree of Fahrenheit -^Vo > m decimals r= O'OOOlOl ; or, expressed in round numbers, 
 one ten-thousandth part of its bulk. 
 
 IV. The Volume of all Gases is increased by increase of If eat. Though the considera- 
 tion of this law is not related, like the two preceding, to the construction of the baro- 
 meter, it is intimately connected with the functions of that instrument, more especially 
 as regards its application to the measuring of elevations; we shall do well, therefore, to 
 consider it at once. 
 
 It has already been remarked, that the rate of expansion of all solid bodies for give n 
 
 Fig. 18. 
 
26 THE ATMOSPHERE PRACTICALLY CONSIDERED. 
 
 increments of temperature is various ; and a similar remark applies to liquids. The 
 rate of expansion of gases was first closely investigated by the immortal Dalton, who 
 arrived at the conclusion that all gases were equally affected by equal increments of 
 heat, expanding to ^^th parts of their volumes at 32 Fah. for each of the 180, 
 between 32 Fah. and 212 Fah. As regards temperature above 212 Fah. and below 
 32 Fah., it was imagined by Dalton that the same law of expansion held good. Various 
 theoretical reasons exist of a nature to create a doubt as to the large generalization of 
 Dalton, and these doubts have been fully substantiated by M. Regnault. He finds that 
 all gases do not dilate to the same extent between equal limits of temperature, neither 
 is the dilatation of the same gas between the same limits independent of its primitive 
 density. These are interesting facts : they prove that to assume one co-efficient of dila- 
 tation for all gases is obviously incorrect ; nevertheless his experiments go to prove that 
 such an universal gaseous co-efficient of dilatation may be adopted for convenience, without 
 appreciable error, within the theoretical limits of ordinary experiment. We must not, 
 however, continue to adopt -^^th as our working gaseous co-efficient of dilatation for 
 every degree of Fah. between 32 and 212, nor 3^-3-, as subsequently adopted by MM. 
 Petit and Dulong, but -^-^y. 
 
 The Atmosphere Actually ez Practically Considered. We have hitherto 
 regarded the atmosphere as a compound or mixture of nitrogen and oxygen gases only, 
 but the reader need not be informed that such an atmosphere is altogether theoretical. 
 Besides nitrogen and oxygen gases, there always exists a portion of carbonic acid, also 
 aqueous vapour, either visible or invisible. These are invariable components of the 
 atmosphere, indispensable to its functions, adapting it to the purpose of this world's 
 economy. The atmosphere contains also other materials, the results of local operations 
 in nature no less than the operations of man. As the subject of our present investi- 
 gations, let us consider the atmosphere as a mixture of the theoretical atmosphere plus 
 aqueous moisture and carbonic acid. 
 
 Limits of the Atmosphere. It follows, from a consideration of the laws of elasticity, 
 that the atmosphere must vary in density for every difference of elevation. The atmos- 
 pheric layer nearest to the earth must be pressed upon by the superincumbent atmosphere 
 above it ; whence the deduction follows, that when we speak of 100 cubic inches, or any 
 definite measure of the atmosphere, weighing a certain number of grains, certain condi- 
 tions and limitations are implied. Some of these have reference to the composition of 
 the atmosphere chemically considered, others have reference to the atmosphere merely 
 regarded as an elastic medium. To the latter consideration alone the reader's attention 
 will be now directed. 
 
 Seeing that the atmosphere, in accordance with the laws of elasticity, must neces- 
 sarily expand the higher we ascend above the normal level of the surface of our globe or, 
 in other words, the level of the sea the first question which arises is this To what extent 
 do the atmospheric limits reach ? does the expansion go on ad infinitum to the farthest 
 realms of space, or are these limits definite ? and if definite, what is the cause, or what 
 are the causes, of limitation ? Two theories have been adopted in reference to this 
 question. According to one theory the atmosphere is illimitable; according to the 
 other it is limited. Some of the arguments for and against I shall now proceed to 
 give. 
 
 If the atmosphere be really illimitable, let us see what should follow, to be in ac- 
 cordant with recognized laws, to which all ponderable matter, or matter subject 
 to gravitating influences, is amenable. Gravitation being directly as the mnsa of 
 
HEIGHT OF THE ATMOSPHERE. 
 
 27 
 
 gravitating bodies, it should follow that, were the atmosphere illimitable, each of the 
 heavenly bodies should be surrounded with an atmosphere proportionate to its mass 
 an assumption which astronomy disproves. Thus astronomy furnishes strong proof 
 in favour of the finite extension of the atmosphere. A consideration of the laws of 
 the atomic constitution of matter lends further, and perhaps stronger, proofs. 
 Chemistry is full of evidence in favour of the atomic constitution of matter ; 
 or, in other words, is full of proofs that all material substances are composed 
 of molecules or particles ; to which extent they can alone be divided, and not 
 beyond. The mathematical reasoning which has been employed against this 
 atomic theory, as chemists term it, is specious at a first glance, but really untenable. 
 To argue that the theoretical space occupied by any material particle may be 
 supposed capable of division, and sub-division, ad infinitum, is really not to the 
 point. Space is one thing, the matter occupying such space is another. The 
 mathematical objection touches the space alone, not the matter ; therefore the chemical 
 evidence in favour of the atomic constitution of matter is unanswered, and is 
 apparently unanswerable. Let us now regard the consequences of this assumption as 
 it relates to atmospheric air. The late Dr. Wollaston was the first person who 
 directed attention to the limitation which should theoretically be imposed on atmos- 
 pheric expansion, supposing the assumption of its atomic constitution to be correct. 
 The atmosphere, like other ponderable 
 material bodies, is subject to gravitating 
 influences, thus imparting a tendency of 
 descent towards the earth. On the other 
 hand, the atmosphere being elastic, its 
 particles are mutually separated from 
 each other by the operation of this 
 force. Now, assuming the atomic con- 
 stitution of the atmosphere to hold 
 good, there must be some finite distance 
 from the earth's surface, at which the 
 force of elasticity would be counter- 
 balanced by the force of gravitation, 
 which distance would correspond with 
 the farthest limits of the atmosphere. 
 The mean distance of this limit is as- 
 sumed to be about forty-five miles, 
 though it must differ for every point Fig. 19. 
 
 north and south, being greatest over the equator, and least at either pole, as indicated 
 by the accompanying diagram (Fig. 119), 
 
 The reason of it being greatest at the equator is immediately referable to the 
 diurnal rotation of our globe on its axis, thus generating a centrifugal force, which 
 has determined the oblate spheroidal form of the earth. Material bodies will be 
 affected by this centrifugal force, c&teris paribus, directly as their attenuation, whence 
 it follows that the atmosphere, being a gas, must be affected to an extreme degree. It 
 will be sufficiently evident, however, that the atmosphere is only affected by the 
 earth's diurnal rotation intermediately, or by friction, the velocity of motion imparted 
 to it by the earth being less considerable than the velocity of the earth itself. "We 
 shall hereafter fiad, when we come to treat of the trade winds, that these permanent 
 
28 OPERATION OF WEIGHING A GAS. 
 
 aerial currents are not altogether referable to atmospheric motion, hut in some degree 
 depend upon the diurnal motion of the earth, 
 
 Determination of the Weight of a given Volume of Atmospheric Air. Nothing can 
 he more easy than the theoretical means of solving this prohlem ; and though certain 
 practical difficulties do interpose, we had better, for the sake of theoretical explanation, 
 consider them absent. 
 
 The case under consideration is g-neral, not specific. The determination- of the 
 weight of a given volume of atmospheric air is accomplished similarly to the deter- 
 mination of the weight of a given volume of any other gas ; nor does it differ in prin- 
 ciple from the process had recourse to when solids or fluids are concerned. 
 
 If, to take the simplest practical case, without reference to cohesive state, it were 
 desired to ascertain the weight of a given bulk of copper or brass say one hundred 
 cubic inches the operator's first care would be to obtain a solid of copper or brass having 
 these cubic dimensions, which having been obtained, no vessel for the purpose of 
 weighing it in would be necessary. This is the simplest case of bulk-weighing which 
 can occur ; nevertheless, a condition has to be regarded which the superficial observer 
 might forget, or perhaps not be aware of. The copper or brass alters its dimensions 
 for every variation of temperature. If heated r it will expand ; if cooled, it will con- 
 tract ' T so that, practically, under no two degrees of temperature has the mass of brass 
 or copper the same size. Practically, so long as solids are concerned, these variations of 
 size, dependent upon variations of temperature, are not of much consequence in ordinary 
 operations of weighing. It is necessary, however, to estimate them for other reasons, and 
 to tabulate these variations. We shall have occasion to refer to this tabulation hereafter. 
 
 Let it now be assumed that the problem before us is to determine the weight of a 
 given bulk of liquid say water. In this case we must have recourse to some vessel 
 of capacity for the purpose of holding the water to be weighed, and further elements 
 of complexity, in addition to that of temperature, are introduced. Firstly, the vessel 
 employed has its own laws of expansion and contraction ; secondly, the water, when 
 poured into the vessel, will not have a perfectly flat surface ; so that, except the mouth 
 of the vessel be small, an error of considerable magnitude will be imparted; neither 
 must it be too small, or the functions of capillary attraction will come into play, and 
 the water will present a higher level than properly belongs to it. 
 
 The chief source of inaccuracy, however, which the operator meets with in operating 
 upon solids and liquids, is that dependent on the variations in bulk referable to thermal 
 increments and decrements ; any alteration due to variations in pressure being practi- 
 cally ignored. So far as atmospheric pressure is concerned, which is the only kind of 
 pressure we need take cognizance of as affecting our subject, it exercises so little influence 
 on the dimensions of solids and liquids, that we may put it altogether out of considera- 
 tion. Far different, however, is it when gases- are concerned. Their attenuation and 
 elasticity are such, that variations of atmospheric pressure exercise the most powerful 
 influence over them ; so that the degree of atmospheric pressure operating at the time 
 of the experiment is, at least, of equal consequence with the degree of temperature. 
 
 "We are now in a position to trace the theoretical steps necessary to be followed in 
 effecting the weight of a given volume of any gas. 
 
 Necessarily, as in the previous case, a vessel of capacity is required ; but, inasmuch 
 as a gas does not admit of being poured into the vessel like water, some practical expe- 
 dient for accomplishing this must be devised. "We had better omit all consideration of 
 this for the present, and assume that the vessel (which will be a globe or flask having a 
 
OPERATION OF WEIGHING A GAS. 29 
 
 neck with stop-cock attached) already filled with the gas to be weighed, at a definite 
 temperature and definite pressure. This accomplished, the operator has only to weigh 
 his flask full of gas, deduct the weight of the flask from the total weight of flask and 
 gas, and the result is gained. Practically, however, many points have to be considered. 
 
 The-Gas must be pure.- "Whether gas, or liquid, or solid, any body, the weight o* 
 which we desire to know, must be pure ; but this precaution applies in the highest 
 degree to gases. 
 
 The Gas must be either dry or its amount of moisture must be definite. The property 
 which gases have of taking up vapours, especially aqueous vapour, is well known ; and 
 it will readily be seen that to the extent of the presence of such vapour will the 
 weight of a given bulk of gas and vapour mixed fluctuate. One of two processes has 
 now to be followed : either the gas must be artificially dried by exposure to one of the 
 hygroscopic bodies used by chemists for that purpose ; or, it mus-t be saturated with 
 moisture to the fullest capacity at some given temperature. 
 
 The further steps of the calculation are based upon a consideration of the ratio 
 between the apecific gravity of steam or vapour, and the specific gravity of dry gas ; 
 and, lastly, the amount of vapour which a gas absorbs at a definite temperature. 
 According to Gay Luesac, the ratio between the specific gravity of aqueous vapour and 
 air under similar conditions of temperature and pressure is 
 0-620 = vapour 
 
 1 = atmospheric air* 
 
 The amount of aqueous vapour which an unit volume of gas can absorb at given 
 temperatures, -has been ascertained and tabulated. 
 
 Applying this knowledge to practice, let us assume that 100 cubic inches of moist 
 air, at 60 Fah. and 30 inches barometer, weigh 31 grains, it is required to know how- 
 much 100 cubic inches of dry air would weigh. 
 
 "We begin by turning to a table indicating the quantity of vapour present in a gas 
 saturated with vapour at any given temperature. Dalton's table gives this quantity 
 for 60 Fah. as 0'524. We next perform the following calculation 
 
 30 : 0-524 : : 100 : 1747 = the volume of vapour in 100 cubic inches of moist 
 air at 60 Fah. 
 
 And as 100 cubic inches of aqueous vapour weigh 19 grains, 1'747 cubic inches weigh 
 0-3 368th of a grain. 
 
 Weight of 100 cubic inches of moist air . 31 
 
 Deduct . 0-3368 
 
 30-6632 
 
 Therefore the weight of 100 1*747 = 98-253 cubic inches of dry air 30'632 
 grains, and 98-253 : 306632 :: 100 : 31-214 grains. 
 
 Whence it follows, according to the foregoing calculation, that the weight of 100 
 cubic inches of dry air at 30 inches barometer and 60 Fah. is 3T214 grains. 
 
 Such, then, are the practical operations by which the weight of a known volume of 
 gas is determined. I have chosen atmospheric air as the subject of illustration, but the 
 processes arc identical whatever the gas may be. 
 
 Although the result of the calculation just effected gives 3T214 grains as the weight 
 of 100 cubic inches of atmospheric air at 30 inches barometer and 60 Fah., and although 
 the number may be accepted for all purposes of meteorologic calculation, nevertheless it 
 
! 
 
 30 THE THERMOMETER. 
 
 must not be viewed in an implicit sense. In point of fact, the exact weight of atmos- 
 pheric air is not yet made out. Probably the determinations of MM. Dumas and Bous- 
 singault are most reliable. According to their experiments 1 litre or 61*02791 cubic 
 inches of air at centigrade and 076 metres barometer weigh 20-065 grains ; whence it- 
 follows that 100 cubic inches, under the same conditions, must weigh 31-093 grains at 
 60 Fah. 
 
 Barometric Pressure at the time of Experiment must be an Element of the Calculation. 
 A consideration of the laws of pressure, as influencing the volume of gases and vapours, 
 will have made the student aware that due allowance requires to be made for variations 
 referable to this cause. Now we are acquainted with amount of expansion and con- 
 traction dependent on variations of pressure. This information is conveyed by a study 
 of the law of Marriotte, which proves that a rule of proportion will furnish the informa- 
 tion required. Thus, for example, suppose we have 100 measures of any gas at a 
 pressure of 29 inches of the mercurial barometric column, and it is required to ascer- 
 tain what volume the gas will fill at 30 inches of the same this being the normal 
 pressure to which all calculations as to the volume of gases are reduced then we say 
 Aa 30 : 29 : : 100 : 96'66 
 
 In other words, the 100 volumes of gas under those conditions would contract into 
 96-66. 
 
 Temperature must be an Element of the Calculation. When it is considered to what 
 extent gases and vapour suffer expansion and contraction by variations of temperature, 
 the necessity of this calculation will be obvious. I have already explained the ratio of 
 thermal expansion to which gases and vapours are subjected by variations of tem- 
 perature. Practically, we have seen at page 466 that this ratio may be considered 
 identical for all of this class of bodies, and to be equal to y^th part of their bulk at 
 32 Fah. and 30 inches barometer for every degree of temperature between 32 Fah. and 
 212 Fah. Let us now apply this information to practice. 
 
 If the temperature of the gas be above 32 Fah., multiply its total volume by 491, 
 and divide the product by 491 plus the number of degrees that the temperature of 
 the gas exceeds 32 Fah. The numeral result of this operation gives us the correct 
 volume the gas in question would occupy at 32 Fah. 
 
 For example, we have 100 cubic inches of gas at 50 Fah. ; it is required to know 
 what volume this gas would occupy if raised to 60 Fah. : thus 
 
 = 96 ' 46 = the volume at 32 Fah< 
 
 And 96-46 + = 101-56 = the volume at 60. 
 
 Recapitulation and Deductions. It appears, then, that the operation of weighing a 
 gas demands in all cases that due allowance should be made for variations of heat and 
 of pressure ; and if the gas be charged with vapour, due allowance has to be made for 
 moisture also. 
 
 The Thermometer. In the course of our preceding investigations relative to 
 the atmosphere, we have seen that temperature is an important element. Not only are 
 the chemical functions of the atmosphere intimately related with temperature, but with- 
 out being able to take cognizance of the expansion produced by increments of heat, the 
 meteorologic observer is unable to comprehend some of the most ordinary physical con- 
 ditions of the atmosphere. "We have already seen that the general effect of heat, as 
 regards alteration of dimensions, is expansion. If the amount of tins expansion be 
 
THE DIFFERENTIAL THERMOMETER. 
 
 31 
 
 determined for any particular range between fixed points, and the linear extension or space 
 thus intercepted be divided into smaller spaces, each of these becomes a representative of 
 heat or temperature. Supposing we assume the temperature at which water freezes 
 to be our starting point or first limit, and the temperature at which water boils to be 
 our other limit ; and supposing, furthermore, we assume the space between the two to 
 be divided into any given number of parts say for example 180 then we may de- 
 scribe any third body the temperature of which is between the temperature of freezing 
 and the temperature of boiling, to be one, two, or any number of parts above the former or 
 below the latter. Thus, by applying these principles, we should have constituted the 
 thermometer or heat-measurer. If the rate of expansion of any one body for given incre- 
 ments of temperature were regular and well determined, there would be no theoretical 
 difficulties in the way of making a thermometer ; practical difficulties there would be, 
 but the hypothetical part of the task would be sufficiently easy. It so happens, however, 
 that the number of expansive agents capable of employment for this purpose is limited. 
 
 The earliest thermometer was that of Sanctorini. Its construction is represented in 
 the annexed diagram. A glass stem, open at one extremity, is terminated at the other 
 by a bulb (Fig. 20). Into the bulb and a portion of the tube is 
 poured a coloured fluid, which being done the stem is inverted 
 into the lower vessel. By virtue of the ordinary laws of hydro- 
 statics, the level of the fluid in the stem will remain constant 
 for every constant temperature ; but inasmuch as every incre- 
 ment of heat will cause the air contained in the bulb to ex- 
 pand, so will it necessarily cause the coloured fluid which latter 
 merely serves as an index to descend in such manner that were 
 the ratios of successive equal linear measures of descent equal, 
 the instrument would be a no less delicate measurer of varia- 
 tions of temperature than it is a delicate indicator of the same. 
 For certain reasons, now to be described, it is not a delicate 
 heat-measurer. Its successive lineal columnar measurements 
 are not comparable among themselves ; whence it follows that 
 the instrument is not a thermometer or heat-measurer, but a 
 thermoscope or heat-indicator. 
 
 Concerning the reasons wherefore the instrument just de- 
 scribed is not a perfect instrument, they readily admit of being 
 made evident. They are immediately referable to the fact that 
 the coloured fluid, which, according to the necessities of the ex- 
 periments should be dynamically passive, is really active. Its 
 activity, moreover, is a variable quantity. If the coloured 
 fluid were merely an index having no dynamical power of its 
 own, then the total increments of expansion and contraction of 
 the air contained in the bulb and part of the stem, would be 
 
 Fig. 20. 
 
 proportionate to the increments of heat and cold within so small a deficit of the truth 
 (see page 458) that the error need not enter into calculation ; but examination of the 
 structure of the instrument will show wherefore this cannot be so. Actually the total 
 expansion of the air in the bulb is the resultant of two forces the force of aerial elasticity 
 due to heat, and the force of pressure or downward tendency of the columnar liquid. 
 Inasmuch, therefore, as the columnar height of the liquid in question varies for every 
 temperature ; and inasmuch, moreover, as the rising and falling of the liquid in the reser- 
 
32 
 
 THERMOMETRIC SCALES. 
 
 voir or lower receiving vessel also confuses the result, the reason will be sufficiently 
 evident wherefore the heat- determining instrument of Sanctorini is not a correct 
 measurer of temperature. 
 
 The Differential Thermometer. Although various forms of air-thermometers are 
 occasionally used in conducting certain specific experiments, their use is rare. Almost 
 the only form of air-thermometer in frequent use is the differential thermometer, an 
 instrument the function of which is to determine the difference between the temperature 
 of any two adjacent bodies, or of the adjacent parts of any one body, without informing 
 the observer concerning the actual temperature of either. 
 
 The differential thermometer is represented by the accompanying diagram (Fig 21). 
 It consists of a glass tube having a bulbular expansion 
 at each extremity, and joined by a stem bent on itself 
 twice at right angles, so that the two bulbs look up- 
 wards. During the process of manufacturing this 
 instrument, whilst one of the bulbs was yet unclosed, 
 liquid was poured into the instrument just enough in 
 quantity to reach a little way up the vertical part of 
 the stems ; each of these vertical parts were now sup- 
 plied with a scale-division of equal lengths or degrees. 
 Looking at this instrument, the observer will readily 
 see that, supposing the tension or expansive force of 
 the atmosphere in each bulb to be equal, the index 
 :fluid in the stem will stand at precisely the same eleva- 
 tion in both vertical stems 4 but supposing the air in 
 one bulb to become heated to a higher degree than the 
 air in the other ; supposing, in other words, its expan- 
 sion to be greater, the corresponding columnar height 
 will be diminished, and necessarily the opposite columnar 
 height will be raised. Hence the difference between the two columnar heights will be that 
 of the temperature of the two bulbs expressed in equal parts of columnar measurement. 
 Thermometer Scales, and Ordinary Thermometers. The liquids ordinarily employed 
 in the manufacture of ihermometers are mercury and alcohol. The former is preferred 
 to all others, when its use is practicable, on account of the comparatively equal f xpon- 
 sion to which it is subject for equal grades of temperature. In the manufacture of 
 thermometers, however, intended to be employed for the measurement of temperatures 
 below the freezing point of mercury, that fluid is necessarily inapplicable. Spirit, or 
 alcohol, has never been frozen by the most intense cold yet produced, therefore it is 
 substituted for mercury on such occasions. I shall now proceed to detail the successive 
 steps in the manufacture and graduation of thermometers. 
 
 Under the head of Barometer the inconvenience was pointed out of using a glass 
 tube of small 'diameter, because of the interference resulting from the expansion and 
 contraction of mercury by heat. Now that which is a cause of embarrassment in the 
 barometer, is the function on which the action of the thermometer depends. It follows, 
 therefore, that in proportion as the bore of a thermometer tube is more small, so is the 
 resulting instruments more delicate, because greater linear increments of expansion 
 will be generated for given amounts of temperature. Necessarily, however, it happens 
 in practice that if the diameter of the tube and the diameter of the mercurial column 
 contained in the tube be s-maller than certain limits, it is difficult to be seen. It is 
 
 Fig. 21. 
 
MANUFACTURE OF THERMOMETERS. 
 
 33 
 
 hardly necessary to indicate, moreover, that the length of linear expansion may be 
 increased to any given limit by increasing the dimensions of the corresponding bulb, 
 or mercurial reservoir. Practice alone can determine the proper relation which should 
 subsist between the bore of a thermometer tube and its corresponding bulb. 
 
 The following directions for the manufacture of a thermometer are not intended to 
 cause the meteorological student to usurp the functions of the mathematical instru- 
 ment-maker ; on the contrary, they are intended to make known to him the defects 
 which he should look for in a thermometer, and which, if discovered, should cause 
 the thermometer to be rejected. 
 
 The tube must be equal in bore throughout. If the bore or diameter of a thermometer 
 tube be not equal throughout, it is evident that the amount of linear expansion cannot 
 be equal, and that the instrument will be absolutely worthless. A very easy means of 
 guaging this equality is the following; Having selected a piece of thermometer tube 
 open at both ends, tie on to one extremity a rigid bottle of india-rubber, and dip the 
 
 other end in mer- 
 
 ciiry, thus (Fig. 
 
 22). Pressing the 
 
 bottle, a portion of 
 
 the atmospheric air 
 
 will be expelled ; 
 
 then allowing the 
 
 bottle to expand, 
 
 some mercury will 
 
 enter, forming a 
 
 mercurial column, 
 
 the length of which 
 
 admits of being 
 
 measured. Let it 
 
 fce measured by 
 
 means of a pair of 
 
 compasses ; then 
 
 let the mercury be 
 
 driven to various 
 
 parts of the tube, 
 
 and the measure- 
 ments repeated. If 
 
 Fig. 22 
 
 Fig. 23. 
 
 the tube be of equal bore throughout, the mercurial column will necessarily be of equal 
 length throughout : this is evident. If the tube stand this test, it may be considered 
 good. Instead of the india-rubber bottle the breath might be employed, but it would 
 be attended with the disadvantage of moistening the tube. However, it is possible to 
 use the breath without prejudice, if the operator take the precaution of blowing through 
 some material absorptive of moisture. The next step in the manufacture of a thermo- 
 meter consists in fusing one end of the tube, and blowing it into a bulb ; this, again, 
 should be effected by means of the caoutchouc bottle, lest moisture be introduced. 
 Mercury has now to be introduced, which is accomplished as follows : 
 
 The thermometer tube having been bent as represented (Fig. 23), its open extremity 
 is immersed in a vessel of mercury. Heat being now applied to the bulb, the air 
 therein contained is expanded, and the heat being removed a partial vacuum results, 
 
34 GRADUATION OF THERMOMETERS. 
 
 to fill which, mercury rushes in. By repeating the operation, the mercury already 
 contained in the bulb is vapourized, and the vapour expanding drives out all the 
 remaining atmospheric air, so that on the removal of heat the whole tube, bulb, stem 
 and all becomes filled with mercury. The bent part of the tube is now broken off, and 
 the final quantity of mercury duly apportioned to the tube. The amount of this 
 apportionment can be only determined by practice ; but, in general terms, it may be 
 described as being such a quantity that, at the boiling point of mercury, it shall nearly 
 extend to the extremity of the tube. 
 
 The next process is one of extreme delicacy. It has for its object the sealing or 
 melting the open extremity of the thermometer tube, without admitting the slightest 
 portion of atmospheric air, the presence of which would materially interfere with the 
 delicacy of the instrument. The operation is conducted as follows : The open end of 
 the tube having been melted in the blow-pipe flame, is drawn out to a fine termination, 
 thereby diminishing still further the internal bore of the tube, and rendering the final 
 
 b occlusion of its orifice more easy. 
 
 The mercury in the bulb is now 
 heated once more, and at the 
 very instant when it is seen to 
 fill the tube and to be in the 
 act of overflowing the capillary 
 extremity, the tube is closed by 
 the fine jet of blow-pipe flame b 
 (Fig. 24). It only now remains 
 "p,. 24 to graduate the thermometer. 
 
 Graduation. The instrument 
 
 just described is, in its present condition, a thermoscope, or indicator of heat; but it is 
 by no means a heat-measurer to convert it into which it requires to be graduated 
 into a determinate number of equal parts. These parts are usually called degrees; but 
 it must be remembered that the so-called degrees have no ratio whatever to any parti- 
 cular unit. They are not like the degrees on the circumference of a circle, each one of 
 which is related to the trigonometrical ratio, or the ratio of the circumference to the 
 diameter. Thermometric degrees are altogether arbitrary, except in so much as certain 
 usages in this respect obtain. This much is, however, invariable and universal the 
 divisional parts or degrees must be established between certain limits fixed by nature. 
 The limits usually imported into the manufacture of thermometers are the boiling and 
 the freezing of water, which phenomena always, under similar conditions, take place at 
 similar respective temperatures. Founded on the bases of these limits three principal 
 scales of graduation have been devised. They are the centigrade scale, or scale of Celsius, 
 the scale of Eeaumur, and the scale of Fahrenheit. The Ltter is mostly used in this 
 country ; the former is chiefly adopted on the Continent. 
 
 Let us commence our illustrations with the centigrade scale, as being most easy. 
 The term centigrade seems to be significant of a hundred divisions. In point of fact, 
 the centigrade scale has for its 0, or zero, the temperature at which water freezes, and 
 for its 1 00 the temperature at which water boils ; the intermediate space being divided into 
 100 equal parts. And if it be desired to carry the graduations above or below the guage 
 limits, this is accomplished by measuring off equal parts by means of a pair of compasses. 
 Reaumur's scale has also its zero point at the elevation of mercurial column corres- 
 ponding with the freezing point of water, but at the other extremity of its scale the 
 
RELATION OF THERMOMETRIC DEGREES. 35 
 
 boiling point of water is considered to indicate 80 ; hence the intermediate space is 
 divided into 80 equal parts. The appended diagram (Fig. 25) will render evident the 
 peculiarities of the ordinarily employed thermometric scales. 
 
 Conversion of One System, of Graduation to Another. This conversion of thermo- 
 metric scales is frequently necessary. The rules for effecting the conversion are 
 evident on reflection ; nevertheless it is well to reduce these rules to general formulae. 
 If all these scales counted their zero from the same point, the method of converting 
 one scheme of graduation into another would be still easier than we find it. Actually, 
 some little confusion at first 
 arises from the circumstance that 
 Fahrenheit's zero is placed not 
 
 at, but below the freezing point Water boils 
 
 -2/2* 
 
 of water. 
 
 Conversion of Fahrenheit to 
 Centigrade Degrees. The propo- 
 sition is evident that one Fah- 
 renheit degree is equal to five- water freezes . . . 
 ninths of a centigrade degree, C old produced by mix- 
 inasmuch as the number of ture of ice and salt . 
 Fahrenheit degrees between the 
 
 freezing and the boiling point *'ig 25. 
 
 is to the number of centigrade degrees for the same space as unity to five-ninths, as 
 is seen to be established by the following proportion : 
 
 F. C. F. C. 
 
 (212 32) = 180 : 100 : : 1 : ifg ; 
 
 whence it appears that we may convert Fahrenheit degrees into their centigrade 
 equivalents by first subtracting 32, which leaves 180 ; then multiplying this number 
 by five, and dividing by nine, as in the following proportion : 
 
 Conversion of Centigrade to Fahrenheit Degrees. Inasmuch as each centigrade degree 
 is longer than a degree of Fahrenheit in the ratio of to one, therefore the former 
 may be reduced to the latter by multiplying by nine, dividing by five, and adding 
 thirty-two. The truth of this operation may be readily demonstrated by working on 
 the number 100, which should give, if the rule just enumerated be correct, 212 : 
 
 ^*_? = 180 + 32 = 212 F. 
 
 The examples just given illustrate the process of calculation when positive degrees 
 or degrees above zero, thus (-J-), are concerned. Exactly the same rule has to be followed 
 when negative degrees ( ) are in question, although the rule, when stated in common 
 terms, appears to be different, inasmuch as following the diction of arithmetic the 
 operator must be told to subtract 32. An example will render this more evident. 
 Suppose we require to represent 5 below zero, or 5 of C., by its equivalent F. 
 Now the number 32, with 9 subtracted, gives 23 for remainder. Viewing all the 
 steps of the calculation involved algebraically, it will be found that the rule of adding 
 32 has been implicitly followed ; a negative 5 however ( 5) yields in the following 
 operation a negative 9 ( 9), which, being added to -f 32, is equivalent to subtracting 
 a positive 9 (-{- 9). For example 
 
36 
 
 THE REGISTER AND BREGUET'S THERMOMETERS. 
 
 5x9 
 
 32 = 
 
 o 5 
 
 The various steps for the reduction of one system of thermometric degrees to 
 another, are comprehended in the appended formula? : 
 
 F. 32 X 5 
 
 Fahrenheit to Centigrade, 
 
 C. X 9 
 
 Centigrade to Fahrenheit, 
 
 F. - 
 
 Fahrenheit to Eeaumur, ^ 
 
 Eeaumur to Fahrenheit, 
 
 = C. 
 
 32 = F. 
 
 32 X 4 
 
 y 
 
 E. x 
 4 
 
 32 = F. 
 
 7%<2 Register Thermometer The greatest difficulty attendant upon the u?e of the 
 thermometer for meteorologic observations, is referable to the necessity of frequent 
 examination. To obviate in some measure the necessity for 
 this, the instrument called the Register Thermometer hns been /^~&~~ > \. 
 
 devised an instrument which depends for its action on the 
 traversing of two steel bars in the bore of the tube, each 
 pressed forward by the .expansion of a liquid column. An 
 instrument of this kind is represented in Fig. 26. The 
 register thermometer is accurate enough in its indications for 
 some rough purposes, but it is by no means adapted to super- 
 sede the use of thermometers of ordinary construction. 
 
 The Thermometer of Breguet. A. very delicate thermo- 
 meter has been invented by M. Breguet. It differs from all 
 which I have hitherto described, in the fact of its dispensing 
 altogether with mercury, or other expansive liquid, and 
 utilizing the expansion or uncoiling of a compound metallic 
 bar. 
 
 The illustration of the different rates of expansion by 
 heat of two different metals (for instance, iron and brass) 
 
 Fig. 27. 
 
 is often made by the following contrivance :a (Fig. 27) is a 
 compound bar of this kind, perfectly straight when cold ; but 
 
 if this same bar be 
 heated, it becomes 
 curved, as repre- 
 sented by Fig. 28. 
 Breguet' s ther- 
 mometer is merely 
 an amplification 
 
 Fig. 28. of the preceding 
 
 experiment ; instead of a straight compound bar, a compound 
 bar twisted into the spiral form is employed. One end of 
 the spiral is fixed, the other end is free, and is attached to an index. The mode of 
 
 Fig. 26. 
 
THE THERMOSCOFE OF KOBILI. 
 
 37 
 
 acdou of the instrument will be obvious. Variations of temperature producing varia- 
 tions of curve, will cause the spiral to unfold, or contract, according as the varia- 
 tions are towards the direction of increased heat or increased cold. The metals 
 employed in making the spiral of Breguet's thermometers are platinum, gold, and 
 silver. Experiment has demonstrated that the needle of this instrument travels over 
 
 -7 7' 
 
 Tig. 29. Fig. 30. 
 
 equal arcs for equal increments of temperature ; hence Breguet's thermometer is 
 not only comparable with itself, but with all other instruments on the same construc- 
 tion. Unfortunately this delicate instrument has no great range of application, its 
 indications heing limited between the freezing and the boiling points of water (Fig. 29) . 
 The Thermoscope of Nobili. By far the most delicate indicator of minute 
 increments and decrements of temperature is an instrument founded on principles 
 totally different to any already described. The electrical thermoscope of Nobili admits 
 
 Fig. si. 
 
 of being thus described : The action of the instrument is based upon the fundamental 
 fact of electro-magnetism, viz., that a magnetic needle, freely suspended and placed in 
 
38 
 
 CONSIDERATIONS RESPECTING THE USE OF THERMOMETERS. 
 
 the vicinity of an electric current, finally arranges itself at right angles to that 
 current. Hence the deflection of a magnetic needle becomes indicative of the existence 
 of such current. Founded on the consideration of this fact, we have the instrument 
 termed the galvanometer, which, in its simplest form, is represented by Fig. 30 ; and a 
 still more delicate construction of which is represented in the woodcut on the previous 
 page (Fig. 31). 
 
 It remains now to remark that heat is a fruitful source of electricity, especially 
 
 when heat is applied to one end of a mechanical 
 arrangement of alternate bars of two metals, such 
 as bismuth and antimony, as represented in the 
 accompanying diagram (Fig. 32) . 
 
 In order to render the combination of metallic 
 bars more compact, they are usually arranged in 
 a bundle as represented below (Fig. 33). 
 fi g- 32. Very few words will now render compre- 
 
 hensible to the reader the structure and functions of the thermoscope of Nobili. A 
 bundle of bismuth and antimony redu- 
 plications, as just described, being 
 placed in communication with a gal- 
 vanometer, the magnetic needle of the 
 latter is ready to be deflected on the 
 first occurrence of an electric current, 
 
 Fig. 33. 
 
 34t 
 
 and such electric current is a direct consequence of the application of heat to one 
 
 extremity of the system of compound 
 metallic bars. The annexed diagram 
 (Fig. 34) represents the thermoscope of 
 Nobili, as employed to indicate small 
 increments of radiant heat evolved from 
 the little lamp on the left, and trans- 
 mitted through the central diaphragm. 
 
 Considerations Respecting the Use of 
 Thermometers. Much that is incorrect 
 passes current respecting the functions of 
 the thermometer the accuracy to which 
 it is susceptible, and the spontaneous de- 
 teriorations to which the instrument is subject. A few words, therefore, on these 
 matters may be advisable. 
 
 Let us assume the thermometer under consideration to be a mercurial thermometer ; 
 let us assume that the mercury used was absolutely pure, that the tube was proved to 
 be of equal bore throughout, that the process of sealing was accomplished without 
 permitting the ingress of the slightest amount of atmospheric air; let us, finally, 
 assume that the tube has been accurately graduated such an instrument may be 
 regarded as free from all errors of construction. 
 
 Nevertheless, it is found that a thermometer thus unexceptionable at first is liable 
 to deteriorate by time, the guage points i.e., 32 and 212 corresponding with higher 
 portions of the tube than they should do, and necessarily, all other degrees. Most 
 probably this result is referable to a gradual contraction of the sides of the tube and of 
 the bulb, the contraction being determined by continuous atmospheric pressure. 
 
THE KIND OF HEAT INDICATED BY THERMOMETERS. 39 
 
 Looking at this contraction, the propriety is suggested of retaining thermometer tubes 
 filled some time previous to graduation. 
 
 The Kind of Heat indicated by Thermometers. The term heat is commonly held to 
 be synonymous with temperature ; but philosophy accepts it in a more extended sense, 
 as comprehending not merely one effect (temperature), but the cause of many effects. 
 The philosophy of latent and specific heat is almost too purely physical for extended 
 examination here ; hence a slight reference to these conditions will suffice. The ther- 
 mometer is not adapted to take cognizance of heat in this latent or specific form. It 
 is only indicative of evident heat or temperature ; nor are its indications in this narrow 
 field so complete, nor the information it conveys so extensive, as is frequently supposed. 
 The thermometer does not even profess to indicate the quantity of calorific heat, but 
 only its degree ; terms which are totally distinct, as will soon be perceived. Let us 
 take the following as an illustration of the difference : A pint of boiling water is as 
 hot as a quart of boiling water, the temperature of both being 212 Fah. ; hence the 
 thermometer, if appealed to, will indicate this identity of temperature. But, neces- 
 sarily, a quart of boiling water must contain twice as much calorific heat as a pint of 
 the same : the deduction is too obvious for comment. 
 
 Again, strictly speaking, the thermometer cannot be said to present us with the 
 correct temperature of anything, inasmuch as the degree of columnar expansion is not 
 the degree corresponding with that of the thing with which the barometer is brought 
 into contact, but the mean of the thing touched, and the bulb which touches it. This 
 objection attains its minimum when the atmosphere itself is the medium, the tempera- 
 ture of which we desire to investigate ; but it becomes of practical importance in al 
 other cases, and its consideration teaches the thermometric observer the necessity for 
 employing instruments the bulbs of which are as small as compatible with well- 
 marked amounts of columnar expansion. This remark especially applies to cases in 
 which the bulk of liquid or solid operated upon is small. As concerns the graduation 
 of thermometers, the most correct instruments are those of which the graduations are 
 effected on the tube of the glass itself. If the graduation be made on a scale of brass, 
 the resulting indications will be very incorrect, except the relative expansibility of 
 brass and glass be taken into consideration, and duly allowed for ; this, however, is so 
 troublesome that it will be too frequently evaded, and errors will creep in. Far better 
 than brass is box-wood ; and slate and ivory are better still. 
 
 Correspondence betu-een Thermometers. Scarcely any two thermometers exactly cor- 
 respond, even though the same materials be employed in their construction, so nume- 
 rous are the points which require attention. Between thermometers constructed with 
 different materials, between air thermometers and mercurial thermometers, for in- 
 stance, or either of these, and spirit thermometers, the discrepancies 1 are still more 
 considerable. 
 
 The experiments of M. Regnault relative to the discrepancies subsisting between 
 mercurial and air thermometers are amongst the latest on this important subject. He 
 found that between 32 and 212 Fahrenheit, there is an almost absolute coincidence 
 between the degrees of the air and the mercurial thermometer. From 212 to 482 
 Fahrenheit, the mercurial and air thermometers remain pretty equal ; but after the 
 latter point the mercurial gains on the air thermometers. 
 
 Hitherto I have treated of the atmosphere, statically considered, in a condition 
 of repose ; but perfect atmospheric quiescence is unknown in nature it is always agi- 
 tated more or less ; hence we are led to the consideration of atmospheric currents or winds. 
 
40 THERMAL CONDITIONS IN THE PRODUCTION OF WINDS. 
 
 So Variable are winds in these northern latitudes, that their incertitude has passed 
 into a proverb ; primary atmospheric currents, nevertheless, are constant in their direc- 
 tion, and are referable to variations of temperature simultaneously existing in different 
 parts of the world. 
 
 Heat in its non-latent condition, or, in other words, that condition of heat which is 
 recognizable by the thermometer, has a tendency to equalize itself. Hence, if two 
 bodies a and b, of which a is hotter than b, be situated in proximity to each other, there 
 is an immediate tendency to equalization of temperature as between the two ; but the 
 thermal conditions which regulate the production of winds will be most readily appre- 
 ciated by reflecting on what takes place when a heated solid is suspended in the atmos- 
 phere by a small chain or wire ; we shall find, on investigation, 
 that a heated solid thus circumstanced gradually becomes cool 
 by the operation of three distributive influences conduction, 
 radiation, and convection. 
 
 Conduction. If the chain by which the heated cannon-ball is 
 represented to be suspended in the annexed diagram (Fig. 35), 
 be examined from time to time by the thermometer, or even by the 
 fingers, it will be found to increase in heat evidently because it 
 removes a portion of temperature from the heated cannon-ball by 
 conducting it away. This function of heat is so well understood 
 and so commonly exemplified, that no further consideration of it 
 Fig. 35. w in b e necessary here. 
 
 Radiation, If a thermometer be held even at the distance of some feet from the 
 heated cannon-ball, the mercurial column will be sensibly affected, thus demonstrating 
 the transmission of heat. But how transmitted ? By contact with the atmosphere? 
 Clearly not, inasmuch as the result just indicated still occurs if the cannon-ball be 
 placed in the vacuum of an air-pump. By experiment it has been determined that 
 the temperature, in the case under consideration, has been given off in the condition 
 of rays, precisely as light is evolved, and hence the propriety of the term radiant heat. 
 Convection Independently of the two processes of heat-distribution already described, 
 there is yet a third. Referring to the suspended hot cannon-ball, we shall find that, if sus- 
 pended in a room from the ceiling, the air near the ceiling becomes hotter than the air 
 below : hence a portion of the temperature of the hot cannon-ball must have become 
 accumulated there, by reason of some cause besides those of conduction and radiation, 
 both of which distribute the temperature equally in all directions, through a homogeneous 
 medium such as the atmosphere for example, in our assumed experiment. The pro- 
 cess of heat-distribution known as convection, is the necessary result of the expansion 
 of liquids and' fluids by heat; for when expanded they are specifically lighter; and, 
 when specifically lighter, they must necessarily ascend. It is by virtue of the pro- 
 cess of heat-distribution, termed convection, that the child's soap -blown bubbles ascend 
 instead of at once falling to the surface of the earth. 
 
 Temperature and pressure being equal, breath evolved from the lungs is heavier 
 than atmospheric air, because it holds more carbonic acid. Nevertheless, inasmuch as 
 it is evolved from the lungs hotter than the surrounding atmosphere, soap-bubbles 
 blown with it ascend. Presently, however, they descend, because the heat acquired 
 from the lungs being evolved, and equalization of temperature with that of the 
 surrounding atmosphere having ensued, the great specific gravity of the gas where- 
 with the bubble is blown causes the latter to sink to the surface of the earth. 
 
THEORY OF WINDS. 41 
 
 It is to the process of heat convection, that we owe the salutary draughts in our 
 chimneys, and also unsalutary draughts in our apartments. No sooner is fuel lighted 
 in a fire-place, than the superincumbent air becomes higher and specifically lighter; it 
 therefore ascends, and cold air rushes in to fill its place. Thus we have, in point of fact, 
 a local wind ; and the causes which determine that wind are exactly comparable to the 
 causes of winds which take place in the grand economy of nature, as will soon be 
 rendered manifest. 
 
 The Trade Winds. Applying the facts just developed, let us now regard the 
 surface of our globe in the aggregate, with reference to the localities of maximum and 
 minimum temperature, and the consequence of such difference of temperature in origi- 
 nating an aerial current. It is evident that the hottest portions of our globe's surface 
 are comprehended within the tropics, and the coldest portions are the arctic regions 
 north and south. 
 
 These circumstances being premised, we are now in a condition to anticipate the 
 direction of the aerial currents or winds which must necessarily ensue. Firstly, an 
 ordinary current of heated air should rise aloft in the tropical regions, then diverge 
 and pass north and south to either arctic circle, thus constituting what may be termed 
 the upper trade current. This current, as it proceeds north and south, gradually 
 becomes cold, in which condition it is rendered specifically heavier, falls to the surface 
 of the earth, and floats along towards the equator, thus generating two principal 
 currents one north, the other south. "Whilst yet in the frigid and the temperate zones 
 these primary currents encounter so many interferences that their primary or funda- 
 mental direction is masked or veiled ; but still proceeding north and south, the per- 
 sistent directive tendency of the trade winds is at length developed. But the currents 
 no longer flow directly north and south. By the operation of a cause which will 
 presently be rendered evident, the directive tendency of either current has acquired a 
 certain impulse towards the west ; or, in other words, both currents come more or less 
 from the east. But at length they blow almost from due east, and finally cease 
 altogether; so that the equator, and a certain space north and south of the equator, are 
 comprehended within what is termed the region of calms. The north trade wind 
 meeting the south trade wind, they are mutually destructive of each other. Two con- 
 flicting aerial forces by mutual impact come to rest, just as two billiard balls, each 
 rolling gently from an opposite point, become quiescent. 
 
 No part of the earth's surface, however, is subject to such capricious and such 
 violent tempests as the so-called region of calms. This is a result which theory 
 would lead us to suspect. The cause has now to be explained wherefore the trade 
 winds do not blow directly north and south. If our globe were at rest, or if its only 
 motion were motion in its orbit, such would be the result; but it revolves on its 
 axis also from west to east, and this circumstance fully explains the deviation from 
 northness and southness of the trade winds. If the lo\ver or returning aerial current 
 which constitutes the trade winds ceased to exist altogether, then our globe's diurnal 
 rotation would generate a current in the apparent direction of east to west. I say 
 apparent direction, because, in point of fact, we may regard the atmosphere under 
 these circumstances as being tranquil or passive, and our globe revolving in the 
 midst of it, in the direction of west to east. Inasmuch as the force representing tins 
 apparent east wind, and the force representing the north and south atmospheric currents, 
 flowing towards the equator from either pole, are simultaneously operating, there occurs 
 a resultant, which is the trade wind. The appended diagram (Fig. 36) roughly illus- 
 
42 
 
 LAND AND SKA BlUiEZtiS. 
 
 trates the points which have heen described. Towards the extreme north and extreme 
 south the great aerial currents, ultimately destined to become the trade winds, are 
 represented fluctuating and variable; gradually, however, they acquire a northern and a 
 
 southern directive tendency respectively; lastly, 
 they come from the point of almost due east, and 
 then cease altogether. 
 
 A consideration of the influences which de- 
 termine trade winds leads to a facile explana- 
 tion of many aerial currents of less extent. The 
 _ heating influence referable to the geographical 
 position of the tropics is not the only influence 
 of this kind. A peculiar condition of the earth's 
 surface, taken conjointly with a favourable 
 condition of the sun's rays, may bring about 
 similar results. In this manner the Mediterra- 
 nean etesian winds, or aerial currents from the 
 north, may be accounted for. Looking at the 
 geographical condition of localities south of the 
 Mediterranean, we find the Sahara, or Great Desert, a region whose surface is strewed 
 with masses of pebbles and sand, and almost totally devoid of vegetation. Such a 
 surface must necessarily become elevated by the solar rays to a high temperature, an 
 atmospheric column must ascend, travel to the north, then fall, and at length return 
 from the north to the Sahara, whence it came. 
 
 Land and Sea Breezes. Many countries near the sea are subjected to winds of 
 diurnal periodicity, known as land and sea breezes. About eight or nine A.M., an 
 aerial current begins to flow from the sea towards the land, and persists until about 
 three p M., when a current in the reverse direction, or from the land towards the sea, 
 takes its place, and continues throughout the night until sunrise next morning, when 
 it ceases, and a calm ensues until the completion of a period of twenty-four hours from 
 the occurrence of the preceding land breeze. These currents, in reverse directions, can 
 be easily accounted for when we consider the heating agency of the sun. Necessarily 
 land becomes hotter than water under an equal power of luminous rays ; whence it 
 follows, that the surface of the ground becoming heated after sunrise, determines the 
 ascent of an atmospheric current vertically ; thence, proceeding oceanward, the same 
 current returns from the sea to the land. No sooner does the sun set, than this current 
 is reversed. 
 
 In the preceding explanations of the cause of winds as being referable to inequali- 
 ties of temperature, it will be observed that reference has alone been made to the 
 calorific effects of the sun's rays. This is, in point of fact, the only source of heat 
 which has to be taken cognizance of in meteoric considerations ; for, although the 
 earth's own temperature gradually increases as we pierce downwards below the sur- 
 face, so imperfect are the conducting powers of the materials of which the crust of our 
 planet is composed, that all consideration of them may be safely omitted in accounting 
 for the present phenomena. 
 
 Process of Reverse Atmospheric Currents. Although the existence of 
 atmospheric currents proceeding in a direction reverse to those we meet with on the 
 earth's surface is forced upon the mind by inferential reasoning, and the fact must be 
 accepted, even though no further evidence of it could be adduced, nevertheless direct 
 
UPPER CURRENT OF TRADE WINDS. 43 
 
 proofs are not wanting. The direction of upper layers of clouds afford their testimony 
 to the truth of the opinion. "Where the trade winds prevail, the higher strata of clouds 
 may be seen taking a direction opposite to that of the wind itself; and travellers, 
 during their ascent of high mountains, have frequently proved the existence of a 
 superior wind pursuing a course opposite to the wind below. This has been espe- 
 cially obvious on the Peak of Teneriffe, a mountain situated in the belt of the trade 
 winds. Here, on the summit of this peak, it has been always found that a south-west 
 wind prevails, whereas the trade wind at the base of the same mountain blows from the 
 north-east. 
 
 A similar remark has been made by travellers who have ascended Mona Kea, in 
 Owyhee, the height of which is 18,000 feet. But perhaps the most striking illus- 
 . tration is the following : The island of Barbadoes lies eastward of St. Vincent, and 
 between the two the trade wind continually blows, and so forcibly that it is only with 
 difficulty, and by making a long circuit, that a ship can sail between the latter and the 
 former. Nevertheless, on one occasion, during an eruption at St. Vincent, dense clouds 
 formed over Barbadoes, and large quantities of ashes fell on the island. A similar result 
 was observed after an eruption of the volcano of Cosoguina, on the shores of the 
 Pacific, in Guatemala, in January, 1835, some of the volcanic ashes falling in Jamaica, 
 more than 800 miles in a direct line distant, and directly opposed to the prevailing 
 lower current. At the same time another portion of ashes was carried westward, or 
 in an opposite direction, falling on Her Majesty's ship " Conway," in the Pacific, 
 more than 1200 miles distant. 
 
 The trade winds are, for the most part, only recognizable at sea ; the solid material 
 of land developing local aerial currents of their own. The extent of prevalence of the 
 trade wind is various. In the Atlantic it prevails from 8 to 28 or 30, but in the 
 Pacific only to 25 1ST. L. In the southern hemisphere, the extent of the trade wind 
 has been less accurately determined. When first the phenomena of trade winds were 
 noticed by Columbus and his associates, they caused the greatest consternation. 
 Accustomed to the fluctuating and irregular breezes of Europe, they regarded the con- 
 tinuance of a wind from the east as emblematic of their perpetual banishment from 
 their native shores. The early Spanish navigators, however, very soon learned to 
 appreciate the value of trade winds, by the aid of which treasure-laden galleons could, 
 setting out from Acupulco, manage to arrive at Manilla almost without changing a 
 sail. 
 
 As respects the upper current proceeding from the equator to either pole, it varies, 
 as might be anticipated, in different localities. Travellers, who have ascended the Peak 
 of Teneriffe, inform us that this upper current is found in that locality at an elevation 
 of 9000 feet ; but Humboldt, during his explorations on the Andes, discovered the 
 eastern trade wind to be blowing at an elevation of 8000 feet above the level of the 
 sea. As the upper, or equatorial current loses its heat, its specific gravity becomes 
 greater, and it sinks lower and lower, no longer manifesting any well-marked directive 
 tendency. 
 
 On the ocean, and between 30 and 40, there is a prevalence of westerly winds, 
 especially in the southern hemisphere. In the Atlantic a similar tendency is manifest ; 
 whence it follows, that the voyage from Europe to America occupies more time than a 
 voynge in the reverse direction. It is difficult to say what may be regarded as the 
 prevailing wind in these isles probably, however, a south-western wind, as stated in 
 the following table : 
 
STORMS. 
 
 Table Representing the Relative Prevalence of Winds in different Countries for a Period of 
 
 1000 days. 
 
 Countries. 
 
 N. 
 
 N.E. 
 
 E. 
 
 S. E. 
 
 s. 
 
 S W. 
 
 W. 
 
 N.W. 
 
 
 
 
 
 
 
 
 
 
 England 
 
 82 
 
 Ill 
 
 99 
 
 81 
 
 Ill 
 
 225 
 
 171 
 
 120 
 
 France 
 
 126 
 
 140 
 
 84 
 
 76 
 
 117 
 
 192 
 
 155 
 
 110 
 
 Germany 
 
 84 
 
 98 
 
 119 
 
 87 
 
 97 
 
 185 
 
 198 
 
 131 
 
 Denmark 
 
 65 
 
 98 
 
 100 
 
 129 
 
 92 
 
 198 
 
 161 
 
 156 
 
 Sweden 
 
 102 
 
 104 
 
 80 
 
 110 
 
 128 
 
 210 
 
 159 
 
 106 
 
 Russia 
 
 99 
 
 191 
 
 81 
 
 130 
 
 98 
 
 143 
 
 166 
 
 192 
 
 North America 
 
 96 
 
 116 
 
 49 
 
 108 
 
 123 
 
 197 
 
 101 
 
 201 
 
 The direction of winds is found, taking the average of many years, to vary 
 according to the season. In Europe south winds are more prevalent than any 
 others during winter ; east winds helong more especially to the spring ; west and 
 north winds to the summer, and towards Octoher the wind usually veers round to the 
 south. Usually the wind is more strong in February and March than at any other, 
 and at all seasons the wind is usually strongest at noon. 
 
 Storms. Whenever the air, from any cause, is thrown into violent commotion, the 
 result will he a storm. The philosophy of storms, notwithstanding the attention which 
 has heen devoted to the subject, is by no means well understood. In point of fact, the 
 causes of storms are numerous and complex. If we reflect on the agency of temperature 
 on the air, one prevalent cause of storms will at least become manifest. If one portion 
 of the atmosphere be suddenly heated, violent commotion must arise there must follow a 
 storm. The laws of latent heat demonstrate that whenever water is suddenly condensed, 
 the surrounding air must be raised in temperature ; and thus we have one of the most 
 frequentlocal causes of storms. According to modern observations storms are, for themost 
 part, circular whirlwinds progressing in a north-eastern direction from the south to the 
 north of the tropic of Cancer. In proportion as a locality is devoid of mountains and near 
 the sea, so is it more liable to be subject to storms. Perhaps the most violent of all Euro- 
 pean storms are those which occur in the south of France during the prevalence of the 
 north-east wind termed mistral; but the most violent storms occur in and near the 
 tropics, and are termed tornadoes, trovadoes, hurricanes, typhoons, &c. Hurricanes are essen- 
 tially tropical ; the West Indies suffer from thorn more than any other region. Hurricanes 
 are of yearly occurrence in the West Indies, but the islands of Trinidad and Tobago, 
 being protected by mountain elevations, usually escape them altogether. The wind, 
 during a hurricane, frequently makes an entire circuit, blowing from every point of the 
 compass ; and it is by no means an unusual occurrence for the wind to cease awhile 
 altogether, and then commence blowing again. Perhaps the most violent hurricane on 
 record is the one which occurred in 1780. It destroyed the fleet of Lord Rodney, and a 
 vast number of merchant ships. It killed no less than 9,000 individuals in Martinique 
 alone, and 6,000 in St. Lucia. It totally destroyed the town of St. Pierre in Marti- 
 nique, and almost as completely the town of Kingston in St. Vincent, only fourteen 
 houses of the latter being left unmolested. Not a few of the West Indian hurricanes 
 extend their ravages northward to the United States ; usually, however, with a violence 
 greatly diminished. 
 
HOT WINDS. 45 
 
 The eastern, western, and southern coasts of Africa are also subject to storms of 
 almost equal violence with the West Indian hurricanes ; these storms, however, in the 
 localities under consideration are called tornadoes. At Sierra Leone and the adjacent 
 parts two or three tornadoes usually usher in the dry season ; they are sometimes accom- 
 panied with rain, and sometimes without ; when of the latter kind, they are called white 
 tornadoes. The eastern coast of Africa, especially towards the south, is also very 
 subject to tornadoes. One of the most violent storms on record occurred in the 
 Mozambique Channel in 1809, extending to the islands of Bourbon, Mauritius, and 
 Rodrigues. The violence of storms at the Cape of Good Hope is proverbial ; the 
 early Portuguese navigators, therefore, called the southern cape of Africa the Cape of 
 Storms. 
 
 Typhoons may be described as hurricanes of the Chinese and Japanese seas ; like 
 hurricanes, they have a rotatory motion, but they are more localized in their action, 
 having no distinct rectilineal progression. 
 
 Hot Winds. Although heat may be regarded as primarily the cause of all winds, 
 it does not follow that all winds must be hot ; indeed, we know that the result is the 
 direct opposite that many winds are very cold. The temperature of a wind is for the 
 most part totally independent of the temperature which caused it, and is determined by 
 the nature of the surface over which it blows. The principal hot winds are those denomi- 
 nated the simoom, the harmattan, the chamsin, the sirocco, and the solano. The term 
 simoom, or samiel, means poisonous, and is derived from a belief of the Arabs that the 
 devastating effects of this wind are attributable to some poisonous emanation which it 
 bears. There is no foundation, however, for this notion. The terms chamsin and har- 
 mattan are little else than Egyptian and negro appellations respectively for the simoom. 
 The Egyptian term chamsin means fifty, and has reference to the duration of the wind 
 fifty days from April 27 to June 18. The simoom is the terror of desert caravans. At its 
 approach the horizon grows dark, the sun's rays scarce penetrate with lurid gleam the 
 atmosphere charged with particles of burning sand. The wind blows with fitful 
 violence, scattering death and desolation in its track, withering the trees and shrubs 
 which it encounters, suffocating animals, and burying them under waves of sand. The 
 camels no sooner perceive the advent of the simoom, than rushing to the nearest tree 
 or bush, or seeking the spur ef some projecting rock, they place their heads in the 
 direction opposite to which the wind blows, and endeavour to screen themselves from its 
 violence. The traveller throws himself on the ground on the lee-side of the camel, and 
 screens his head from the fiery blast within the folds of his robe. Too frequently all 
 .these precautions are unavailing, both man and beast falling a prey to the terrible 
 simoom. The idea, however, of the wind being poisonous is not founded on fact. In 
 the western parts of Asia, more especially in Arabia, the simoom only blows in the 
 summer months, and with maximum violence in July. It occurs only in the day 
 time, and for the most part only lasts a few hours. In Lower Egypt, the direc- 
 tion of the simoon is from the south-west ; in Mecca, it comes from the east ; in 
 Surat, from the north ; in Bassora, from the north-west ; in Bagdad, from the west ; 
 and in Syria, from the south-east ; in every case proceeding from the neighbour- 
 ing desert where the air has suffered rarefaction. The simoom, far from being 
 poisonous, is in some localities beneficial to health, by drying up aqueous exhal- 
 ations, which, if not removed, would give rise to fevers and other diseases. This is 
 particularly the case on the western coast of Africa. It is, nevertheless, always in- 
 jurious to vegetation. 
 
46 
 
 COLD WINDS AND WHIRLWINDS. 
 
 The Italian sirocco and the solano of Spain may be regarded as European con- 
 tinuations of the harmattan or simoon of "Western Africa. The sirocco, although 
 usually restricted to Malta, Sicily, and southern Italy, sometimes extends into 
 Germany and Switzerland : in the latter locality it is denominated the fohn. The 
 fb'hn, although prejudicial to trees, develops to a surprising degree the vegetation of 
 young plants, and can hardly be regarded as a calamity. It is most prevalent in 
 Switzerland, near the Lake of the Four Cantons, Its period of duration does not 
 usually exceed a few hours, though sometimes this period is exceeded, and it rarely 
 occurs in winter. 
 
 The southern part of Australia is subject to a hot north wind, presenting such a 
 marked resemblance to the sirocco that geographers are led to the natural inference 
 that the unknown interior of the Australian continent is a desert of sand and rock, 
 like the Sahara and the wilds of Arabia Petrooa. 
 
 Cold Winds. These winds are less noticeable and fewer in number than those 
 already mentioned. Their low temperature is usually referable to the circumstance of 
 their passing over mountain ranges covered with snow. The most considerable winds 
 of this kind exist in Mongolia, Beloochistan, and the Russian steppes. To this class 
 also belongs the mistral, a north-east wind prevalent in southern France, and which is 
 exceedingly prejudicial to vegetable life. 
 
 Whirl-winds. When two violent winds meet, the result is a whirlwind, so called 
 from its rotatory character. If a whirlwind occurs at sea, or over water, it 
 
 elevates a large column of water aloft, 
 sometimes to the height of many hun- 
 dred feet, thus giving rise to the meteo- 
 ric phenomenon termed a water-spout. 
 If a whirlwind occurs on land it lifts up 
 dust, boughs, the roofs of houses, and 
 other solid matters, producing a column 
 of well-defined shape. These whirlwind 
 columns, whether they consist of water 
 or solids, present the same general for- 
 mation and contour. They consist of a 
 hollow cone, sometimes straight, but 
 more frequently curved or horn-shaped, 
 its upper portion proceeding from a 
 cloud; its lower part consisting of an 
 aggregation of water or of sand and 
 dust, according to the locality. The 
 upper and lower portions of these co- 
 lumns are so much denser than the 
 remainder, that they are generally 
 opaque, whereas the middle portion is 
 generally transparent. The tint of 
 these colours is various sometimes 
 gray, sometimes brown or nearly black, 
 and occasionally fiery red. 
 
 Independent of the circular or axial 
 
 Fig. 37. 
 
 motion of these whirlwind columns, they pursue an onward course, sometimes straight, 
 
INFLUENCE OF WINDS ON THE BAROMETER. 
 
 47 
 
 at other times curved. The velocity of this course differs within wide limits. Some- 
 times a man on foot can readily keep pace with it, whilst at other times they pro- 
 ceed at the rate of nine or [ten 
 miles an hour, sometimes more. 
 
 Whirlwind columns, whether 
 they eventually "become water- 
 spouts or not, always originate 
 on land, or in the vicinity of land 
 where the winds and temperature 
 are mutable. They are usually 
 attended with thunder, lightning, 
 and other electrical phenomena ; 
 and they constitute the centre 
 of an aerial commotion, all around 
 the focus of which a profound 
 calm prevails. Bodies which they 
 have taken up are not readily 
 deposited, but carrie-l along in 
 their onward course. Sometimes 
 ^hey are quite in the clouds, at 
 other times on the surface of the 
 earth or water, and their forma- 
 tion may be prevented. Even when 
 already formed, they may fre- 
 quently be destroyed by some vio- 
 lent aerial commotion, such as 
 that produced by the discharge of 
 a piece of ordnance a fact well Fig. 33. 
 
 known to seafaring men. The size and height of these whirlwind currents is various ; 
 occasionally they present a diameter of no more than two feet, while the diameter of 
 some has been estimated at two hundred, or even more. Again, the height of some 
 is no more than thirty feet, whereas others have been known the height of which 
 was no less than three thousand feet. 
 
 Water- Spouts. Of these columnar whirls the water-spout is less damaging than 
 the dry whirl, probably because the weight of fluid which it carries diminishes the 
 violence of its rotatory motion. Not the least extraordinary amongst the many curious 
 circumstances relative to water-spouts, is the well-attested fact that, although occurring 
 at sea, they have been occasionally known to break and deluge a ship with a torrent of 
 fresh water. 
 
 Influence of Wind on the Barometer. Although the barometer has hitherto been con- 
 sidered in reference only to the pressure of a tranquil column of air, its variations are 
 influenced by many other circumstances, which we must not omit to consider. Amongst 
 the most important of these are winds. Having regard to the ultimate cause of winds 
 it will be evident that the existence of a wind bespeaks the condition of different tem- 
 perature in two different places. Hence, every wind necessarily varies to some degree 
 the temperature which would have subsisted at any given place under a perfectly 
 tranquil atmosphere. Now, inasmuch as the atmosphere expands by heat and contracts 
 by cold, varying to a corresponding degree its density or specific gravity, so it follows 
 
48 ATMOSPHERIC MOISTURE. 
 
 that the height of the barometric column will be influenced by winds. In Europe it 
 will be generally found that a fall of the barometer corresponds with a rise in the 
 thermometer ; this rule also prevails for the tropics, nevertheless it is subject to many 
 variations. The barometer may rise and fall without any corresponding change in the 
 thermometer or both may rise and fall together. 
 
 The application of the barometer as a weather-glass is altogether collateral and 
 secondary, nevertheless its indications in this respect are, for the most part, worthy of 
 confidence ; generally the barometric column sinks the day before rain occurs, and 
 rises during its prevalence. The barometric column is much agitated during the exist- 
 ence of a storm, owing to the conflict which then ensues between atmospheric currents 
 tending towards opposite directions. 
 
 Other Variations of the Barometer. Besides the elevation of barometric mercury due 
 to direct atmospheric pressure and to aerial currents, there exists other fluctuating 
 causes, both diurnal and annual. The former are scarcely noticeable in temperate, but 
 very conspicuous in the torrid zone. Every day the barometric column twice attains 
 a maximum, and as often a minimum. The two periods of maximum elevation occur 
 between 8^ and 10| A.M. (say an average of 9h. 37m.) ; and between 9 and 11 P.M. 
 (say an average of lOh. llm.) The two periods of minimum elevation are between 3 and 
 5 A.M., average 3f ; and between 3 and 5 P.M., average 4h. 5m. During winter and 
 the rainy tropical season, the diurnal variations of the barometer are least, and they 
 assume their maximum in April. The variations are much less on elevated mountains 
 than in the plains below. 
 
 Mean Barometric Condition of a Place. It was formerly assumed that everywhere at 
 the level of the sea the barometric condition for the same time was identical. This 
 opinion is fallacious, latitude having a well-determined influence in this respect. It is 
 least of all at the equator, whence it increases north and south, attaining its maximum 
 about 30 or 43 of latitude ; it then decreases to between 60 and 70. "Within the polar 
 circle it would appear to reascend, but further experiments for this locality are a 
 desideratum. 
 
 Longitude also appears to exert some influence over the elevation of the barometric 
 current. It is greater in the Atlantic than in the Pacific, by a small but readily 
 perceptible quantity. 
 
 Causes of Periodical Barometric Variations. Various opinions have been advanced 
 to account for these periodical barometric variations. To say they are attributable to 
 difference of temperature is to advance a cause too remote from the result. Many philo- 
 sophers have attributed these variations to the existence of veritable atmospheric tides ; 
 but the most plausible explanation of diurnal barometric variations would seem to be 
 that of Dove, who assumes them to depend upon the varying amount of aqueous vapour. 
 Aqueous vapour and atmospheric air are possessed of different specific gravities, and the 
 barometric height of a column of mercury for any time will be the sum of pressure of 
 dry atmospheric air and associated moisture; as the relative amount of the two varies, 
 so will vary the height of the barometric column. 
 
 Atmospheric Moisture and its Derivatives. When treating at 'page 469 
 of the means to be employed for weighing a gas, the facility wherewith gaseous 
 bodies absorb moisture was adverted to. Some idea then may be gained of the 
 amount of moisture present in the atmosphere, seeing that the latter is ever in con- 
 tact with large expanses of water. The atmosphere, in point of fact, is never dry, or 
 in any way near dryness. Even when the air seems parching hot, drying the skin 
 
ATMOSPHERIC MOISTURE. 49 
 
 and withering vegetables, it is easy to demonstrate, by the aid of chemical agents, 
 the existence of aqueous moisture ; without the presence of which neither the functions 
 of animal or of vegetable life could be maintained. Even when the air approaches the 
 condition of dryness, within very remote limits, breathing is difficult and symptoms of 
 feverish restlessness speedily sets in. The natural craving of the lungs for moisture 
 is demonstrated by the presence of a close stove in a small room. The sensations, 
 which are very unpleasant, can always be alleviated by placing a small dish of.water 
 on the stove, so that evaporation may go on continuously. It is of the utmost import- 
 ance, then, to be enabled not only to demonstrate the existence of atmospheric moisture, 
 but to determine its quantity. A few experiments for effecting this demonstration I 
 shall now detail. 
 
 Experiment 1. The accompanying diagram (Fig. 39) represents a balance or pair of 
 scales, into one pair of which there has been placed a small dish of oil of vitriol, and into 
 the other a counterpoise. If the apparatus be exposed to the air, even when the earth 
 is hottest and dryest, nevertheless the equilibrium of the pair of scales will soon be 
 destroyed. Some ponderable increase will have been acquired by the pan containing 
 the oil of vitriol, and analogy demonstrates the increase in question to be due to the 
 absorption of \vater. Founded on this 
 property, oil of vitriol is frequently em- 
 ployed by the chemist for desiccating 
 substances which could not be heated 
 without damage. Accordingly, if a pan 
 of oil of vitriol and a moistened sheet of 
 paper be enclosed together, under an in- 
 verted glass, the paper will in course 
 of time become dry. Far more rapid 
 and powerful is the operation of the 
 oil of vitriol when, instead of being 
 placed together with the substance to 
 be dried, and in a mere bell-glass, the 
 two are placed under the receiver of an air-pump, and the air exhausted. Under these 
 circumstances an atmosphere of aqueous vapour alone soon fills the air-pump receiver, 
 and the absorptive operation of the oil of vitriol being continuous, the water is 
 speedily evaporated. 
 
 Experiment 2. Instead of oil of vitriol, carbonate of soda, or chloride of calcium 
 and, in an inferior degree, common salt (chloride of sodium) may be used ; for these 
 bodies are all hygrometric that is to say, they have the property of absorbing water 
 from the air. Of the three bodies mentioned, chloride of calcium is the most hygro- 
 metric, and is of constant application by the chemist. Founded on the hygrometric 
 quality of common salt, and other saline materials contained in sea water, is the 
 property which certain sea weeds have of becoming moist in damp weather, and of 
 indicating by their dry crispness an opposite atmospheric condition. 
 
 Although aqueous vapour be always present in the atmosphere, it is not always 
 visible. Frequently it is quite transparent, and only demonstrable by the process of 
 getting it out ; but at other times it aggregates, becoming vesicular, and forming clouds, 
 fog, dew, rain, snow, hail, or sleet. 
 
 Dew. Although the philosophy of dew is now perfectly well understood, no 
 atmospheric phenomenon before the happy researches of Dr. "Wells was more im- 
 
50 
 
 THEORY OF DEW. 
 
 perfectly explained and involved in greater mystery. The formation of dew is 
 immediately referable to the function of radiation, concerning which it will be 
 proper to make a short explanation in addition to that which has been already stated 
 at page 480. 
 
 In that place the general indication only has been made that a heated body for 
 example, a cannon-ball if suspended in space, darts off heat cognizable on temperature 
 under the condition of rays. It remains now to be stated that the function of radiation 
 is determined as to its extent by the surface of bodies : rough metallic surfaces radiate 
 more than those which are smooth; glass surfaces radiate more than metallic surfaces; 
 plants radiate more than the earth ; grass and leaves more than bushes and trees ; loose 
 gravelly land more than hard soil. 
 
 To demonstrate tLa effect of surface on radiation, many instructive experiments may 
 be performed by means of the differential thermometer and a cubical canister of tin 
 plate. If such a canister be taken, and one side of it scratched, another polished 
 smooth, another painted white, and the fourth black, a mixture of lamp-black and size 
 being used by preference for the latter purpose if the canister be now filled with hot 
 water and held between the two bulbs of a differential ther- 
 mometer as represented in the accompanying diagram (Fig. 
 40), each side of the canister will represent and indicate a 
 different amount of radiating influence, as shown by the 
 complementary disturbance of the two mercurial columns. 
 It will be found thut the polished side has the minimum, 
 and the blackened side the maximum, radiating effect. It 
 will soon be perceived that these deductions concerning the 
 property of radiation are intimately connected with the phi- 
 losophy of dew. 
 
 Until the experiments of Dr. Wells, which will be soon 
 adverted to, the most erroneous notions prevailed concerning 
 the theory of dew. According to some it fell from the sky, 
 according to others it rose from the ground, both which 
 I<1: ' 40t theories are altogether untenable. 
 
 It is a sufficient answer to the proposition that dew falls from the sky, to say that 
 dew never occurs when nights are cloudy ; and it is a sufficient answer to the 
 statement that it rises from the ground, to remark that a slight screen thrown 
 on the ground, or elevated above the ground, is incompatible with the formation of 
 dew. 
 
 The theory of dew is hardly explained by a consideration of the laws of radiant 
 heat. The starting point of the investigation is the atmosphere. Now the atmosphere 
 always contains moisture, as I have already explained, and the amount of this moisture 
 will, cceteris paribus, be correlative wiih the degree of atmospheric heat at the time. Ii', 
 then, the atmosphere being raised to its fullest point of saturation for any given degree 
 of temperature, that temperature should by any chance fall, the result will necessarily 
 be a deposition of moisture. Let us now apply these principles to the conditions of 
 a heat-radiating surface of the earth and a clouded sky. In this case no dew occurs, nor, 
 according to theory, should any occur, inasmuch as the clouds perform the functions of 
 a second radiating surface. The earth radiates heat owing to the clouds ; but the clouds 
 in their turn radiate heat back again to the earth ; whence it follows that the earth 
 practically does not lose heat, and its temperature not falling below the temperature 
 
FORMATION OF DEW. 
 
 51 
 
 of the circumambient atmosphere, no atmospheric moisture can be deposited ; in other 
 words, no dew can occur. 
 
 For these facts we are indebted to the late Dr. "Wells. They are demonstrated by 
 thousands of natural conditions, and bear the test of any properly-devised ex- 
 periment. 
 
 The following diagram (Fig. 41) is intended to show the manner in which a screen 
 will prevent the occurrence of dew. Two plates of glass are represented as supported 
 
 over an expanse of grass. 
 Underneath the glass plate 
 not the slightest dew will be 
 foundjthough the grass around 
 will be dewed heavily. 
 
 A very pretty illustration 
 of the conditions which 
 regulate the formation of 
 dew, will frequently be sup- 
 plied by a sheep lying down 011 the grass, on a clear, tranquil, cloudless night, when, 
 to use a popular but incorrect expres- 
 sion, dew is falling; it will be found 
 that the upper part or aspect of the 
 wool of a sheep, is completely drenched 
 with do\v, although the under part or 
 aspect of the animal is dry, as represented 
 in the accompaning diagram (Fig. 42). 
 
 The explanation of this phenomenon 
 will be so obvious that no further remark 
 concerning it is necessary. 
 
 Consideration of the laws of radiant heat will render manifest the reason wherefore 
 some surfaces are more bedewed than others. The amount of dew will depend, cceteris 
 paribus, on. two circumstances firstly, on the kind of surface ; and secondly, at its angle 
 
 Fit:. 43. 
 
 of inclination. Reference has already been made to the comparative facility wherewith 
 certain bodies found in nature favour the deposition of dew upon them, and the most 
 
52 
 
 AMOUNT OF DEW DETERMINED. 
 
 casual observer cannot fail to be struck with the difference. In all cases, the bodies 
 
 which indicate heat are the most 
 favourable to the deposition of dew upon 
 them. Few, if any, objects naturally 
 occurring are so solicitous of dew as 
 spiders' webs; and no object present the 
 phenomena of dew under a guise so 
 beautiful. Not unfrequently a thin 
 filament of cobweb, so small that it 
 would be invisible to the naked eye, 
 presents itself to the vision on a dewy 
 morning as if it were strung with little 
 pearls* (Figs. 43, 44). 
 
 That angular inclination of a body 
 should influence, and be intimately 
 connected with, the function of dew 
 formation directly follows from a con- 
 sideration of the laws of radiant matter ; 
 and it may be readily illustrated by a 
 diagram. Let us be assured that a 
 
 YW> -- 
 
 Fig. 44. 
 
 screen of glass be supported over the radiating surface of the earth in one case 
 horizontally, in the 
 other case at an an- 
 gular inclination, as 
 represented by the 
 accompanying dia- 
 grams (Figs. 45,46). 
 It will be evident 
 that the horizontal 
 glass in Fig. 45 will Fig. 45. 
 
 radiate back more heat than the diagonal glass in Fig. 46. 
 
 Determination of the Amount of Dew. Although the laws which regulate the forma- 
 tion of dew are perfectly well known, and a rough method of determining the amount 
 of dew deposition not difficult, yet no correct means of estimating its actual amount 
 has yet been devised. Instruments for determining the amount of dew are called dro- 
 someters. A drosometer is a balance, suspended to one arm of which is a plate, to the 
 other a pan containing weights exactly proportionate to the weight of the plate, so that 
 both may be in equilibrio. Supposing dew to be deposited on the plate, evidently the 
 latter will increase in weight by the amount of such deposition, and a deviation of the 
 beam from the horizontal will ensue. The'priuciple of the instrument is unimpeach- 
 able, but in practice it is imperfect. Instead of the plate as described, recourse may be 
 had to a lock of wool or eider-down, or one of a large choice of hygrometric materials. 
 Bodies of this kind were employed by Wells. "Wilson and Flangergue had recourse to 
 a plate, but it seems that the materials employed by Dr. Wells should have the pre- 
 ference. 
 
 A very instructive experiment relating to dew formation, and one which may be 
 regarded as presenting a summary of the whole matter, is as follows: 
 
 If on a clear, cloudless night, when dew is being deposited, a glass ball (Fig. 47) be 
 
HONEY DEW. 
 
 53 
 
 suspended in the open air some height from the ground, dew-drops will form on the 
 ball ; not, however, equally on all portions of its surface. 
 Firstly, its upper aspect will be bedewed, then its sides ; 
 but only rarely, and in extreme cases, the inferior aspect 
 of the glass globe becomes covered with dew-drops, and 
 when they do occur they are very small ; indeed, a com- 
 plete gradation of size is manifest, the dew-drops decreas- 
 ing in size as the upper aspect of the globe is departed 
 from. 
 
 Generalizations. The following generalizations re- 
 lating to the phenomena of dew may now be appended ; 
 they will, for the most part, be seen to be directly dedu- 
 cible from a consideration of the laws of radiant heat: 
 Clouds and brisk winds are both inimical to the formation 
 of dew ; the former, because of their own radiating power ; 
 the latter, because of the removal of cool air, its place being 
 supplied by air already warmed. If the night be cloudy, 
 and the wind still, very little dew results. If clouds and 
 wind occur together, dew is totally absent. Screen-like 
 objects interposed between the sky and the radiating sur- 
 face produce an effect identical with clouds, hence bodies 
 freely exposed to the atmosphere, cesteris paribus, are most 
 freely bedewed. Morning and evening are not the times, as 
 commonly supposed, when dew is formed most copiously* 
 It is deposited at all hours of the night, but most copi- 
 ously rather after midnight. It sometimes occurs even 
 before night, late in the afternoon. Dew is not deposited with equal readiness in all 
 parts of the world, but attains its maximum in warm lands near the margin of the sea, 
 rivers, or lakes ; as, for example, near the Red Sea, the Persian Gulf, the coast of 
 Coromandcl, at Alexandria, and in Chili. It is quite absent in very arid regions, in the 
 interior of continents such, for example, as central Brazil, the Sahara, and Nubia; 
 neither does it frequently occur at sea, because of the bad radiating quality of a surface 
 of water. 
 
 The imperfect radiation of a surface of water is well illustrated by the following 
 striking experiment: Glass is a good radiating surface; whence a piece of glass 
 freely exposed in an atmosphere when dew is forming soon becomes covered with dew. 
 If, however, the glass have its surface wetted previously to exposure, instead of 
 becoming more wet it becomes dry, simply because radiation is impeded, and evapo- 
 ration takes place unchecked. Cccteris paribus^ the amount of dew produced will be 
 proportionate to the amount of aqueous vapour present in the atmosphere, and thus 
 readily explains the fact that a copious production of dew is frequently the precursor 
 of rain. 
 
 Honey-Dew. Occasionally a sweet, damp, sticky moisture attaches itself to leaves 
 during the night, and does not disappear throughout the day. The term honey-dew 
 has commonly been applied to it, though, in point of fact, it is not dew at all, being 
 merely an excretion from certain insects termed aphides. 
 
 Hoar-Frost. Hoar-frost differs only from dew in the circumstance of temperature. 
 One is deposited and remrans uncongealed ; the other, becoming consolidated by the 
 
54 THEORY OF FOGS. 
 
 agency of freezing cold, is converted into ice. Every meteorological observer knows 
 that the existence of hoar-frost is held to be indicative of coming rain, and in most 
 instances the opinion is verified. The phenomena of hoar-frost are even more beautiful 
 than the corresponding ones of unfrozen dew. Upon leaves and vegetable stems the 
 deposition of hoar-frost is particularly beautiful. If hoar-frost be examined microsco- 
 pically, or sometimes even attentively by the naked eye, a crystalline structure will be 
 evident. The crystals belong to the same crystallographic system (the rhombohedric) 
 as those of snow, but their general appearance is somewhat different. 
 
 Fogs. Fogs may be regarded as clouds which form close to the earth's surface ; 
 hence we might discuss their peculiarities under the general head of clouds. They are 
 characterized by some peculiariaties, however, chiefly dependent upon the vesicular 
 aqueous vapour of which they are composed, embracing and retaining volatile particles 
 evolved naturally or from the operations of man. In this way it is well known that 
 London fog is anything but pure aqueous matter. One of its very important consti- 
 tuents is the condensible part of smoke. As regards the production of fog it is usually 
 referable to one of two circumstances -. either non- visible aqueous vapour may be con- 
 verted into the visible or vesicular form by decrease of atmospheric temperature imme- 
 diately, or by the cooling agency of the earth's surface depressing the temperature of 
 the layer of air next to it below the dew point. In this country, and in Europe gene- 
 rally, fogs are of most frequent occurrence in spring and autumn. There are regions, 
 however, in which fogs prevail throughout the year. The coasts of California are 
 almost constantly veiled in fog ; and the same remark applies, in a minor degree, to the 
 western coast of the American continent, even so far south as Peru. Newfoundland, 
 Nova Scotia, and Hudson's Bay, are all subject to frequent and dense fogs, attributable 
 in these localities to the condensation of vapour which arises from the hot gulf-stream 
 by contact with neighbouring and colder air. Fogs do not occur so frequently on level 
 plains as on mountainous regions. In Arabia and the arid table-land of Persia they arc 
 almost altogether wanting. London and Amsterdam have acquired a somewhat evil 
 character for fogs ; but this meteoric condition applies to many other European locali- 
 ties with an almost equal amount of propriety. At Antwerp fogs are very prevalent, 
 and the navigation of the lower and middle Rhine is sometimes impeded for weeks 
 together by the occurrence of this pest of the sailor. Neither can Paris boast of much 
 immunity from fogs ; they are somewhat less dense and less frequent than our own 
 London fogs, it is true, but are, nevertheless, far from contemptible. Amongst the 
 regions which are likely in future to be celebrated for the prevalence of fogs, the Black 
 Sea may be enumerated. Until recent events that locality was comparatively unknown 
 to us ; and since the conveying of stores to our troops in the Crimea has necessitated 
 its continuous navigation, the embarrassment of fogs has only been too apparent. 
 
 Dry Fogs. Under this name has been described a dull opaque appearance which the 
 atmosphere of certain regions occasionally assumes, deadening the fiery beams of the 
 sun, and dulling that luminary so that he may be looked fit without pain by the naked 
 eye, and embarassing respiration. 
 
 The dry fog is most common in certain parts of North America, during the period 
 known as the Indian Summer. It also occurs in Germany, and more rarely in Eng- 
 land. There can be no doubt that many atmospheric opacities, different in character as 
 well as in cause, have been summarily classed under the denomination of dry fog. When 
 the phenomenon occurred locally, it can generally be traced to such causes as the burn- 
 ing of extensive districts of turf or of forests. When its prevalence is mere general, the 
 
DIVISION OF CLOUDS. 55 
 
 most rational explanation would seem to be that which attributes it to volcanic erup- 
 tions. Some meteorologists have invoked electricity as the cause of this phenomenon, 
 but it is not easy to see in what way the assumed cause could produce the effect in 
 question. Electricity has long been to meteorologists what the class radiata was 
 to Cuvier ; namely, the receptacle for things unknown or unexplained. 
 
 Clouds. Clouds are perhaps the most beautiful of all aerial phenomena. All the 
 charms of changeful variety of colour, of form, and of motion are theirs ; nor is their utility 
 inferior to their beauty. Without clouds there would be neither rain, nor snow, nor hail ; 
 the consequences of this deprivation may be anticipated, or they may be readily learned, 
 by turning to the geographv of countries where rain, and snow, and hail are unknown. 
 All regions thus circumstanced, provided irrigation be impossible, and that the altogether 
 exceptional condition of copious dews be absent, are, despite the most favourable con- 
 ditions of climate and of soil, barren wastes. 
 
 Notwithstanding the thousandfold varieties of clouds their protean shapes, their 
 manifold colours, and other distinctions when the observer comes to regard them 
 with a scrutinizing eye, he will not fail to recognize broad distinctions between them, 
 admitting of being made the basis of a philosophic classification. Thus, some clouds 
 are devoid of outline, their edges merging away into circumambient air ; some aro 
 black and massive, almost conveying the idea of a hard substance; some are white 
 and fleecy ; others extended like a pennon. All these are forms of cloud which present 
 manifest distinctions amongst themselves. Mr. Howard was the first who effected a 
 regular classification of clouds. This classification is now generally adopted, I shall, 
 therefore, present the reader with an outline of his system. According to this meteoro- 
 logist, there are three elementnry and four secondary forms of cloud. 
 
 Primary Forms. The first primary form is the cirrus, consisting of feathery expan- 
 sions, and which is only seen in clear weather. 
 
 The second primary form of cloiid is the cumulus, composed of large hercisnheroidal 
 masses superiorly and apparently resting below on a horizontal base. This form of 
 cloud chiefly occurs in summer. 
 
 The third primary form of cloud is stratus, composed of horizontal layers, the smaller 
 layers being underneath. It is this form of cloud, more than any other, which presents 
 itself under a variety of beautiful colours. It chiefly appears at sunset. 
 
 Secondary Forms. The secondary forms of cloud-? are (1) Cirro cumulus. It is a 
 mixture, as its name indicates, of cirrus with cumulus, nnd is made up of an aggre- 
 gation of small white clouds, which have been compared to a flock of sheep. (2) Cirro 
 stratus. A compound cloud, which is formed of the two primary clouds embodied in its 
 name. (3) Cumulo stratus. This compound cloud chiefly appears towards night in dry 
 windy weather, and is of a leaden colour. (4) Nimbus, or rain cloud. This cloud is 
 seen in greatest perfection during a thunder-storm. All the varieties of clouds 
 described are represented in the appended diagram (Fig. 48). 
 
 It will be readily anticipated that clouds are frequently so mingled and con- 
 founded, that they are not always susceptible of the precise classification just 
 announced ; nevertheless, a prevalence of one type of cloud over another will be 
 generally seen to prevail. 
 
 Relation beticeen Clouds and the Weather. People who are in the habit of narrowly 
 studying the phenomena of clouds, are enabled. to draw conclusions of much accuracy 
 respecting the coming weather. Thus cirro cumuli ore for the most part indicative of 
 serene fair weather ; the prevalence of wind subsequently to the appearance of much 
 
CLOUDS DESCRIBED. 
 
 extended and highly-coloured stratus clouds, is a matter of popular experience. The 
 appearance of nimbus clouds proclaims the advent of rain ; and the cirro stratus 
 
 Fig. 43. 
 
 which sometimes colours the sky as with a veil, all well-defined form being absent, is 
 almost a sure forerunner of bad weather. 
 
 Composition of Clouds. That clouds are composed of water in some condition does 
 not require to be demonstrated ; but some explanation must be given of the circum- 
 stances enabling water, a material so much heavier than atmospheric air, to remain 
 suspended frequently in very elevated regions, where the atmosphere, thin though it 
 be on the earth's surface, is still more attenuated. This is a matter which it must be 
 confessed is still veiled in considerable obscurity; but perhaps the most rational ex- 
 planation of cloud formation is the following : Firstly, aqueous vapour is diffused, or 
 rather absorbed, invisibly throughout the air. The laws of the absorption or diffusion 
 are perfectly well known. The amount differs, as we have already seen, for different 
 temperatures, being proportionate to the temperature. Assuming, then, that the upper 
 regions of the atmosphere at any time are saturated with atmospheric moisture in its 
 invisible condition, let us now contemplate the effect of cooling that atmosphere by 
 any cause. It is not difficult to furnish reasons for these cooling agencies ; they are 
 numerous and varied such as sudden variations of electric condition, sudden varia- 
 tions in the direction of winds. If, then, from any cause the atmosphere is cooled 
 below its capacity for holding vapour, in the invisible form, aqueous deposition occurs. 
 If this deposition take place on the earth's surface, the result is dew ; if aloft in the 
 air, we have a cloud. 
 
FORMATION OF CLOUDS. 57 
 
 Thus far th steps of each succeeding change are evident ; hut the remaining points 
 of cloud-formation are more ohscure, the circumstance of chief difficulty heing to find 
 an explanation of the aerial permanence of clouds, seeing that the material of which 
 they are composed is so much heavier than air. Probably the most consistent expla- 
 nation is this : Atmospheric moisture, when it changes from the invisible to the 
 visible form, assumes the physical condition of spheroids or vesicles minute bubbles of 
 water, in point of fact, each bubble filled with air. If these vesicles be exposed to the 
 sun's rays, it is evident, from consideration of known laws, that they must become 
 specifically lighter than the surrounding medium ; and thus affected, they would float, 
 for the same reason that a soap-bubble floats whilst it is yet warm, notwithstanding that 
 the air which it contains is heavier than the surrounding atmosphere. We seem, there- 
 fore, in a position enabling us to account for the upper part of cloud strata. Directing 
 our attention now to the lower part of these strata, it seems rational to assume that the 
 vesicles of which they are composed should descend. Probably they do so ; continuing 
 their descent until they come in contact with an atmosphere sufficiently warm to dis- 
 solve their aqueous coating, and convert their water once more into the vaporous or 
 invisible form. Thus it may be, and most probably is, that a cloud which looks per- 
 manent to the eye, is really exposed to the continued operation of resolution and re- 
 formation : or, rather, that the two opposing causes, which are here spoken of 
 as producing active changes, balance themselves, and give rise to a condition of 
 equipoise. 
 
 Although it has hitherto been taken for granted that clouds are formed of unfrozen 
 water, we know that such is not invariably the case. If the temperature of a nimbus 
 cloud sinks to freezing point, or 32 F., its contents freeze, and snow is the result. 
 Many philosophers, indeed, but more especially Kaemtz, are of opinion that the cirrus 
 the cloud which soars in the highest regions, frequently at an elevation not less than 
 20,000 feet above the earth's surface consists of particles of snow or ice. Assuming 
 this to be the case, it is not easy to advance the reason of such molecules remaining aloft 
 in a medium so attenuated as is atmospheric air in a position so elevated. 
 
 Position of Clouds. It is somewhat remarkable that every known form of cloud 
 assumes more or less the horizontal position. Vertical clouds are very rare ; and if we 
 choose to except water-spouts, not recognizing them as clouds, perhaps we may say un- 
 known. The horizontality of clouds, warrants their being spoken of as composed of 
 strata ; generally, indeed, these strata are very well defined. MM. Paytier and 
 Hoffard have carefully examined the thickness of these cloud-strata on the Pyrenees, 
 and have found their average variation to be between the limits of 3400 and 1600 feet. 
 The maximum observed thickness was 5000 feet, although this measurement is un- 
 doubtedly in some cases greatly exceeded. 
 
 Height of Clouds. Several meteorologists, amongst whom Eiccioli, "Wrede, Kaemtz, 
 and Arago must be particularly mentioned, have set themselves to the task of discover- 
 ing the height of clouds. The methods by which these investigations were made have 
 been various. Eiccioli determined their height by placing two observers a certain and 
 known distance apart ; "Wrede by making use of their shadows, and then reducing the 
 computation of height to the solution of a problem in trigonometry. Eiccioli states the 
 maximum height of clouds to be 25,000 feet, and Lambert takes their minimum height 
 at 13,000, whilst their maximum height, according to the same, another is from 15,000 
 to 20,000 feet. Gay Lussac, when he acquired in his balloon an elevation of 21,600 feet, 
 perceived small clouds floating still much above him. Perhaps the statements of 
 
58 
 
 HEIGHT OF CLOUDS. 
 
 Kaemtz, relative to the height of clouds above the earth, are the most trustworthy. 
 He believes the usual range of cumulus to be from 3000 to 10,000 feet; of cirrus from 
 10,000 to 24,000 feet; of nimbus, or thunder-cloud, between 1500 and 5000 feet. That 
 very accurate physicist Pouillet, as the result of certain experiments performed in 1840, 
 
 states, that he 
 has proved the 
 existence of 
 clouds at an 
 elevation from 
 about 22,300 
 to 38,000 feet, 
 and probably, 
 as the general 
 result of all re- 
 corded trust- 
 worthy obser- 
 vations on tho 
 elevation of 
 clouds, we may 
 arrive at the 
 conclusion that 
 cirrus does not 
 descend below 
 
 40. 
 
 2000 or 3000 feet, whilst nimbus occasionally descends so low that it approaches the 
 
 earth within a few hundred feet of surface. The maximum mean elevation of clouds 
 
 seems, in low 
 
 latitudes, evi- 
 
 dently on ac- 
 
 count of the 
 
 greater capa- 
 
 city of the 
 
 atmosphere to 
 
 absorb and dis- 
 
 solve aqueous 
 
 vapour. It 
 
 should here be 
 
 remarked that 
 
 the elevation 
 
 of a cloud 
 
 cannot be de- 
 
 termined by 
 
 reference to its 
 
 apparent place 
 
 in the sky ; 
 
 and, except 'the distance be known, neither can its actual size. These remarks a 
 
 illustrated by the accompanying diagram (Fig. 50), where the same cloud will be seen 
 
 atdifferent times under very different angles by the same observer, as reference to the angles 
 
 A E C and B E D will testify, whence the height would differ for two different statures. 
 
THEORY OF EAIX. 59 
 
 And again, in the subjoined diagram it is demonstrated that the observer at A will 
 see clouds which are quite invisible to another observer at B. 
 
 Rain. When from any cause cloud- vesicles aggregate into drops, and these drops 
 fall, the result is rain. Rain would appear, therefore, to be necessarily dependent on a 
 cloud; and in a large majority of instances we find this to be the case. Still, the phe- 
 nomenon of rain without clouds is well attested. The amount of rain which falls at 
 any place, and at any stated interval, may be readily estimated by means of an instru- 
 ment termed a pluviameter, or rain-gauge. The principles on which this instrument is 
 founded are of the simplest kind. If, for example, a cup or basin, of known cubic 
 capacity and known orifice, be exposed so that it may receive the fallen drops of rain, 
 the cup or basin would be a rough rain-gauge. "Were it not that the collected water 
 thus exposed would be continually evaporating, thus apparently diminishing the total 
 .fall of rain, no better rain-gauge need be desired ; but in practice it is necessary to 
 provide against such evaporation ; and it is usually accomplished by forming an instru- 
 ment of such construction that one part may be destined to collect the fallen rain, and 
 another part to diminish, within the smallest limits, the amount of evaporation. In 
 expressing the amount of rain which falls at any particular spot, it is necessary to 
 express the height also at which the observation is made. The annual fall of rain increases 
 as land acquires elevation ; nevertheless, at one and the same place the amount of rain 
 decreases with elevation, for the reason that each drop of rain throughout its descent 
 goes on collecting moisture and becoming larger. Dalton appears to have been the 
 first to notice this feet. He proved, on comparing two sets of observations, one set 
 made at the base, the other at the summit, of a high tower, that the amount of rain at 
 the top to that at the bottom was as two to three. Similar observations made at the 
 Paris Observatory have led to similar results. This variation between the amount of 
 rain at diiferent elevations in one locality, is the more considerable as the point of 
 aerial saturation is more nearly attained ; for which reason it is less in summer than in 
 winter. The heaviest rains usually occur in the tropics, and during the hot season. 
 There the fall of rain is enormous, sometimes amounting to an inch per hour ; nay, 
 Humboldt has related that in South America no less than five inches of rain fell in one 
 hour. In these islands we can hardly say that any one season merits the designation 
 of the rainy season ; but in tropical regions, except the belt of calms, and in the sub- 
 tropical regions, the separation between the dry and rainy seasons is well marked. 
 In the continental portions of the torrid zone, the rainy season sets in when the summer 
 heat attains its maximum, and continues during four or five months, the atmosphere 
 being clear and bright throughout the remaining portion of the year. Near the equator 
 there are two wet seasons, sometimes separated from, each other by a totally rainless 
 period, but at other times demarcated only by periods of maximum and minimum fall of 
 rain. Dutch Guiana furnishes the well-known illustration of a country having two well- 
 marked rainy seasons : one, and that the chief, commences in April, and lasts till June ; 
 the other, or minor rainy season, commencing in the middle of December lasts till 
 the middle of February. The drops of tropical rain attain a magnitude never seen 
 in the tamer showers of these northern regions ; their weight is so considerable, and the 
 force with which they descend so great, that their splash or stroke leaves a smarting 
 sensation on the skin. The region situated between the influence of the two trade- 
 winds, and commonly known as the region of calms, is devoid of periodic rains, although 
 the fall of rain there is frequent and heavy. 
 
 Rainless Portions of the Earth. There are some localities in which rain never 
 
60 
 
 ANNUAL DISTRIBUTION OF KAIN. 
 
 occurs: for example, Egypt, the Desert of Sahara, the table-lands of Persia and Mon- 
 golia, the rocky flat of Arabia Petraa, &c. Rain is generally the most abundant near 
 mountain ranges ; but there arc exceptions, one of the most remarkable of which is 
 presented by the part of Spain south of the Sierra Nevada. 
 
 Condition of Europe with regard to Rain. Europe, considered in relation to the 
 prevalence of rain, admits of being divided into three districts the South European, 
 the Middle European, and the Swedish. In Portugal and the larger portion of southern 
 and central Spain, there is an almost total absence of rain during summer ; but north of 
 the Pyrenees, rain occurs at variable times throughout the whole year. All the portions 
 of Europe north of the Alps and Pyrenees are subject to the Middle European and the 
 Swedish pluvial conditions. The characteristic of the Middle European climate as 
 regards rain is, that the latter chiefly occurs during westerly winds; whereas the 
 Swedish climate is characterized by the prevalence of rain during both easterly winds 
 and westerly winds, which bring rain to the whole of Central Europe, and deluge our 
 isles with wet, leaving the bulk of their moisture by the mountainous Scandinavian 
 range which separates Norway from Sweden. St. Petersburg and Moscow cannot bo 
 said to belong either to the Central or Northern European climate ; these places lie- 
 on the confines of both ; hence neither westerly nor easterly winds are there prevalent.. 
 In England, the maximum number of rainy days throughout the year occurs in Corn- 
 wall and Devonshire ; passing thence east into Central Europe, the total number of 
 rainy days per annum continually declines. If we assume the annual amount of rain 
 which falls at St. Petersburg to be nearly three, the corresponding annual amount for 
 the West of England will be 2-1 ; in Central England, 1-4 ; in Central Germany, 1-2. 
 This statement assumes an average of some special localities to have been taken into 
 consideration: special places present many deviations. In describing any place as 
 subject to rain, or rainy, distinction must be made between the actual quantity of rain 
 per annum which falls, and the total average number of rainy days. Understanding 
 by the latter term every day on which rain, much or little, falls, the number of rainy 
 days increases in Europe from south to north. The mean average for Southern Europe 
 may be taken as 120, in Central Europe as 146, and in Northern Europe as 180. The 
 following statement indicates the total number of rainy days per annum for a few places 
 specifically named :-~ 
 
 Buda 
 
 Warsaw .... 
 Germany, average of 
 Caiisniho .... 
 Tagcrnsco .... 
 Munich .... 
 Stutteardt . 
 
 112 
 
 138 
 150 
 174 
 170 
 1-19 
 127 
 
 Ratisbon 115- 
 
 Rotterdam 187 
 
 Paris ........ 160 
 
 Poitiers 99 
 
 St. Petersburg 168 
 
 Moscow . 205 
 
 Annual Distribution of Main. The time of year at which raii> is most prevalent, is 
 subject to much variation for different countries. Throughout Central Europe rains 
 are most prevalent in summer, but in Southern Europe the preponderance is on the sido 
 of winter rains. Norway is subject to copious winter rains ; whilst in Sweden they aro 
 almost entirely wanting. Sweden, in point of fact, although placed so near the sea, has 
 a climate altogether like that prevalent in continental regions. The reason wherefore 
 Norway is subject to winter rains, and Sweden is deprived of the same, hinges upon the- 
 explanation already made that western winds (which predominate in Scandinavia during 
 
SHOWERS OF FISHES, STONES, ETC. 61 
 
 "winter) lose their moisture in passing over the Scandinavian range. Although summer 
 rains are in many places rare, yet when they occur they are generally more copious 
 than rains at any other period. 
 
 Rain which falls in summer at different places, taking the rain which falls on a 
 winter's day at the corresponding place as unity :- 
 
 England 1'07 
 
 Western France . T03 
 
 Gcimany 1'76 
 
 St. Petersburg 2'17 
 
 Central France 1*57 
 
 from which statement it appears that the prevalence of summer rain increases towards 
 vthe east. 
 
 Peculiarities of Rain- Water. Fresh-falling rain-water, collected far from towns or 
 other sources of local contamination, is vciy nearly pure ; nevertheless, modern chemical 
 observation has succeeded in discovering the presence in rain-water of many substances 
 present in small quantities. Nitric acid and nitrate of ammonia arc by no means unusual 
 constituents ; and iodine has been frequently recognized. As regards the sources of "these 
 and other extraneous bodies, much still remains to be discovered. Nitric aeid is most 
 probably formed in the atmosphere by the agency of electricity ; and the ammonia may be 
 referable to exhalations from decomposing matters on the earth's surface. M{iny of the 
 extraneous bodies, especially salts, sometimes recognized in rain-water, are un- 
 
 questionably due to the action of winds upon finely-divided ocean-spray. 
 
 Showers of FisJt,cs, Stones, $c. Instances are on record of whole shoals of iishes, 
 and numerous collections of other animals also stones, &c. being cast on the earth by 
 showers. At one time these phenomena were regarded mysteriously, and referred to 
 occult causes. At present they are deprived of their mystery, and referred to the 
 previous elevation of the fishes, &c. by aerial currents, whirlwinds, and water-spouts. 
 
 Snow. If the temperature of a cloud should fall at any time to 32 F. or lower, 
 instead of rain the result is snow. Much that is beautiful and beneficent is seen in 
 this divided form of frozen water. In our own temperate clime we do not comprehend, 
 except by reflection, the true value of snow in the economy of nature. Its fall amongst 
 us is uncertain and exceptional ; we know not when it is to come, or how long it is to 
 remain. We therefore make no provision for it regard it as a condition to bo tolerated 
 regret that it interferes with our locomotion that it impedes our railway trains, and 
 wets our feet ; and wish it away. Nevertheless, even in these isles, the farmer, from 
 
 experience, is not insensible to the value of snow. He says it keeps his winter crops 
 warm ; and the thoughtless passer-by, wrapped in his own self-conceit, laughs at him 
 for making a statement so apparently grotesque. The philosopher, however, who is 
 aware of the low heat-conducting power of snow, and who can appreciate the evil con- 
 sequences of frost on vegetation, indorses the farmer's statement. 
 
 If we would desire to recognise the full benefits of snow, we must direct our atten- 
 tion to northern climes to Sweden, to Russia, and Canada. There the advent of snow 
 is looked forward to as a blessing ; and when it comes, the period of its duration admits 
 of being predicted with tolerable accuracy. No sooner is the ground covered with 
 sufficient snow, than wheeled-carriages, which but yesterday wqre sticking up to the 
 axlctrce in mud and wet, are put aside, and sledges supplied in their stead. Market- 
 places, which before the snow had fallen were naked and iinworthy, now teem with 
 good things brought from hundreds of miles away. Snow has all at once laid clown a 
 far-stretching railroad, over which the sledges glide almost with the ease and velocity 
 of a railway-train. 
 
62 
 
 THE SXOW LINE. 
 
 Form of Snow Flakes. In certain conditions of temperature snow falls as a pul- 
 verulent body, in other conditions as a flaky amorphous mass ; but if very dry snow be 
 microscopically examined before it has been broken up, indications of crystalline 
 structure will be recognizable. Sometimes these crystalline snow-flakes attain such 
 large dimensions, that they are quite evident to the naked eye. The crystalline forms 
 thus developed are numerous, but they are all referable to one crystalline system, the 
 rhombohedric or rhombohcdral ; the characteristic of which is that crystals belonging to 
 it have three axes crossing each other at the angle of sixty, and one axis at right 
 
 angles to these. Scoresby, who has mi- 
 nutely examined these snow-flakes, de- 
 scribes five principal forms of snow 
 crystals : 1st, crystals having the form, 
 of thin plates, which are the most 
 abundant; 2nd, surfaces or spherical 
 nuclei, with ramifying branches in dif- 
 ferent planes ; 3rd, fine points, or six- 
 sided prisms ; 4th, six-sided pyramids. 
 The latter form is the least frequent of 
 
 all. The accompanying diagram (Fig. 51) represents the principal crystalline varieties 
 of snow-flakes. 
 
 The Snoiv-line. Inasmuch as the upper regions of the atmosphere are intensely 
 cold, there is an elevation for every latitude at which atmospheric moisture is changed 
 into snow. This elevation corresponds with what is termed the snoio-Une. At the 
 equator the snow-line is elevated from 11,000 to 12,000 feet above the sea-level. As 
 we proceed towards the north, the elevation of the snow-line will evidently be lower. 
 Snow does not fall on level ground in Europe farther south than Central Italy ; but in 
 Asia and America the region extends nearer to the equator. Through Florence passes 
 the isothermal line of 59 F., and it may be regarded as the southern limit of the 
 region in which snow falls on level places. Snow does not usually fall at the time of" 
 maximum cold ; some meteorologists say it never does but this is an error. After 
 snow has fallen the weather generally increases in severity. We are usually in the habit 
 of assuming that the total quantity of snow which falls increases as we reach either pole 
 an assumption, however, which is only correct within certain limits. Thus, taking tie 
 northern hemisphere, for instance, the fall of snow increases from the isothermal of 
 59 F. to the isothermal of 41 F., which latter cuts the town of Drontheim, in Norway. 
 Passing still further north, the quantity of snow goes on diminishing, evidently because 
 in the polar regions the temperature of the air is too cold to retain much moisture, and 
 atmospheric moisture must necessarily be the antecedent to either rain or snow. 
 
 The atmospheric condition during the fall of snow may vary from the limits of 
 almost complete tranquillity, to the other extreme of most violent perturbations. In 
 Germany, and other countries having a corresponding latitude, the fall of snow is 
 usually tranquil, except during the months of February and March. In high latitudes 
 snow usually occurs during violent tempest-gusts, almost equal sometimes to the West 
 Indian hurricane or the Chinese typhoon. In Norway these storms are very frequent, 
 also in Kamtschatka ; in which latter region they are called purga. They are veritable 
 thunder-storms, as is completely proved by the intense electrical condition of the atmo-- 
 sphere. On mountainous elevations snow-storms are commonly prevalent, irrespective 
 of latitude. 
 
HAIL AND ITS PHENOMENA. 63 
 
 Coloured Snow. Red and green snow have been frequently described by travellers. 
 The cause of these phenomena is now referred to the presence of minute algse the 
 protococctis nivalis in the so-called red snow, and the protococcus viridis in the green 
 variety. 
 
 Hail. Frequently cloud-vesicles become aggregated and frozen into lumps of vari- 
 ous sizes and shapes, sometimes opaque, sometimes transparent, and occasionally, though 
 not very often, containing neuclei of solid foreign matters. Meteorologically, aggre- 
 gations of this kind constitute hail. In most parts of the world where rain occurs hail 
 is known, but certain localities are particularly subject to hail-storms. Generally hail 
 falls by day ; indeed an opinion prevails that hail-storms are unknown by night. This 
 supposition is, however, erroneous. The form of hail is various, though for the most 
 part they assume a spherical, spheroidal, paraboloidal, or pyriform contour ; and still 
 more frequently they are rounded, flattened, or angular. According to Delcross, the 
 most common form of hail is that of a three-sided spherical segment, resulting from the 
 comminution of larger spheres. 
 
 The diameter of hail-stones at a mean latitude is, according to Muncke, not usually 
 greater than one and a-half or one and three-fourths of an inch, although on some 
 occasions blocks of ice of enormous dimensions have fallen. For example, in 1719, 
 there fell at Kremo, hailstones weighing not less than six pounds ; and at Namur, in 
 1717, others weighing not less than eight pounds. Again, it is stated that, in 1680, 
 masses of ice fell in the Orkneys twelve inches thick; and in 1795, hailstones fell in 
 New Holland from six to eight inches long and two fingers thick. It is recorded, but 
 on doubtful testimony, that there fell in Hungary, on May 28, 1802, a piece of ice 
 three feet square by two feet thick, and the weight of which was 1100 pounds. But 
 even this is much exceeded by a statement that, in the latter part of the reign of Tippoo 
 Saib, a lump of ice fell at Seringapatam as large as an elephant. The size of the 
 elephant, however, is not mentioned. 
 
 With regard to foreign substances existing in hailstones, they are described as various. 
 In 1755 there fell in Iceland hailstones containing sand and volcanic ashes ; others which 
 fell in Ireland in 1821 contained a metallic nucleus, which proved, on analysis, to be 
 iron pyrites (sulphide of iron) ; a similar phenomenon occurred in Siberia in the year 
 1824. The presence of small pieces of straw in hail has been frequently demonstrated. 
 
 The largest hailstones fall in summer during thunderstorms. Storms of this kind 
 are most frequent in Juno and July ; they are more rare in May, August, and September, 
 and still more so in April and October. They usually occur at the close of long periods 
 of calm, sultry weather. Hail-clouds are much lower in the sky than rain-clouds ; and 
 are generally recognisable by a peculiar ragged or jagged contour, and by their lower 
 portions being marked with white streaks, the other portions of the cloud being inky 
 black. Previous to the occurrence of hail, the barometer sinks very low ; and, what is 
 unusual before rain, the thermometric column suffers a corresponding depression. The 
 thermometer during a hail-storm has even been known to sink through 77 F. A pecu- 
 liar rustling sound in the air is also indicative of speedy hail, by a darkness resembling 
 that dependent on an eclipse of the sun. Hail-storms are very seldom of long duration, 
 usually lasting a few moments only seldom longer than a quarter of an hour. The 
 rapidity with which hail-storms travel is very great : one which occurred in Central 
 France in 1788 travelled at the rate of forty miles an hour. The force of hailstones is 
 sometimes dangerously great, not only breaking windows and shattering tiles, but killing 
 herds and men and animals, cutting off branches of trees and herbage, and, in short, 
 
64 ATMOSPHERIC MOISTURE HYGROMETERS. 
 
 desolating all save the largest vegetable growths. The hail-storm in France of 1788, 
 already adverted to, extended its devastations over 1039 parishes, destroying property to 
 the amount of 25,000,000 of francs. Although hail-storms often extend very far in a 
 linear direction, their breadth is usually inconsiderable, often but a few hundred, or at 
 most a few thousand feet, though the linear extension has been known to exceed 
 four hundred miles. 
 
 It has been already mentioned that wherever rain-clouds rest, hail may occur; 
 nevertheless, latitude and local conditions determine the frequency of the phenomenon. 
 Eain seldom occurs on the level land of tropical countries ; and it is rare in the extreme 
 north. The hail-belt, pre-eminently so considered, is comprehended between 30 and 
 60, and to elevations less than 6000 feet. Even within this belt, and below the limit 
 of elevation just assigned, there are certain localities where the occurrence of hail is a 
 very rare phenomenon. Certain of the Swiss valleys may be cited as a well-known 
 illustration of this remark ; more especially in the Valais and its allied dales. It has also 
 been well determined that hail more rarely occurs at the base of mountains than in 
 localities a short distance removed. Perhaps no country, upon the whole, is more subject 
 than France to the ravages of hail-storms, and in no country are the effects more serious. 
 It has been ascertained that the average annual number of hail-storms in France is 
 about fifteen. They are especially prejudicial to the vine and the olive, sometimes lay- 
 ing whole districts under desolation. Having regard to the highly excited electrical 
 condition of the atmosphere as the rule during the occurrence of hail-storms, great hopes 
 were once entertained that they might be prevented, or their ravaging power diminished, 
 by means of suspended conductors. The idea of using such conductors appears to have 
 been first suggested by Guenaut de Montbeillard in 1776 ; and hail- conductors have 
 been extensively tried, but hitherto without any amount of practical benefit to justify 
 their longer continuance. In 1820 a peculiar kind of hail-preventor was suggested by 
 La Postolle, and subsequently by Thollard. The instruments consisted of straw ropes 
 in which a metallic wire was interwoven, and suspended by means of pointed rods simi- 
 lar to lightning-conductors ; but, like instruments having a similar object, and which pre- 
 ceded them, they were found to be unavailing. 
 
 Methods of Determining the amount of Atmospheric Moisture. - 
 Having described the numerous forms under which aqueous moisture may exist in the 
 atmosphere, it now remains to indicate and describe certain instruments which have 
 been devised by different experimenters for determining its amount. These instru- 
 ments, founded on different principles, as will be seen, are termed hygrometers. 
 
 It is a matter of common experience that many bodies are affected as regards their 
 dimensions, more particularly their linear dimensions, by mutations of atmospheric 
 moisture. "Wood is a very common example of this property ; more particularly a stick 
 of wood cut transversely to the grain. Founded on this property of wood, the late Mr. 
 Edgeworth constructed a very ingenious toy, which, though it be not a hygrometer, 
 inasmuch as it does not measure the amount of atmosphere prevalent in the air, is at 
 any rate a hygroscope. It is related that the somewhat eccentric philosopher just named 
 once laid a wager that a certain toy a wooden horse constructed by himself, should, 
 after the lapse of some time, walk across his room. The horse was accordingly made, 
 and placed at one end of a chamber ; the door of the chamber was then locked, and the 
 key deposited in safe keeping. In process of time the horse did indeed arrive at the 
 other end of the room ; and the manner in which this was accomplished will now be 
 made evident. 
 
CONSTRUCTION OF HYGROMETERS. 65 
 
 Underneath, each hoof was a claw, long enough to stick into the flooring, and there 
 take hold. The horse itself was made out of a piece of wood cut transversely to the 
 .grain; the consequence was that when the weather was dry, the linear dimensions 
 of his hack contracted, and when the weather was wet his hack again elongated. Now, 
 hearing in mind the construction of the feet of this toy-horse, it is evident that these 
 alternate contractions and expansions must necessarily result in a forward motion. 
 
 Again, the condition of human hair illustrates the effect of varying amounts of 
 moisture in the atmosphere. Every lady knows that she cannot retain her hair in curl 
 during wet weather so well as when the weather is dry, because of the moisture present, 
 which causes the hair spirals to relax and unfold. In point of fact, each hair contracts 
 and elongates alternately hy every mutation of dryness and moisture ; so that if only 
 the exact ratio of contraction and expansion could be determined and applied, the 
 meteorologist might construct a hygrometer, having for its basis of actuation a human 
 hair. The hair-hygrometer of Saussure takes advantage of this principle. 
 
 To construct this hygrometer, a soft human hair is boiled for a short time in a 
 solution of sulphate of soda, afterwards for a few minutes in pure water ; it is then 
 well washed to free it from all adhering salt, and dried in a shady place. Next, one 
 extremity of the hair is fastened to the extremity of a little tongue, and the other end is 
 wound round a small pulley having two grooves. The second groove is for the purpose 
 of retaining a filament of silk, from which a weight is suspended for the purpose of 
 retaining the hair in a constant state of tension. To the pulley is fastened an index, 
 traversing a graduated arc, whenever the pulley turns in any direction by the contrac- 
 tion or elongation of the hair. The graduation of the instrument is thus effected : It is 
 placed in a receiver holding chloride of calcium, or concentrated oil of vitriol, the air is 
 exhausted from the receiver, and the place where the index then stands is marked. This 
 mark corresponds with the point of greatest dryness, or of the scale. We have next to 
 determine the point of greatest saturation, which is thus effected : The instrument is 
 next placed in a receiver containing a dish of water ; so that as the hair elongates the 
 index turns, and finally coming to rest, the point at which it stands is marked. This 
 mark corresponds to the maximum of moisture, which of course may be indicated by 
 any number arbitrarily selected ; this number being usually 100. Finally, it remains 
 to divide the interval between the two extremes into 100 equal parts, and the instrument 
 is complete. Notwithstanding all the care which may be devoted to the construction of 
 this instrument, it is, after all, scarcely deserving the name of a hygrometer it is little 
 better than a hygroscope. 
 
 Occasionally certain helical vegetable fibres have been used as hygrometers in a 
 way which the accompanying diagram will render manifest. 
 
 Let A represent a circular card or other flat disc, and B a vegetable filament heli- 
 cally coiled it is evident that the helix will unravel in a damp, and tighten its coil in 
 a dry, atmosphere. An instrument of this kind has been some- 
 times employed for the purpose of determining whether a bed be 
 moist or dry. The instrument is a very good hygroscope ; but, inas- 
 much as the coiling and uncoiling of the helix is most comparable 
 with equal arcs, the instrument can hardly be termed a hygrometer. 
 We are under obligations to the ingenuity of the Dutch for 
 another hygrometer, or rather hygroscope for, like the instrument 
 52 - just described, it is not a true indicator of the quantity of moisture 
 
 present in the atmosphere. The instrument is of this kind . A piece of catgut is sus- 
 
66 DEW-POINT HYGROMETERS. 
 
 pended from one extremity, and to the other, or lower extremity, is fixed transversely a little 
 horizontal bar. On one extremity of the bar, a lady in gay summer attire is represented, 
 on the other a man dressed appropriately for a rainy day ; finally, the catgut and its toy- 
 appendages are surrounded with a case having two openings, and in such fashion that 
 only one of the toy images can be visible at a time. Now, it is evident that the fibres 
 of the suspended catgut will partially untwist under the influence of moisture, and re-twist 
 as the atmosphere becomes dry ; whence it follows that the lady will appear under the 
 latter circumstances, and the man under the former. This instrument, though something 
 more than ingenious, for it is a good hygroscope, does not merit the dignified term of 
 hygrometer. 
 
 The Dew-point Hygrometer of Daniell. If a wine-bottle be taken from its bin, it will 
 frequently be found covered with moisture ; and in proportion as the air is saturated 
 with moisture, so will the depression of temperature be at which this moisture begins to 
 be deposited on the bottle. In this manner, if we had the means of regulating the tem- 
 perature of the bottle at our will, depressing it at pleasure, we might ascertain the 
 exact temperature at which moisture would begin to be deposited ; and thus noticing 
 the variations of temperature at different times, we might establish and tabulate a 
 correspondence between each particular temperature at which moisture was deposited, 
 and the corresponding amount of moisture contained in the atmosphere. Now, the 
 degree of temperature at which moisture begins to be deposited in this way, is called 
 the deiv-point; and hence the propriety of the appellation dew-point hygrometer which 
 has been given to the instrument presently to be described. It consists of a doubly- 
 bent exhausted glass-tube, each end terminating in a bulb. One bulb is covered 
 with a coating of thin gold or platinum foil, the other with a fine linen rag. The 
 former bulb is partially filled with ether, and holds a small thermometer, the graduated 
 portion of which passes up the tube. If ether be dropped on the second bulb, eva- 
 poration rapidly ensues, and the bulb is cooled, thereby condensing the vapour of 
 ether which it contains, and permitting a new evolution from the ether in the bulb. 
 This evolution of ether cools the bulb, and causes dew to be deposited on its surface. The 
 inclosed scale indicates the dew-point. The following reasoning explains how the determi- 
 nation of the dew-point can indicate the amount of aqueous vapour in the atmosphere : In 
 proportion as the temperature of the air is elevated, will it be capable of holding more 
 moisture. Hence, by cooling the air, its power of holding moisture is diminished ; a 
 portion of moisture, therefore, becomes condensed in the form of dew- vesicles. The 
 greater the moisture contained in air, the more readily will condensation ensue for a 
 given reduction of temperature. 
 
 The dew-point hygrometer of Daniell, though a great advance upon the rude in- 
 struments just described, is attended with some imperfections. Its construction has 
 been improved upon by Doberemer and by Regnault ; but the instrument, in its most 
 perfect form, still leaves much to be desired. Not only does its employment necessi- 
 tate the use of a large amount of ether ; but, what is of more consequence, when the 
 weather is extremely dry the deposition only takes place with great difficulty. A far 
 more effective instrument, though based on different conditions, is that now about to be 
 described. 
 
 The Psychrometer. The psychrometer consists of two thermometers mounted on the 
 same frame, the bulb of one thermometer being naked, whilst the bulb of the other is 
 enveloped in muslin or other similar absorbent texture, from which there extends a 
 wick-like absorbent stem, terminating in a cistern of water. From a consideration of 
 
METEOROLOGIC RELATIONS OF I3IPOKDERABLE AGENTS. 67 
 
 the structure of this compound instrument, it will be evident that the mercurial column 
 of the naked or uncoated bulb will stand higher than the mercurial column of the 
 second or wetted bulb. The reason of this is obvious. The process of evaporation 
 lowers the temperature : and it follows, that under one condition, and only one, can the 
 readings of the pair of thermometers which constitute the psychrometer correspond 
 namely, when the atmosphere is saturated with moisture to such an extent that it is 
 unable to take up more. By an extension of this reasoning it will be now evident that 
 the mercurial readings of the pair of thermometers will continually vary, according to 
 the amount of dryness or moisture of the surrounding atmosphere. The variation, in 
 point of fact, is in an inverse ratio to the amount of moisture ; so that by means of 
 formula) we can easily connect the indications of the psychrometer with the dew- 
 point. 
 
 Diurnal Variation of ^itmospheric Moisture. The amount of moisture present in air 
 varies at different times of the day. There appears to be two maxima and two minima. 
 The first maximum occurs about 9 A.M., the second at 9 P.M. ; the first minimum shortly 
 before sunset, the second about 4 A.M. Popularly, the air is said to be most damp at 
 sunrise, and in the sense of dew or palpable moisture the popular expression is correct ; 
 but, provided the air be hot enough, we have already seen that it can absorb large 
 quantities of moisture, retain it invisibly, and impart no sensation of moisture ; indeed, 
 pure steam is no more wet than a pure gas is wet. 
 
 Monthly Variation of Atmospheric Moisture. The fact needs no comment, that all 
 months throughout the year are not equally moist. It appears that at London, Paris, 
 Geneva, and Great St. Bernard, the absolute amount of vapour in these places attains 
 its maximum in January, and. its minimum at the end of July or the beginning of 
 August ; but the relative moisture is greatest at London, Paris, and Geneva in December, 
 and least in May. 
 
 Like the ferae aerial atmosphere, atmospheric aqueous vapour continually varies as to 
 its amount of tension or elasticity. According to Dove, the amount of tension is less 
 during north and south winds than during eastern and western winds, an observation 
 which has also been confirmed by Kaemtz. Necessarily, too, the direction whence the 
 wind blows must influence the quantity present of aerial vapour. North and north- 
 east winds are, at least in these latitudes, less moist than winds blowing from the 
 opposite direction. 
 
 Inasmuch as the aqueous vapour dissolved invisibly in air assumes the condition of 
 vapour whenever the air is cooled below the dew-point, the influence which mountain 
 ranges exercise in robbing winds of their moisture and producing rain will be readily 
 evident. By an extension of the same reasoning, it will be also evident that winds 
 which have reached into continents far distant from the ocean, and lost considerable 
 bulks of water, must be necessarily dry. Such are the indications of theory, and 
 observation fully confirms them. 
 
 The Meteorologic Relation of Imponderable Agents. In our preceding 
 investigations, the meteorologic relation of the imponderables has been almost unnoticed. 
 Incidentally, some few of the leading properties of heat have been treated of, but no 
 general statement of the meteorologic relations of these agents has been offered, this 
 being a subject of such vast importance that it merits a treatment of itself. 
 
 The expression, imponderable agents, or imponderable forces, is now commonly 
 applied to indicate the cause or causes, whatever they may be, which give rise to the 
 phenomena of light, heat, electricity, magnetism ; we must now also include actinism, or 
 
68 THEORIES OP LIGHT. 
 
 the radiant influence of the sun, being neither light nor heat, to the operation of which 
 photographic pictures are due. 
 
 The imponderable agents have always presented a field of great interest to the 
 student ; but especially interesting is the study at this time, seeing that it is a tendency 
 of modern philosophy to refer all these imponderable agencies or forces to various modi- 
 fications of one grand cause. The correlations between light and heat, electricity and 
 magnetism, is so intimate and so well marked, that assent can hardly be refused to the 
 assumption that they must all be due to a modification of one common agent. Never- 
 theless, even though the cause of heat and light, magnetism and electricity, be granted 
 as one and the same, the functions of these four results arc so diverse that no generali- 
 zation of treatment will include them all ; they must be held distinct, and treated each 
 under its own head. 
 
 Light. This treatise not having chemistry for its primary object, but merely cm- 
 bracing chemistry as a collateral adjunct, it may be sufficient to comprehend under the 
 general appellation light all non-calorific radiant influences. Strictly speaking, we 
 ignore by this arrangement the existence of a peculiar radiant influence termed ac- 
 tinism ; but so long as the exclusion be noted, and a reason assigned for the omission, 
 no prejudice to scientific truth will result. A sufficient reason is, that the function of 
 actinism concerns the meteorologist in a minor degree ; that it has already been dis- 
 cussed in the treatise on Chemistry ; and that all its meteorological relations may be 
 with convenience included under the general treatment of light. 
 
 Theories of Light. Except for the completeness of description, it is rarely worth while 
 to quote the opinion of philosophers of Greece and Home on any matter of natural 
 science. The wonderful acumen, the quick perception, the subtle reasoning faculty of 
 the Greeks though the source of a noble literature, of a sculptured embodiment of all 
 that is beautiful in living forms, of a terse logic and wonderful geometry was perhaps 
 disadvantageous to the development of experimental science. Minds that could venture 
 so far in the region of speculation, were not the most likely to invoke the tedium of pro- 
 tracted experiment which the cultivation of physics demands ; accordingly, all that has 
 reached us relating to this branch of knowledge was crude and unreliable. The first theory 
 of light which is on record is, I believe, that of Plato, who assumed that light consisted 01 
 certain emanations evolved from the eyes of animals. Subsequently the prevalent notion 
 was that light consisted of emanations from luminous bodies, a theory which was adopted 
 by Newton, and has been designated the theory of corpuscles, for a reason which will 
 speedily be evident. Even previous to the time of Newton, a theory of light, called the 
 undulatory theory, had been advanced ; but at that period its sway was short, though 
 subsequently the theory has been revived, and is at the present time accepted almost 
 universally. Of the corpuscular theory, the remark may be made that it affords a rough 
 and gross explanation of the greater number of common luminous phenomena, for which 
 reason it was long universally accepted; it is totally incompetent, however, to deal 
 with some of the uncommon and very interesting phenomena of light, especially those of 
 double refraction and polarization. The undulatory theory of light (so called from 
 undula, a little wave) assumes that the property of function which affects the optic 
 nerve, and which we agree to call light, is a result of the vibrations of certain waves 
 occurring in an alternated medium far too subtle for chemical analysis, and which is con- 
 ventionally termed ether. Now in strict truth it must be admitted that there is something 
 opposed to the Baconian code of induction, in beginning with the assumption that a 
 fluid ether of the kind indicated exists ; and it must also be admitted that the direct 
 
VELOCITY OF LIGHT. 69 
 
 evidence in favour of the existence of such ether is but slight. There does exist, how- 
 ever, a presumptive evidence favouring the existence of the agent, independently of the 
 arguments analytically deduced from the petitio principii that light is really the result of 
 waves. Astronomers have remarked that the motions of the heavenly bodies in space are 
 subject to certain retardations, indicative of their travelling through some impeding 
 medium ; and this is, perhaps, the strongest argument which can be produced. If such 
 medium be a reality, we have the petitio principii granted, which the undulatory theory 
 of light demands. 
 
 The study of the agencies of light involves a consideration of less theoretical 
 reasoning than is demanded by a corresponding study of the other imponderable agents. 
 In studying the phenomena of electricity and of magnetism, we can hardly make one 
 step without continually employing theory of some kind as a rallying-point for our 
 ideas, and a stepping-stone by which we rise to our deductions. Not so in the matter 
 of light. "Whatever be the ultimate cause of this agency, whether corpuscular or un- 
 di Jatory, the function or impression of light is exercised in straight lines, except in a few 
 special cases, which Avill be brought under notice hereafter. 
 
 Velocity of Light. Astronomical observations of two distinct kinds furnish us with 
 very precise observations, relative to the velocity of light. Its rate of travelling is, 
 firstly, deduced from certain phenomena of Jupiter's satellites ; secondly, from the 
 aberration of light. As the result of both these kinds of investigation, light is found to 
 travel at the rate of about 192,000 miles in a second of time. The rapidity, as 
 will be seen, is enormous ; yet it is far exceeded by the rapidity of the passage of 
 electricity. 
 
 Determination of the Velocity of Light by the First Method. Astronomical calcula- 
 tions inform us when each particular eclipse of Jupiter's satellites should take place, and 
 the instant any such eclipse does occur may be seen by observation. Now, considering 
 that the diameter of the earth's orbit is 190,000,000 miles, it is evident that our planet 
 must at one time be 190,000,000 miles nearer the planet Jupiter than at another time. 
 Hence, the visual indication of such an eclipse must come to us through a path at one 
 time 190,000,000 miles nearer than at another time, inasmuch as 190,000,000 miles is 
 the measure of the diameter of the earth's orbit. From comparative observations of 
 this kind, it is determined that light occupies about sixteen minutes and twenty seconds 
 in traversing the distance of 190,000,000 miles, which gives the velocity of light per 
 second at about 192,000 miles. 
 
 Determination of the Velocity of Light ly the Second Method Aberration of Light. 
 It will be convenient now to adopt the common expression "ray," as indicative of a 
 luminous agency exercised rectilinearly. Whatever theory of light bo adopted, this 
 definition will hold good. 
 
 Having premised this definition of the term luminous ray, we may now say that 
 luminous bodies are rendered evident to us by reason of rays of light darted off in 
 straight lines. Provided, then, that a luminous body darting off these rays, and the 
 observing eye which receives these rays, be both at rest, and provided that all interfering- 
 causes were absent, the eye would refer the luminous body to its true point in space. If, 
 however, the case be varied by assuming the observer to be in motion, or the luminous 
 body to be in motion, or both, then the eye would not refer the luminous body to the 
 correct point in space, because of what is termed the aberration of light. Inasmuch as 
 our planet is in motion, and the heavenly bodies are in motion, we never see the latter 
 in their true positions, but in the positions which they respectively occupied at some 
 
70 
 
 ABERRATION OF LIGHT. 
 
 anterior period proportionate to the time occupied by light in travelling from them to the 
 eye. From these considerations it will be evident that if we know the position in space 
 of a heavenly body at any instant of time, and the space travelled over by the observer 
 in a similar time, also the direction of motion in the two cases, the necessary elements 
 are furnished for calculating the velocity of light. 
 
 The following diagram (Fig. 53) is intended to illustrate the foregoing proposition. 
 
 In the diagram I is a luminous body in space, 
 a o, o a' represent portions of the earth's orbit 
 travelled over by the observer in given equal 
 portions of time seconds, for example. The 
 direction of luminous rays is indicated by I a, 
 I o, I a'. It is now demonstrable that if light 
 do not occupy time in travelling, the luminous 
 object I will appear in its true position for each 
 time of observation, however much the observer 
 may have progressed. If, on the contrary, time 
 be really occupied by light in travelling, then 
 the observer's eye, supposed to move in the 
 direction a a', will not see the luminous body I 
 by reason of the rays which emanate from it at 
 the period of observation, but by reason of the 
 rays which started on their journey at some 
 anterior period; and, seeing that the direction, or angular position, of luminous objects 
 is determined by taking cognizance of their luminous rays, it follows that I will never 
 be seen by the moving observer in its true position. Let us suppose the observer's eye 
 to be at , then I will not appear in its true position, namely, the position indicated by 
 the line I a, but in some antecedent position, which the diagram does not represent. 
 Supposing the observer to have arrived at o, the apparent position of I would correspond 
 to p (-i.e., parallel to I a', as indicated by the dotted line op). It follows, then, that the 
 distance of a luminous body being known, also its true position and its apparent position, 
 the rate of travelling of light can be determined by trigonometric calculation. 
 
 Consideration of Primary Optical Laws. The meteoric relations of light being 
 almost exclusively optical, it will be proper to enumerate a few primary optical 
 laws. 
 
 Ray of Light. Definition : A ray of light is a rectilinear agency of the luminous 
 essence for any given transparent medium. This definition is equivalent with the ordinary 
 
 Fig. 54. 
 
 expression that light travels in straight lines so far as that expression is correct. Taken 
 without limitation, however, the expression is not correct. The agency of light in 
 
LUMINOUS REFRACTION. 
 
 straight lines is only maintained for one homogeneous transparent body, and not those 
 in some peculiar cases. 
 
 Law I. The intensity of light varies inversely to the square of the distance. The 
 operation of this law is illustrated by the preceding diagram, wherein the figures 
 1234 represent four distances from a luminous object ; the intensity of light at 2 will 
 be one-fourth the intensity of the same at 1 ; at 3, one-ninth ; and at 4, one-sixteenth. 
 If a in the preceding body be conceived to stand for an opaque screen, having determi- 
 nate square divisions say one foot and 234 other opaque screens, having the respec- 
 tive dimensions of four, nine, and sixteen feet, then at position 1 the one-foot screen 
 will intercept all the light, at 2 the four-feet square screen, &c. 
 
 Law II. When a ray of light falls on a reflective surface, the reflected and the 
 incident ray are both in one plane. Thus, in the annexed diagram, 
 m ip represents a reflective plane, d an incident ray, n a reflected 
 ray, and i the point of impact ; thus the rays d and n lie in one and 
 the same plane. 
 
 Law III. The angle of incidence and the angle of reflection 
 are equal. Thus, referring to the subjoined diagram (Fig. 55), where 
 d i represents the incident ray, impinging at i and reflected at n, the 
 ray di makes the angle with a line b i perpendicular to the reflecting 
 plane m ijp, which is equal to the angle made by the reflected j^ 55> ' 
 
 ray * n. 
 
 Law IV. When a ray of light passes from one transparent medium into another of 
 different density to the first, refraction ensues. If the density of the second body be 
 more considerable than the density of the first, it is refracted in a direction towards a 
 perpendicular to the plane of the refractive surface. If the 
 order of relative density be reversed, the ray of light is re- 
 fracted from the perpendicular plane of the refracting surface. 
 
 The annexed diagram (Fig. 56) is intended to illustrate the 
 law just enunciated. The diagram represents two refractive 
 media. The upper one is indicated by four dotted lines, in- 
 cluding a rectangle ; the lower one by four plain lines, also 
 including a rectangular space. We may assume, for the con- 
 ditions of our argument, the upper rectangular space to be 
 filled with atmospheric air ; the lower rectangular space cor- 
 responding with a block of glass. Assuming, now, r to stand 
 for a ray of light passing in the direction r id r', we remark 
 that on entering the glass the ray bends towards the perpendicular^, because glass is the 
 denser medium. On leaving the glass, the ray bends from p', and assumes its original 
 course. Hence, the law is satisfied. Hereafter we shall discover that a full appreciation 
 of the action of lenses depends upon a previous cognizance of the law. 
 
 Many natural instances continually present themselves in exemplification of this 
 law. The salmon-poacher well knows, that if he would succeed in spearing the fish, 
 which he sees lying on the bed of a river, he must not strike in the apparent direction 
 of the object, but he must make allowance for the refraction of light caused by the 
 water. 
 
 A very common experiment, illustrative of the refraction of light, is performed with 
 a basin containing some small object, which latter, for a certain position of the 
 observer, is only rendered evident when the basin is filled with water. Thus, in the 
 
 rig. 56. 
 
72 
 
 ATMOSPHERIC BEFKACTION. 
 
 annexed diagram (Fig. 57), the basin is supposed to contain a boy's marble. The latter 
 ^_ would be invisible to the eye at E by a ray of light 
 passing in the direction of E K, although visible 
 by the same ray when bent in the direction of 0, as 
 would be the case provided the vessel were filled 
 with water. 
 
 Far more important to the meteorologist is tho 
 following illustration of the law of luminous re- 
 fraction by the atmosphere. "We have already seen 
 that the atmosphere is to be considered physically 
 as made up of concentric shells of elastic matter of 
 varying density. Practically, then, each successive 
 atmospheric layer (to assume the existence of layers where the blending is complete) has 
 a different refractive power for a ray of light. 
 
 The accompanying diagram (Fig. 58) is intended to represent the effect of atmo- 
 spheric luminous refraction. For the purpose of illustration, three atmospheric shells 
 
 or zones are depicted. Let us now trace the re- ,.-- --^ 
 
 fractive effects of these zones on a ray of light. 
 
 Fig. 57. 
 
 S is the sun or other luminous heavenly body, r 
 a ray of light proceeding from the latter to the 
 earth, and, of course, passing the atmosphere of our 
 planet. Now the ray r, instead of going straight 
 on as it would have done if the transparent me- 
 dium the atmosphere were of one uniform unvarying density, takes the course r r'. 
 The outermost zone of air refracts it a little ; the second zone being denser, refracts it 
 still more; and the third, or innermost zone, denser still, effects a third and final 
 amount of refraction ; so that the sun would be rendered visible to an observer at r', 
 although really below the horizon. 
 
 Chromatics. Hitherto, light has been treated of without reference to colour ; but 
 certain phenomena of light, involving the production of colour, are especially interesting 
 to the meteorologist. The rainbow the tint of clouds, and aurora or morning dawn 
 the manifold hues of sky and sea, may be cited as familiar examples. 
 
 To the immortal Newton we are indebted for the first consistent theory of colours ; 
 a theory which, slightly modified, is generally accepted at the present time. The fact 
 is almost too familiarly known for comment, that Newton effected the decomposition 
 of light by means of a triangular prism, causing a white ray to split into coloured 
 rays, and demonstrating that white light consisted of the prismatic colours blended 
 together. 
 
 According to Newton, the primary or prismatic colours were seven, as follow : 
 1. Red; 2. Orange; 3. Yellow; 4. Green; 5. Blue ; 6. Indigo; 7. Violet; violet 
 rays being most, -and red rays least refrangible. Subsequent experimenters, however, 
 have reduced the number of primary rays to three, namely, red, blue, and green. This 
 demonstration cannot be satisfactorily arrived at by prismatic decomposition ; but by 
 
ANALOGY BETWEEX LIGHT A.2TD SOL'XD. 
 
 employing glasses of different colours, and absorbing certain tints of light, the demon- 
 stration is easy. 
 
 The subject of meteorology scarcely demands that I should enter upon the discussion 
 of spherical or chromatic aberration, still less on the consideration of doxible refraction 
 and polarized light. I shall, therefore, conclude this short theoretical exposition of the 
 functions of light by presenting a summary of the arguments for and against the cor- 
 puscular or emissary, as well as the undulatory theory. 
 
 It has already been remarked that the wave theory of light i.e., the undulatory 
 theoiy originated at a period anterior to the corpuscular theory introduced by Sir Isaac 
 Newton. The great objection to the wave theory was this : if light be the result of 
 waves, it was argued, how is it that light will not travel round a corner after the manner 
 of sound ? which latter was known to be the result of vibrations in the air or other 
 medium. This question was deemed to present an impossibility to the comprehension 
 of the advocates of the wave theory of light. Curiously enough, the modern philosopher 
 may accept the corner-test as one of the strongest proofs in favour of the wave theory of 
 light. 
 
 As the most striking similarity prevails between light and sound in many of their 
 relations, let us establish a comparison between the two, in respect of turning a corner. 
 Every one knows that sound can turn a corner ; but the fact is scarcely less patent that, 
 in the act of turning a corner, a portion of the soniferous impulse is deadened or lost. 
 The proof of this assertion is so common, that we scarcely require the performance of an 
 artificial experiment. Who is there who has not noticed the sudden deadening of the 
 rumbling noise of carnage-wheels, immediately the carriage has turned to the right or 
 left down a side-street ? "Who that has been present during artillery practice some- 
 times standing near the muzzle of a cannon, sometimes behind it has not been 
 made cognizant of the difference between the violence of the report of the cannon's 
 discharge ? Thousands of instances of this kind must have presented themselves to 
 everyone. 
 
 The following experiment simply, yet satisfactorily, illustrates the same propositions. 
 If a tuning-fork be struck and held at arms-distance from the ear, its prevailing note 
 will be heard with a certain distinctness. If, now, a card be interposed between the 
 tuning-fork and the ear, the former may be brought very near the latter without 
 making the listener cognizant of a sound equal in intensity to that which he heard 
 when the tuning-fork was more distant, but when no card intervened. Indeed, what- 
 ever test be devised for demonstrating the deadening of sound by necessitating its 
 travelling round a corner, the result is invariably of one kind it is affirmative of the 
 proposition. 
 
 In endeavouring to establish the existence of a similar property in respect of light, 
 due allowance must be made, of course, for the difference of degree between sonorous and 
 luminous vibrations. I shall very soon demonstrate that, whatever may be the cause of 
 light, each particular colour corresponds to a certain size of something, either of wave 
 or of particles. Assuming these measures to be the measures of waves, then we are in 
 the condition to prove that the difference between the size of sound-waves and of light- 
 waves is enormous ; and being enormous, we cannot expect that light should be able to 
 turn a corner to a similar extent and with the same facility we find sound to do. But 
 that it can turn a corner within certain limits is unquestionable ; therefore, we establish 
 an analogy in an important particular between light and sound, and furnish an answer 
 to the argument which Newton thought to be unanswerable. 
 
74 INTERFERENCE OF LIGHT. 
 
 The simplest demonstration of the fact that light turns the corner is furnished by 
 the phenomena of shadow. What artist is there who does not perfectly well know 
 that the edge of a shadow is somewhat more illuminated, or less dark, than the 
 centre of the shadow ? and granted that the fact be so, does it not prove that light 
 and sound, in the matter of turning a corner, present a complete analogy ? 
 
 Another illustration, not so familiar as the last, but equally expressive, is this : If a 
 minute perforation be made in a card, of course a certain amount of light can be caused 
 to pass through the perforation. If, now, an object be viewed through the aperture, the 
 object will be represented magnified, just as it would if a magnifying lens were fixed in 
 the aperture. Now, on the assumption of the passage of light through the aperture, 
 merely subjected to the law of diminished intensity, inversely as the square of the dis- 
 tance, we cannot account for this magnifying power ; but if we grant that the light, 
 by friction (to use a comprehensible but not unexceptionable term) against the edge of 
 the aperture, is bent outwards, then the result is accounted for. 
 
 Interference of Light. Some of the most beautiful arguments in favour of the undu- 
 latory theory have reference to what is called the interference of light. "What is meant 
 by this term may be thus summarised. It is possible, by certain optical arrangements, 
 to produce darkness by causing one ray of light to impinge against another ray in a 
 certain distance ; whereas, varying the distance, it is also possible to make the lumi- 
 nosity of the first ray more powerful by the extent of the luminosity of the second. How 
 beautifully, how completely does this accord with the phenomena of musical sounds ! 
 Let the following experiment be performed by a person whose ear is moderately sensible 
 to harmony, and the effect noted : Let two musical instruments wind-instruments are 
 the best (and perhaps two organ pitch-pipes are preferable to any other) be caused to 
 produce simultaneously discordant notes. That there is discord the ear can hardly fail 
 to recognize ; but to recognize the cause of discord will require a little attention. The 
 two series of pulsations will be heard clashing mutually against each other ; all idea of 
 musical tone will have departed ; and even the sound, regarded as a mere noise, will have 
 become less than the sum of the two sounds. 
 
 If, however, the two organ pitch-pipes be now set in harmony, as the musicians 
 term it ; and, more especially, if one be set to produce a tone one octave above the other, 
 and both simultaneously sounded ; not only will the result be harmonious, satisfying the 
 musician's ear, but the power of the sound will be equal to the sum of the power of the 
 two notes. 
 
 Now the scale-value of musical notes can be demonstrated to correspond with, and 
 be dependent upon, waves of definite size for any particular medium ; and it admits of 
 demonstration that musical harmony depends on an accordance between sonorous vibra- 
 tions ; moreover, that discord results when soniferous waves clash in different phases of 
 their vibrations. All this, however, will be more evident if we seek an illustration in a 
 case involving the production of visible waves ; as, for example, the illustration fur- 
 nished by the waves which can be made to arise on the surface of water. If a stone 
 be thrown upon the surface of water in a pond previously unruffled, a series of concen- 
 tric waves will be developed, originating in the centre, or point of impact of the stone, 
 and extending outwards concentrically. If, now, a second stone be thrown on the sur- 
 face of the pond, evidently a second series of concentrically expanding rays will be pro- 
 duced, which, meeting the first series, will give rise to one of two opposite effects, 
 according to the manner in which the waves strike each other. If the crest or swell of 
 one wave happens to correspond with the crest or swell of a second, then the two will 
 
I)TMI-:\STONS OF LOIIXOUS WAA'ES. 75 
 
 blend, and the result will be a wave larger than either. If, however, the crest of one 
 wave happens to strike the depressed curve of another, the result will be a diminution of 
 the size of both waves nay, the absolute destruction of both, provided they were 
 originally of precisely the same dimensions. 
 
 The meaning of the expression, " correspondence of pliase of vibration" will now be 
 evident. If the crests of two waves strike and coalesce, both curves pursuing the same 
 direction, they are described as meeting in the same phase of their vibration ; if they 
 I meet, the curves of each wave pursuing poposite directions, that is to say, one rising 
 Avhile the other is falling, then the waves are said to meet in opposite phases of their 
 vibration. 
 
 I have selected the surface of water as furnishing a visible illustration of wave 
 interference, in every way comparable to the interference of sonorous waves as demon- 
 strated ; and to the interference of luminous waves as assumed on the strongest grounds 
 of probability. 
 
 Let us proceed now to develop the principles from the consideration of which 
 the size of an unseen wave may be demonstrated. Sonorous waves are of this 
 kind. 
 
 Firstly, supposing atmospheric air to be the medium of soniferous waves, we require 
 to know the velocity of sound through this medium. This has been determined to be 
 about 1,120 feet per second at a mean temperature and pressure. Considering then that 
 the velocity with which sound travels has been determined, and that each particular 
 tone of the musical scale corresponds with a determinate number of undulations or 
 vibrations in a given tone, we may ascertain the dimensions of sonorous waves corres- 
 ponding with any particular tone, provided we know the number of vibrations for any 
 given tone. M. Savart accomplished this by a very ingenious instrument, the con- 
 struction of which will be best introduced by the following reference to a circumstance 
 frequently occurring. Perhaps it may have happened to the reader that on some occa- 
 sion when briskly passing along near a long range of iron or wooden railings, he has 
 unconsciously touched them with the end of his cane, which necessarily will have struck 
 each bar of the railing with lesser or greater amount of velocity, according to the rate at 
 which the pedestrian may have been walking along. Now it will scarcely fail to have 
 been noticed that for every degree of rapidity of impact, there will have been produced 
 a different sound, the tone becoming more and more shrill in proportion as the velocity 
 of impact is greater. 
 
 On this principle is founded the instrument of M. Savart a spiked wheel, capable of 
 being set in motion with a determinate velocity. Inasmuch as this machine furnishes a 
 known velocity for a known sound, we have two of the data for ascertaining the size of 
 a sonorous wave ; the remaining datum is the velocity of sound, already mentioned. 
 The size of a sonorous wave, corresponding with any particular musical note, may now 
 be determined by application of the subjoined formula : 
 
 Let S =. velocity of sound per second, 
 
 ^ = number of vibrations per second necessary to produce any given note, 
 "W = length of wave corresponding to that note ; 
 then I W. 
 
 From the application of this formula are determined the lengths of organ-pipes, cor- 
 responding to different notes, as given in the following table : 
 
76 
 
 MEASUREMENT OF LUMINOUS WAVES. 
 
 NUMBER OF VIBRATIONS PER SECOND PERFORMED BY WAVES OF AIR CORRESPONDING 
 TO CERTAIN MUSICAL NOTES, AND LENGTH OF THE RESPECTIVE WAVES. 
 
 Notes of the 
 Organ. 
 
 Length of 
 Pipe. 
 
 No. of vibrations 
 per second. 
 
 Lenqth of 
 Wave. 
 
 
 Lowest C 
 
 32 
 
 16 
 
 70 
 
 or ^R 
 
 C 1 
 
 16 
 
 32 
 
 35 
 
 or Hi- 
 
 C2 
 
 8 
 
 64 
 
 17-5 
 
 or ^r! Q 
 
 C 3 
 
 4 
 
 128 
 
 8-75 
 
 or J i\fs (? 
 
 C 4 
 
 2 
 
 256 
 
 4-375 
 
 or -Wo" 
 
 & 
 
 1 
 
 512 
 
 2-1875 
 
 or^ 
 
 Fig. 59. 
 
 Determination of the Size of Sonorous Waves. Having illustrated some of the phe- 
 nomena of waves by reference to a surface of water, and shown in what manner the 
 size of sonorous waves may be determined let us now proceed to apply a parallel course, 
 of reasoning to a determination of the size of luminous waves, corresponding with any 
 particular colours, assuming, of course, as we are obliged to assume, that the sensation of 
 light be referable to the existence of waves. The accompanying diagram (Fig. 59) will 
 
 illustrate the manner by which this can be effected : 
 it represents a plano-convex lens, laid with its 
 convexity downwards upon a flat glass surface. 
 / If this be done, and if the two be pressed together 
 
 with a certain degree of force, the space of air 
 lying between the flat glass and the convexity of 
 the lens at every point, with the exception of the centre, will be tinted with the primary 
 colours. The outermost band of colour, represented in the diagram by the letters a a\ 
 will correspond with red light ; and the innermost or central band, circumscribed around 
 b', will correspond with violet light ; and between the two all the prismatic tints will 
 appear in their ordinary scale of gradation. Let us contemplate the beautiful deduc- 
 tions which flow from this simple experiment. Firstly, it is evident, that for each 
 coloured band there is a corresponding definite thickness of air ; so that whatever the 
 cause of light may be whether particles, or waves, or anything else the space cor- 
 responding with each particular colour is the measure of that on which the colour 
 depends, to the same extent that the cleft between two rocks, into which a fish has 
 become fixed, is a measure of the size of the fish : so if we can ascertain the size of 
 this aerial space for any given point, we can ascertain the size of the cause of light ; 
 and granted that waves of different dimensions arc the cause of different colours of 
 light, we can speak confidently of the size of each particular wave. These measure- 
 ments do not admit of being determined by rough direct measurement of rule and line ; 
 but they can easily be measured by obvious trigonometrical calculations, inasmuch as 
 the curve of the lens is the solid formed by the rotation of an arc, of a circle of 
 known diameter. 
 
 Knowing the velocity of light, and knowing that it travels at the rate of 190,000 
 miles per second, we are now in a position to determine that a wave of the extreme red 
 of the solar spectrum has a length of -00000266th part of an inch, and that it vibrates 
 458 million times per second; that a wave of extreme violet light has a length of 
 
DECOMPOSITION OF WHITE LIGHT. 77 
 
 00000467th part of an inch, and accomplishes 727 millions of vibrations in a second. 
 The steps of the calculation, as will be observed, are the exact parallels of the steps 
 already given above for the calculation of the size of sonorous -waves, and are based on 
 a knowledge of the velocity of light per second, and the actual space corresponding to 
 each particular tint. Inasmuch, however, as the subject of light is somewhat abstruse, 
 perhaps the following parallel cases will render the train of reasoning, by which the 
 size of the waves of light are determined, more obvious : 
 
 PARALLEL CASES. 
 
 Light travels at the rate of 190,000 miles A man travels at the rate of 60 yards per 
 
 per second. minute. 
 
 Length of waves of red light, -00000266th (Assumed) length of the man's strides, 
 
 part of an inch. 1| yard each. 
 
 Query. How many vibrations docs a ray Query. How many strides docs the man 
 
 of red light make per second ? make in a minute. 
 
 Formula for solving the query. Formula for solving the query. 
 
 Velocity of light per sec. Xo. of vibra- Velocity of man per niin. Xo. of strides 
 
 Length of wave of every tions per sec. Length of stride per min. 
 
 colour 
 
 Or, Or, 
 
 Answer. Answer. 
 
 11)60(40 strides per minute. 
 
 Atmospheric Decomposition of White Light. Coloured light has been demonstrated 
 to be a component of white light ; and, indeed, there is very little white light in nature. 
 Owing to the various decomposing agencies to which light is subject, the result is 
 generally coloured. The tints of natural objects are generally determined by their 
 quality of luminous absorption: If a surface absorb red and blue light, merely reflecting 
 yellow rays, then the tint of the body in question will be yellow. If it absorb yellow 
 and red, merely reflecting blue rays, then the tint of the body will be said to be blue ; 
 and so on for the remaining case. An absolutely white body should of course reflect 
 the rays of every colour, but absolute whiteness is rare. An absolutely transparent body, 
 again, should transmit rays of all colours ; but this absolute transparency can be scarcely 
 .said to exist. Even the atmosphere, transparent though it seem to be, obstructs much 
 light, though the blue colour of the atmosphere is not owing to the blue colour of its 
 particles, but to the reflection of blue rays. 
 
 Water, again, is said conventionally to be transparent ; but we have only to look 
 through a mass of water, or to look upon the sxirfacc of a mass of water, to be convinced 
 that this liquid has the property of absorbing some colours more than others. The con- 
 sequences which flow from the imperfect transparency of the atmosphere are very 
 curious and important. If it were perfectly transparent, the heavens would appear black 
 to us, and the heavenly bodies would be seen brilliantly shining as if in a framework of 
 jet. By its absorptive power the atmosphere becomes to a certain extent visible ; the 
 light of the heavenly bodies is mellowed and softened down ; and the beautiful pheno- 
 mena of twilight and morning dawn are determined. "Were the atmosphere perfectly 
 transparent, the surface of the earth would be illuminated in a way totally unadapted to 
 our necessities. Wherever the direct rays of the sun might fall, the surface would be 
 illumined with a blaze of light ; but all other spots, provided light did not chance to be 
 
78 MAGNETIC PHENOMENA. 
 
 reflected upon them, would be quite dark. Aeronauts, and travellers who ascend elevated 
 mountains, confirm the indications of theory relative to the agency of the atmosphere 
 in diffusing or scattering luminous rays. In proportion as the elevation above the 
 earth's surface is greater, so does the sky appear more dark. 
 
 Magnetism. The magnetic-needle, when freely poised on a pivot or freely sus- 
 pended, is known to assume a directive tendency ; and that tendency is popularly 
 described as being north and south. Actually, however, the direction of north and south 
 is only correct for certain parts of the earth's surface. In by far the greater number of 
 places, the line of true north and south is more or less departed from. In certain localities 
 the departure is very great ; thus, for example, in Greenland the magnetic-needle actually 
 points east and west ; and Parry even found that in one part in the west of Greenland 
 the north pole of the magnetic-needle actually turns to the south. In France the mag- 
 netic meridian, or line in which the freely-suspended needle comes to rest, is nearly 
 coincident with the astronomical meridian, only differing from it by 22 towards 
 the west ; thus giving rise to what is called magnetic declination, or variation. A few 
 centuries ago there was an absolute coincidence between the magnetic and the astrono- 
 mical meridians there was, in point of fact, no magnetic deflection or variation. 
 
 Independently of its northern and southern directive tendency, the magnetic-needle 
 is sxibject to the influence of what is called the magnetic-dip. The dipping-needle con- 
 sists of a magnetic-needle poised in such manner that it can move in a vertical plane, 
 when the influence of dip will be rendered manifest in a way best illustrated perhaps by 
 the following case : Premising that a new magnetic steel-needle may be readily mag- 
 netized by contact with a magnet, it is evident that such a non-magnetized needle may 
 be so poised on a pivot, or suspended by a string, that it shall lie in a perfectly horizontal 
 direction. If, whilst lying thus, it be suddenly magnetized by contact, and if the opci-a- 
 tion be performed anywhere in the northern hemisphere, the horizontality of the mag- 
 netic-needle will be immediately departed from, and the northern extremity or pole will 
 be depressed. If the experiment be performed anywhere in the southern hemisphere, 
 then the horizontality of direction will also be departed from ; but it is the southern 
 extremity now of the magnetic-needle that will be depressed. The amount of the 
 depression or dip is various for different parts of the world. At a certain line, not 
 quite correspondent with the terrestrial equator, the needle lies horizontally there is no 
 dip ; this line corresponds with what is called the magnetic equator. The average 
 amount of dip for this part of Europe is about 70 north. Proceeding towards 
 the north, the dip continually increases ; and it attains its maximum at the north magnetic 
 pole, which, however, is not coincident with the terrestrial north pole, but some distance 
 removed from it. 
 
 As the northern magnetic-dip goes on increasing as we pass towards the north, so 
 docs the southern magnetic-dip go on increasing when we travel in the opposite direc- 
 tion ; but the magnetic phenomena of the southern hemisphere have not been so 
 accurately studied as the phenomena of the northern. Here it should be remarked that 
 some confusion has arisen for want of accurately defining the meaning of the term mag- 
 netic pole. Sometimes it has been held to signify the two points of greatest magnetic 
 energy of the whole earth, independently of the collateral energy due to the operation of 
 local causes ; at other times it has been held to be synonymous with the point exhibiting 
 the greatest influence of these local causes. Adopting the latter idea, some writers describe 
 two northern magnetic poles one in Siberia, the other in North America ; but this seems 
 an unphilosophical application of the term, polarity. Accepting the term magnetic pole in 
 
VARIATIONS OF TERRESTRIAL MAGNETISM. 79 
 
 its first sense, it -will be easy to see that one of the northern magnetic poles, at least, cor- 
 responds with the point of greatest cold. 
 
 Variations of Terrestrial Magnetism. 'These may be divided into regular and irregular. 
 The former are dependent on determinate causes ; and though the causes of the latter be 
 not determinate, analogy furnishes us with a plausible explanation of them. Referring 
 to magnetic variations of the determinate kind, they will be found to present a correlation 
 with variations of temperature. Some time during the morning the north polar extre- 
 mity will have attained its greatest variation towards the east, and shortly past noon it 
 will have deviated towards the west of its normal meridian ; it then returns eastward 
 again, and about midnight it will have assumed a deviative position almost similar to 
 that which it had in the morning. These oscillations have not the same amplitude at 
 all times of the year ; they are greater in summer than in winter greater on a warm 
 and cloudless, than on a cold and clouded day : circumstances which point to the thermal 
 origin of the variations. 
 
 The amount of dip also manifests horary variations ; a circumstance which leads to 
 the inference that the magnetic poles are not fixed spots like the geographic poles of the 
 earth, but are subject to deflections. This is exactly what should result, if the thermal 
 theory, of terrestrial magnetism be adopted. 
 
 The regular variations of the magnetic-needle are so clearly referable to the normal 
 effects of solar heat operating upon the surface of our globe, that philosophers have 
 universally agreed to refer them to that cause. But the influences of solar heat are occa- 
 sionally abnormal; besides which, specialities of locality, the effect of casual flows of 
 tepid oceanic water, the influence of volcanoes, &e., furnish a sufficient basis for an 
 a priori assumption that other magnetic variations besides those which admit of being 
 predicted will come into operation. Hence, other phenomena of magnetic aberration will 
 be determined they have been studied more especially by M. Gauss ; and their 
 dependance upon what may be termed an abnormal-thermal condition of our globe is 
 rendered the more probable that they are coincident with irregular variations of the 
 barometer. 
 
 Three systems of lines are employed to represent on charts the three different values 
 of magnetic declination, inclination, and intensity. These lines were denominated by 
 Humboldt isogonic, isoclinic, and isodynamic lines. 
 
 Isogonic lines are such as connect those parts of the earth possessing equal magnetic 
 declinations ; and charts are formed in accordance with these lines, called decimation 
 charts. Inasmuch, however, as the amount of magnetic declination for any one place 
 is never permanent, but continually changing, these charts are only reliable for short 
 successive periods, and require to be frequently altered. There are, nevertheless, a few 
 points on the earth's surface where the amount of decimation is permanent, or very 
 nearly so ; the most remarkable of these are Spitzbergen and the wertern parts of the West 
 Indies. The most important and most remarkable of all the isogonic lines is that coin- 
 ciding with places where the magnetic and astronomical meridians exactly coincide ; 
 and where, consequently, the needle points due north. In the year 1657, this line passed 
 tlrrough London, and two years later throxigh Paris. 
 
 Isoclinic lines are such as connect parts of the earth which are characterized by the 
 same amount of magnetic inclinations. Charts of these lines exist under the name of 
 inclination charts. The most impoi'tant of these lines is the one corresponding with the 
 magnetic equator ; and at which, as before explained, the dipping-needle has no inclina- 
 tion. The magnetic and the terrestrial equators cut, as was before explained, and the 
 
80 CAUSE OP TERRESTRIAL MAGNETISM. 
 
 points of intersection continually vary. In 1825 one point of intersection of the two 
 equators occupied a position near the Island of St. Thomas, on the western coast of 
 Africa, being distant about 188| from the South Sea node ; between 1825 to 1837 the 
 former node shifted 4 westward. On the Brazilian coast the magnetic is situated 15 
 south of the terrestrial equator. About one-fifth of the magnetic equator cuts the sur- 
 face of the ocean. 
 
 Places at which the intensity of magnetic force is equal are said to be isodynamic, 
 and on charts are represented as being cut by isodynamic lines. Generally the intensity 
 of magnetism may be said to increase from the equator to either pole ; nevertheless, 
 isodynamic magnetic lines neither run parallel to the magnetic nor to the terrestrial 
 eqxiator. The minimum of intensity occurs near the coast of Brazil. The maximum 
 of magnetic intensity is about twice the minimum. 
 
 Cause of Terrestrial Magnetism. Reference has already been made to the influence 
 of heat in causing perturbations of the magnetic-needle. "We shall presently find that 
 the very existence of terrestrial magnetism is ultimately referable to heat. 
 
 When treating of the thcrmoscope of Nobili, some mention was made of the function 
 of thermo-electricity, and the conditions necessary for bringing it into operation. It will 
 now be desirable to discuss the properties and conditions of this function more closely. 
 
 Nothing can be more certain than the correlation which subsists between heat, elec- 
 tricity, and magnetism. It is impossible to produce an elevation or a depression of 
 temperature without producing at the same time a manifestation of electrical excitement. 
 This will be demonstrated hereafter under the head of Electricity. For present purposes 
 it will be sufficient to establish the law of electro-magnetic excitation, and to show in 
 what manner it is in some cases ultimately referable to the operation of heat. Long 
 before the precise dependance of magnetism upon electricity was known or suspected, 
 the connection subsisting between them was assumed. Pieces of steel were frequently 
 rendered magnetic during thunderstorms ; the polar tendency of magnets already existing 
 was reversed by the same cause ; and at a later period in the progress of electrical experi- 
 ment the discovery was made that a small steel-needle might be converted at pleasure 
 into a magnet by subjecting it to the. influence of a wire helix, acting the part of an 
 electrical conductor. 
 
 The experiment in question is very easy, and as follows : Within the coils of a wire 
 helix (see Fig. 60) a needle, enveloped in paper or some equivalent imperfectly-con- 
 ducting material, is laid. An electric discharge then being transmitted through the 
 conductor, and the needle removed, it will be found to have acquired magnetic pro- 
 perties, indicated by its quality of attracting iron -filings at either end ; by its north pole 
 or end repelling the north pole or end of a known magnet, attracting the opposite, and 
 vice versa. 
 
 Looking at the conditions of this experiment, it will be perceived that, whatever 
 connection between the producing electric current and the produced magnet there may 
 be, the electricity has been acting tangentically to the long axis of the needle. This is 
 an important point, for we shall hereafter see that the same tangential relation con- 
 tinues to subsist in all subsequent experiments resulting in the formation of electro- 
 magnets. 
 
 Common or frictional electricity does not furnish the operator with a means of 
 proceeding far in his examination of the relations subsisting between electricity and 
 magnetism. The aid of voltaic electricity is required, as in the following experi- 
 ments : If a metallic wire be twisted into a helix, as represented in the following 
 
VOLTAIC COXDUCTING-WIE.ES MAGNETIC. 
 
 81 
 
 . 61. 
 
 diagram (Fig. 60), and a bar of iron, previously enveloped in paper, inserted within the 
 helix ; if now a current of voltaic electricity be transmitted through the wire, the iron 
 bar will for the time-being be converted into a magnet. This experiment is, indeed, 
 similar to that already indicated, with the exception that an iron bar instead of a steel 
 needle is employed ; the fact being that, though 
 steel, when once rendered magnetic, retains the 
 magnetic influence with much permanence, iron 
 
 is more readily amenable to the same influence, Fi &- 60 - 
 
 and is therefore commonly employed in these and similar experiments. Instead of 
 using a plain wire, it is better in practice to employ a 
 wire covered with flax, or cotton, or silk fibre, or with a 
 layer of gutta-percha, when the necessity which previously 
 existed of enveloping the bar in paper ceases. Coated 
 wire of this description is extensively prepared by manu- 
 facturers of gutta-percha. 
 
 The influence of voltaic electricity in determining mag- 
 netism is still more evidently manifested when the iron bar, 
 instead of being straight, is bent into the horse-shoe form 
 as represented in the accompanying diagram (Fig. 61) ; 
 and the power of the developed magnet is greater if the 
 number of helical turns be increased. Usually, therefore, 
 in practice, several wires are soldered together at cither 
 extremity, as represented in the following woodcut (Fig. 
 62). In all these instances, the circumstance will not fail to have been noticed that the 
 magnetic power is developed at right angles, or tangcnti- 
 cally to the direction of the passing electric current. 
 
 Voltaic Conducting Wires are themselves Magnetic. 
 If a wire conducting voltaic electricity be brought in 
 proximity to iron- filings, it attracts the latter just as a 
 magnet would do ; thus begetting a strong a priori assump- 
 tion that siich conducting wire, for the time being, is itself 
 magnetic. Now, if the assumption be well founded, of 
 course it should exercise the usual influences of attraction "&% 62. 
 
 and repulsion when a freely-poised or freely-suspended magnetic-needle is brought into 
 its vicinity. Such influence is found to be manifested ; and thus we shall presently see 
 that all the hitherto anomalous tangential influences are lucidly explained. Firstly, let us 
 not discard the illustration presented by the attraction of iron-filings with the conducting 
 wire, without deducing from these phenomena an important corollary. 
 
 Assuming A 13 (Fig. 63) to represent such a wire, and assuming, as is really the 
 
 fact, that the iron-filings 
 are attracted all round the 
 S surface of the wire, it is 
 evident that the magnetic 
 polarity is developed in 
 lines at right angles to A B 
 that is to say, in the direction of c d ; whence it follows that such conducting wire 
 is a magnet at right angles to its length. 
 
 This point is very strikingly illustrated by the following contrivance : Z C (Fig. 64) 
 
 c 
 
 Fig. 63. 
 
82 
 
 LAWS OP DEVELOPED MAGNETISM. 
 
 are respectively plates of zinc and copper, communicating with a wire in such manner 
 that a current of electricity passing from one plate to the other 
 must necessarily pass along the "wire. Now, the instrument 
 here represented may be caused to swim, by means of a cork- 
 float, in a basin of dilute acid, when it becomes an active 
 voltaic combination ; and, inasmuch as the liquid support 
 admits of the free motion of the instrument, allowing it to 
 rig. 64. turn in any direction, the vertical plane cut by the wire 
 
 ring should, provided magnetism be developed, assume the direction of the magnetic 
 
 meridian, i.e. north and south. This it is found to 
 
 do again illustrating the proposition, and more 
 
 plainly than before, that a wire conducting voltaic 
 
 electricity is magnetic at right angles to its long 
 
 axis. Not only does this remark hold good, but 
 
 the two magnetic polarities have a definite rela- 
 tion to the transverse section of the wire ; one 
 
 definite side of the plane always pointing north, 
 
 whilst the other necessarily points south. If the 
 
 conducting wire, instead of being bent into a mere 
 
 Fig. 65. 
 
 ring, be twisted helically, as represented in Fig. 65, the magnetic conditions developed 
 will be still more evident. 
 
 This was a form of wire devised by Ampere, for the purpose of demonstrating the 
 magnetic character of wire in the act of transmitting electric currents. The forms of 
 apparatus in question may be delicately suspended or converted by means of pieces of 
 cork and metallic plates into floating arrangements. Thus arranged, the flat spirals will 
 always arrange themselves in the direction of north and south ; one definite side of the 
 coil always corresponding to one invariable direction of the electric current. In confor- 
 mity with principles already enunciated, each coil of the flat helices may bo regarded as 
 a. separate magnetic pole. 
 
 If, now, we carefully investigate the law of this developed magnetism by means 
 of a freely-poised magnetic-needle, it will at a first glance appear to be altogether 
 anomalous. Thus, if a voltaic conducting-wire be held in suc- 
 cessively different relations to a magnetic-needle, the needle will 
 seem to be deflected according to no recognizable rule ; but further 
 examination demonstrates not only the existence of a law, bu demon- 
 strates the seemingly irregular motions to be still due to the ope- 
 ration of the tangential force already described. 
 
 The following simple rule will, under all circumstances, serve to 
 fix the direction of the polarity of a magnet developed by electrical 
 agency in the mind : Let the reader assume that he, himself, is 
 the electrical conductor; that the one fluid theory of electricity is 
 adopted ; and, finally, that the current passes in at his head and 
 emerges at his feet. Let him now conceive that he holds in his 
 hand, and directly in front, a freely-poised magnetic-needle, under 
 which circumstances the north pole of the magnet would always 
 be deflected towards his right hand. In accordance with these facts, 
 dnd in illustration of them, the following diagram (Fig. 66) has been devised. It 
 is a soldier, who holds a musket in his hand; the bayonet of which musket is 
 
 Fig. 66. 
 
DEFLECTING POWER OF MAGNETIC COILS. 
 
 83 
 
 assumed to stand for the north magnetic pol 
 to pass in at his head, and 
 emerge at his feet, the bayonet 
 would invariably be directed 
 towards his right hand. 
 
 We have next to consider the 
 effect of causing an electric 
 current to encircle a freely-sus- 
 pended magnetic-needle in a 
 plane coincident with its axis, as 
 represented in the accompanying 
 diagram (Fig. 67). Reflection 
 on the circumstances of this 
 case will render manifest, that 
 
 Supposing, then, a current of electricity 
 
 Fig. 67. 
 
 each part of the wire thus arranged will tend to produce the same general result 
 namely, deflection of the needle to a position at right angles to the vertical plane in 
 which the wire lies. 
 
 If instead of one turn the wire be caused to encircle the magnetic-needle twice (Fig. 
 
 68), then the needle will be deflected with 
 an energy double that effected by the pre- 
 vious combination; and generally in pro- 
 portion as the number of coils is greater, so 
 will the deflecting power be more con- 
 siderable. 
 
 On an appreciation of these principles 
 depends the instrument termed the galva- 
 nometer, which teaches the force of a voltaic 
 current in motion, by causing a deflection 
 of a greater or lesser axis of the suspended 
 magnetic-needle. 
 
 The galvanometer, in its simplest form, is 
 given at page 277, Fig. 30. It will be seen 
 to be nothing more than a close mechanical adaptation of the principles developed in the 
 two preceding operations. By increasing the number of coils in a galvanometer, it 
 necessarily follows that its power of deflecting a suspended magnetic-needle will be in- 
 creased also. Accordingly, delicate galvanometers are always formed with a compound 
 coil, and are, moreover, covered by a glass shade, as represented at page 277, Fig. 31. 
 
 In all these experiments I have assumed, for the sake of not complicating the matter 
 needlessly, that an ordinary single magnetic-needle has been employed. The use of 
 such a needle, however, is attended with this important disadvantage, namely, the 
 earth's magnetic tendency is a force to be overcome by the magnetic energy artificially 
 established. For example, the tendency is, as we have constantly seen, that a fiecly- 
 poised magnetic-needle shall place itself at right angles to the direction of a passing 
 electric current. If then the electric current should be caused to pass in the direction of 
 north to south, the magnetic-needle should, in accordance with the principles developed, 
 arrange itself cast and west. Such is the tendency, and such is the direction the mag- 
 netic-needle would assume, provided the voltaic current be sufficiently powerful ; but it 
 is not difficult to conceive a case where, the electric current being weak, the natural 
 
 Fig. 68. 
 
84 DIRECTIVE TENDENCY OF MAGNETIC NEEDLE. 
 
 directive tendency of the needle i.e. north and south in obedience to the earth's mag- 
 netic influence, should overpower it. To obviate this interference, the astatic needle has 
 been devised. The astatic needle may be described as being a double magnetic combi- 
 
 N-- . ^~^S nation of two needles mounted on one pivot, the 
 
 5 '* ' N north pole of one needle being opposed to the 
 
 Fi 69 south pole of the other, as in Fig. 69. 
 
 If the two magnets be of exactly equal power, 
 
 then, evidently, the compound instrument would possess no natural directive tendency. In 
 practice, this absolute balancing of forces is undesirable ; therefore, usually one of the 
 needles is rather stronger than the ofchcr, so that the slightest possible amount of direc- 
 tive tendency may be maintained. 
 
 Cause of the Directive Tendency of the Magnetic- Needle. The experiments already 
 described, and the principles deduced from them, furnish a rational explanation of the 
 directive tendency of the magnetic-needle. Firstly, it is granted that any variation of 
 temperature always develops a current of electricity. This proposition the reader will 
 accept as authority for the present ; but, hereafter, under the head of electricity, the 
 demonstration will be made plain. Secondly, it is granted that wherever there is an 
 electric current set up, there will always be a mag- 
 netic energy developed at right angles to the electric 
 current. If we now assume, as the proximate 
 cause of the magnetic-needle's north and south 
 directive tendency, that it docs so because the 
 earth itself is a magnet in the direction of north and 
 south, we have only to discover the cause of an 
 electric current at right angles to this direction, and 
 the mystery is explained. Kow it is evident that 
 our globe is diurnally heated in the direction of 
 cast to west by the sun's rays ; whence, according 
 to the result of artificial experiments, there should 
 also exist an electric current in the same direction ; 
 and, this being so, the earth itself becomes a vast 
 magnet, the one pole of which is northern, and the other southern. 
 
 In illustration of this, the following experiment has been devised : A hollow paper 
 globe (Fig. 70) has been lined internally with a revolving copper wire, so arranged that 
 it should serve as an electrical conductor. If electricity be passed through this copper 
 wire, and a freely-poised magnetic-needle be placed in various successive positions on 
 its surface, all the variations of deflection and dip which naturally occur may be indi- 
 cated in the clearest manner imaginable. 
 
 Eda-magnetism. For a long time it was imagined, and even in this day the idea 
 is popularly entertained that iron alone, of all bodies, is susceptible of magnetism. 
 Experimenters eventually admitted that the metal nickel participated with iron in the 
 property in question, and the notion began to be entertained that yet other bodies 
 might be included in the same category. It was by no means easy, however, to subject 
 the opinion to the test of experiment, considering the known difficulty experienced in 
 banishing every trace of iron from the materials operated upon. Magncticians even 
 argued that the magnetism of nickel might be only apparent, the quality being attri- 
 butable to the presence of iron. At length M. Biot set these doubts at rest definitively. 
 He caused some nickel to be freed from all traces of iron by that distinguished chemist 
 
BIA-MAGNETISM. 
 
 85 
 
 M. Thonard ; tie then caused magnetic-needles to be made of this nickel ; he not only 
 determined the existence of their power of attraction, but their polar directive ten- 
 dency ; and he finally discovered the ratio of their polarity by comparison with an 
 ordinaiy steel magnetic-needle. On the result of his inquiries he established the fact, 
 that the directive force of the nickel was about one-third of that of the steel needle. 
 The needles with which he conducted his experiments were eight inches in length by 
 two-tenths of an inch wide, and the weight of each was about five grains. M. Cavallo 
 followed in this line of demonstration, by proving that other substances besides nickel 
 were susceptible of ordinary magnetism brass, for instance ; especially brass rendered 
 hard by hammering. 
 
 The term ordinary magnetism requires to be explained, and the explanation will at 
 once introduce the phenomena of dia-magnetism. The property of being subject to 
 magnetic influence long known to bo manifested by iron, and subse- 
 quently proved to be participated in by nickel and brass, is perhaps iini- 
 versal ; but the kind of influence differs. Assuming iron to represent 
 the normal kind of influence, let us consider what takes place if we 
 suspend a needle of that metal in what is called the magnetic field 
 namely, the space between the two extremities or poles of a horse- 
 shoe magnet. Under these circumstances, the iron needle would 
 assume what is termed an axial position, one end being attracted to 
 the north pole, the other end to the south pole of the horse-shoe 
 magnet, as represented in the annexed diagram (Fig. 71). Fig. 71. 
 
 Now, M. Cavallo committed the following important mistake : He fancied that he 
 had demonstrated all the bodies proved by him as being magnetically endowed, to bo 
 magnetic in the same sense as iron and nickel are magnetic ; that is to say, that sup- 
 posing a needle of either of these bodies to be suspended in the magnetic field, one end 
 of the needle should be attracted towards the north pole, the other end to the south pole 
 of the magnet. If such were the case, the function now to come under notice as the 
 function of dia-magnetism could have no existence. Before entering more into detail 
 concerning the properties and functions of dia-magnetism, it must be premised that the 
 position which an iron needle naturally assumes when hung in the magnetic field is said 
 to be an axial position, or it is said to arrange itself axially. If it were to assume a 
 position at right-angles to the same, its position would then be described as being 
 equitorial. Now, M. Cavallo believed that all bodies endowed with a magnetic tendency, 
 in any degree, would manifest that tendency by assuming the axial position when freely 
 suspended between the poles of a horse-shoe magnet. This, as I have already remarked, 
 is an error : some bodies arrange themselves axially under these circumstances, and are 
 therefore said to be magnetic ; whilst others arrange themselves cquatorially, and are 
 said to be dia-magnctic. 
 
 The knowledge of dia-magnetism may be said to have originated with Becqucrel, 
 though Coulombe and the Abbe Haiiy had both laboured in the same direction. The 
 investigation has been followed up with great ardour by Professors Faraday and Tindall 
 in England, and Professor Pliicker of Bonn, to whose published papers on this subject I 
 must refer the student who desires further information relative to dia-magnetism than 
 accords with the nature of this treatise. It must be remarked, however, that when bars 
 arc made of different substances, and submitted to the influence of the magnetic field, 
 those of iron, nickel, and cobalt, point axially : most probably those of titanium, palla- 
 dium, and platinum, are in the same category ; but needles of all other metals assume 
 
86 DEVELOPMENT OF ELECTRICITY. 
 
 the equatorial direction in the magnetic field, and are therefore dia-magnctic. The fol- 
 lowing table presents a list of magnetic and dia-magnctic bodies : 
 
 MAGNETIC BODIES. DIA-MAGXETIC BODIES. 
 
 Iron Cerium Bismuth Cadmium Silver Uranium 
 
 Nickel Titanium Antimony Sodium Copper Rhodium 
 
 Cobalt Palladium Zinc Mercury Gold Iridium. 
 
 Manganese Platinum Tin Lead Arsenic Tungsten 
 
 Chromium Osmium 
 
 From a consideration of the preceding remarks, it will be observed that the functions 
 of heat, magnetism, and electricity are intimately allied, more especially the two latter. 
 Instead, therefore, of entering upon the discussion of metcorologic phenomena, due or 
 attributed to magnetism, in this place, it will be desirable to present the reader with a 
 short exposition of electrical science and phenomena. 
 
 Electricity. Definition and Derivation of Term. The term electricity is applied 
 to comprehend a large class of phenomena, which are related in various ways with the 
 operation of an invisible force, to which, founded on speculative considerations, the 
 appellation electric fluid has been given; not that such fluid can be proved to exist, or 
 that even it is at this time, by the majority of philosophers, supposed to exist, although, 
 for the sake of convenience in illustration, the expression, electric fluid, is still popularly 
 retained. 
 
 The science of electricity is one of the most recent ; nevertheless, the primary phe- 
 nomena on which the science is based arc of very ancient date. Thcophrastus and 
 Pliny were aware that the substance amber, if rubbed with silk, flannel, &c., became 
 endowed with the property of influencing the motions of certain light bodies, such as 
 feathers, attracting them under certain circumstances, and repelling them under other 
 circumstances ; but there the investigation of this class of phenomena ended. About 
 the middle of the last century, however, the subject was returned to, recommencing 
 from the starting-point of Theophrastus and Pliny ; and from the simple fact of the 
 peculiar excitation of amber under certain circumstances of treatment, to build up the 
 interesting and important science now known as that of electricity. 
 
 Development of Electricity. The first development of electricity was accomplished by 
 the friction of one particular substance amber, as we have seen; but when the 
 attention of modern philosophers was directed to the science, they soon found that amber 
 only furnished one particular cause of a result far more general. Many other bodies, in 
 addition to amber, Avere discovered which, on being submitted to friction, became 
 electrically excited, or electrical; and to these the general term electrics was given. 
 Furthermore, it was discovered that the bodies thus capable of electrical excitation were 
 not capable of conveying away electricity ; whence they were also called non-conductors 
 of electricity. Great as was the advance thus made on the crude notions of Theo- 
 phrastus and Pliny, it fell far short of the truth, modern electricians having proved that 
 no real or functional difference subsists between conductors and non-conductors, only a 
 difference of degree ; consequently, bodies do not admit of division into the classes of 
 non-conductors and conductors, except in a conventional sense, and as a matter of 
 practical convenience. Experiments fully illustrative of the propriety of this view will 
 bo furnished hereafter. 
 
 Though friction be the earliest observed cause of developed electricity, and though 
 it constitute the principle on which the ordinary electrical machine is founded, never- 
 
150DIES AVH1CII ACT AS ELECTRICS. 
 
 87 
 
 theless it is only one cause, and perhaps the least important, of those which the meteor- 
 ologist has to take cognizance of as coming within the scope of his science. It is diffi- 
 cult to say what alteration of matter, chemical and mechanical, is unattended hy the 
 development of electricity. In all probability there is none of this kind, though it 
 happens that in most cases special, and sometimes very refined, contrivances are neces- 
 sary for rendering electrical excitation evident. A notable illustration of this . is fur- 
 nished by the hydro-electric machine, familiar now to many people by its exhibi- 
 tion at the Polytechnic. This machine constitutes the most powerful instrument for 
 developing electricity by artificial means known ; yet if the glass legs on which it 
 stands were removed, the instrument would become inoperative, and the existence of all 
 the vast force of electricity which it generates would remain unknown. . 
 
 Having premised these general remarks concerning electricity, it will be desirable 
 now, before taking cognizance of the operation of this force in nature, to present the 
 student with the fundamental causes or propositions on which the science of electricity 
 depends. In doing this, I shall avoid, as much as possible, having recourse to the elec- 
 trical machine, or any complex electrical arrangements, which, though indispensable to 
 the full illustration of secondary electrical facts, are rather perplexing than otherwise 
 so long as fundamental principles alone are concerned. 
 
 Definition of the term Electric. Any body which after having been rubbed acquires 
 the property of attracting light substances, after the manner of amber, is an electric. 
 
 What bodies or class of bodies are Electrics ? Inasmuch as the act of friction will be 
 involved in our experiments having reference to this demonstration, it necessarily follows 
 that only one physical division of bodies namely, solids admit of being readily sub- 
 mitted to our notice ; for though liquids and gases can be subjected to friction, yet the 
 contrivances for effecting this, consistent with the requisite electrical demonstrations, 
 are so complex that they cannot be taken cognizance of at present. 
 
 I shall assume that our present observations are limited to three bodies glass, 
 sealing-wax, and a metal ; each, for convenience of manipulation, fashioned into the 
 form of a stick. I shall assume, moreover, that the rubbers, 
 or body wherewith friction is effected, arc of flannel and of 
 silk. By the employment of these simple materials some 
 important results will bo arrived at. 
 
 Experiment I. If the stick of sealing-wax be brisldy 
 rubbed, and held at some distance (not too far) from a sus- 
 pended feather, represented in Fig. 72, the latter will be 
 attracted towards the sealing-wax will attach itself to the 
 latter : but the attachment will not be permanent. After 
 a time it will leave the sealing-wax, and be repelled. 
 
 Experiment II. If the previous operation be repeated 
 with a stick of glass, the feather will be first attracted, and 
 afterwards repelled, in a precisely similar manner as be- 
 fore. Hence, for aught we at present see, the kind of 
 influence developed by friction on glass is similar to that 
 developed by friction on sealing-wax. 
 
 Experiment III. Let the glass rod be excited by friction, and held near the feather 
 as before. The feather will necessarily be first attracted, then repelled. If the rod or 
 sealing-wax be similarly excited, and held near the feather, which has refused to be 
 further attracted towards glass, it wiU, nevertheless, be attracted toward the scaling- 
 
 Fig. 72. 
 
88 CONDUCTORS AND NOX-COXDUCTOUS. 
 
 wax, and vice versa. This experiment demonstrates that, whatever be the nature of 
 electricity, this force is susceptible of two manifestations : it is, in point of fact, a dual 
 or polar force, similar in this respect to magnetism. It admits of being demon- 
 strated that, in either of the preceding experiments, the rubber or body wherewith 
 friction is excited always assumes an electrical polarity different to that of the body 
 rubbed. 
 
 Designation of the two Electric States. As the two polarities of magnetic energy 
 are designated respectively north and south, without which, or some equivalent de- 
 signation, it would be impossible to describe the peculiarities of magnetic phenomena ; 
 so, in like manner, it will be necessary to designate the two electrical functions already 
 proved to exist. Accordingly, the terms, positive and negative, or vitreous and resinous 
 electricities, have been long employed. The words, positive and negative, have reference 
 to the theory of Franklin, that all the phenomena of electricity depended upon the 
 operation of one electric fluid. A certain class of electrical phenomena he assumed to 
 depend upon an excess of this fluid another, or opposite class of phenomena, to depend 
 on a diminution of the same. The first class of phenomena he termed positive, the 
 second negative. 
 
 The origin of the terms, vitreous and resinous, will perhaps have been anticipated 
 from a consideration of the teachings of Experiments I. and II. Glass, it has been seen, 
 when rubbed, gives rise to the development of one function of electricity, and sealing- 
 wax of another. Now, glass and sealing-wax are. in this respect, only the types of all 
 other bodies. 
 
 Conductors and Non- Conductors. Referring to Experiments I., II., and I'll., three 
 distinct stages of electrical condition may be observed. Firstly, the feather, before it 
 has been subjected at all to the influence of the excited glass or scaling-w r ax, presented 
 the condition of electric neutrality. Secondly, it presents two conditions of excitement, 
 namely, attractive excitement and repulsive excitement. If, whilst the feather is under 
 either of the latter conditions, it be touched with various substances successively, 
 certain important results will be observed. If it be touched with the finger, all electri- 
 cal excitation will be at an end. This effect is best demonstrated by touching the 
 feather when in the repulsive phase of excitation. A similar result will ensue if, instead 
 of the finger, the excited feather be touched with any metal or wood, or one of numerous 
 other bodies to which the term electrical conductor is conventionally applied I say con- 
 ventionally, because the cimunstance has been already indicated that the distinction 
 between conductors and non-conductors is purely one of degree, not of kind. Never- 
 theless, in practice it is useful to divide bodies into electrical conductors, and electrical 
 non-conductors. A table, representing this division, is appended. The terms non- 
 conductor and insulator are, it is necessary to observe, synonymous. 
 
 CONDUCTING BODIES, PLACED IN THE ORDER OF THEIR CONDUCTING POWER. 
 
 All the metals. 
 Well-burnt carbon. 
 Plumbago. 
 Concentrated acids. 
 Dilute acids. 
 Saline solutions. 
 Metallic ores. 
 
 Spring water. 
 
 Eain water. 
 
 Ice above 13 Fah. 
 
 Snow. 
 
 Living vegetables. 
 
 Living animals. 
 
 Flame. 
 
 Animal fluids. | Smoke. 
 
 Sea water. | 
 
 Vapour. 
 
 Salts soluble in water. 
 Rarefied air. 
 Vapour of alcohol. 
 
 of ether. 
 Earths and moist rocks. 
 Powdered glass. 
 Flowers of sulphur. 
 
ELECTRICAL TEEMS. 
 
 89 
 
 INSULATING BODIES. PLACED IX THE ORDER OF THEIR INSULATING FACULTY. 
 
 Dry metallic oxides. 
 Oils (the heaviest are the 
 
 best). 
 
 Ashes of vegetable bodies. 
 Ashes of animal bodies. 
 Many dry transparent 
 
 crystals. 
 
 Ice below 13 Fah. 
 Phosphorus. 
 Lime. 
 Dry chalk. 
 
 Native carbonate of baryta. 
 Lycopodium. 
 Caoutchouc. 
 
 Camphor. 
 
 Some silicious and argil- 
 laceous stones. 
 
 Dry marble. 
 
 Porcelain. 
 
 Dry vegetable bodies. 
 
 "Wood that has been strongly 
 heated. 
 
 Dry gases, and air. 
 
 Leather. 
 
 Parchment. 
 
 Dry paper. 
 
 Feathers. 
 
 Hair, wool. 
 
 Dyed silk. 
 
 White silk. 
 
 Eaw silk. 
 
 Transparentprecious stones. 
 
 The diamond. 
 
 Mica. 
 
 All vitrifactions. 
 
 Glass. 
 
 Jet. 
 
 Wax. 
 
 Sulphur. 
 
 The resins. 
 
 Amber. 
 
 Gum lac. 
 
 Gutta-percha is one of the most perfect insulators, but its exact place in the above 
 table is yet undetermined. 
 
 The terms conduction) non-conduction, insulation^ and, indeed, most other terms of 
 electrical science, have reference to the idea of an electrical fluid or fluids ; and, indeed, 
 however much we are constrained to refuse our sanction to the probability of the 
 existence of such fluids, nevertheless we cannot proceed far in electrical investigations 
 without recognizing the convenience of many terms suggested by the assumption. 
 
 Division of Bodies into Electrics and Non-electrics untenable. If the experimenter try 
 to render a bar of metal electrical, as he succeeded in rendering a bar of sealing-wax 
 and of glass respectively electrical, he would not succeed. The earlier electricians, 
 having noted this result, termed the metals, and indeed all conducting bodies, non- 
 electrics ; but if the conducting property of the hand and of metals be considered, this 
 division, so arbitrarily made, cannot fail to seem premature. No legitimate conclusion, 
 as regards the electric or non-electric property of bodies, can evidently be arrived at 
 until the body rubbed has been held by a non-conducting handle ; for .otherwise, even 
 though electricity should be excited, it would readily pass away. If the experiment of 
 rubbing a metallic bar be tried after such bar has been provided with a non-conducting 
 handle, it is rendered electrical, its electrical excitation being made evident by the usual 
 tests of attraction and repulsion. In conducting experiments of this kind, very satis- 
 factory insulating handles may be made by enveloping one end of the bar to be operated 
 on with a piece of gutta-percha, previously warmed by the fire, or softened by dipping 
 into boiling water. The gutta-percha should be wrapped round and about the extremity 
 of the bar, and trimmed whilst yet warm by a pair of wet scissors. 
 
 Induction. In strict propriety of language, no one electrical function or set of 
 functions can be referred exclusively to induction. Electricians of the last century 
 were in the habit of thinking differently; they spoke of induction as if it were a 
 function that might be exercised at will, whereas, in point of fact, no such exclusive 
 electrical function exists. 
 
 If an insulated conductor, charged with either condition of electricity, be brought 
 near to another conductor ; the second conductor, if examined, will be found to be in 
 the opposite electrical condition, which condition was sure to be induced. The term 
 induced, though still employed, conveys a very different meaning to that formerly 
 accepted ; but a discussion of this point is hardly consonant with the requirements of 
 
90 ATMOSPHEEIC BEFEACTION. 
 
 tliis treatise ; wherefore I must direct the reader who would know more concerning it to 
 the volume of this series on Chemistry. 
 
 Electrical Generalizations. In the few remarks on the imponderable agents already 
 made, it is not proposed to present the reader with more than a faint outline of their 
 general nature and correlations. All- important though they he to a meteorologist, that 
 importance is paradoxical, though the statement may serve as a sufficient justifica- 
 tion for treating very casually on them in a treatise like the present. The objects of 
 meteorology are so numerous, and its topics so varied, that to devote more space to a 
 consideration of the imponderable agents would be injudicious. Let us summarize, then, 
 what has already been remarked concerning them, so far as relates to the subject of 
 meteorology. Probably light, heat, electricity, and magnetism arc all effects of one 
 cause, differently modified. Between heat, electricity, and magnetism the alliance is 
 marked ; so is the alliance between light and heat. 
 
 Electricity is, perhaps, the most stupendous imponderable agent with which the 
 meteorologist has to concern himself, and it is the one most amenable to human control. 
 Not less wonderful than the energy of electricity is its universality : not a drop of 
 water can be evaporated by the sun, not a current of water can flow, not a leaf can 
 move or reed bend, not a breeze can skim the surface of the earth, without developing 
 this wonderful force. Very short, indeed, is the task of specifying the material causes 
 of electrical energy. We have only to include every known case of mechanical motion, 
 and every known cause of chemical action, and the task is complete : it is one of 
 universal inclusion. 
 
 In discussing the meteoric relations of the imponderables, it matters little with 
 which we begin. Already certain meteoric functions of heat have been brought colla- 
 terally under the reader's notice; I purpose now considering the imponderable agents, 
 not secondarily, but primarily. 
 
 Phenomena of Atmospheric Refraction. A sketch of the laws of refraction 
 has already been given, and what may be called the normal function of atmospheric 
 refraction has been announced. Eeferring to that announcement, it will be seen that 
 the amount of refraction is due to inequality of the density of the air, determined by 
 pressure alone. But inequality of density, and therefore inequality of refractive power, 
 may be the result of varying amounts of expansion, referable to the operation of varying 
 degrees of heat ; and thus arise what may be termed the abnormal effects of atmospheric 
 refraction. 
 
 Every one must have noticed the peculiar, tremulous condition of the air in sum- 
 mer-time over an ignited brick-kiln or near a red-hot bar of metal, or even on the 
 surface of the ground, provided the weather be sufficiently hot. This tremulous appear- 
 ance is referable primarily to the expansion of air near a hot surface, and immediately 
 to the diminished refrangibility attendant on such expansion. These local sources of 
 heat set up local currents, each being composed of air of a different density from that of 
 neighbouring currents, whence each has a different refractive power. That which an 
 ignited brick-kiln, or a glowing metal bar, can accomplish on the small scale, is accom- 
 plished on a larger scale by many natural causes, giving rise to phenomena both 
 striking and delusive. Pictures of ships and towns inverted, the vain semblance of 
 lakes of water in the midst of burning sands where no water really exists, aerial cities, 
 spectral forms of men and animals. all these, and many more, arc the phenomena of 
 atmospheric refraction and reflection. 
 
 One of the most common effects of irregular atmospheric refraction is the twinkling 
 
THE MIEAGE. 
 
 91 
 
 of the stars. This appearance is strictly conformable with all the teachings of theory 
 in reference to the laws of refraction, and is due to the fluctuations of variously-heated 
 currents of air. When these small aerial currents, having different temperatures, are 
 numerous, had weather is likely to supervene ; hence an explanation of the increased 
 twinkling of stars before bad weather sets in, a phenomenon which has been very 
 commonly noticed. 
 
 Extreme examples of a tmospheric phenomena are, for the most part, only seen in 
 hot climates ; but there they are frequent. The mirage is an atmospheric phenomenon, 
 in part attributable to refraction and in part to reflection ; it occurs in Egypt, and gives 
 rise to the impression in a stranger's mind, of a lake or tranquil expanse of water, though 
 the region is only a waste of sand. The explanation of the phenomenon is this : The 
 villages throughout Lower Egypt are usually built on elevated mounds; hence the 
 houses are to some extent elevated above the general level of the earth, which level 
 becoming intensely hot, imparts heat to the atmosphere placed in contact with it, and 
 alters the refractive power of that portion of atmosphere. An optical illusion now 
 ensues the lower or heated atmospheric layer assumes a tremulous appearance, like the 
 surface of a lake, on which the images of the buildings of the village are seen reflected, 
 whilst the direct image of the village is still evident in its true position. The Egyptian 
 mirage is so deceptive, that a stranger seeing it for the first time can hardly be con- 
 vinced that the semblance of water is only an optical delusion. The term mirage is 
 peculiar to India ; yet the phenomenon to which it refers is common in many other hot 
 regions, especially in Central India and the Sahara. 
 
 The visual inversion of objects, a phenomena not at all uncommon in hot places, is 
 partly due to refraction and partly to reflection, for, in point of fact, the function of 
 reflection may be demonstrated to be only an extreme case of refraction. 
 
 The localcis supposed to be a hot, sandy region, and a date palm is the object seen inverted, 
 the explanation of which phenomenon is as follows : The eyes of the observer being at p, 
 will first see a direct image of the palm-tree by rays which come straight in the direc- 
 tion of the line hp; simultaneously he will see an inverted image of the palm-tree. Let 
 us examine how this happens. Referring to the illustration, several parallel lines will 
 be seen, c c' c" c'". These are 
 intended to denominate atmos- 
 pheric layers of different amounts 
 of density. Tracing the ray of 
 light h *', let us now examine 
 what becomes of it. Firstly, it 
 impinges on the upper atmosphe- 
 ric layer, which is more hot, and 
 consequently more expanded, 
 than the next layer above; the 
 ray h i is, therefore, refracted 
 from the perpendicular, according 
 to the law mentioned at page oil. 
 Passing on to the next layer, it 
 is refracted still more from the 
 perpendicular; and this refractive 
 
 Fig. 73. 
 
 gradation is repeated on the ray hi until it arrives at m, at which point, the tendency to 
 fly from the perpendicular still remaining, this tendency is manifested not under the con- 
 
92 
 
 FATA MORGANA. 
 
 Jition of refraction but reflection ; consequently, the ray hm is directed towards the eye of the 
 observer, and, along with the other rays upon which a similar operation has been effected, 
 gives rise to the appearance of an inverted object. In order that the phenomenon should 
 occur, however, there must be the following conditions besides those already mentioned : 
 "Not only must the solar heat be considerable, but the air must be calm, so that the lower 
 atmospheric layers may retain a density less considerable than the density of those above 
 them. 
 
 As, under the peculiar circumstances just mentioned, refraction may pass into reflec- 
 tion, and the reflection may be excited upwards from below, so may the operation be 
 reversed ; in which case the inverted images of terrestrial objects will be seen in the air.- 
 
 The celebrated fata morgana, sometimes observed on the Calabrian coast, and more 
 especially at Rcggio, is a celebrated example of this kind. At certain times the whole 
 city of Messina and its environs are reflected downwards from an upper stratum of the 
 air; thus presenting an appearance sufficiently curious, but by no means the striking 
 and well-defined character which the records of early travellers would lead us to 
 suppose. 
 
 The correlation between atmospheric refraction and atmospheric reflection, and, at 
 the same time, a rationale of the peculiar aerial visions which may occur in certain 
 atmospheric states, is furnished by the diagram (Fig. 75) suggested by M. Biot. The 
 line b t e is supposed to be a ray of light proceeding from J, passing thence downwards - 
 
REFRACTIVE ABERRATIONS. 
 
 93 
 
 to the point t, -whence it is reflected to the observer's eye at c. Now the optical con- 
 ditions of this arrangement are such that any rays proceeding from b below the ray b t c 
 represented, would he invisible to the observer at c, whilst two images will be seen of all 
 objects above this line. Supposing the object in question to be a man, suppose, further, 
 the man to be walking from 
 the observer, he would be pre- 
 sented to the latter under the 
 successive forms seen in Fig. 
 75. 
 
 In these atmospheric opti- 
 cal delusions, involving the 
 appearance of two images, one lg ' 5 * 
 
 of them inverted, both natural and inverted images have occupied a horizontal plane. 
 Occasionally, however, the reduplication of image has been projected on vertical planes, of 
 'which phenomenon the following is an example. On June 17, 1820, whilst MM. Soret and 
 Turinc were in the second story of a house on the lake of Geneva, they looked towards 
 a ship two miles off, and making for the harbour. Immediately the vessel in ques- 
 tion arrived at q she appeared reduplicated on a vertical plane ; when she came to r the 
 reduplication still continued, but the second image was further removed than before, an I 
 
 both were distorted; lastly, 
 when the vessel arrived at 
 the point s, the reduplicated 
 image had receded to a dis- 
 tance still farther aAvay, and 
 both images, though distorted, 
 presented an appearance of 
 distortion very different from 
 before ; they were apparently 
 drawn out, elongated both as 
 to the hull and rigging, as re- 
 presented in the accompany- 
 ing diagram (Fig. 76). The 
 following is the explanation 
 of the phenomenon as sug- 
 gested by the two observers above-mentioned, and there seems no reason to doubt its 
 correctness. 
 
 The letters, ABC, represent an outline of the eastern bank of the lake of Geneva; the 
 air over that bank had, at the time of observation, been long under the shadow thrown 
 upon it by the mountains of Savoy, whilst, contemporaneously the western bank had 
 been strongly heated by the sun ; hence, from the conjoint operation of these two causes, 
 there were two vertical layers of atmosphere of different temperatures, and consequently 
 of different densities; hence, they were of two different refractive and reflective 
 powers. 
 
 All that is necessary to determine aerial reflections is a sufficient difference between 
 the temperatures of any two adjacent atmospheric layers. In the instance already men- 
 tioned, this difference has been occasioned by portions of the ground being hotter than the 
 strata of air with which they are in contiguity. The reverse of these conditions may, 
 however, obtain, and phantastic atmospheric delusions may be the result. This latter 
 
94 
 
 EEFKACTIVE ABEBBATIOXS. 
 
 
 case generally presents itself at sea, and by no means exclusively in warm localities. 
 Thus, for instance, it is prevalent enough in the Northern Ocean. Sometimes the 
 atmospheric delusion has merely the effect of prolonging the appearance of an object 
 really belo\v the horizon; sometimes not only is the appearance prolonged, but the body is 
 
 seen double. "What- 
 ever the appearance, 
 the class of atmos- 
 pheric delusions now 
 under consideration 
 are usually seen near 
 the horizon a posi- 
 tion where the optical 
 powers of the atmos- 
 phere attain their 
 greatest intensity. 
 
 The Rainbow. 
 The most beauti- 
 ful of all luminous 
 meteorologic pheno- 
 mena is the rainbow, 
 which results from 
 the decomposition of 
 light by refraction 
 through drops of 
 rain, and subsequent 
 reflection. Rainbows 
 are of two classes, 
 solar and lunar ; the 
 latter, however, are 
 rare, and even when 
 they do occur the bow 
 is seldom coloured. 
 
 The chief condi- 
 tions under which a 
 solar rainbow may 
 
 occur are the follow- 
 ing : The sun must 
 not have less than 
 42 of angular ele- 
 Fig. "". vation ; the back of 
 
 the spectator must be 
 
 towards the sun, and rain must be falling from a highly illuminated cloud. The rain- 
 bow is usually double, and the theory of its formation may be thus explained. Let it 
 be assumed that a straight line passes from the eye of the observer through the sun ; 
 then this line will constitute the axis of a cone, the base of which will be the rainbow, 
 and its vertex the eye of the observer. If the bow be at the horizon, and the place of 
 observation be a level plain, then the rainbow will appear as a perfect circle constituting 
 the base of the cone. This complete circular appearance is, however, rare, the rainbow 
 
THE EAIXBOW. 
 
 95 
 
 being far more generally, as its name implies, a mere arc of coloured light. The expla- 
 nation is now evident of the fact that the rainbow cannot appear when the sun has at- 
 tained an elevation greater 
 than 45. The rainbow, 
 though still depicted, is de- 
 picted below the horizon, 
 and is, therefore, invisible. 
 It follows, then, that the 
 size of the visible rainbow 
 is inversely to the eleva- 
 tion of the sun above the 
 horizon. 
 
 The annexed diagram 
 (Fig. 78) illustrates the for- 
 mation of the rainbow. The 
 straight line A P is supposed 
 to be drawn between the sun 
 and the eye of the observer, 
 passing through the latter. 
 Through this line a vertical Fig. 78. 
 
 plane is supposed to be drawn. If the line A # be drawn through A so that the anglo 
 P A # shall amount to 42, the rain-drops will reflect coloured drops to the eye. 
 Assuming the line A x to rotate, a cone will evidently be generated, part of which, 
 lying below the horizon, will be invisible. It is the sxu-face of the cone thus generated 
 which is the reflecting surface, and to which, therefore, the rainbow is due. Inasmuch 
 as every colour has a refractive quality peculiar to itself, each drop only represents one 
 tint to the eye. An arc having the breadth of about 2 is sufficient to include all the 
 prismatic colours ; 2 therefore is about the breadth of the rainbow. 
 
 The colours of the rainbow are partly due to refraction and partly to reflection, as 
 has been observed. The first eifcct of light on the drops of rain is refraction, by the opera- 
 tion of which white light arrives at the posterior side of each drop of rain, decomposed or 
 dissected into the primitive colours of which it is composed. At the posterior aspect of 
 each drop of rain the dissected colours are reflected unto the eye, and a coloured image 
 is presented. 
 
 Such is an explanation of the theory of the primary rainbow besides which, the 
 surrounding rainbow requires to be noticed. The secondary rainbow is outside the pri- 
 mary, and is larger than it, but also much fainter. Its angular position is denned by the 
 limits 50 59' and 54 9', measured with reference to the axis Ax. The secondary rain- 
 bow has all the colours of the primary, but less completely defined, and in a reverse 
 order. Its existence may be explained by the statement that it is the result of light 
 twice decomposed, whereas the true rainbow is the result of light only once decomposed. 
 The secondary rainbow, then, is produced by drops of water very far oif. 
 
 Inasmuch as any angular elevation of the sun above 45 is incompatible with the 
 existence of a rainbow, it is evident that this beautiful meteor can never occur in the 
 south. It may occur, however, either in the east, west, or north. 
 
 As concerns lunar rainbows, they are, as I have before remarked, exceedingly rare, 
 and are very seldom coloured. Nevertheless, in northern latitudes, where the moon 
 shines with a brilliancy unknown to us, coloured lunar rainbows are occasionally seen. 
 
96 
 
 HALOS AND PAEHELIA. 
 
 Kalos and Parhelia. These meteoric phenomena are far more rare with us 
 than in more northern latitudes, where they are continuously visible for long periods of 
 time, and give rise to phenomena of extraordinary beauty. The term halo is applied to 
 a luminous circle occasionally seen around luminous bodies, more particularly the sun 
 and moon, and is partly due to the refraction of light by vaporous water, sometimes in 
 the form of true clouds, sometimes not ; and partly to the properties of light termed 
 diffraction and interference. As respects diffraction, the circumstance has been already 
 announced that when light passes through a minute orifice such, for example, as a 
 small aperture punctured in a card the edges of such light bend : this is termed diffrac- 
 
 Fig. 79. 
 
 tion. The term, interference of light, is used to explain the phenomena of colour, or 
 alterations of luminous condition generally, which result from the assumed jarring 
 impact of luminous waves meeting in different phases of their vibration. 
 
 Solar halos frequently exist, though unnoticed, the sun's light being so powerful that 
 the eye of the observer cannot withstand its impressions. By the aid of a sheet of glass 
 rendered dull by smoke, these halos are frequently rendered visible. 
 
 Although halos, and also the phenomena next to be described, are referable to the 
 action of atmospheric moisture on luminous rays, yet it is evident that the condition of 
 that moisture will vary according to temperature ; in other words, the aerial moisture 
 which would be mere cloud- vesicles at temperatures above the freezing-point, would, if 
 depressed below 32 F., be converted into snow, or spiculoe of ice. The alteration which 
 these are capable of effecting on luminous rays being far greater than mere uncongealed 
 vesicular water can effect, the resulting optical phenomena are far more brilliant and 
 remarkable. Hence in northern latitudes the phenomena of halos and parhelia, as arcs 
 of light appearing near the sun, and sometimes intersecting each other, are called, are 
 
PARHELIA. 
 
 97 
 
 brilliant and impressive beyond anything which corresponding phenomena occurring in 
 this region would lead us to conceive. These luminous arcs, sometimes intersecting 
 each other, are as often occasioned by the moon as by the sun. As the phenomena in 
 question, when referable to the latter cause, are demonstrated parhelia, so when de- 
 pendent on the former cause they are termed paraselene. 
 
 Frequently parhelia 
 and paraselenae consist, 
 not only of the inter- 
 secting arcs just men- 
 tioned, but of circular 
 luminous meteors to 
 which the term mock 
 suns are especially ap- 
 plicable. Associated 
 with parhelia, and some- 
 times included under 
 the same name, is a 
 luminous band, passing 
 horizontally through 
 the sun, and not un- 
 frequently making a 
 circuit of the whole 
 heavens. Where this 
 luminous band and the 
 inner parhelion cross, a 
 mock sun usually ap- 
 pears, as represented in the accompanying diagram (Fig. 80). 
 
 Aurora Borealis. It has already been indicated, under the head of electricity, 
 that various circumstances materially operate to produce the condition termed electrical. 
 By far the best-studied of atmospherical electrical phenomena are thunderstorms ; but 
 it is an error to suppose that the atmosphere contains at the time of a thunderstorm 
 its maximum of electricity ; the experiments of Faraday have sufficiently made out 
 this point. 
 
 Eescrving the consideration of thunderstorms for the present, I shall introduce here 
 the subject of electrical phenomena by a description of the aurora borealis a phe- 
 nomenon sometimes said to be magnetic, inasmuch as the magnetic-needle is strongly 
 affected during its prevalence, but which, nevertheless, seems more naturally to belong 
 to electricity. 
 
 The term aurora borealis, or northern light, is applied when the phenomenon to be 
 presently described occurs in the north ; and the term aurora australis is applied when 
 it occurs in the south. But the former has been seen as far south as 45 of southern 
 latitude ; and the latter has more than once been visible in Britain. Nevertheless, the 
 beautiful phenomena of northern and southern lights are most prevalent towards the 
 north and south poles respectively. 
 
 Northern and southern lights, when in their greatest perfection, consist of a well- 
 defined arc of white light, and luminous streams of coloured light flowing therefrom. 
 The arc is not permanent as in the rainbow, but bends and twists in all directions 
 like a ribbon agitated by the wind. The intensity of the aurora varies within ex- 
 
98 
 
 AURORA BOREALIS. 
 
 tensive limits: when faint, the light is only recognizable at night by careful examination ; 
 
 but when highly developed, the aurora borealis, 
 or australis, can be seen during broad sunshine. 
 These phenomena may occur at any season, 
 but they are most prevalent in the months of 
 March, September, and October, or about the 
 period of the equinoxes. The aurora borealis 
 and australis are sometimes said to be magnetic 
 storms ; a more reasonable foundation is required 
 for the remark than is supplied by the tur- 
 bulent agitations of the magnetic-needle with 
 which they are attended ; but we have already 
 seen that the functions of magnetism and electri- 
 city are so nearly connected, that it is impossi- 
 ble in some cases to distinguish between the 
 two. 
 
 There is a very common electrical experiment 
 which furnishes an artificial phenomenon very 
 nearly resembling the northern and southern 
 light, in all respects except in the shape of the 
 illuminated body, which is a luminous arc. 
 
 The experiment consists in exhausting, by 
 means of an air-pump, all the air out of a glass 
 tube furnished with a metallic point at each end, 
 looking internally, and placed in the electric 
 current, as represented in Fig. 81. When a stream of electricity, of adequate in- 
 tensity, is passed through the apparatus from -|- to , the whole interior of the tube 
 becomes illuminated with flashes of light, very similar iu appearance to the flashes of 
 the aurora. 
 
 The phenomenon of the aurora borealis was noticed by Aristotle and Pliny, although 
 neither philosopher could have seen it to advantage. Gassendi first originated the 
 term aurora borealis, to indicate the phenomenon of this kind observed by him on 
 September 12, 1621. These phenomena appear to be subject to some laws of secular 
 variation not yet understood. That they have appeared in certain years, and certain 
 groups of years more than others, is certain. According to Mairan, twenty-six 
 occurred between A.D. 583 and 1354; thirty- four between 1446 and 1560; sixty -nine 
 between 1561 and 1592 ; seventy between 1593 and 1633 ; thirty-four between 
 1634 and 1684 ; two hundred and nineteen between 1685 and 1721 ; nine hundred 
 and sixty-one between 1722 and 1745 ; and twenty-eight between 1746 and 1751. 
 
 After 1790, auroras became unfrequent, but since 1825 they have been on the in- 
 crease. A very remarkable aurora borealis occurred in the autumn of 1847 : it was 
 conspicuous not only in England, but even so far south as Italy and Spain. 
 
 Height of the Aurora. As a proof of the doubt which exists concerning the height 
 of the aurora, they have been variously estimated from 3000 or 4000 feet to several 
 miles. The probability is, that the conditions on which the aurora depends vary in the 
 altitude of their operation ; but the truth is, that, notwithstanding the electrician by his 
 artificial experiments can imitate the light of the aurora borealis and australis, notwith- 
 standing the prevalence of the phenomena in question near the magnetic t>oles seems to 
 
PHENOMENA OF THE ATTEORA. 
 
 99 
 
 point to magnetic agency as the cause, our real knowledge concerning the aurora 
 borealis and australis is very slight. 
 
 It may be as well here to present the reader with a summary of the various opinions 
 which have prevailed at different times relative to the phenomenon in question. Many 
 early writers referred the appearances presented to mere optical causes, considering 
 them to be due to the reflection of the sun's light thrown up wards by a mirror of snow 
 
 and ice, and a subsequent reflection downwards by atmospheric agencies. De Mairan, 
 the observer who, perhaps more than anyone else, is entitled to be considered the chroni- 
 cler par excellence of the phenomena of the aurora, attributed them to the penetration of 
 our planet at certain periods into the solar atmosphere. On this supposition it will be 
 remarked, that the solar atmosphere must be assumed to extend to the orbit of our 
 planet, an hypothesis totally irreconcilable with the teachings of optics and astronomy. 
 The celebrated Euler, the philosopher who could deal so satisfactorily with the abstrations 
 of number and quantity, seems to have offered a most crude and improbable theory ex- 
 planatory of the aurora. Adopting the molecular theory of light, he assumed that the 
 solar rays, striking against the particles of our atmosphere, actually carried particles of 
 the latter iip into the heavens to a height of more than four thousand miles, the height 
 at which Euler believed auroras to exist. Some philosophers, of whom Volta may be 
 regarded the Coryphaeus, adopted a chemical theory of auroras, referring them to the 
 ignition of hydrogen gas spontaneously generated on the earth, and rising by its light 
 specific gravity to the higher atmospheric regions. It was assumed by these philoso- 
 phers that the evolution in question took place in the tropical regions chiefly, and that 
 
100 
 
 OX THUNDERSTORMS. 
 
 it was wafted by the upper current of air treated of in connection with the trade- wind- 
 to the north and south poles respectively. 
 
 Halley was the first, I believe, who suggested that the phenomena of aurora borealis 
 and australis might be due to the passage of magnetism from one magnetic pole to the 
 other ; and the -theory of Halley is so far retained, that the aurora is assumed to be in 
 some way connected with electricity and magnetism, but in what manner is beyond the 
 competence of observers to decide. 
 
 On the Phenomena of Thunder and Lightning. Perhaps no meteorologic 
 phenomena are now so well understood as these ; though, before electrical science had 
 been studied by the phlosophers of the last century, and the crowning experiment of 
 Franklin performed, the phenomena of thunder and lightning were so mysterious, that 
 even philosophers were content to refer them to the operation of an occult cause. 
 
 The intimate study of electrical science opens a field of somewhat abstruse matters 
 for consideration; the field is far too wide and too abstruse to be dealt with satisfactorily 
 here. In the treatise on Chemistry of the Imponderable Agents, belonging to this 
 series, it has been treated somewhat in detail ; and to this I must refer the reader who 
 desires to know more on this subject than strictly belongs to the necessities of what I 
 may term practical meteorology. 
 
 With this explanation I shall not hesitate to adopt the term electric fluid, although 
 the reader has already been made aware that no such fluid is at all likely to exist. Let 
 us now contemplate the phenomenon of that electrical excitation, the solution of which 
 is lightning, under the simplest conditions that the phenomenon can assume. Let A and 
 
 Fig. 83. 
 \ 
 
 B (Fig. 83) represent two clouds, which, being made up of watery vesicles, are neces- 
 sarily electrical conductors; and being surrounded by the atmosphere on all sides, are 
 necessarily insulated. For the sake of our illustration, it will now suffice to assume 
 that neither of the clouds here represented is electrically excited at this period of 
 the description, and hence that the marks -f- and are for the present misplaced. Let 
 it now be assumed that the cloud A becomes positively electrified, that is to say, 
 charged with positive electricity, owing to some natural cause unnccssary here to 
 explain ; and let the results of this condition be traced out. Firstly, there is not in all 
 nature, and there cannot be, such a condition as that of independent electric excitation ; 
 in other words, there cannot be one body positively excited without the co-existence of 
 another body negatively excited. Hence, if cloud B were away and cloud A positively 
 
LIGHTXIXG-COXDUCTORS. 101 
 
 excited, the air circumjacent to A would assume the second or negative function ; but 
 if the cloud B is present, it therefore becomes negative, and the two clouds A and B are 
 mutually attracted, because opposite electricities attract each other. Hence they ap- 
 proach until the space of air between the two is insufficient to restrain their mutual 
 electric tension : this condition having arrived, a discharge takes place, precisely analo- 
 gous to the discharge of a Ley den jar. Under the postulates of our experiment, the 
 discharge, or lightning flash, takes place between the two clouds A and B. 
 
 It follows, however, from the consideration of known electrical laws, that just as 
 the two oppositely electrified bodies may be two clouds as assumed, so also may they be 
 one cloud, and the surface of earth or water, or conductors placed upon either one or 
 the other, under which conditions a downward discharge will take place ; and generally 
 electricity will always take the nearest path between any two bodies oppositely charged, 
 the conducting facilites being equal. 
 
 Lightning- Conductors. It is almost unnecessary in these days to announce 
 that Franklin, in the year 1752, first demonstrated the nature of lightning by drawing 
 electric sparks from the string of a kite, previously caused to ascend into the region of a 
 thunder-cloud. This experiment performed, the connection between lightning and 
 electricity could no longer be doubted, and a means of drawing off a surcharge of the 
 electric fluid by lightning-conductors was immediately suggested. The most important 
 instruments were not adopted, however, tintil after numerous and varied conflicts. 
 Firstly, the argument was adduced by some that lightning-conductors could not be 
 adopted without impiety, being intended to contravene the will of Providence. An 
 argument so fallacious was no sooner abandoned than lightning-conductors were ex- 
 posed to another ordeal, founded on an erroneous practical estimation of a truth in 
 theoretical electricity. I allude, as the electrician will perceive, to the contest between 
 the advocates of spherical, and of pointed, terminations for electrical conductors. 
 
 Now, regarding the question of points or spheres abstractedly, it is easy to see that 
 preference should be given to the former, inasmuch as points draw off and give issue 
 to the electric fluid in silence, whereas spheres draw off and give issue to electricity 
 in sparks ; but, inasmuch as the largest spherical termination ever used, or ever likely 
 to be used, for the upper extremity of a lightning-conductor, is virtually a point in 
 comparison with the enormous surface of the smallest thunder- cloud, the dispute, 
 though violent and prolonged, never had the practical significance which was at one 
 time taken for granted, 
 
 The history of lightning-conductors furnishes a remarkable illustration of the 
 difference between the mere knowledge of a fact, and the confidence or conviction 
 resulting from that knowledge and justifying its practical application. The whole 
 theoiy of lightning-conductors was almost as well known half a century ago as now ; 
 yet it is only within the last few years, and owing to the unflagging perseverance of 
 Sir "W. SnoW Harris, that due effect has been given to the theory, and lightning-rods 
 have been fearlessly applied. 
 
 ElectriiCtil Principles connected with Lightning-conductors. The electrical principles 
 on which the efficiency of lightning-conductors depend are few and simple ; they all 
 admit of being readily demonstrated by electrical experiments artificially performed, 
 and they have been universally justified by the result of three practical applications : 
 
 j[l.) Electricity cceteris paribus folloivs its course through the best conductors which 
 happen to be in its path. Thus, Tor example, if a Ley den jar be charged, and the elec- 
 tric connection between its external and its internal coating be completed by three 
 
102 THE LEYDEX JAK. 
 
 lineai substances of equal length say, for example, silk, wire, and linen thread thin 
 wire being the best electrical conductor of the three, will transmit the whole of the 
 electricity to the exclusion of the silk and the linen thread. It is assumed, however, 
 in the performance of this experiment, that the wire is sufficiently large to convey the 
 whole electric energy. 
 
 (2.) Provided the conductor be good, and its sectional area adequate, the electric Jluid or 
 energy is conveyed harmlessly away. No point in the whole of electrical science can be 
 
 more satisfactorily established than this. It 
 admits of being illustrated by numerous experi- 
 ments, amongst which the following may suf- 
 fice : 
 
 The diagram (Fig. 84) represents a common 
 Ley den jar, represented in the act of being dis- 
 . charged by the ordinary discharging instrument. 
 That instrument is, as usual, of brass, all save 
 the handle, which is of glass, and therefore a 
 non-conductor of electricity. Those who arc 
 conversant with the form and construction of 
 pi 8 .i the discharging instrument, are aware that its 
 
 two terminal balls admit of being unscrewed. 
 
 Assuming one of the balk, viz. the upper one in the diagram, to have been unscrewed, 
 the liberated brass stem to be passed through amarroon,or box holding gunpowder, and 
 the ball to have been again replaced, the conditions will have been fulfilled which the 
 diagram represents. It is evident that the Ley den jar, as represented, will be dis- 
 charged. It is, moreover, evident that the whole of the charge wiU be transmitted 
 through the gunpowder contained in the marroon, yet that gunpowder will not inflame. 
 If, however, instead of the conditions of the last experiment, a very fine metal wire (a 
 steel wire by preference) be passed through themarroon,or rather through some combina- 
 tions of explosive materials less potent than a marroon, which would now be dangerous, 
 . and electricity transmitted as before, the wire, not presenting a sufficient amount of 
 transverse area of surface to convey the electricity, melts, and the explosive compound 
 is inflamed. 
 
 (3.) Lateral discharge must be provided against. The meaning of lateral discharge 
 will be illustrated by the following experiment : The diagram (Fig. 85) represents, as 
 before, a Ley den jar readily arranged for being discharged through a metallic wire, one 
 end of which has already been brought into contact with the outside of the jar, while the 
 other end can be brought into contact with the knob communicating directly with the 
 inside of the jar. The hand is represented in the act of holding a glass rod, around which 
 one end of the wire is coiled, and the extremity of the wire is finished off with a ball. 
 All these arrangements, the electrician will perceive, are necessary for giving effect to 
 the efficient discharge of the Leyden jar through the conducting wire. The chief poin 
 for observation, however, is the band of the wire, by means of which one part is caused 
 very closely to approach another part, .as represented at a 6. Now it is possible by 
 choosing a wire sufficiently small, and causing the two bands to pass sufficiently near, to 
 determine the passage of an electrical spark from a to b, instead of proceeding through 
 the entire length of the wire, from the internal to the external coating of the jar. This 
 is what electricians call the lateral discharge, and it requires to be studiously guarded 
 against in the construction and arrangement of lightning-conductors. 
 
ELECTRICAL DEDUCTIONS. 
 
 103 
 
 Application of the Foregoing Deductions. Perhaps the deductions already arrived 
 at -will suffice for practical guidance in the matter of lightning-rods, though these 
 deductions by no means exhaust the science of 
 the subject. Firstly, the fact may be considered 
 as proved, that all bodies, even the most dan- 
 gerous and inflammable, all edifices, all living 
 beings, may be shielded from the evil conse- 
 quences of lightning, by the safeguard of light- 
 ning-conductors. The conductors, however, 
 must present a sufficient external area ; and the 
 point has been made out by numerous trials, 
 that a copper rod a square inch in sectional dia- 
 meter will convey away the utmost fury of tho 
 most highly-charged thunder-cloud ever proved 
 to exist. Copper is one of the best electrical 
 conductors amongst metals ; but, by providing a 
 sufficiently increasing sectional area to compen- 
 sate for inferior conducting power, any metal 
 may be made to perform the function of copper. 
 Whatever be the conductor, its upper ex- 
 tremity should project considerably above the 
 
 Fig. 85. 
 
 edifice to be protected ; and if pointed theoretically, all the better, though practi- 
 cally the bluntest termination could be only as a point by comparison with the enor- 
 mous mass of a thunder- cloud. Far from preventing contact between the building to 
 be protected and the conductor, as is sometimes done by the interposition of glass or 
 earthenware guards, a lightning-conductor cannot be brought into too intimate metallic 
 connection with every part of the edifice to be protected. The conductor should 
 branch and ramify over the surface of the building, and should be brought into contact 
 with every important system of metal line work, such as the iron pipe which fre- 
 quently runs down the side of a wall ; finally, the conductor should at its lower ex- 
 tremity be brought into contact as efficiently as possible with some good electric 
 conductor, such as the system of gas or water-pipes which run underneath most houses, 
 especially under the streets of most civilized towns. As regards the number of con- 
 ductors necessary to be supplied to one building, that will depend, firstly, on tho 
 shape of the building, whether it be composed of many elevations, or whether, like a 
 column, it has only one. Perhaps the best practical testimony on this point is gleaned 
 from the fact, that a ship having three masts, one of which only was protected by a 
 lightning-conductor, the unprotected masts have been shattered by the effects of a 
 thunder-storm, while the other remained untouched. If a column be surmounted by a 
 metallic statue, it is worse than useless to disfigure the head of the statue by a pro- 
 jecting metallic spike as the beginning of a lightning-conductor ; nothing more is 
 requisite in this case than to provide sufficient metallic conduction for the electricity 
 downwards into the ground. Lightning-conductors, it should be remembered, do not, 
 as they are commonly said to do, attract electricity. They no more attract electricity, 
 than a gutter attracts water. They merely open a channel for electricity to pass through. 
 Before the demonstrations of Sir William Snow Harris had taken effect, marine light- 
 ning-conductors were something more dangerous than lightning itself : consisting merely 
 of chains, which were only elevated aloft after the thunderstorm had come on. Marine 
 
104 PHENOMENA OP AEROLITES. 
 
 lightning-conductors are now fixtures on the masts ; they are made of copper, running 
 band-like down the mast, and embedded in the latter in such manner that, whether the 
 masts be elevated or lowered, perfect metallic contact between any two pair of masts 
 remains uninterrupted. 
 
 Relative Prevalence of Thunderstorms. The phenomena of thunder and 
 lightning are nowhere so violent or so frequent as in the so-called region of calms ; 
 but they are very prevalent throughout the torrid zone, more especially during the rainy 
 season. Thunder and lightning are almost always absent in the polar regions ; and even 
 at a spot no farther north than Bergen, in Norway, the annual number of thunderstorms 
 does not average more than six. As the rule, thunderstorms chiefly occur in hot 
 weather, winter thunderstorms being comparatively rare. Iceland and the western 
 coast of North America are remarkable for the predominance of thunderstorms in 
 winter. In Sweden, however, winter thunderstorms are almost unknown ; thus fur- 
 nishing another example of the circumstance already noted, that the Scandinavian range 
 of mountains eifect a remarkable difference between the general climate of Sweden and 
 Norway, though the two, geographically considered, are so close together. 
 
 Aerolites Shooting Stars Meteoric Stones. The beautiful phenome- 
 non of shooting stars is common enough ; but at certain periods it is peculiarly remark- 
 able, the whole sky being filled with these fleeting meteors. The beginning of August 
 and the beginning of November are noticeable for their connection with shooting stars ; 
 more especially have they been recorded between the 9th and the 14th of August. The 
 bouquet of shooting stars observed at this period in North America has been sometimes 
 called the Shower of St. Lawrence. 
 
 For our knowledge respecting the periodicity of the phenomenon of falling stars, we 
 are indebted to Quetelet Besenberg and others; but Muschenbrock, so long back as 
 1762, first directed attention to the so-called shoiver of St. Laiorence. In addition to 
 the August and November phenomena of the kind under consideration, other periods 
 have been noticed for instance, in April, and from the 6th to the 12th of December; 
 but these bouquets of shooting stars which thus occur are less considerable and less 
 regular than the former. Besides these showers of shooting stars, the periodicity of 
 which is well attested, single meteors of this kind are frequently noticed, and they occur 
 at all seasons ; therefore, whatever may be the cause of shooting stars, this cause must 
 be regarded as continuously operating. 
 
 Sometimes the luminous meteor termed a shooting star attains a large magnitude ; 
 observers then speak of it as a fire-ball. The identity of shooting stars and fire-balls is 
 now well established, though formerly they were treated of as distinct. Fire-balls are 
 sometimes seen alone, but more frequently in connection with shooting stars. Their 
 light and their bulk are frequently so considerable, that they can be seen in broad day- 
 light. Their velocity through the heavens, or rather through the upper layers of our 
 atmosphere, is various ; but it generally exceeds that of the earth. 
 
 As regards the nature of shooting stars, we have only theory and analogy for our 
 guidance; but our knowledge of fire-balls is far more accurate, and there seems no 
 reason to doubt that their teachings illustrate the nature of falling stars. Numerous 
 fire-balls dart through our atmosphere, become luminous, and disappear, no one knows 
 whither. Others, though passing near to our atmosphere, fail to enter it, and 
 therefore are not rendered visible. A third division not only come within our atmo- 
 sphere, shine, and burst with a loud report, but they fall ; yet, falling either into the sea 
 or upon an unfrequented spot of land, the locality of their fall remains unknown. Whilst 
 
ORIGIN OF SHOOTING STAES. 105 
 
 a fourth division of fire-balls may be seen to fall, dug out, and examined ; thus sup- 
 plying data for an investigation of fire-balls in general. 
 
 Masses of this kind are termed aerolites, and their connection with fire-balls has been 
 placed beyond all doubt. Fire-balls have been seen to fall, and aerolites have been 
 extracted from the place whereon they have fallen. Nevertheless, some aerolites have 
 fallen upon the earth without the assumption of a previous appearance of luminosity. 
 Cases, though rare, are well attested of an aerolite suddenly falling from a small cloud, 
 attended with a noise resembling the discharge of cannon ; others, again, have fallen 
 silently, and out of the clear air, not the slightest trace of cloud being visible at the time. 
 
 Testimony concerning showers of stars and the fall of aerolites has been handed 
 down to us from all periods, but it is only since the time of Chladni that the occurrence 
 of these phenomena has been placed beyond doubt. 
 
 Amongst the best-attested examples of the fall of aerolites are the following : On 
 the 16th of June, 1794, a shower of stars fell at Sienna ; and in the following year, 
 December 13, an aerolite, weighing no less than fifty-six pounds, fell in England. Three 
 years afterwards, and remarkably enough also on December 13, a fire-fall split up and 
 discharged round stones. A very large shower of stones fell April 26, 1803, near Aigle, 
 in France ; the occurrence is particularly interesting on account of its having been 
 noticed and verified by M. Biot. Ten such meteoric showers were observed in France 
 in twenty-six years i. e., between 1790 and 1815. The meteoric shower at Aigle in 
 1803, which poured its contents over a surface of two and a half French miles long, by 
 one in breadth, consisted of 2,000 fragments of different sizes, some weighing not more 
 than two drachms, others near twenty pounds. Aerolites are sometimes much larger 
 than this ; one fell at Agram on the 26th of May, 1751, having a diameter not less than 
 eighteen feet, and weighing seventy-one pounds ; but the largest known aerolite fell in 
 Mexico, and weighed between 30,000 and 40,000 pounds. 
 
 As to shape, aerolites are generally prismatic, or angular, rarely smooth ; and almost 
 always sheathed in a crust of pitchy blackness. Their specific gravity is various, some 
 being sufficiently porous to absorb water with rapidity, others being dense and metallic. 
 Looking at the specific gravity of aerolites in the aggregate, it may be said to vary 
 between 1*94 to 4'28, thus presenting a mean of about 3'5. All the heavier varieties of 
 aerolites are made up of iron, holding a little nickel ; traces also of cobalt, manganese, 
 chromium, copper, arsenic, tin, and other well-known elementary bodies, are found. 
 
 Origin of Fire-balls and Shooting-stars. Various opinions have been 
 advanced to account for these bodies. One of the earliest, if not the very earliest, of 
 these hypotheses, originated in 1660, and assumed fallen aerolites to be mineral masses 
 originally projected from lunar volcanoes ; and calculations were made, having for their 
 object to demonstrate what volcanic force might be sufficient to project aerolites of a 
 given mass into the sphere of attraction of the earth's atmosphere. Unfortunately for 
 the probability of this theory, the moon's surface appears to be altogether devoid of active 
 volcanoes. Then followed the chemical hypothesis, according to which it was assumed 
 that aerolites were nothing more than aggregations of metallic vapours, which had risen 
 to the upper region of the atmosphere, aggregated there, and fallen. The opinion of 
 Chladni is now, however, generally received ; he regards aerolites to be of cosmical 
 origin to be so many planets, or planetary fragments, which revolve in orbits of their 
 own, variously inclined to the orbit of the earth ; that our planet encounters periodic 
 shoals of these little worlds, some of which, becoming entangled in the earth's gravi- 
 tating system, pass into our atmosphere, become heated by friction against its particles, 
 
106 OCCULT METEOROLOGICAL EMANATIONS. 
 
 and ultimately fall to tne ground. Although the discovery of aerolites is comparatively 
 rare, the meteors, of which they are the final result, are by no means so. It has been 
 calculated that the average annual fall of aerolites is not less than 700, or about two 
 daily. 
 
 Ifteteorologic Result of Occult Emanations. When the utmost powers of 
 a refined chemistry have been applied to the analysis of atmospheric constituents and 
 conditions, much still remains to be unveiled. There are atmospheric causes, whatever 
 they may be, of epidemic and endemic diseases, and perhaps other agencies which our 
 philosophy little suspects or dreams of. I have ventured to include these undetermined 
 agencies under the general expression occult emanations. It is not difficult to point out 
 objections to this designation in some of its applications. Perhaps it is not strictly 
 philosophical to speak of emanations thus hypothetically ; perhaps this may be only a 
 repetition of the error of assuming the existence of an electric fluid ; perhaps the number 
 of influences due to allotropism and to polarity is greater than we imagine ; but, at any 
 rate, the term " occult emanations" may be accepted as a rallying-point for a certain class 
 of facts, until the time arrives when their true significance shall be correctly made out. 
 
 "Without invoking the hypotheses of allotropism and polarity, there are undoubtedly 
 some atmospheric agencies to which the expression occult emanations is applicable, and 
 concerning which the only thing occult about them is the insufficiency of ordinary 
 chemical examinations to demonstrate their existence, though that existence admits of 
 being demonstrated by extraordinary chemical means. Thus, for example, it is a well- 
 authenticated fact, that the atmosphere of localities in which fever is endemic, usually 
 contains minute traces of hydrosulphuric acid, and an odorous animal matter substances 
 which ordinary chemical processes fail to detect, but which, nevertheless, by the adoption 
 of refined methods of investigation can be proved to exist. The late Professor Daniell 
 was of opinion that the much- dreaded fever of Western Africa was augmented by the 
 diffusion, through the atmosphere of that coast, of minute traces of sulphuretted 
 hydrogen. And the circumstances under which African fever originates, are perfectly 
 consonant with the above theory. The disease only prevails now on the coast, its 
 ravages being limited to a small belt, partly of land and partly of sea, Central Africa 
 being comparatively exempt from its inflictions. Now Professor Daniell assumes the 
 hydrosulphuric acid to be the result of decomposition, under a powerful sun, of matter 
 borne seawards by the Niger and other great rivers, in connection with certain sulphates 
 of sea- water. Be this theory true or the contrary, there can be no doubt as to the 
 truth of the assumption which refers fever, when endemic in certain localities, to a 
 vitiated condition of atmosphere ; which vitiation may be generally summed up as con- 
 sisting of minute traces of hydrosulphuric acid, and of undetermined animalized matter. 
 
 Amongst other occult emanations, we can hardly refuse to admit the cause, whatever 
 that cause may be, of intermittent fevers. The ultimate, if not the proximate, cause of 
 this class of disease is so well known, that we may almost produce or banish intermittent 
 fevers at pleasure. Given heat and moisture continuously, ague almost invariably sets 
 in, and continues its ravages as long as the conditions of heat and moisture coexist. 
 
 What is the occult emanation here ? What is the proximate cause of intermittent 
 fever ? Are we to attribute the disease to the conjoined evaporation of moisture and 
 heat directly, or to some further emanation to which these conditions give rise ? Some 
 pathologists have assumed that light carbonetted hydrogen, or marsh-gas as it is called, 
 determines the disease ; but the notion hardly coincides with known facts in relation to 
 this disease. The horizontal demarcation of altitude above which the influence of 
 
IGXIS FATT7US. 
 
 107 
 
 ague cannot extend, is one of the most remarkable circumstances in connection with 
 the disease ; a difference of no more than ten feet in perpendicular height frequently 
 corresponding with the region of fever and the region of salubrity respectively. 
 
 The mention of light marsh-gas naturally suggests the curious meteorologic phe- 
 nomenon called "Will-o'-the-wisp, or Jack-o'-lantern; of which gas ignited, or, according 
 to some, phosphuretted hydrogen gas, it is believed to consist. The Will-o'-the-wisp is 
 not a very frequent phenomenon anywhere ; but it is chiefly seen in marshes and 
 
 Fig. 8G. 
 
 churchyards, the latter locality apparently adding to the hypothesis that it is nothing 
 more than ignited phosphuretted hydrogen gas. 
 
 Scarcely less accurately demonstrated than the locality of ague, is the locality of 
 yellow-fever, the focus of which may be considered to be Vera Cruz. Strange to say, 
 the yellow-fever is totally unknown on the Pacific coast of Mexico ; it extends north as 
 far as New Orleans, but rarely further. This condensation, so to speak, of febrile energy, 
 points to some local cause most probably of atmospheric origin ; but chemistry has 
 been unable to determine its nature. 
 
 Reflections similar to the above are suggested by the contemplation of several con- 
 tagious diseases ; of which the plague is a notable example. This scourge, although 
 capable of spreading from its normal focus into regions wide apart, is in certain spots de- 
 termined conditionally by the existence of some unknown conditions ; and these are most 
 probably atmospheric. The plague never originates in very hot or very cold regions. Its 
 focus of development is Egypt, Turkey, and the Levant, from which spots it can never 
 be said to be absent altogether ; but it only appears with violence at intervals of eight or 
 
108 
 
 CLIMATOLOGY. 
 
 ten years. Still more extraordinary than any of the preceding, are the varied conditions 
 which give rise to Asiatic cholera. Unlike the plague and the yellow-fever, and in- 
 termittents, this fell destroyer seems independent of region, recognizable limits, or 
 other conditions of demarcation. From east to west it has extended its ravages, under 
 every vicissitude of season and of clime. Much as there is mysterious in this absolutely 
 ignorant though we be of the proximate influences by the operation of which cholera 
 and other epidemics are generated, important facts have, nevertheless, been made out ; 
 and their consideration tends to allay the extreme fear wherewith epidemics were 
 formerly regarded. The most important fact in connection with this subject is, that 
 epidemic influences, whatever their nature may be, only as a rule prevail over the 
 weak, exhausted, ill-fed, or mentally broken-down individuals of a community; whence 
 it happens, that here in our metropolis the medical statician is enabled to lay his finger 
 upon the regions of epidemic virulence, as he would on the locality of a mountain or a 
 coal-field ; and with equal satisfaction can he point to localities where epidemics formerly 
 raged, but whence they have been banished by the art of man. 
 
 Climatology. Many of the effects of heat, in its meteorological relations, have 
 already been incidentally considered ; but reference has not yet been made to the sources 
 of heat, and to the means of its distribution over the surface of our planet. The term heat, 
 as applied to the matter now under investigation, may be regarded as synonymous with 
 elevation of temperature ; and inasmuch as such elevation necessarily presupposes a 
 condition of antecedent depression, we may, without impropriety, comprehend the meteo- 
 rological effects of high and low temperatures (heat and cold) under one and the same 
 generalization. 
 
 Central Heat of the Earth, The hypothesis was first propounded by Leibnitz, that 
 the whole of our planet was once a molten mass, which by the operation of cooling, un- 
 interruptedly going on in successive ages, has become superficially encrusted over, the- 
 crust having become adapted to the necessities of animal and vegetable life. Various 
 circumstances may be adduced in favour of this notion, more particularly the gradual 
 increase of terrestrial heat downwards, the heat of deep springs, and the evidences ot 
 fusion in what geologists term the igneous rocks. Whether the idea of Leibnitz hold 
 good in its entire acceptation, that is to say, whether the centre of our planet be one 
 molten mass or not, there can be little doubt that all positions of the globe, at a sufficient 
 distance below the surface, have at some period been submitted to fusion. Nevertheless,, 
 at this time, the earth's central heat may be altogether ignored as tending to influence, in 
 any manner, the climatic temperature of our globe. Primarily, the sun's direct rays 
 determine the climatic temperature ; those portions of the world's surface being most 
 strongly heated on which the sun shines at the greatest angle a remark which of course 
 applies to the tropics ; while those are least heated on which the direction of solar rays is 
 most oblique a remark which of course applies to the arctic and antarctic regions. But 
 the latitude of a region has less connection with its climatology than might at first seem 
 probable. The varying conditions of insular and continental sea level, or elevated table- 
 land, valley or mountain, and still more the influence of thermal oceanic currents, have 
 much to do with the climatic result. The high table-land of Mexico is strongly illus- 
 trative of the effect of mere elevation. The traveller who disembarks first on the- 
 Atlantic coast, and wanders inland, soon finds himself amidst all the luxuriance of a 
 tropical forest, and surrounded by all the dangers of tropical existence. Still wending 
 his way inland, he ascends a mountain elevation, and finds himself suddenly transported 
 to a region, where, on account of its elevation, the climate has totally changed, and with 
 
INFLUENCE OF CURRENTS ON CLIMATE. 109 
 
 .it the vegetation. So marked is the change, that the high table-land of Mexico is -well 
 adapted to the growth of wheat, which refuses to grow anywhere in tropical lowlands. 
 
 Of all the causes which influence the climate of a region, that attributable to oceanic 
 currents has been hitherto least studied; yet there is none which deserves to be scru- 
 tinized more narrowly. Looking at the enormous amount of oceanic surface in com- 
 parison with that of the land taking into consideration the mobility of water, its sus- 
 ceptibility of thermal impressions, and the effect of the configuration of capes, head- 
 lands, and lines of coasts the contemplative observer soon arrives at the deduction, that 
 the ocean presents to him, at least, as wide a field for investigation as the atmosphere, 
 and one scarcely less interesting. " The fauna and the flora," says Maury, " of the sea 
 are as much the creatures of climate, and are as dependent for their well-being upon 
 temperature, as are the fauna and the flora of dry-land. Were it not so, we should find 
 the fish and alga?, the marine insect and the coral, distributed equally and alike in all 
 ^parts of the ocean. The polar-whale would delight in the torrid zone, and the habitat 
 of the pearl-oyster would be also under the iceberg, or in frigid waters colder than the 
 melting ice." The particles of water being mobile, the ocean, and indeed aqueous col- 
 ' lections generally, are amenable to the same law of conviction as was described when 
 treating of the cause of winds. Hot water, being specifically lighter than cold water, 
 must necessarily come to the surface , and for every current in one direction there must 
 be a counter-current in the reverse direction, precisely in the same manner as occurs in 
 the development of a wind. 
 
 Contemplating the ocean in its relation to the effects of heat and motion, our original 
 ideas concerning that vast collection of waters are modified and expanded. Instead of 
 regarding the ocean as one shapeless aggregation of briny water, it presents itself to us 
 as an assemblage of many streams, a network of mighty rivers, each following its own 
 course, each having its own temperature, its own flora, its own animals ; and though 
 devoid of palpable banks, scarcely less accurately defined on that account. Amongst all 
 these oceanic currents, that denominated the gulf-stream is the largest in size, the most 
 important in the functions it subserves. The gulf-stream is so far from being an im- 
 agining of mere theory, that the dark blue alone of its waters suffices to point out its 
 limits and define its course. 
 
 All hypotheses as to the cause of the gulf-stream are as unsatisfactory as the direction 
 of the stream itself, and its benign influences are evident. The first idea relative to the 
 gulf-stream was, that it originated in the impetus given to the ocean by the disembogue- 
 ment of the Mississippi ; but placing out of consideration the inadequacy of this assumed 
 cause, on account of the comparatively small amount of water which even a river so 
 vast as the Mississippi can pour forth, it follows that, if really the cause of the gulf- 
 stream, the whole Gulf of Mexico should, in process of time, be found to contain only 
 fresh, or at the most brackish water. This, it is scarcely necessary to remark, is not the 
 case. Franklin advanced the theory, that the gulf-stream is referable to the pressure 
 of an inordinate amount of water against the coast of the Gulf of Mexico by the trade- 
 winds, an idea which is scarcely more tenable than the last. 
 
 Whatever the cause of the gulf-stream may be, the direction of its current is 
 obvious. Setting out from the hot regions of the Mexican Gulf and the Caribbean Sea, 
 it proceeds northward to the great fishing-bank of Newfoundland, and thence to the 
 shores of Europe, yielding up its heat to the genial west winds, and thus transferring a 
 portion of the superfluous heat of the tropics to our colder shores. The greatest heat of 
 the oceanic water of the Mexican Gulf is about 86, or about 9 above the ocean tern- 
 
110 THE GULF-STREAM AJSD ITS CUEEEXTS. 
 
 perature due to latitude alone. After it has ascended to 10 of north latitude, the gulf- 
 stream has still only lost 2 of the original heat with which it set out. Ascending north- 
 wards a distance of three thousand miles from its first origin, this mighty oceanic 
 river still preserves the heat of summer even in winter time. It now crosses in a 
 westerly direction, in a line coincident with about the fortieth degree of north latitude, 
 spreads itself out, and imparts to Europe a genial temperature ; which mere latitude 
 could never give. The gulf-stream now pauses in its course ; it is split into two 
 divisions by the British Isles, and two gulf-streams are formed. Of these, one tends 
 northward in the direction of Spitzbergen, whilst the second enters the Bay of Biscay 
 imparting temperature to each, and causing a soft mantle of vapour to arise, which,, 
 wafted landward, in its turn disperses the heat of the gulf-stream far inland. Very 
 little is known concerning the depth to which the gulf-stream extends. 
 
 Lieut. Maury, of the United States naval service, assumes that depth to be two 
 hundred fathoms ; and arguing on this assumption, he calculates that the amount of 
 heat led away from the Gulf of Mexico by this oceanic torrent raises, on a winter's 
 day, the whole atmosphere which hovers over France and the British Isles from the 
 temperature of 32 F. to about 79; in other words, from winter-cold to summer-heat. 
 But the genial influence of the gulf-stream on the British Isles is more than this. 
 Every western breeze that blows towards us crosses the mighty gulf-stream, robs it of a 
 portion of its heat, and comes towards our shores charged with warmth and laden 
 with balmy moisture, clothing Ireland in a suit of green, and imparting a mildness to 
 both England and Ireland which can be best appreciated when we consider that the 
 coasts of Labrador, on the American side, and under the same parallel of latitude as 
 England, are rigid with ice. In 1831, the harbour of St. John's, Newfoundland, was 
 closed with ice as late as the month of June ; yet the harbour of Liverpool, though 2 
 further north, is never ice-locked even in the severest winters. By referring to any chart 
 of isothermal lines, the current of the gulf-stream, as just described, may be readily traced. 
 
 Although the climatic effect of the gulf-stream is so advantageous to Western Europe, 
 more especially to these Isles, it is scarcely less advantageous to the regions whence it 
 originates. If the gulf-stream be the channel along which an amount of heat so con- 
 siderable passes westward, we may speculate on the consequences that would have arisen 
 had the amount of temperature now conveyed away remained in the gulf itself. Even 
 now the coast-line of this region is extremely hot and unhealthy ; how much more hot 
 and unhealthy would it have been had the gulf-stream not existed ! 
 
 Under-Current of the Gulf -Stream. As the trade-winds.are only an under atmospheric 
 current, passing in an opposite direction to a current above ; so the gulf-stream is only 
 the counterpart of an inferior current of cold water flowing back to compensate for that 
 which has departed. Not only does theory proclaim this, but it is borne out by experi- 
 ment. At a mean depth of two hundred and forty fathoms, an under-current of water 
 flows into the Caribbean Sea ; and the temperature of this current has been found as low 
 as 48, whilst the sxirface-water had a temperature of 85. At the depth of three hundred 
 and eighty-six fathoms the temperature had fallen to 43 ; and at the very bottom of the 
 gulf-stream the temperature was only 38; hence the existence of the returning cold cur- 
 rent is fully borne out. It comes, there is little reason to doubt, from the arctic circle ; 
 presenting the closest analogy to the lower aerial current which constitutes the trade- 
 wind. 
 
 The course and extent of the gulf-stream were not generally known until the cele- 
 brated Dr. Franklin drew attention to the subject. The history of this event is worthy of 
 
FIRST CHART OF THE GULF-STRE AM . Ill 
 
 narration, illustrating as it does the discriminating and logical mind of that extraor- 
 dinary individual. 
 
 Happening to be in London in 1770, his opinion was demanded respecting a memorial 
 presented by the Board of Customs at Boston to the Lords of the Treasury, stating that the 
 Falmouth packets were generally a fortnight longer on their voyage to Boston than common 
 traders were from London to Providence, Rhode Island : whence their request that the 
 Falmouth packets might be sent to Providence instead of to Boston. " Franklin could 
 not understand the reasonableness of this request, inasmuch as London was much further 
 than Falmouth, and from Falmouth the routes were the same, so that the difference should 
 have been the other way. Desiring a solution of his difficulty, he consulted Captain 
 Folger, a Nantucket whaler, who chanced to be in London at the time. The whaler 
 explained that the difference arose from the circumstance that Rhode-Island captains 
 were acquainted with the gulf-stream, while those of the English packets were not. The 
 latter kept in it, and were driven back sixty or seventy miles a-day ; while the former 
 avoided it altogether." Maury. 
 
 The manner in which the old whaling captain had been made acquainted with the 
 existence, the extent, and the direction of this gulf-stream, is curious enough in its way. 
 His instructors were the objects of his search, the arctic whales animals which, having 
 a dislike to warm water, never enter the gulf-stream, though they swim close up to it 
 on both sides. 
 
 Franklin having extracted this intelligence from the whaling captain, got him to 
 draw a chart of the gulf-stream to the best of his ability. The chart was drawn, Frank- 
 lin had it printed, and copies were sent to the Falmouth captains. They, however, were 
 foolish enough to pay no heed to its teachings ; nor did they profit for many years after 
 by the knowledge of the gulf-stream. Though the date of Franklin's discovery was 1775, 
 yet a knowledge of the gulf-stream was not generally diffused and acted upon until 
 fifteen years later. Not the least extraordinary fact in connection with the gulf-stream 
 is the sharpness of its line of demarcation. No river imprisoned between two scarped 
 rocky banks could flow in channel more defined. " If," remarked the American author 
 Jonathan Williams, " these strips of water had been distinguished by colours of red, 
 white, and blue, they could not be more distinctly discovered than they are by the 
 thermometer." " And, he might have added," remarks Maury, "nor could they have 
 marked the position of the ship more clearly." 
 
 The notion prevails amongst sailors that the gulf-stream is the great storm-breeder 
 of the Atlantic the father of storms ; and, indeed, the tempests which follow in its 
 course or on its borders warrant that designation. What are the indications of theory 
 in this respect ? Had the Atlantic been still an untravelled waste, and the existence of a 
 gulf- stream, such as we now know it, been propounded as the basis of discussion, would 
 not the theorist have predicted that storms must originate in the meeting of the hot, 
 moist atmosphere which hovers over the ocean tract of seething waters from the fiery 
 shores of Mexico and the Caribbean Sea, mixed with the chilling blasts of the north ? 
 What torrents of water must result from the condensation of the tepid mists what stu- 
 pendous electrical force must be brought into operation ! 
 
 If the gulf-stream, by its impulsive flow, sometimes impedes the mariner and drives 
 his ship from the desired course, it nevertheless affords a compensation, not only in 
 assisting to propel ships sailing in the direction of its course, but in affording a genial 
 climate to the weather-beaten mariner, frozen and benumbed by the shivering blasts 
 of the regions outside its channel. " No part of the world," says the writer to whom I am 
 
112 OTHER OCEAN CUEEEXTS. 
 
 largely indebted for much that in these pages concerns ocean currents and ocean clima- 
 tology, " no part of the world," remarks Maury, " affords a more difficult or dangerous 
 navigation than the approaches of our (the American) coast in whiter." Before the 
 warmth of the gulf-stream was known, a voyage for this reason from Europe to New 
 England, New York, and even to the capes of the Delaware or Chesapeake, was many 
 times more trying, difficult, and dangerous than it now is. In mailing this part of the 
 coast, vessels are frequently met by shore-storms and gales which mock the seaman's 
 strength, and set at naught his skill. In a little while his bark becomes a mass of ice, 
 and his crew frosted and helpless. She remains obedient only to her helm, and is kept 
 away for the gulf-stream. After a few hours' run she reaches its edge, and almost at the 
 next bound, passes from the midst of winter into a sea at summer-heat. Now the ice 
 disappears from his apparel ; the sailor bathes his stiffened limbs in tepid water ; feeling 
 himself invigorated and refreshed with the genial warmth about him, he realizes out 
 there at sea the fable of Antaeus and his mother Earth. He rises up and attempts to 
 make his port again ; and is again as rudely met, and beaten back from the north-west ; 
 but each time that he is driven off from the contest, he arises forth from this stream, 
 like the ancient son of Neptune, stronger and stronger ; until after many days his 
 freshened strength prevails, and he at last triumphs, and enters his haven in safety, 
 though in the contest he sometimes falls to rise no more, for it is often terrible. Many 
 ships annually founder in these gales ; and I might name instances, for they are not 
 uncommon, in which vessels bound to Norfolk or Baltimore, with their crews enervated 
 in tropical climates, have encountered, as far down as the Cape of Virginia, snow-storms 
 that have driven them back into the gulf-stream, times and again ; and have kept them 
 thus out for forty, fifty, and even for sixty days, trying to make an anchorage. 
 
 Nevertheless, the presence of the warm waters in the gulf-stream, with their summer- 
 heat in mid- winter off the shores of New England, is a great boon to navigation. At 
 this season of the year especially, the number of wrecks and loss of life along the Atlantic 
 sea-board are frightful. The month's average of wrecks has been as high as three 
 a-day. How many escape by seeking refuge from the cold in the warm waters of the 
 gulf-stream, is a matter of conjecture. Suffice it to say, that before this temperature 
 was known, vessels thus distressed knew of no place of refuge short of the "West Indies ; 
 and the newspapers of that day Franklin's Pennsylvania Gazette among them inform 
 us, that it was no uncommon occurrence for vessels bound to the capes of Delaware in 
 winter to be blown off, and to go to the West Indies, and there wait for the return of 
 spring before they would attempt another approach to this part of the coast. 
 
 The gulf-stream is the largest known oceanic current ; it has been perhaps more fully 
 studied than any other, and its teachings may therefore be appropriately regarded as the 
 type of the rest. We have seen how powerful and extensive are its effects ; we have 
 seen a few of the purposes to which it ministers. The ocean is full of streams similar to 
 this, each taking its well-defined course, carrying its own temperature, clad with its 
 own fauna and flora, peopled with its own denizens. Thoughts like these prove how 
 false and unfounded is the expression, ocean waste, so commonly applied. The ocean has 
 its regions, its valleys, and its mountains climates and varied inhabitants- no less than 
 the earth. 
 
 Other Oceanic Currents. The Mediterranean. It has long been known that 
 an upper or sailing current constantly sets into the Mediterranean through the Straits 
 of Gibraltar. What, then, becomes of the water of the currents ? That water must 
 either be dissipated by evaporation, or there must be a second or back-current. 
 
CURRENTS OF THE SEA. 113 
 
 The existence of this under-current was first demonstrated very curiously in 1712. 
 At that time, France being at war with Holland, M. L'Aigle commanded a French 
 privateer, called the Phoenix of Marseilles. Near Ceuta this privateer gave chase to a 
 Dutch ship bound to Holland, come up with her, delivered one broadside, when the Dutch 
 ship immediately went down. A few days later, the sunken ship, with all her cargo of 
 brandy and oil, came to light again ; but this took place on the coast of Tangier, at least 
 four leagxies westward of the place where the ship went down, and in a direction quite 
 opposite to that of the upper or navigable current. This well-authenticated case was 
 communicated to the Royal Society in 1724. 
 
 Currents of the Red Sea,. Precisely similar to the Mediterranean currents, just 
 described, are those of the Red Sea. The necessity of a free change of waters here is 
 even more necessary than in the Mediterranean ; the sea is not only shallower, and sub- 
 jected to a more powerful evaporation, but its waters are not freshened by the afflux of 
 any rivers. It has been calculated by Dr. Buist, that, taking into consideration the mean 
 evaporation on every part of the surface of the Red Sea, a sheet of water, eight feet 
 thick, and equal in superficial area to the whole extent of the surface of the Red Sea, will 
 be raised in vapour annually. When this enormous rate of evaporation is considered, 
 the necessity for a continuous interchange of water between the Red Sea and the 
 external ocean will be evident. If the Red Sea outlet were choked up, so that ingress 
 and egress were no longer possible, the evaporation of about a thousand years would, 
 it is calculated, completely dry it up. 
 
 Currents of the Indian Ocean. Many thermal currents originate in the Indian Ocean. 
 Amongst the foremost of these is the Mozambique current ; another of these currents, 
 first escaping from the Straits of Malacca, and being swollen by warm streams from the 
 Java and Chinese Seas, flows out into the Pacific between the Philippines and the Asiatic 
 shores. Passing thence towards the Aleutian Isles it ultimately loses itself on the north- 
 west coast of America. Meteoric conditions like those which mark the course of the 
 gulf-stream also mark the course of this. Fogs and mists follow in its track, and storms 
 are also generated on the bank of this oceanic river. 
 
 For a fuller account of oceanic currents than is consistent with the limits and objects 
 of these pages, I must refer the reader to treatises which deal with the special matter, 
 and above all to the work of Lieutenant Maury, of the United States service, to whom 
 meteorologists are under deep obligations for his contributions to their knowledge of 
 ocean phenomena. 
 
 In contemplating these oceanic currents, of which the gulf-stream may appropriately 
 be considered the type, we cannot fail to be impressed with an evidence of design, where 
 .at first no design would seem to exist. These oceanic currents originate in, and are deter- 
 mined by, a peculiar conformation of the land. Now what can be more seemingly 
 irregular or capricious than the shape of land ? If dropped down into the ocean at 
 random, or elevated by a subterranean power equally capricious, the crust line of islands 
 and continents, the solid blocks of our planetary crust, could not well be more irregular 
 than they are ; and yet how practically harmonious is the relation between land and water : 
 how well adjusted the powers of each, how well adapted to the mutual benefit of mankind. 
 It appears an unimportant mattar when in the map of the world we skim our eye over 
 the southern hemisphere of the terrestrial globe ; there we behold the limits of the 
 African and American continents in their furthest extent ; but how terribly would the 
 locomotive faculty of mankind have been impeded had either continent expanded itself 
 to the soutli pole, or even traversed much further south than is actually the case ! 
 
114 COLD AND ITS EFFECTS. 
 
 In connection with the subject of ocean currents, the effects of aqueous temperature 
 on the ocean's denizens deserves a passing word of remark further than has already 
 been devoted to it. In the Caribbean Sea, and other ocean caldrons, where stores of 
 heat are accumulated for distribution in regions far away, in the hot currents which 
 originate in these tepid sources of oceanic rivers, there are a fauna and a flora : the fauna 
 no less marked than we see on tropical lands. There grows the coral, there swims the 
 shark ; and thousands of shelled mollusca, of gorgeous colour and enormous size, revel- 
 ling in oceanic forests of rank and bulky growth, represent the land forests and their 
 denizens of corresponding climes. But though grandeur and beauty be the characteristics 
 of these warm ocean spots the thermal ocean-tropic (if the propriety of that expression 
 be allowed) it is the colder oceanic currents which support the form of animal life most 
 useful to man. Strange though the circumstance may appear, it is no less true, that the 
 fish of the hot parts of the world are always indifferent as food. This fact is strikingly 
 illustrated in the Mediterranean. The temperature of the Mediterranean water is 
 usually four or five degrees above the temperature of the external ocean, and what a 
 difference in the fish ! Whoever has compared the edible fish of the Atlantic with those 
 of the Mediterranean, will be at no loss to admit the vast superiority of the former. 
 The naturalist does not require to be informed, that not only are Mediterranean fish 
 inferior to those of the Atlantic, but they are for the most part of different species. 
 
 Let us now take a glance at the locality of the principal fishing regions. Limiting 
 ourselves to two of these, they would unquestionably be the fishing grounds of New- 
 foundland and Japan. The former is the better known of the two, though, perhaps, the 
 latter is the more considerable, seeing that the natives of Japan are debarred from the 
 use of animal food by their religion, although the eating of fish permitted, they are all 
 ichthyophagi ; and, notwithstanding the dense population of the Japanese islands, their 
 inhabitants, though fish-eaters, are abundantly fed. 
 
 Between the gulf-stream and the coast is a narrow band of cold water : here are the 
 fisheries of Newfoundland. Between the China current, as it is denominated, and 
 flowing in an opposite direction, is the cold current, in which the Japanese fisheries are 
 prosecuted. These are the two most prominent examples, and they will be found to 
 present the type of many others. 
 
 Cold, and its Effects. Under the heads of snow, hail, and hoar-frost, some of 
 the meteorologic effects of cold have been already described. The phenomena due to 
 this powerful agency are, however, so numerous and so important, that it may be well 
 to make the subject of cold a matter of special contemplation. If, casting our eye over 
 the field of nature, we endeavour to select the most prominent results of cold, they will 
 be found to relate to the departure from an ordinary law, which Nature has made in 
 the expansion of water during the act of freezing. 
 
 The term freezing is but a general expression for the act of solidification by cold. 
 Popularly, the term is only applied to solidified water ; but this is altogether a conven- 
 tional acceptation of the word. "We are justified, then, in comparing the act of solidifi- 
 cation of water, with the act of solidification of any other fluid, and seeing to what 
 extent the conditions which regulate one also regulate the other. 
 
 When the temperature falls to 32 F. water ceases to be liquid, and becomes ice ; the 
 weather is said to be frosty, and water is said to be frozen. Whatever water be con- 
 tained in the atmosphere at the freezing temperature, is deposited in the solid form of 
 hoar-frost, the particles not being irregular, but bounded by definite mathematical out- 
 lines frequently giving rise to forms of great beauty, especially on window-panes, 
 
ICE PICTUKES. 
 
 115 
 
 blades of grass, and leaves (Fig. 87). These forms are similar, in general contour, to 
 
 Fig. 87. 
 
 those of well-formed snow-flakes (Fig. 88), but far more beautiful, and, like snow- 
 flakes, prove that frozen water is a crystalline body, and that it crystallizes in forms 
 belonging to the rhombohedral system. 
 
 But the most important point connected with the freezing of water, and without 
 
 Fig. 88. 
 
 which our globe would cease to be habitable, is this : Water, during the act of 
 freezing, expands, and thus becomes specifically lighter. Ice, therefore, swims on 
 water ; it cannot sink. How stupendous are the consequences of this departure from 
 the law of freezing ! Had it so happened that frozen water, like frozen mercury, was 
 more heavy than the corresponding liquid material, each frozen sheet of water would 
 sink as soon as formed ; and thus, being far removed from the melting influence of 
 solar rays, the production of ice would have been accumulative, and the ocean, long ere 
 this, would have been completely ice-locked. 
 
 In tracing, hypothetically, an assumed aberration of Nature to its consequences, the 
 mind is speedily entangled and lost in the chaos of jarring effects. The speculative 
 meteorologist finds himself unable to thread the labyrinth of causation here involved. 
 Having shown that the ocean would have been full of ice, had the ordinary law of 
 solidification not been departed from in the case of water, it is perhaps unnecessary to 
 follow the development of our hypothetical case further. That ocean life must have 
 been destroyed is evident ; that the sea's liquid highway would have ceased to be, is 
 only a figurative expression for a frozen ocean. But would what is now the solid land 
 have then served the purposes of animal life ? "Where could the rivers have flown, had 
 the ocean been a block of ice ? or would not the rivers have remained frozen too, seeing 
 the vast cooling power of a frozen ocean ? It is easy to see that, under such circum- 
 stances, our planet would have been totally unfit to be a resting-place for its present 
 denizens had the freezing of water not assumed a departure from a law, though it be 
 impossible to imagine all the consequences that would have resulted. 
 
116 CLIMATE AND ORGANIC DEVELOPMENT. 
 
 The same expansive force of water during the act of freezing, by the operation of 
 which it is rendered specifically lighter than water, subserves many important purposes 
 in the world's economy, besides floating the ice. Water percolating into the fissures of 
 rocks, and being subsequently frozen there, displays its irresistible force by splitting 
 large rocks into fragments, and disintegrating their fragments. In this way hard 
 and sterile districts become covered with useful soil, in which low forms of vegetable 
 life can take root ; and by their subsequent decay, contribute the elements necessary to 
 the support of higher forms of vegetation. 
 
 Relation of Climate to Organic Development. The naturalist who, in his desire to 
 see as in a dioramic picture the wonderful characteristics of animal and vegetable 
 types, sighs to think that a vision so glorious will never pass in reality before his gaze, 
 may at least console himself with the assurance of that great master of philosophic 
 travel, Humboldt, that it is in the power of man's creative faculty, aided by philosophy, 
 to imagine those striking types in the vividness of their truth, gladdening the closet 
 with ideal images of the living features of Nature. 
 
 Even in the narrow region of European travel, the intelligent observer will not fail 
 to see distinctive physiognomies. Passing from the cold green-sward and modest vege- 
 tation of our own Isles to the Mediterranean shore, a striking change in the aspect of 
 nature meets the view. The sturdy oaks and elms of our own forests disappear ; the 
 absence of smaller grasses removes the green carpet of our meadows; tall graminacesc 
 spring up ; the aloe and the prickly pear bespeak a mixed condition of heat and drought ; 
 and the date-palm, barely acclimatized, gives some faint notion of what the characteristics 
 of a tropical forest must be. " It would be an enterprise worthy of a great artist," says 
 Humboldt, " to study the aspect and the character of all these vegetable groups, not 
 merely in hot-houses, or in the description of botanists, but in their native grandeur in 
 the tropical zone. How interesting and instructive to the landscape painter would be a 
 work which should present to the eye, first separately, and then in combination and 
 contrast, their leading forms ! How picturesque is the aspect of tree ferns, spreading 
 their delicate fronds above the laurel-oaks of Mexico ; or the groups of plantains over- 
 shadowed by arborescent grasses. It is the artist's privilege, having studied these 
 groups, to analyze them : and thus in his hands the grand and beautiful form of nature 
 which he would portray resolves itself, like the written works of men, into a few simple 
 elements." 
 
 When the meteorologist has exhausted his knowledge in the laying out of climatic 
 groups when he has placed in correlation conditions identical, as he thinks, in every 
 respect the growth of vegetable forms demonstrates his inability to comprehend many 
 hidden secrets of nature, which their delicate organization makes known. European 
 olive-trees grow luxuriantly at Quito, but they bear neither fruit nor flowers ; and a 
 similar remark applies to walnut-trees and hazel-nuts in the Isle of France. In India, 
 the bamboo flowers luxuriantly ; but in South America, where it flourishes equally well, 
 so far as general aspect of growth is concerned, so rare an event is the infloration of the 
 bamboo, that during a four years' residence in South America, Humboldt was only 
 enabled to obtain blossoms once. But perhaps a still more remarkable example of luxu- 
 riant growth without inflorescence is furnished by the sugar-cane. The West Indies 
 have come to be considered as the region par excellence of the sugar-cane ; yet it seldom 
 bears flowers there nor indeed does it in any part of the American continent; thus 
 furnishing a strong presumptive argument in favour of the theory which asserts that 
 no variety of the sugar-cane is indigenous to the New World. 
 
PKACTICAL METEOEOLOGY. 
 
 117 
 
 LA-WSON'S THERMOMETER STAND. 
 
 PBACTICAL METEOEOLOGY. 
 
 So many amateur meteorologists are now springing up in every direction, that it may 
 be desirable to point out the precautions and reductions necessary in order to render the 
 raw observations useful to science. The present paper is, therefore, written in con- 
 tinuation of Dr. Scoffern's Meteorology, to enable the amateur, with as little trouble as 
 possible, to reduce his observations to useful results. 
 
 To the non-meteorologist such terms as adopted mean temperature, elastic force of 
 vapour, reduced barometer, &c., unexplained, serve rather to perplex than to enlighten 
 
118 
 
 METHODS OF OBSERVING. 
 
 or point out that care has been bestowed upon the observations, and that the requisite 
 reductions and precautions had been adopted. 
 
 The first duty of an observer is to procure good instruments, and to place them in 
 such positions as may be deemed least likely to be affected by local circumstances, 
 such as heat from a fire, draughts round a building, &c. Much time is frequently 
 thrown away by using imperfect instruments. "When a person purchases a thermo- 
 meter, he is too apt to consider it correct ; whereas, in many instances, it is far from 
 being accurate. Mr. Hartnup, of the Liverpool Observatory, in his last report, mentions 
 that among the thermometers used by the captains of the merchant service, an error of 
 4 or 5 is quite common ; even a thermometer fitted up for taking the temperature of 
 water at different depths, and professing to have been made with care, was found to be 
 8| in error in one part of its scale. The most laughable instance, however, was a 
 -barometer which had the following errors : 
 
 At 28 inches was 2'22 inches of the standard. 
 
 At28 
 At 29 
 At29f 
 At 30 
 At30 
 At 31 
 The whole yearly range of pre 
 
 1-88 
 
 073 
 
 0-30 
 
 0-02 
 + 0-33 
 + 1-07 
 
 sure seldom reaches two inches, whilst the range in 
 error of this instrument was 3-29 inches. 
 
 Meteorological Observations. In order to place this portion of the subject in 
 as clear a light as possible, let us suppose that daily observations are made of the baro- 
 meter and its attached thermometer, of the temperature of the air, of the wet bulb 
 thermometer, of self-registering maximum and minimum thermometers, of the amount 
 of rain, amount of ozone, temperature on the grass and in sunshine, state of electricity, 
 amount and class of cloud, direction and force of wind, and state of the weather. Let 
 us further suppose that two observations are taken daily, the hours of observation being 
 9 A.M. and 10 P.M. 
 
 The Pressure of the Air. The barometer is the instrument by which the alterations 
 in the weight of the air are ascertained. The gravitation of the earth exerts a certain 
 pressure which would always be alike, were it not that lateral disturbances had the 
 power of removing a portion of that pressure and adding it elsewhere. Thus, suppose 
 the average pressure to be 29| inches, if the barometer is seen to rise to 30 inches, it is 
 certain that in another portion of the earth a corresponding fall has taken place to 
 account for this change ; however, as it is not our object to enter into the physical and 
 local changes of the weather here, we shall at once proceed to describe the reductions 
 that are requisite. 
 
 It is essential to correct observation that the barometer used be a standard instru- 
 ment. The ordinary instruments are useless, owing to the friction of the mercury 
 against the sides of the glass in small tubes, the impossibility of applying the reduction 
 necessary to correct for the alteration of the height of mercury in the cistern with the 
 common wheel barometer, owing to a rise and fall in the tube ; and, further, because 
 if errors occur in this barometer, they are increased by the circumstance of the index 
 of the wheel barometer being a long-arm worked by a small wheel, thus multiplying 
 all the errors. 
 
 The Standard Barometer should have a tube of not less than \- -ths of an inch in 
 
NEWMAN'S THERMOMETER. 
 
 119 
 
 771 
 
 diameter, and the nearer it approaches to T^ths the better, as the 
 correction requisite for the friction against the sides of the glass 
 rapidly diminishes with an increase in the size of the tube. This 
 is the correction for capillarity, which is additive. The base of 
 the tube should be plunged into a cistern of pure mercury, and the 
 scale of inches should have a rod attached, terminating in an ivoiy 
 point, so contrived as to be moved by rack- 
 work until the point just touch the surface 
 of the mercury. This is requisite, in order 
 that the measurement may be made from the 
 surface of the quicksilver in the cistern. Sup- 
 pose we either neglect this operation, or the 
 barometer is one not capable of having it 
 applied ; and let us further take a reading 
 without reference to this correction after a 
 sudden change in the barometer, this reading 
 may be represented as 30'036 inches; on 
 bringing the ivory point down to the mer- 
 cury, this second reading may be 30'074 
 inches : thus exhibiting an error, from the 
 want of this precaution, of '038, or nearly four- 
 hundredths of an inch. The thermometer fitted 
 for use should have its bulb plunged into the 
 cistern of mercury, otherwise it will not give 
 the temperature of the mercury itself, but 
 merely show the heat of the apartment in 
 which it is placed. Such an instrument is 
 made by Newman, the optician in Eegent 
 Street ; it is the same as is made for the Royal 
 Society, Greenwich Observatory, Admiralty, 
 and the British Colonial Observatories. 
 
 Description, a is the mahogany board to 
 affix against the wall; b, brackets which 
 support the barometer, between which it is 
 capable of being revolved so as to observe 
 the light on the surface of the mercury ; c, 
 a vase, which unscrews to allow of the socket 
 d being removed to receive the upper end of 
 the barometer; e, the adjusting screws for 
 shifting the lower centre, by which the baro- 
 meter is to be made exactly perpendicular : 
 to accomplish this, the ivory point is to be 
 adjusted to the surface of the mercury, the 
 barometer gently turned between the two 
 brackets; and if in any position the point 
 should be elevated from the surface, or de- 
 pressed into the mercury, the screws must be 
 altered accordingly, until the point coincides 
 
120 
 
 THERMOMETER STANDS. 
 
 in every position ; /, the key by which the ivory point is adjusted the ivory point 
 being a termination of the brass scale marked off at the temperature of 32, and which 
 is adjusted by means of a tangent screw ; g, the glass part of the cistern, through which 
 the surface of the mercury and ivory point are seen ; A, the cistern ; , the screw which 
 is to be loosened when the barometer is fixed, to admit the atmospheric pressure ; , 
 the moveable part of the cistern, on which the index t is engraved ; I, the key by 
 which the vernier is adjusted; m, the thermometer dipping into and showing the 
 temperature of the mercury. 
 
 This barometer is necessarily expensive. Modifications of this standard are made by 
 Mr. Barrow of Oxendon Street, and Messrs. Negretti and Zambra of Hatton Garden ; 
 and these are more reasonable in price, answering remarkably well, though not equal 
 to Mr. Newman's Standard. Having procured a barometer, it should be compared with 
 the Standard, in order to ascertain its index error. The apartment in which it is kept 
 must not be subject to great changes of temperature ; one with a window facing the 
 north is to be preferred ; and the instrument should hang where there is a strong light, 
 but an outer wall should be avoided. 
 
 Thermometers. There are various constructions of instruments for ascertaining 
 the temperature of the air ; of these the mercurial ones are the best. Thermometers for 
 comparison require to be placed upon a proper stand, as the errors arising from 
 peculiarity of situation, from radiation, absorption of heat, &c., will alter the reading 
 very materially. There are two forms of stand, the one constructed by the late Henry 
 Lawson, Esq., F.R.S., the other by James Glaisher, Esq., F.R.S. 
 
 Thermometer Stands. In comparing the readings of one thermometer with any 
 
 other, it is requisite that each instrument should be placed, as much as possible, in a 
 
 similar manner ; without this uniformity no deductions can be drawn with any claim to 
 
 accuracy. In looking to the situation of instruments used by persons who have not 
 
 s is s ns been aware of the necessity of providing 
 
 V r 
 
 themselves with a thermometer stand, 
 some will be found facing the north, 
 others the south, south-east, north- west, 
 and, in short, every point of the compass. 
 Again, some are placed from one to five 
 feet from the ground, others ten to 
 twenty feet ; some in the angle of a large 
 building, others exposed to the sun's rays 
 either during the morning or the even- 
 ing ; some touching a wall, others at a 
 distance from one; in short, the situations 
 vary in as great a degree as the tempera- 
 ture deduced from the observations made 
 by them. To obviate these sources of 
 inaccuracy, the late Mr. Lawson con- 
 structed his " Thermometer Stand," a 
 ^^ sketch of which is given at the head of 
 
 this chapter. This stand consists of a frame, which has been found to answer the 
 intended purpose very well. It is composed of white deal boards, and can be constructed 
 by any carpenter. It consists of an oblong trunk T, 12 inches by 8 inches outside 
 measure ; to the opposite side of which trunk are nailed boards b b, at the distance of 
 
 1. 
 
 Fig. 2. 
 
GLAISHER'S THERMOMETER-STAND. 
 
 121 
 
 three quarters of an inch, and projecting about six inches from the trunk towards the 
 north. Outside of these are nailed other thin boards c c, full half an inch distant, and pro- 
 jecting about four inches beyond the last-mentioned boards, also towards the north. 
 These sides or shades being multiple, prevent the sun from heating the interior of the 
 stand where the thermometers are placed. The top, or pent board, P, is made double, 
 and the boards are placed full three-quarters of an inch distant from each other, and 
 come forward so as to overhang, by a full inch, the Night Index Thermometer, 
 placed immediately beneath, for the purpose of preventing rain or dew from falling per- 
 pendicularly upon the bulb of the thermometer. The legs, L L, of the stand are merely 
 the continuation of the sides of the trunk. The board or feet, F F, are loaded or fixed 
 to the ground, to sustain the force of the wind. The interior, T, is blackened to prevent 
 strong reflections of light. 
 
 Fig. 2 is a ground-plan of the machine, which will prove sufficiently clear to any 
 intelligent workman for its construction. The sides (and wood- work generally) are of 
 half-inch white deal. The distance or space between the sides of the trunk T and 
 the board or inner side, i s (Fig. 2), is three-quarters of an inch ; and the distance 
 from that board to the outer side, o s (Fig. 2), is fuU half an inch. The narrow boards, * * 
 (Fig. 2), are to be nailed, with studs inter- 
 vening, to the middle board or side * s, and 
 are for the purpose of preventing the sun 
 from shining between the trunk and the sides 
 o s and i s when near the meridian. The 
 sides are fixed, one upon the other, at the 
 required distance (viz. three-quarters of an 
 inch and half an inch), by numerous wooden 
 studs, about three-quarters of an inch dia- 
 meter; and the nails or screws passed through 
 the sides and studs, fixing the whole firmly 
 together. The whole is to be painted white, 
 
 and no other colour, except the face of the 
 trunk T, which may be black, to prevent 
 strong reflections of light. 
 
 This thermometer-stand can be placed in 
 any eligible spot that may suit the conve- 
 nience of its owner ; its four sides should face 
 the cardinal points, commanding therefore a 
 true north and south aspect. It can be visited 
 on every side, and be free from all sur- 
 rounding objects. The thermometers used 
 can be read off with the greatest facility, 
 and the whole will be at a known distance 
 from the gnmnd. Those instruments placed 
 on the south face will have the meridian sun, and those on the north face will be always 
 in the shade, in consequence of the projecting wings. It can be employed by anv 
 meteorologi 't, wherever residing ; it is of a determinate form, height, anci size ; it is 
 not costly, but firm, and can be placed on any open spot that may be thought eligible for 
 its use. The instruments may be read off with the greatest promptitude, so as to prevent 
 or reduce errors arising from the person of the observer being too long in the vicinity of 
 
 Fig. 3. Glaisher's Stand. 
 
122 MAXIMUM AND MINIMUM THERMOMETERS. 
 
 the thermometers. By the general adoption of this stand, instruments placed upon it -will 
 all be used or observed under similar circumstances, and deductions from them be more 
 correctly drawn than where there is no stand. It follows that observations made by 
 individuals wherever residing, either in Europe, Asia, Africa, or America, if drawn 
 from instruments thus similarly placed, can be compared one with the other with far 
 less chance of error than has hitherto been the' case. 
 
 Mr. Glaisher, F.R.S., has likewise constructed a stand differing from the one just 
 described, but which is also an excellent contrivance. 
 
 Rutherford? s Thermometer. The mercurial thermometer of Rutherford's construction 
 pushes a registering-pin before it as the mercury expands by heat ; on the air becoming 
 cooler, the mercury contracts, leaving the registering-pin at the point of maximum 
 heat. One objection to this instrument will always be felt, i.e. the pin is not 
 unlikely to become entangled with the mercury, especially if the mercury should 
 oxidize. 
 
 It is very desirable to incline Rutherford's maximum thermometer from the hori- 
 zontal position, so that the bulb shall be slightly the highest, in order that the ther- 
 mometer may have the assistance of gravitation in pushing the pin forwards. "Within 
 the last fifteen years many of Rutherford's construction have at the Highfield Hotise 
 Observatory become useless, owing to the registering- needle becoming entangled with the 
 mercury. Two thermometers, however, of this construction deserve to be noticed, the one 
 purchased of Dollond and the other of Bennett : they have done their duty satisfactorily, 
 and are at the present time in perfect working order. 
 
 Negretti's Patent Maximum Thermometer. Too much praise cannot be given to this 
 ingenious invention, which, from the circumstance of doing away with the registering- 
 needle, prevents the possibility of the instrument getting out of order. Since the invention, 
 half a dozen of these thermometers have been constantly employed here without any 
 derangement. The principle of the thermometer is this : A small piece of enamel is 
 pushed into the thermometer tube near the bulb, and the tube is then bent so as to secure 
 it in its place ; if the thermometer act once, it will continue to act until the instrument is 
 broken. As the temperature rises, the mercury will flow over this enamel ; yet, on the 
 air becoming colder, the contraction will only take place below this point, the whole 
 column of mercury in the tube being left to mark the maximum heat. It is only 
 requisite to turn the thermometer in a vertical position, and give it a gentle shake, when 
 the mercury will descend and flow over the obstruction, which, in the horizontal posi- 
 tion, had prevented it from returning into the bulb of the thermometer. 
 
 Phillips' s Construction of the Maximum Thermometer. This, the invention of Professor 
 J. Phillips, P.R.S., also records without a registering-needle. A small bubble of air is 
 passed down the tube, and a portion of mercury is made to remain above this bubble ; 
 as the air increases in temperature the whole column rises, yet, when it cools, the mer- 
 cury only falls from below where the bubble of air is situated. It is an instrument 
 liable to get out of order in travelling ; yet, when once properly fixed, does its work well 
 In consequence of the bubble of air expanding with an increase of temperature, a slight 
 error will be occasioned, and the thermometer will read a little too high in warm 
 weather, and the reverse when cold. 
 
 The Minimum Thermometer, until lately, has been filled with spirit instead of mercury, 
 with a slender glass pin floating in the liquid. This pin is carried down with it to the 
 lowest point; on the temperature rising, this pin is left behind, and thus marks the coldest 
 point. Like the maximum thermometer, gravitation should be allowed to assist the 
 
MERCURIAL MINIMUM THERMOMETERS. 
 
 descent of the pin, which is accomplished by slightly lowering the bulb from the hori- 
 zontal position. The best that I have received have been from Messrs. Negretti and I 
 Zambra ; in these the pin is much longer and more slender, and they very rarely get out 
 of order. A disadvantage will always be felt with spirit thermometers ; their action 
 differs from those filled with mercury. 
 
 The Mercurial Minimum Thermometer. Meteorologists have for some time urged the 
 opticians to invent a mercurial minimum thermometer, an invention which long seemed 
 almost to be an impossibility. However, such an instrument now exists, thanks to 
 Messrs. Negretti and Zambra. A thermometer with a large tube is placed in a vertical 
 position, in which is a slender, pointed needle, which is brought down to the surface of 
 the mercury ; quicksilver being heavier than the needle, it is held above it; yet, the 
 needle being pointed, plunges a small, but sensible, distance in the mercury, which it 
 invariably does by the side of the glass of the thermometer tube. This being the case, 
 the needle will descend to the lowest degree of cold ; on the thermometer rising, the 
 mercury presses the needle to the glass, and rises up by the side of it, instead of 
 raising the needle. Four months' working of this thermometer has proved it to 
 be a valuable invention. The instrument must come into general use amongst 
 meteorologists. 
 
 The Solar and Terrestrial Radiation Thermometers are made entirely of glass, the 
 scale being engraven upon the bulb itself. For thermometers continually exposed to 
 damp, the swelling of the wood and the obliteration of the index thereon has been felt a 
 great annoyance ; consequently, the substitution of glass for wood has been hailed with 
 pleasure by meteorologists, independently of its great improvement in a scientific point 
 of view. 
 
 The white enamel placed along the back of the thermometer tube, an invention of 
 Negretti and Zambra' s, is now becoming generally adopted ; the improvement is at 
 once manifest on inspecting two instruments, the one with and the other without the 
 enamel. 
 
 The Wet and Dry Bulb Thermometer. The dry bulb is the ordinary thermometer, 
 and the wet bulb differs only in having the bulb enclosed in a muslin bag, with a cotton- 
 wick conductor to a cup of water, so that it shall always be wet from the capillary action 
 of the cotton conveying the water constantly to it. If the muslin bag were attached 
 to self-registering, instead of ordinary thermometers, the greatest heat and cold of 
 the wet bulb would be obtained an important addition to the meteorological instru- 
 ments, yet one almost unknown. The muslin and cotton should be changed every 
 month. 
 
 . Bennett's Photographic Wet and Dry Bulb Thermometer is an additional evidence 
 of the close relationship between heat and light. In this most ingenious contrivance 
 light is incessantly writing down the heat of the air. It is simply requisite to place a 
 sheet of paper upon the roller once a day in order to record every change in temperature 
 of the wet and dry bulb for the twenty-four hours. This is, indeed, a triumph of 
 science. 
 
 Evaporators. Having had constructed gauges of various sizes, I am enabled to 
 speak with confidence as to their working. The water in gauges under eight inches in 
 diameter becomes too warm, owing to the small quantity that can be contained in them ; 
 consequently, an excess of evaporation results. There are two gauges to be recom- 
 mended ; the one Newman's, and the other Negretti' s. These gauges work well. New- 
 man's is a very convenient and ornamental instrument, having a graduated glass tube 
 
124 
 
 RAIX-GATJGES. 
 
 30 
 
 40 
 
 [t consists of a short cylinder twelve inches in diameter, having connected with it, 
 
 oy means of a stopcock, a glass tube graduated to huudredths, and terminating in a 
 
 lower vessel, which will contain a sufficient 
 quantity of water to be raised by artificial pres- 
 sure into the upper one for exposure to the 
 atmosphere. 
 
 To use the apparatus, pour water into it 
 until it rises to the zero in the glass tube, then 
 by means of a syringe force the air through the 
 tube X (Fig. 4) into the lower vessel, so as to 
 raise the water into the upper one to any height, 
 you please. Now shut off the stopcock beneath 
 to retain the water in the upper vessel ; then, 
 having exposed the apparatus for any length of 
 time required, open the cock, the water will run 
 into the lower vessel, filling it and pail of the 
 glass tube, the divisions of which will now indi- 
 cate the quantity of water evaporated. Ne- 
 gretti's is the cheaper, being simply a cylinder 
 of the diameter of eight inches, which fits into 
 a wooden box filled with wet sand ; this keeps 
 the outside of the metal cool, and prevents that 
 excessive evaporation which would result from 
 the heating of the metal by the sun. Where 
 the diameter of the gauge is large, and the 
 water several inches deep, the effect of the sun- 
 shine on the metal sides is not felt. 
 
 Rain Gauges. There are several con- 
 Fig. 4. structions, yet none so good as Negretti and 
 
 Zambra's, which is simple, and at the same time prevents any loss by evaporation, an 
 
 important point, which has been too much overlooked. Another contrivance is that of a 
 
 cylindrical vessel of brass or zinc (Fig. 5) ; the latter 
 
 would be the cheapest, and answer all purposes equally 
 
 well. Into this cylinder, a funnel, with its tube bent, fits 
 
 tightly ; the diameter should be eight inches, and the tube 
 
 about an inch in length. The object of this bended tube 
 
 is to prevent evaporation taking place from the surface of 
 
 rain collected in the rain gauge, for a few drops of water 
 
 will hermetically seal the opening from the escape of 
 
 vapour, and most frequently the evening dews will deposit 
 
 sufficient moisture for this purpose, which the heat of the 
 
 day will scarcely have time to dissipate before night brings 
 
 a fresh supply. 
 
 The readiest mode of measuring the amount deposited 
 
 in the gauge, is by procuring another cylindrical vessel, or measure, which is exactly 
 
 four inches in diameter, and four inches deep; this, when quite full, will just contain 
 
 an amount equal to the deposit of an inch of rain as collected in the eignt-inch gauge. 
 
 Parts of an inch can be measured by plunging a rule (Fig. 6) perpendicularly to tho 
 
 bottom of the measure, the portion wetted by the water being the decimal part of an inch 
 
ELECTROMETERS. 
 
 125 
 
 65 E- 
 
 60 
 
 required. Thus, having made a rule exactly four inches in length, and divided it into 
 ten equal parts, and each division being subdivided into ten others, a measure is obtained 
 which will read off the hundredth of an inch ; and as the divi- 
 sions are tolerably wide, it is not difficult to estimate even to 
 thousandths of an inch. Thus, for example, as used in the 
 measure, the rule four inches long, divided into 100 parts, 
 represents one inch of rain fallen ; the score at twenty-five, or 
 one inch, represents a fall of a quarter of an inch, and so on. 
 
 Electrometers. Atmospheric electricity has been much 
 neglected by meteorologists ; it is an important item of meteo- 
 rological investigation. There are several methods of studying 
 the subject: the most simple is Glaisher's electrometer, which 
 being portable, should become generally adopted. "Where there 
 are the conveniences for having exploring wires, properly in- 
 sulated, the results are more satisfactory ; and when this plan is 
 adopted, the following electrometers should be used : 
 
 1. De Saussure's Electrometer, which consists of two fine 
 wires, each terminated by a small pith-ball, their expansion 
 being measured by a graduated scale. 
 
 2. Volta's Electrometer, consisting of two thin stems of 
 about two inches in length, and fitted to a metal rod by small 
 rings. 
 
 3. Singer's Electrometer, consisting of two slips of gold- 
 leaf. For stronger electricity, a pair of Dutch-gold leaves 
 become a useful addition. 
 
 4. Zamboni's Dry-pile Electrometer. It is a single gold- 
 leaf suspended from the conducting-rod between two dry piles, 
 the negative pole of the one and the positive of the other 
 being uppermost. This shows whether the electricity is posi- 
 tive or negative. 
 
 For powerful electric storms, the self-registering apparatus 
 belonging to the atmospheric recorder is very useful ; whilst a 
 nicely-arranged electrical bell will be a means of warning the 
 observer that his presence is required in the electric-room. 
 
 The effect of a thunder-storm, when three or four miles 
 distant, as shown on these electrometers, is exceedingly interest- 
 ing. It is not only possible to witness the instantaneous con- 
 vulsion caused by a flash of lightning at least twenty seconds 
 before the peal of thunder occasioned by it is heard ; but it is 
 also possible to know, several seconds before a flash takes 
 place, that one is about to occur. 
 
 The beneficial effects of electricity on the vegetable king- 
 dom are of a character so apparent, that any extended researches Fig. 6. 
 upon the branch of Meteorology calculated to throw additional light upon the subject, is 
 very desirable. 
 
 The Gimbal Vane. This is a wind-vane exactly balanced and hung on gimbals, 
 having vertical fans to carry it in the direction of the wind, and horizontal fans to 
 enable it to tip up or down, to show the angle at which the air is blowing. 
 
 55 
 
 35 
 
 30 
 
 W 
 
 
126 
 
 OZONOMETEES AND WIND-VANES. 
 
 The Rain Angle is a simple contrivance for showing the angle at which the rain 
 descends. It consists of a rod, which moves along a graduated arc. 
 
 The Distance Measurer. A modification of the carpenter's rule, having an elevated 
 eye-hole fixed vertically in the joint, and a slender pin screwed vertically on each leg of 
 the rule. The rule itself slides on a graduated arc. In order to measure approximately 
 the distance of a peculiar cloud, or other celestial phenomenon, from any fixed object, the 
 legs of the rule are opened until one of the vertical pins covers the object to be measured, 
 and the other the object to which it is referred. The distance will be seen on the 
 graduated arc. 
 
 Ozonometer A chemical contrivance for ascertaining the amount of ozone in the 
 atmosphere. Ozone was discovered by Dr. Schonbein of the University of Bale, in the 
 year 1848. Test-papers are hung in a situation where there is a free current of air, but 
 not in sunshine ; the north face of a thermometer-stand is a convenient situation. 
 These papers will remain colourless if there be no ozone present, and will be more or less 
 tinged with blue, as the ozone is more or less powerful. A scale of diiferent tints, marked 
 from to 10, is furnished with the test-papers, to which the slips are referred after ex- 
 posure to the atmosphere, immediately after immersion in water for the space of one 
 minute. 
 
 These test-papers are made in the following manner : 
 
 200 parts of distilled water, 
 10 of starch, 
 
 of iodide of potassium, 
 
 boiled together for a few seconds, and then slips of bibulous paper dipped into the solu- 
 tion ; when dry, they are re&dy for use. If there be any ozone present in the air, it seizes 
 on the potassium the blue colour left being iodide of farina. Other test-papers have 
 been prepared by Dr. Moffat of Hawarden, which do not require to be plunged into 
 water in order to bring out the proper colour. Dr. Mofiat's test-papers must be kept in 
 the dark. They are placed in a box, the bottom of which is perforated with holes foi 
 the free passage of air. These test-papers, if kept in the dark, will last for several years, 
 retaining the amount of discoloration. 
 
 The Wind Vane should be so contrived as to move with the least amount of friction ; 
 otherwise, in calm weather the changes will not be seen to take place in the direction 
 of the wind at the proper time. 
 
 The Atmospheric Recorder, although too expensive for the ordinary meteorologist, 
 still cannot be passed over in silence. Having had one in work for a year, I can speak 
 of its value. Every change in the atmosphere is written down by pencils at the precise 
 moment of occurrence. It is merely requisite to supply the cylinders with sufficient 
 paper ; after which, if the clock is occasionally wound up, and the pencils kept capable 
 of drawing a clearly-defined line, no further care or trouble is needed. This machine is 
 incessantly writing down the force of every gust of wind, the extent of every change in 
 its direction, the commencement and termination of every shower, the quantity of rain 
 which has fallen, and the amount of evaporation and electricity. The temperature of 
 the air, the pressure and hygrometrical condition, are recorded every quarter of an hour. 
 The discussion of these records cannot fail in being productive of much good to meteor- 
 ologists. Mr. Dollond makes this machine. 
 
 The Actinometer. An instrument for ascertaining the force of solar radiation. It 
 consists of a large hollow cylinder of glass, soldered at one end to a thermometer tube, 
 
ANEMOMETERS AND HYGROMETERS. 127 
 
 terminated at the other end by a ball drawn out to a point, and broken off so as to leave 
 the end open. The cylinder is closed at the other end. It is filled with a deep blue 
 liquid (ammonio-sulphate of copper). The cylinder is enclosed in a chamber which is 
 blackened on three sides. This instrument requires careful manipulation, and is but 
 little used. 
 
 Intensity of Light. The gauge for ascertaining this consists of a long narrow box, with 
 a window at one end facing the north. The box is painted black inside, with a white 
 belt of a couple of inches in width, graduated up to 100. The brighter the day, the 
 further can the figures be seen up this box. 
 
 The Transit Instrument is a useful addition. It has been well described by Mr. 
 Breen in his treatise on astronomy. A very close approximation to correct time may 
 be obtained by the use of Dollond's Portable Transit. 
 
 A good astronomical clock, with a mercurial pendulum, ought to be found in every 
 observatory, and a good watch in the pocket of every observer ; each should be provided 
 with the seconds-hand. The watch in use here was supplied by Mr. Bennett of 
 Cheapside ; it has performed its work to my entire satisfaction. 
 
 WheweWs Anemometer, invented by the Rev. Dr. "WTiewell, is an ingenious self- 
 registering anenometer, which gives the amount of horizontal movement in the air. A 
 system of wheels is worked by windmill-sails, according to the velocity of the wind ; 
 and these carry a pencil, which is constantly recording the direction and velocity of the 
 wind. 
 
 Osier's Anemometer constantly records the force and direction of the wind, and 
 also the amount of rain. An admirable invention. Unfortunately, such instruments 
 as this and Dr. WhewelTs are too expensive for the majority of meteorological 
 observers. 
 
 The Electrical Observatory at Kew, under the able management of Mr. Eonalds, 
 lias become a most important meteorological station, being, in fact, a collection of all 
 requisite electrometers; a brief description of which will be found in Dr. Drew's 
 " Practical Meteorology." 
 
 Daniel's Hygrometer. An instrument for ascertaining from direct observation the 
 temperature of the dew-point. This is an interesting contrivance, as a ring of dew is 
 precipitated at the temperature of the dew-point. It consists of two glass balls, com- 
 municating with each other by means of a bent tube. The one ball is of black glass, and 
 the other transparent. A thermometer is fixed with its bulb within the blackened ball ; 
 and as soon as three-fourths of this ball is filled with sulphuric ether, it is immersed. The 
 air having been exhausted, the tube is hermetically sealed. The transparent ball is 
 covered with muslin. A duplicate thermometer for ascertaining the temperature of the 
 air is attached to the stem of the instrument. To find the temperature of the dew-point, 
 all the ether must be made to run into the black ball ; ether is then poured from a phial 
 on the muslin ; this produces rapid evaporation, and the temperature is cooled down to 
 that of the dew-point ; at this temperature, a ring of dew is formed round the black 
 bulb, and at this instant the immersed thermometer must be read off : rapidity of 
 observation being necessary, as the temperature will continue to fall below this point, and 
 the ring of dew also to increase in width. In a few seconds the ring will gradually 
 disappear, and at this moment the thermometer should be read a second time. The mean 
 j of the two readings will give the temperature of the dew-point. It is necessary that the 
 1 ether should be very good. 
 
 The results obtained by actual observation and by calculation (from the wet and 
 
128 THinSTDEE-STORMS. 
 
 dry-bulb thermometers) are very nearly identical, as the following illustration, taken 
 to-day at noon (May 17), will show : 
 
 By Observation. 
 Daniel's hygrometer, temperature of air . . . . . .56-0 
 
 Daniel's hygrometer, temperature of dew-point when ring was formed . 48 3> 1 
 
 Daniel's hygrometer, temperature of dew-point when ring disappeared , 48'2 
 
 IJaniePs hygrometer, mean temperature of dew-point . . . 48 'I 
 Daniel's hygrometer, temperature of dew-point below temperature 
 
 of air 7 3 -9 
 
 By Calculation. 
 
 Temperature of dry bulb 56 *0 
 
 Temperature of wet bulb . . 'v.V-'M v <! vW*J ;:-i i, *?.;.; ,>." . . 52 3 -0 
 
 Calculated dew-point %j I .a ?x...7 . 48'-2 
 
 Temperature of dew-point below temperature of air . . . 7 -8 
 
 Eegnaulfs Hygrometer. Another instrument for ascertaining the dew-point from 
 direct observation. It differs considerably in its construction. Some meteorologists 
 prefer it to Daniel's hygrometer. Messrs. Negretti and Zambra have constructed a 
 modification of Regnault's hygrometer, and from them it can be procured at the same 
 price as that usually charged for Daniel's hygrometer. 
 
 Oonnett's Hygrometer is a third instrument for ascertaining the dew-point from 
 observation. In this the temperature is lowered to the dew-point by means of an ex- 
 hausting syringe. The wet and dry bulb thermometer, however, answers every purpose, 
 and to the ordinary observer is not so liable to be incorrectly read off. . 
 
 The Oy ammeter consists of a flat ring, divided into 53 equal parts, and numbered 
 from to 52, the being white, and 52 very dark blue ; the other numbers are painted 
 every intermediate tinge from nearly white to deep blue. By its use the colour of the 
 sky can be ascertained by observation. 
 
 General Directions. The following occasional phenomena require a few words. 
 The most uninstructed observer may give useful information on these subjects with 
 comparatively little trouble to himself, by making known the particular features desirable 
 to observe. It is desirable to note in 
 
 Thunder-storms, the direction in which they move, the point of the hoiizon in 
 which they were first noticed, and that in which they disappeared ; the time when 
 thunder was first and last heard ; the colour of the lightning ; the number of seconds 
 elapsing after a flash before thunder became audible (noted at different periods daring 
 the storm's continuance) ; the commencement and termination of rain ; the direction of 
 the wind before, after, and during its continuance ; the time when the electrical breeze 
 springs up (this is a peculiar violent breeze noticed in most thunder-storms) ; whether 
 hail falls ; and any other feature that may appear remarkable or be deemed desirable to 
 be recorded. 
 
 Aurora Borealis. Its position amongst the stars, whether merely a low auroral arch, 
 or accompanied by coruscations. If a brilliant display, whether a cupola or dome is 
 formed a little south of the zenith ; and if formed, whether oscillatory among the stars. 
 The hour of its occurrence when seen as a diffused light ; if there be floating patches of 
 luminous haze or cloud, &c. 
 
HYGROHETRICAL TABLES. 
 
 129 
 
 Solar and Lunar Saks. When visible, the quarter of the heavens where they 
 appeared, and how long they remained visible. 
 
 Mock-suns and Complicated Circlet of Light. Their form and position with respect 
 to the sun or moon ; whether prismatic. 
 
 Meteors, or Falling-stars. Their apparent size, shape, colour, path amongst the 
 stars, velocity and duration ; whether accompanied by a streak of light, or separate frag- 
 ments ; if large, whether a streak of light remained after the meteor itself had disap- 
 peared ; and after bursting, whether any noise of explosion be heard if so, how many 
 seconds after the meteor itself had burst ; the time of appearance, &c. These observa- 
 tions should more especially be attended to from the 6th to the 16th of August, and from 
 the 9th to the 14th of November. 
 
 Gales of Wind. Their direction, and estimated force ; when they commenced and 
 terminated ; the height of the barometer during its continuance. 
 
 Snow. Note when it fell, how deep in inches on the ground, and the form of the 
 snow-crystals. To sketch the crystals, a magnifying-glass is requisite. 
 
 Hail. The shape of the stones, &c. 
 
 The times of breaking up of long dry periods, and frosts ; the termination of rainy 
 periods, the commencement and duration of fogs, wind changes, &c. 
 
 Solar Eclipses. During their continuance, and before and after, record the tempera- 
 ture in sun and shade repeatedly. Expose for ten seconds every five minutes slips of 
 Mr. Talbot's sensitive-paper, to ascertain the effect of the diminution of sunlight on 
 this paper. 
 
 Requisite Tables of Reduction. 1st. Glaisher's Hygrometrical Tables (2nd 
 edition). These splendid tables enable the observer, with comparatively little labour, to 
 calculate the temperature of the dew-point, the elastic force of vapour, the weight of 
 vapour in a cubic foot of air, the additional weight of vapour required to saturate a 
 cubic foot of air, the degree of humidity ; the whole amount of water in a vertical column 
 of the atmosphere ; the weight of a cubic foot of air, and also to separate the pressure 
 due to vapour from that due to the gases, for any temperature from 10 to 100 F. 
 On page 5 there is also a table for reducing the readings of the barometer to the level 
 of the sea. Mr. Glaisher has calculated this table from the fact determined by M. 
 Regnault, that air expands 45 j; i^th part for every increase of 1 of heat. 
 
 2nd. Table of the Corrections for Temperature to Reduce Observations to 32 F. for 
 Barometers with Brass Scales, by J. Glaisher, Esq. It is absolutely requisite to reduce 
 the readings of the barometer to a certain acknowledged temperature, otherwise the 
 true pressure of the air could not be ascertained ; for it must be remembered that, 
 besides the pressure of the air on the mercury, the mercury itself obeys the same law 
 which is pointed out to us by the thermometer, i.e. expands by heat and contracts by 
 cold. Thus, suppose the actual pressure of the air to be stationary, the barometer will 
 be seen to rise or fall, if there be an increase or decrease in the temperature of the air. 
 In like manner, the metal scale of the barometer is subject to expansion and contraction 
 by an increase of heat or cold ; and this, as Mr. Glaisher says, explains the apparenl 
 anomaly that, although the readings are said to be reduced to 32 (or the freezing-point), 
 the point of no correction is 28|. As metal bars will vary in length with every degree 
 of temperature, it is apparent that a certain temperature should be determined upon at 
 which the standard unit of measure must be referred to. 
 
 Now this temperature has been fixed at 62 F. ; therefore above 62, as the metal 
 will expand, this will make the divisions on the scale of the barometer too large, and 
 
130 EXAMPLES OF READING FOE TEMPERATURE. 
 
 consequently the barometer -will read lower than it should do ; on the contrary, below 
 62, the metal contracting, will bring the index divisions closer together, and the baro- 
 meter will read too high, unless corrected. Thus, owing to this cause alone, at the 
 temperature of 32 the barometer is made to read '009 of an inch too high. 
 
 The great use of the reduction for temperature will be at once apparent when an 
 example is given : thus, suppose a person has two barometers, one in a room heated 
 artificially and the other as cold as possible : 
 
 Reading of barometer at temperature of 90 . 30'200 
 Correction for temperature . . *.. .; { 0'165 
 
 Reading corrected for temperature . . . 30'035 inches. 
 
 Reading of barometer at temperature of 40 . 30'066 
 Correction for temperature .' : '."' J . *' . 031 
 
 Reading corrected for temperature . ' ' ;{ .' s . 30-035 inches. 
 
 The difference '134 inch between the two readings being due to the expansive action 
 of heat. 
 
 These tables have been calculated from Schumacher's formula, which is here 
 copied : 
 
 ~~ ^ 1 I A /# OOO\ 
 
 1 -j- in (t 62, ) 
 z =. reading of barometer. 
 
 m the expansion in volume of mercury for 1 F. = 0'OOQ1001. 
 t =. the temperature of the mercury and the scale. 
 
 * . the expansion of the brass scale in length for 1 F. = 0*000010434 (the 
 normal temperature being 62). 
 Thus the formula becomes 
 
 0-0001001 X (t 32) 0-000010434 x (t 62) 
 1 -f 0-0001001 x (t 32) 
 
 3rd. Table of Corrections to be applied to Meteorological Observations for Diurnal 
 Range, prepared by the Count?! of the British Meteorological Society. These tables are 
 of the utmost importance, as they enable an observer, from one, two, or three readings 
 daily, to find from them the true monthly means ; in fact, to make his observations represent 
 a reading taken every hour day and night. Thus, if a reading of the barometer is made 
 daily at 3 A.M. in March, the mean will be '023 too low, or, at 11 A.M., '015 too high. 
 The necessity of this reduction becomes very evident from hourly readings of the ther- 
 mometer; for, suppose the readings are made in June, at 4 A.M., the mean will be 9 0- 3 
 too low, or, if at 2 P.M., 8'6 too high. 
 
 The only correction requisite for the reduction of meteorological observations not 
 found in the three above-mentioned tables is that for capillarity the capillary action of 
 the tube of a barometer depressing the mercury by a quantity inversely proportional to 
 the diameter of the tube. The following table will be found sufficient for this reduction : 
 it is copied from the work published by the Committee of Physics and Meteorology of 
 the Royal Society : 
 
CORRECTION OF BAROMETRIC READINGS. 131 
 
 Correction to be added to barometer readings for capillary action. 
 
 Diameter of tube. Correction for unboiled tubes. Correction for boiled tubes. 
 
 Inch. Inch. Inch. 
 
 0-60 . . . . + 0-004 ... . 4- 0-002 
 
 0-50 . . . + 0-0 7 .... 0-003 
 
 0-45 .... 0-010 .... 0-005 
 
 0-40 . .... 0-014 .... 0-007 
 
 0-35 .... 0-020 .... 0-010 
 
 0-30 .... 0-028 .... 0-014 
 
 0-25 .... 0-040 .... 0-020 
 
 0-20 .... 0-060 .... 0-029 
 
 0-15 .... 0-088 . . . . 0-044 
 
 0-10 .... 0-142 .... 0-070 
 
 The following reductions for meteorological observations will supply examples of 
 every reduction necessary : 
 
 Barometer Reductions. To find the mean pressure of the barometer for the 
 month of February, 1856 (height above the sea-level, 281 feet). 
 
 Sum of all the readings made at 9 A.M., 867640 ; at 10 P.M., 867528. 
 Divide the above by the number of observations. 
 
 29)867640(29-918 inches. 29)867528(29-915 inches. 
 
 58 58 
 
 287 287 
 
 261 261 
 
 266 ' 265 
 
 261 261 
 
 54 --42 
 
 29 29 
 
 250 138 
 
 252 145 
 
 7 
 
 Sum of all the readings of the attached thermometer at 9 A.M., 13435 ; at 10 P.M., 
 13745. 
 
 29)13435(46-3 temp, of mercury. 29)13745(47'4 temp, of mercury. 
 
 116 116 
 
 183 . 214 
 
 174 203 
 
 95 .115 
 
 87 116 
 
132 CORRECTIONS FOR TEMPERATURE. 
 
 Mean pressure at 9 A.M. * = 29'918 
 
 Correction for temperature of 46 3 '3 . . . 'V . . = 047 
 
 Mean pressure corrected for temperature . . . . = 29 '8 71 
 
 Index error .' **'. . '002 
 
 Mean pressure further corrected for index error . . " ".' 29 '869 
 
 Corrections for capillarity, the mercury being boiled . . -|- 002 
 
 Correct reading for 9 A.M. . . . . ' . . . =29-871 
 
 Correction for diurnal range for February . . . . 008 
 
 Approximate mean pressure . . . . . . =29'863 
 
 Mean pressure at 10 P.M = 29*915 
 
 Correction for temperature of 47*4 <> " ! '" 7 V % "v- . = 050 
 
 Mean pressure corrected for temperature .... 29 '865 
 
 Index error 002 
 
 Mean pressure further corrected for index error . r,vi ".Ivi =29-863 
 Correction for capillarity, the mercury being boiled . . -\- 002 
 
 Correct reading for 10 P.M. .... ' "-! " ' ''*' 29-865 
 
 Correction for diurnal range for February .... 007 
 
 Approximate mean pressure 29-858 
 
 Ditto ditto 29-863 
 
 Sum of the two observations . ... . . 2)59*721 
 
 Adopted mean pressure for the month = 29-8605 
 
 To reduce the mean pressure of the month to the sea-level, the adopted mean tem- 
 perature of the air being 36'0, and the cistern of the barometer 281 feet, the adopted 
 mean pressure 29*860 inches. 
 
 In Table 2 of Glaisher's Hygrometrical Tables (page v.), showing the volume of a 
 mass of dry air after expansion from heat for each degree of Fahrenheit's scale, it will 
 be seen that a stratum of air 90 feet in thickness will balance a column of mercury 
 O'l inch in height. 
 
 The factor for 36 is 1-008 
 
 Multiply this by 90 feet 90 
 
 feet. 
 
 90-720 = 90-7)281-0(-3098ofaninch. 
 2721 
 
 8900 
 8163 
 
 7370 
 7256 
 
 114 
 
DREW '8 THEOREM. 
 
 133 
 
 Adopted mean pressure for altitude of 281 feet 
 Correction to reduce to sea-level . 
 
 At sea-level the mean pressure is . 
 in. feet. 
 
 29-860 
 4- 310 
 
 30-170 inches. 
 
 As 90-7 
 
 :: 281 : = 
 
 281 X 
 
 90-7 
 
 =: 0-310 
 
 Another method, based upon the theorem of Sir George Shuckburgh and the calcu- 
 lations of Regnault, has been described by Dr. Drew, of Southampton, who has con- 
 structed a table showing the height, in feet, of a column of air equivalent in weight to a 
 column of mercury one inch in height, at different temperatures, under a pressure of 
 thirty inches of mercury. This table is copied from Dr. Drew's " Practical Meteor- 
 ology." 
 
 Temp. 
 
 Feet. 
 
 Temp. 
 
 Feet. 
 
 Temp. 
 
 Feet. 
 
 Temp. 
 
 Feet, 
 
 Temp. 
 
 Feet. 
 
 30 
 
 865-1 
 
 40 
 
 882-8 
 
 50 
 
 900-5 
 
 60 
 
 918-2 
 
 70 
 
 935-8 
 
 31 
 
 866-8 
 
 41 
 
 884-5 
 
 51 
 
 902-2 
 
 61 
 
 919-9 
 
 71 
 
 937'5 
 
 32 
 
 868-5 
 
 42 
 
 886-2 
 
 52 
 
 903-9 
 
 62 
 
 921-6 
 
 72 
 
 939-3 
 
 33 
 
 870-3 
 
 43 
 
 888-0 
 
 53 
 
 905-7 
 
 63 
 
 923-4 
 
 73 
 
 941-1 
 
 34 
 
 872-1 
 
 44 
 
 889-8 
 
 54 
 
 907-5 
 
 64 
 
 925-2 
 
 74 
 
 942-9 
 
 35 
 
 873-9 
 
 45 
 
 891-6 
 
 55 
 
 909-3 
 
 65 
 
 927-0 
 
 75 
 
 944-7 
 
 36 
 
 875-7 
 
 46 
 
 893-4 
 
 56 
 
 911-1 
 
 66 
 
 928-8 
 
 76 
 
 946-5 
 
 37 
 
 877-5 
 
 47 
 
 895-2 
 
 57 
 
 912-2 
 
 67 
 
 930-6 
 
 77 
 
 948-3 
 
 38 
 
 879-3 
 
 48 
 
 897-0 
 
 58 
 
 914-7 
 
 68 
 
 932-4 
 
 78 
 
 950-1 
 
 39 
 
 881-1 
 
 49 
 
 898-8 
 
 59 
 
 916-5 
 
 69 
 
 934-1 
 
 79 
 
 951-8 
 
 For any temperature above that given in the table, the addition of 1/7 feet for every 
 degree; and for any temperature below that given in the table, the subtraction of 1- 7 
 feet for every degree, will give the factor required. 
 
 To work out the addition required to reduce to sea level, let T represent the tabular 
 number opposite the temperature of the air, z the reading of the barometer at/ feet 
 above the sea-level, and x the correction required ; then 
 
 * = T X 3b 
 
 Dr. Drew gives the following practical example : The barometer being 29 '500 
 inches, the temperature being 50, and the height above sea- jvel 60 feet ? 
 
 60 29-5 
 X ~ 900-5 * ~30~ ~ * correction required. 
 
 .*. 29-500 -f- 0-065 =. 29-565 inches (the pressure reduced to the sea-level). 
 
 Thermometer Reductions. To find the mean temperature for the montk of 
 December, 1855, from observations made at 9 A.M. and 10 P.M., and from self-register- 
 ing thermometers. 
 
134 
 
 MEAN COEEECTIONS. 
 
 Sum of all the readings made at 9 A.M. 10915 ; and at 10 P.M. 11017. 
 
 31)10915(35-2 mean at 9 A.M. 
 
 93 -jl o-9 cor. for diurnal range. 
 
 161 36 0> 1 approx. mean for month. 
 155 
 
 31)11017(35-5 mean at 10 P.M. 
 
 93 -f- 0-5 correction for diurnal range. 
 
 171 36-0 approx. mean for month. 
 155 
 
 167 
 155 
 
 3 12 
 
 Sum of all the readings of a self-registering minimum thermometer, 9365 ; and maxi- 
 mum thermometer, 12759. 
 
 31)9365( 30-2 mean minimum temp. 
 93 -j-0'2 index error. 
 
 65 30'4 corrected for Jiadex error. 
 62 
 
 31)12759(41-2 mean maximum temp. 
 124 0-2 index error. 
 
 41'0 corrected for index error. 
 
 35 
 31 
 
 49 
 62 
 
 13 
 
 50 '4 mean minimum corrected. 
 41 '0 mean maximum corrected. 
 
 2)71'4 sum of the two series. 
 
 35'7 mean of the two series. 
 0-0 correction of diurnal range. 
 
 357 approximate mean from self-registering instruments. 
 36' 1 approximate mean from hourly observations at 9 A.M. 
 36*0 approximate mean from hourly observations at 10 P.M. 
 
 3)107-8 sum of the three series. 
 
 35-9 adopted mean temperature for December, 1855. 
 
 To find the mean temperature of the wet bulb thermometer, for the month of 
 December, 1855, from daily observations made at 9 A.M., and 10 P.M. 
 Sum of all the readings at 9 A.M., = 10509 ; at 10 P.M., = 10658. 
 31)10509( 33-9 mean at 9 A.M. 31)10658( 34'4 mean at 10 P.M. 
 
 93 -4" '6 cor. for diurnal range. 93 -\- '2 cor. for diurnal range. 
 
 120 
 93 
 
 279 
 279 
 
 34 -5 approximate mean. 
 
 135 
 
 124 
 
 US 
 124 
 
 34 -6 approximate mean. 
 
 ..6 
 
MEAN TEMPEKATTTRE OF DEW-POINT. 135 
 
 Approximate mean from observations made at 9 A.M. = 34 '5 
 
 at 10 P.M. = 34 -6 
 
 Sum of the series 2)69-1 
 
 Adopted mean temperature of the wet bulb . . = 34'5 
 
 To find the mean temperature of evaporation. 
 
 Adopted mean temperature of the air from dry bulb . . = 36'0 
 Adopted mean temperature of the wet bulb . . . = 34 '5 
 
 Difference = OV5 
 
 Adopted mean temperature of air =z 35 -9 
 
 Difference, or mean temperature of evaporation . . 34-4 
 
 To find the mean temperature of the dew-point, the mean dry bulb* being 35 '8, 
 and the mean wet bulb 34 5. 
 
 Example 1. In the hygrometrical table for 35, 
 
 The dew-point opposite to 34 wet bulb is . . 32'4 . 32-4 
 
 The dew-point opposite to 35 wet bulb is . .35-0 
 
 Difference on the increase in dew-point for an in- 
 crease of 1 in wet .... . 2'6 
 Proportional part of the increase for 0' 5 is . . .' . . -f- 1*3 
 
 Temperature of the dew-point corresponding to 35 dry, and 34 0< 5 wet, is 33'7 
 In the fourth column the decrease of dew-point for an increase of 1 
 
 in the dry bulb is 1'5, the proportional part for 0'8 is . 1*2 
 
 The adoptedtemperature of the dew-point for 35-8 dry, and 34'5 wet, is = 32*5 
 
 Example 2. To find the mean temperature of the dew-point for the month of 
 December, 1855, dry bulb being 36'0, and wet bulb 34-5. 
 In the hygrometrical table for 36, 
 
 The dew-point opposite to 34 wet bulb is . . 31*0 . . 31 -0 
 The dew-point opposite to 35 wet bulb is . .33-5 
 
 Difference .2-5 
 
 Proportional part of the increase for 0'5 is . . . , + 1*2 
 
 Mean temperature of dew-point . . . . . . . = 32 -2 
 
 The same tables, and the same manner of reduction, will find " the elastic force of 
 vapour/' "the weight of vapour in a cubic foot of air," " the additional weight required 
 to saturate a cubic foot of air," and the " degree of humidity." To render the tables 
 clear, one example of each will be worked out for a temperature of 35 0< 8 dry bulb, and 
 34-5 wet bulb. 
 
 * The hourly readings of the thermometer, which are made at the same time as those of the wet 
 bulb, are called the ' dry bulb.' 
 
136 
 
 WATER CONTAINED IN A COLUMN OF AIE. 
 
 To find the elastic force of vapour of the temperature of 35-8, the wet bull 
 being 34-5. 
 
 In the hygrometrical table for 35, 
 
 Inch 
 0-184 
 
 The elastic force of vapour opposite to 34 -wet bulb is 
 The elastic force of vapour opposite to 35 wet bulb is 
 
 Difference, or the increase for an increase of 1 in wet 
 Proportional part of the increase for 0> 5 is 
 
 Inch 
 0-184 
 0-204 
 
 0-020 
 
 -f 0-010 
 0-190 
 
 Elastic force of vapour corresponding to 35 dry, and 34 0< 5 wet is 
 In the 6th column, the decrease of the elastic force of vapour for an in- 
 crease of 1 in the dry bulb, is O'OIO ; the proportional part 
 forO-8is -0-008 
 
 Mean elastic force of vapour . ,. . . . ' " . . Q-182 
 
 To find the whole amount of water in a vertical column of the atmosphere, i.e. from 
 the surface of the earth to the top of the atmosphere. 
 
 This is found by multiplying the elastic force of vapour by the constant 1383. 
 
 1383 constant. 
 182 
 
 2766 
 11064 
 1383 
 
 2-51706 inches. 
 
 Thus the whole amount of water in a vertical column of the atmosphere, if precipi 
 tated on the earth at one time, would be in this instance 2 -5 or 2~ inches. 
 
 To find the weight of vapour in a cubic foot of air. 
 
 In the hygrometrical table for 35, 
 
 The weight of vapour in a cubic foot of air opposite 34 Grain8 - 
 
 wet bulb is ....... 2'1 
 
 The weight of vapour in a cubic foot of air opposite 35 
 
 wet bulb is . . . . . . . 2'4 
 
 Difference, or the increase in weight of vapour for an 
 
 increase of 1 in wet is 0'3 
 
 Proportional part of the increase for 0"5 
 
 Weight of vapour in a cubic foot of air corresponding to 35 dry and 
 S4-5 wet is 
 
 In the 8th column, the decrease of weight of vapour for an increase of 1 
 in the dry bulb is 0-2, the proportional part for 0- 8 is 
 
 The adopted weight of vapour for 35-8 dry and 34-5 wet ia 
 
 Grain*. 
 2-1 
 
 -f 0-1 
 
 2-2 
 
 0-2 
 2-0 
 
 To find the additional weight of vapour required to saturate a cubic fort J air. 
 In the hygrometrical table for 3-5', 
 
WEIGHT OF A CUBIC FOOT OF AIE, 137 
 
 The dry bulb being 35-8 and the wet bulb 34'5. 
 
 Additional weight of vapour required to saturate a cubic Grain. Grain. 
 
 foot of air opposite 34 wet bulb is . . . . 0'3 . . 0'3 
 Additional weight of vapour required to saturate a cubic 
 
 foot of air opposite 35 wet bulb is . . . O'O 
 
 Difference, or the decrease in amount required to saturate 
 
 a cubic foot of air for an increase in 1 in wet bulb is 0'3 
 Proportional part of the decrease for 0> 5 0*2 
 
 Additional weight of vapour required to saturate a cubic foot of air cor- 
 responding to 35 dry and 34 0< 5 wet is . . . . . = O'l 
 
 In the 10th column, the increase in the weight of vapour required to 
 saturate a cubic foot of air for an increase in 1 dry is -\- 0'3, the 
 proportional part for 0'8 is + 0'2 
 
 The adopted additional weight of vapour required to saturate a cubic foot 
 
 of air for 35 -8 dry and 34 -5 wet is . . V" = 0'3 
 
 To find the degree of humidity, the temperature being 35 0- 8 and the wet bulb 34 :> '5, 
 complete saturation = 100. 
 
 In the hygrometrical table for 35. 
 
 Degree of humidity opposite 34 wet bulb is . . .90 . . 90 
 
 Degree of humidity opposite 35 wet bulb is . . .100 
 
 Difference, or the increase of humidity for an increase of 
 
 1 in wet is 10 
 
 Proportional part of the increase for 0' 5 is +5 
 
 Degree of humidity corresponding to 35 dry and 34'5 wet is . . 95 
 In the 12th column, the increase in the degree of humidity for an increase 
 
 of 1 in the dry bulb is 9, the proportional part for J '8 is . . 7 
 
 Adopted degree of humidity for 3 5 -8 dry and 3 4 '5 wet is . . . = 88 
 
 To find the weight of a cubic foot of air, the mean pressure of the barometer being 
 29742 inches, dry bulb 35'8, and wet bulb 34'5. 
 In the hygrometrical table for 35. 
 In column 13, the weight of a cubic foot of air (the barometer being Grains. 
 
 29-0 inches) opposite 34 wet is 543'4 
 
 In column 14, the decrease of weight for an increase of 1 in dry is l'l, 
 
 the proportional part for 0' 8 is 0'9 
 
 542-5 
 In column 14, the increase of weight for an increase in the reading of 
 
 the barometer for one inch is -|- 18 '7 grains. 
 
 Opposite 7 in the ta^ under 18-7 in last column, is . . . . 13'1 
 Opposite '04 in the t^/.e, in right-hand corner of the page, is . . 07 
 
 Che adopted weight of a cubic foot oi air is 556 '3 
 
138 
 
 TERRESTRIAL AND SOLAR RADIATION. 
 
 To find the mean pressure of dry air, or that due to the gases when separated from 
 the water contained in the air. 
 
 Subtract the adopted elastic force of vapour (which is the pressure of the water 
 contained in the air) from the adopted pressure of the barometer. 
 
 Example. If the mean pressure of barometer is . . . = 29 '742 inches, 
 And the elastic force of vapour ia . >. .= 0' 186 inch, 
 
 The pressure of dry air (of that due to the gases) will be . = 29'556 inches. 
 
 To find the monthly range of temperature, &c., deduct the coldest temperature from 
 the hottest, observed during the month. Thus 
 
 Hottest temperature . ; j ? ^ : .u?-?: . . 50*8 
 Coldest temperature iW*. V& "*' ^' . 12-0 
 
 Monthly range of temperature 
 
 To find the mean daily range of temperature, deduct the mean of all the readings of 
 the minimum thermometer from the mean of all the readings of the maximum ther- 
 mometer. 
 
 Mean maximum temperature . . . 41'0 
 Mean minimum temperature ' . . 30 0- 4 
 
 Mean daily range of temperature 
 
 = 10'G 
 
 To find the amount of terrestrial radiation, deduct the reading of a minimum ther- 
 mometer placed on the grass from that of a minimum thermometer placed four feet 
 above the grass. Thus 
 
 Greatest cold four feet above the grass . . 36 0- 5 
 Greatest cold on the grass .... 27 '8 
 
 Amount of terrestrial radiation 
 
 = 8-7 
 
 To find the amount of solar radiation, deduct the reading of a maximum ther- 
 mometer (with a blackened bulb) placed in full sunshine from the greatest heat in shade. 
 
 Thus 
 
 Greatest heat in sunshine . . ' . . r= 59*3 
 
 Greatest heat in shade = 34'l 
 
 Amount of solar radiation 
 
 25-2 
 
 To find the greatest heat and greatest cold of the wet bulb thermometer. This is 
 I obtained by attaching the muslin and cotton conductor to self-registering thermometers, 
 
 instead of to the ordinary thermometer. Thus 
 
 Greatest cold of dry bulb 
 Greatest cold of wet bulb 
 
 Difference . 
 
 34-0 
 
 Greatest heat of dry bulb . 467 
 Greatest heat of wet bulb . 42'8 
 
 Difference . 3 "'9 
 
CLASSIFICATION OF CLOUDS. 139 
 
 To find the range of temperature of the wet bulb thermometer, deduct the greatest 
 heat from the gi-eatest cold recorded. Thus 
 
 Greatest heat 42-8 
 
 Greatest cold 32-5 
 
 Range of wet .:.... 10-3 
 
 To find the mean amount of cloud. 
 
 This, by practice, is accomplished very accurately by estimation, 10 being considered 
 an overcast sky and a cloudless sky. It must be remembered, in making the estimate, 
 that, with a partially covered sky, the forms of the clouds, and the space they cover, 
 are only correctly seen in the zenith ; the nearer the horizon we approach, the more 
 obliquely do we look upon them ; and, consequently, near the horizon the sky will 
 appear to be more overcast than it is in reality. The observer's judgment should, there- 
 fore, be more especially confined to the upper half of the sky. 
 
 Observations made at 9 A.M. and 10 P.M., December, 1855. 
 
 Sum of all the estimates at 9 A.M., 2158 ; and at 10 P.M., 2024. 
 31)2158( 6-96 mean at 9 A.M. 31)2024( 6'5 mean at 10 P.M. 
 
 186 0*1 cor. for diurnal range. 186 -|~ 0'2 correction for diurnal range. 
 
 298 6 -86 corrected amount. 164 6^7 corrected amount. 
 
 279 155 
 
 190 "9 
 
 186 
 
 ..4 
 
 corrected reading at 9 A.M. 6 -9 
 corrected reading at 10 P.M. 6'7 
 
 sum of the two series 2) 1 3 6 
 adopted mean amount of cloud . . 6 0< 8 
 
 Classes of Clouds. Mr. I/uke Howard, the well-known meteorologist, was the 
 first to classify the clouds ; and these classes have been very conveniently abbreviated by 
 Mr. Glaisher. They are 
 
 Cirrus =: ci . . . feathery-looking, the most lofty cloud. 
 Cumulus =. cu . . . mountainous-looking. 
 Stratus =2 st . . . the ground- cloud forms at sunset and disappears at 
 
 sunrise. 
 Cirro-cumulus ci-cu . . rounded masses, or woolly tufts. 
 
 Cirro-stratus = ci-st . . horizontal masses. 
 
 Cumulo-stratus = cu-st . . an accumulation of cumuli, sometimes fungus-shaped. 
 Nimbus = ni . . . rain-cloud. 
 
 Scud =. so . . . broken, flying nimbi. 
 
 In recording the class of clouds, the interest is increased by also recording the direc- 
 tion in which the clouds are moving, the colour of the clouds, and their height and 
 velocity. The height may be conveniently estimated by supposing 6 to represent very 
 
140 ESTIMATED FORCE OF WIND. 
 
 high clouds, and those floating along the ground ; he velocity, by supposing 6 to 
 represent those moving at the greatest speed, and those motionless. 
 
 To find the mean daily amount of rain. Whole amount of rain collected during the 
 mouth of December, 1855, 0764 of an inch. 
 
 31)0-764(0-025 of an inch being the mean daily amount. 
 62 
 
 144. 
 155 
 
 11 
 
 To find the mean daily amount of evaporation for December, 1855. Whole amount 
 evaporated, 0-610 inch. 
 
 31)0-610(0-0197 
 31 
 
 300 
 
 279 
 
 210 
 217 
 
 --7 Therefore, 0-020 inch is the daily amcunt. 
 
 To find the amount of evaporation in rainy weather. 
 
 Suppose TOGO inch of water is placed in the evaporator, and that a rain-gauge, of the 
 same diameter as the evaporator, and placed at the same level, has recorded 0'510 inch 
 of rain since last observation, whilst the evaporator is fourtd to contain 1 '444 inches. 
 Then 
 
 1'444 inches the amount in evaporator. 
 0-510 amount of rain. 
 
 0-934 difference. 
 
 1-000 amount of water placed in the evaporator. 
 
 0-066 of an inch is the amount due to evaporation. 
 
 To estimate the force of wind. 
 
 Anemometers being expensive, the majority of meteorologists estimate the wind's 
 force. It is recommended that represent a calm and 6 a hurricane, for this estimate 
 is easily converted into Ib. pressure on the square foot; the square of the estimate 
 will represent the Ib. pressure on the square foot ; viz. 
 
 Olb. 
 1=1 
 2=4 
 3=9 
 
 4 = 16 
 
 5 = 25 
 
 6 = 36 
 
 By practice a near approximation to the truth can he obtained. 
 
DEDUCTION OF THE DEW-POEST. 
 
 141 
 
 Mr. Belville, in his Manual of the Barometer, gives the following concise table of 
 factors for deducing the temperature of the dew-point, from the temperature of the air 
 and that of the temperature of evaporation ; this table originally appeared in the Green- 
 wich Magnetical and Meteorological Observations for 1844. The dew-point deduced 
 in this manner, is a close approximation to that obtained by Glaisher's hygrometrical 
 tables. 
 
 Temperature. 
 
 Factor. 
 
 Temperature. 
 
 Factor. 
 
 Temperature. 
 
 Factor. 
 
 28 to 29 
 
 57 
 
 34 to 35 
 
 2-6 
 
 55 to 60 
 
 1-9 
 
 29 30 
 
 5-0 
 
 35 40 
 
 2-4 
 
 60 70 
 
 1-8 
 
 30 31 
 
 4-6 
 
 40 45 
 
 2-3 
 
 70* 80 
 
 1-7 
 
 31 32 
 
 3-6 
 
 45 50 
 
 2-2 
 
 80" 85 
 
 1-6 
 
 32 33 
 
 3-1 
 
 50 55 
 
 2-1 
 
 85 90 
 
 1-8 
 
 33 34 
 
 2-8 
 
 
 
 
 
 Multiply the difference between the wet and dry bulb by the factor corresponding 
 to the temperature of the dry bulb, and subtract the product from the dry bulb, which 
 will be the temperature of the dew-point. T the temperature, "W wet bulb, / factor, x 
 the product, and D the dew-point. 
 
 T Wx/=* 
 and T x = D. 
 Thus : temperature 50 and wet bulb 45, 
 
 50 45 = 5 
 5 X 2-1 = 10 -5 
 50 10-5 = 39'5 the dew-point. 
 By Mr. Glaisher's tables this dew-point is 397. 
 
 Recommendations and Precautions. Preserve, as much as possible, the con- 
 tinuity of the observations. It is desirable not to change the positions of the different 
 instruments, nor even to alter the method of reading and registering. As it is probable 
 when two persons are employed in taking observations, that each will read slightly 
 different, a series of simultaneous readings should be made, in order to find the personal 
 error of the observers. If, from any cause, the continuity of the register should be 
 broken, on no account attempt to fill the blank so caused by estimation. Be punctual 
 to the hours determined upon for observations, and read off the instruments in the same 
 way every day ; by doing this, it is less likely that an observation of any one instru- 
 ment shall be overlooked. It is convenient to rule and mark off the form of observation 
 upon a slate, to be transferred to the observatory book, after the whole observations have 
 been made. Before calculating the means, it is recommended to examine each column, 
 to ascertain that no evident error of entry has been made ; an inch in the reading of th^ 
 barometer is a common error. The maximum reading of the thermometer is also some- 
 times entered in the wrong column, being placed in that of minimum ; and vice versa. 
 Decimal arithmetic shbuld always be used. 
 
 Before fixing the barometer, it should be ascertained that the space above the mer- 
 cury is free from air. Incline the instrument slightly from its vertical position ; if the 
 mercury, in striking the upper end of the tube, produce a sharp rap, the vacuum is per- 
 fect; if the tap be dull, or not heard, there will be air above the mercury, which must 
 
142 
 
 NEWMAN'S STANDARD BAROMETER. 
 
 be driven into the cistern by inverting the instrument and then tapping it gently. II 
 the observer does not succeed in producing this sharp report by tapping, the instrument 
 will require the aid of the maker. In fixing the barometer, adjust the tube vertically 
 by the aid of a plumb-line. In reading the instrument, place the eye on the exact 
 level of the top of the column of mercury, so that each side of the index, and the top of 
 the column, shall be in the same horizontal plane. 
 
 The thermometers should be protected from rain ; and in making a reading the 
 observer should do it quickly, and whilst doing so avoid touching, breathing on, or in 
 any way warming the thermometer by the near approach of his person. 
 
 Sir John Herschel recommends that every meteorologist should take an observation 
 every hour throughout the twenty -four, on four stated days in the year ; viz. March 21, 
 June 21, September 21, and December 21, excepting when one of these days occurred on 
 Sunday, then to substitute for this date the 22nd. The observations to commence at 
 6 A.M., and terminate at 6 A.M. next morning. 
 
 Thermometers should be frequently compared with a standard instrument, in order 
 to ascertain whether the freezing-point has remained at the temperature as marked off 
 on the scale. It is a well-known fact that the zero point moves, ascertained from the 
 circumstance that after a great change in temperature, the glass requires a considerable 
 time to enable it to return to its normal condition. 
 
 Newman's Standard Barometer form of Rutherford's thermometer differs in 
 the following manner from that already described : A platinum tube is drawn over a 
 steel wire, which is said to prevent the index from fixing by oxidation. Not having 
 had this instrument at work, I am unable to speak of its advantages from practice. An 
 improvement has also just been accomplished with Negretti and Zambra's rain-gauge. 
 I conceived it was liable to have the water in the canal surrounding it emptied into 
 the graduated glass with the rain to be measured. At my suggestion this has been 
 altered; the canal is now placed much lower down the gauge, and there seems no 
 possibility of an erroneous measurement. The gauge may, therefore, be now said to 
 be perfect. 
 
 Of late years two new barometers have appeared, viz. the Aneroid and JBurdon's. 
 They are both good indicators of atmospheric pressure, but cannot take the place of a 
 Standard barometer. In the first place, the metal is influenced by temperature ; and in 
 the second, there is a chance of the box of the Aneroid and the tube of the Bur don losing 
 their vacuum. As a household instrument they are preferred to the common wheel- 
 barometer ; but I have not had an opportunity of testing their respective merits. 
 
 At the time of the Great Exhibition of 1851, when the jury were examining the 
 meteorological instruments, a remark was made that a more perfect maximum and 
 minimum thermometer was required, both of which should be mercurial ; and on the 
 counsel-medal being presented to Messrs. Negretti and Zambra, their attention was 
 called to this suggestion. Not many months had elapsed before the patent maximum was 
 produced, and within four years the patent minimum. The latter was sent for trial to a 
 few meteorologists six months ago, and one of these instruments came to the Beeston 
 Observatory. The experiments made with it have boen perfectly successful ; indeed, so 
 much so, as to have astonished all who have used it. The ordinary minimum ther- 
 mometers do not work uniformly with this new instrument, owing to the alcoholic 
 vapour contained in the upper portion of their tube, and which is more or less developed 
 according to the temperature. Two such important improvements have not taken place 
 
METEOEOLOGICAL APPARATUS. 143 
 
 since the invention of the thermometer ; and it is creditable to them that both should 
 have emanated from the same persons who invented the enamelling of fine tubes. 
 
 Professor C. P. Smyth, the Astronomer Royal of Scotland, has caused the electric 
 telegraph to work in mete orology. A wind dial at the one extremity of a wire is mado 
 to turn another simultaneously at the other extremity. The time will come when al] 
 large towns will have buildings devoted to these observations, and in which dials will 
 be seen in every direction, some labelled Edinburgh, others Liverpool, Dublin, London, 
 Paris, York, c., and where the public will be enabled to see the direction of the wind at 
 the same instant at most remote places. The benefit to the farmer and the navigator will 
 be great from such an arrangement. "Were such stations to be thickly scattered through- 
 out the country, every change of wind, and every shower, could be traced and recorded, 
 and a knowledge imparted, the benefit of which could not be sufficiently appreciated. 
 
 To be enabled to announce the approach of a thunder-storm, however, at a. time 
 when the sky is free from clouds, and to ascertain its speed so as to foretel when it may 
 be expected in any given place, would afford the farmer an opportunity of so benefiting 
 by the information that he would gladly pay a small rate in order to take advantage 
 of it. The world is slow in appreciating any new invention until its usefulness is 
 experienced; let us, however, hope that one or two such stations may speedily be 
 established, and we venture to predict that, at no very distant period, eveiy conspicuous 
 eminence will have its road station, to impart the information collected at the principal 
 establishments, where the electric wires are made to record the changes as they occur. 
 Our knowledge of meteorology would then make rapid advances ; laws of the weather 
 would be unfolded ; and predictions of coming changes, which are now mere guesses, as 
 often wrong as right, would be based upon truth. 
 
 It is a common expression that nothing is more changeable than the weather ; yet 
 that all these changes are governed by certain laws, is as certain as that gravitation 
 binds the heavenly bodies together. "Were there no laws to keep the changes within 
 certain limits, we should at one period experience cold, and at another heat so intense, 
 that existence would be intolerable ; the earth would be deluged with rain, and anon 
 parched up with drought ; the sky would be cloudy for months, perhaps years together, 
 and then cloudless for as long a period ; in short, the laws which govern the weather 
 keep the extreme changes within proper bounds. 
 
 As the more complicated machinery of a meteorological observatory cannot be ex- 
 pected to be found except in our principal establishments, it will be requisite to mention 
 what instruments are absolutely necessary for the ordinary observer. These are 
 
 A standard barometer. 
 
 A wet and dry bulb thermometer. 
 
 A maximum and a minimum ther- 
 
 A rain-gauge. 
 
 An evaporator. 
 
 A thermometer-stand. 
 
 A wind- vane. 
 
 mometer. 
 
 These would cost from 17 to 30, according to which barometer was selected. 
 The following additional instruments are also desirable (the expense would be 4 or 
 5) : The solar and terrestrial radiation thermometers, Glaisher's electrometer, an 
 ozonometer, and an extra rain-gauge. 
 
 Snow Gauge. The gauge used here consists of a thin metal cylinder, eight inches 
 in diameter and twelve inches deep, graduated upon one side to a quarter of an inch. 
 This cylinder will penetrate through the snow, scarcely disturbing it, and the depth in 
 inches is at once seen. By careful manipulation, if the cylinder is turned round, all 
 the enclosed snow can be lifted from the ground. It is desirable to melt it in a wide- 
 
144 CALENDAR OF NATURE. 
 
 mouthed bath, being previously corked to prevent evaporation, as it frequently happens 
 that snow is blown out of the mouth of the rain-gauge before it has had time to melt ; 
 consequently, the result of melted snow, as shown by the rain-gauge, will be too little 
 in amount. 
 
 Calendar of Nature. Every meteorologist should endeavour, as much as pos- 
 sible, to record the arrival and departure of migratory birds, the dates of trees coming 
 into and losing their leaves, the blooming of plants, the ripening of fruit and seeds, the 
 building of birds' -nests, the first appearance of various insects, diseases amongst animals 
 and plants, the appearance of abundance or otherwise of crops of fruit, corn, &c. If 
 such registers were extensively kept and carefully recorded, the effect of the weather 
 upon the animal and vegetable kingdoms would be well seen. It is extremely desirable 
 that every precaution should be taken, in order that, year after year, the same object 
 shouH be the special one on which the remarks are based, and that one epecies is not 
 mistaken for another. The following examples will show that it is essential to use the 
 utmost care : 
 
 First, " the elm is said to lose its leaves on a certain date." Such an observation is 
 useless. It is requisite to mention the particular kind of elm ; thus, the broad-leaf elm 
 is the first tree to become leafless, which it frequently does in September ; the Siberian 
 elm, on the contrary, will remain green after all other trees have become leafless, 
 sometimes it is in leaf as late as December. It will also be found that the same species, 
 in a group whose boughs touch each other, will come into leaf at different dates. In 
 no tree is this more strikingly exhibited than in the beech ; two beeches growing close 
 together may be seen to vary a couple of weeks in their period of coming into leaf. 
 The age of the -tree also causes a difference to occur. In the different kinds of lilacs 
 and laburnums, there will be a range of some days in their time of coming into bloom. 
 Amongst herbaceous plants, none that have been transplanted should be the objects of 
 record. 
 
 In migratory birds, the swallow, sand-martin, and white-martin will appear, at 
 different dates, in places so near together, that a flight of one minute would enable 
 them to reach it. The swallows are seen near the Trent some days previous to coming 
 here, although only a mile distant. The cuckoo and the landrail are invariably heard 
 earlier in the season three or four miles west of this place. 
 
 Amongst the observations on the ripening of fruits, the strawberry will serve as an 
 example : the variety called black prince will be ripe before Kean's seedling, Kean's 
 seedling before British Queen, and British Queen before the Elton pine. 
 
 In entering the flowering of plants, it is advisable to give the dates when they first 
 come into bloom, as well as the dates when in full glory of flower, and when the 
 blooming is over. 
 
 The most conspicuous objects should be entered in a book, space being left which 
 may occupy the remarks for several years. Every observer who, for a course of years, 
 pursues such a course of observation, will find that he has done some good to his kind ; 
 and if any plan could be devised for recording and bringing together such a body of 
 cibservations, they would form a valuable collection of facts for the naturalist and 
 meteorologist to generalize upon.