EM? 
 
THE LIBRARY 
 
 OF 
 
 THE UNIVERSITY 
 OF CALIFORNIA 
 
 PRESENTED BY 
 
 PROF. CHARLES A. KOFOID AND 
 MRS. PRUDENCE W. KOFOID 
 
AN ESSAY 
 
 ON 
 
 D E W , 
 
 AND 
 
 SEVERAL APPEARANCES CONNECTED 
 WITH IT. 
 
 BY WILLIAM CHARLES WELLS, M.D., 
 
 F.R.S.L.&E. 
 
 PHILADELPHIA : 
 HASWELL, BARRINGTON, AND HASWELL, 
 
 293 MARKET STREET. 
 
 1838. 
 

CONTENTS. 
 
 PAGE 
 
 INTRODUCTION, 5 
 
 PART I. 
 
 OF THE PHENOMENA OP DEW. 
 
 SECT. I. Of circumstances which influence the production of Dew, . 6 
 
 SECT. II. Of the old connected with the formation of Dew, . . 16 
 
 PART II. 
 
 OF THE THEORY OF DEW. 
 
 Former Theories, 27 
 
 A NEW THEORY PROPOSED. Dew is the Production of a preceding Cold 
 
 in the Substances upon which it Appears, . . 28 
 That Cold precedes the formation of Dew, ascertained by experiment, . , 29 
 This fact applied to explain several Natural Appearances: 
 
 1. The variety in the quantities of Dew on different Bodies exposed to 
 
 the Air during the same time of the night, but ia different situations, 31 
 2. The Cold connected with Dew not being always proportional to the 
 
 quantity of that fluid, ib. 
 
 3. The production of Heat by the formation of Dew, . . . ib. 
 4. The fact of more Dew being acquired in very calm nights, by sub- 
 stances placed upon a raised board, than by others of the same kind 
 on the grass; and that of a slight agitation of the atmosphere, when 
 very pregnant with moisture, increasing the quantity of Dew, . 33 
 5. The fact of Dew never being formed in temperate climates upon 
 
 the naked parts of a living and healthy human body, . . . ib. 
 6. The fact of Hygrometers formed of Animal and Vegetable sub- 
 stances, when exposed to a clear sky at night, marking a degree of 
 moisture beyond what is actually resident in the Atmosphere, . ib. 
 
 The Cold which produces Dew is itself produced by the Radiation of Heat 
 
 from those Bodies upon which Dew is deposited, . . ib. 
 
 The Cold produced by the radiation of Heat from substances upon the sur- 
 face of the earth, is compensated or over-balanced in the day-time by the 
 Heat from the Sun, and lessened at night by various causes, . . .35 
 
 The Cold originating in the nightly radiation of Heat from Bodies upon the 
 surface of the Earth, though lessened by various causes, is often very con- 
 siderable, .... 36 
 
 Some of the useful effects of the radiation of Heat from the Earth at night, 38 
 
 M363JL81 
 
4 CONTENTS TO ESSAY ON DEW. 
 
 PAGE 
 
 Observations upon, or explanations of, the under-mentioned circumstances: 
 
 1. The prevention, wholly or in part, of Cold from radiation, in sub- 
 stances on the ground, by the interposition of any solid body between 
 them and the sky, 39 
 
 2. The prevention, wholly or in part, of Cold from radiation, in sub- 
 stances on the ground, by the interposition of clouds, . . . ib. 
 
 3. The prevention, wholly or in part, of cold from radiation, by fogs, 40 
 
 4. The prevention, wholly or in part, of cold from radiation, by conduc- 
 tion from warmer substances in contact with the radiating sub- 
 stance, 42 
 
 5. The effect of wind in compensating the Cold from radiation, and 
 sometimes in lessening, and sometimes in increasing, the production 
 of Dew, 43 
 
 6. The Cold from radiation, of a thermometer placed on a board, being 
 less diminished than that of one suspended in the air, . . . ib. 
 
 7. The hurtful effects of Cold occurring chiefly in hollow places, ac- 
 cording to a remark of Theophrastus, . . . . . ib. 
 
 8. Frost being less severe upon hills than in neighbouring plains, in 
 calm and serene nights, . . . . , . . .44 
 
 Reasons assigned for believing that Air is actually heated by the sunbeams 
 which enter it, and that it not only absorbs, but radiates Heat, . . . ib. 
 9. The leaves of trees often remaining dry throughout the night, while 
 those of grass are covered with Dew, ...... 49 
 
 10. Bright metals exposed to a clear sky in a calm night being less 
 dewed on their upper surface than other solid bodies; and those 
 metals which radiate Heat most, being most attractive of Dew, . ib. 
 11. The difference between black and white bodies with respect to 
 
 radiation, when exposed to the sky at night, ..... 52 
 
 Whether Dew is the product of vapour emitted during the night by the 
 earth and plants upon it, i;^*r'-. : . 53 
 
 PART III. 
 
 OF SEVERAL APPEARANCES CONNECTED WITH DEW. 
 
 1. Of the greater moisture sometimes observed in winter mornings upon 
 the insides of the panes of glass in windows covered with inside shut- 
 ters, than upon those not covered by them, 57 
 
 2. Of the greater sensation of Cold, which is sometimes experienced upon 
 exposure to the sky in a clear night, than is to be explained by the tem- 
 perature of the Atmosphere, . . 58 
 
 3._Of the effect of those means employed by gardeners to protect tender 
 
 plants from Cold during the night, which screen them from the sky, . 59 
 
 4. Of the effect of a covering of snow, or of other matters, during still 
 and serene nights, in protecting vegetables from Cold, . . . .61 
 
 5. Of the putrefaction which has been supposed to take place in animal 
 substances exposed to Moonshine, .62 
 
 6.- Of the formation of Ice during the night, in Bengal, when the tempera- 
 ture of the air is above 32, 63 
 
 CONCLUSION, 70 
 

 ESSAY ON DEW 
 
 INTR ODUCTION. 
 
 I was led, in the autumn of 1784, by the event of a rude experi- 
 ment, to think it probable, that the formation of dew is attended with 
 the production of cold. In 1 788, a paper on hoarfrost, by Mr. Patrick 
 Wilson of Glasgow, was published in the first volume of the Transac- 
 tions of the Royal Society of Edinburgh, by which it appeared, that 
 this opinion had been entertained by that gentleman before it had oc- 
 curred to myself. In the course of the same year, Mr. Six of Canter- 
 bury mentioned, in a paper communicated to the Royal Society, that, 
 on clear and dewy nights, he always found the mercury lower in a ther- 
 mometer laid upon the ground, in a meadow in his neighbourhood, 
 than it was in a similar thermometer suspended in the air, six feet 
 above the former ; and that, upon one night, the difference amounted 
 to 5 of Fahrenheit's scale. Mr. Six,however,did not suppose, agree- 
 ably to the opinion of Mr. Wilson and myself, that the cold was 
 occasioned by the formation of dew ; but imagined, that it proceeded 
 partly from the low temperature of the air, through which the dew, 
 already formed in the atmosphere, had descended, and partly from 
 the evaporation of moisture from the ground, on which his ther- 
 mometer had been placed. The conjecture of Mr Wilson, and the 
 observations of Mr. Six, together with many facts, which I after- 
 wards learned in the course of reading, strengthened my opinion ; 
 but I made no attempt, before the autumn of 1811, to ascertain by 
 experiment if it were just, though it had, in the mean time, almost 
 daily occurred to my thoughts. Happening, in that season, to be 
 in the country on a clear and calm night, I laid a thermometer on 
 grass wet with dew, and suspended a second, in the air, two feet 
 above the other. An hour afterwards, the thermometer on the 
 grass was found to be 8 lower, by Fahrenheit's division, than the 
 one in the air. Similar results having been obtained from several 
 similar experiments, made during the same autumn, I determined 
 in the next spring to prosecute the subject with some degree of 
 steadiness, and with this view went frequently to the house of one 
 of my friends, who lives in Surrey. At the end of two months, 
 I fancied that I had collected information worthy of being published 
 but fortunately, while preparing an account of it, I met by accident 
 with a small posthumous work of Mr. Six, printed at Canterbury 
 in 1794, in which are related differences observed on dewy nights, 
 OCT. 1838. Q 2 
 
6 ESSAY ON DEW. 
 
 between thermometers placed on grass and others in the air, that 
 are much greater than those mentioned in the paper presented by 
 him to the Royal Society in 1788. In this work, too, the cold of 
 the grass is attributed, in agreement with the opinion of Mr. Wilson 
 altogether to the dew deposited upon it. The value of my own 
 observations appearing to me now much diminished, though they 
 embraced many points left untouched by Mr. Six, 1 gave up my 
 intention of making them known. Shortly after, however, upon 
 considering the subject more closely, I began to suspect, that Mr. 
 Wilson, Mr. Six, and myself, had all committed an error, in regard- 
 ing the cold, which accompanies dew, as an effect of the formation 
 of that fluid. I, therefore, resumed my experiments, and having, 
 by means of them, I think, not only established the justness of my 
 suspicion, but ascertained the real cause both of dew, and of several 
 other natural appearances, which have hitherto received no sufficient 
 explanation, I venture now to submit, to the consideration of the 
 learned, an account of some of my labours, without regard to the 
 order of time in which they were performed, and of various con- 
 clusions which may be drawn from them, mixed with facts and 
 opinions already published by others. 
 
 PART I. 
 
 OP THE PHENOMENA OF DEW. 
 SECTION I. 
 
 OP CIRCUMSTANCES WHICH INFLUENCE THE PRODUCTION Oi 1 DEW. 
 
 ARISTOTLE* and many other writers have remarked, that dew 
 appears only on calm and serene nights. The justness of this 
 observation, however, has not been universally admitted. For 
 Musschenbroekt says, that dew forms in Holland, while the surface 
 of the country is covered with a low mist ; but, as he mentions at 
 the same time that it is deposited upon all bodies indiscriminately, 
 the moisture, of which he speaks, connot properly be called dew, as 
 will be more distinctly seen hereafter. Other writers of considerable 
 reputation have also regarded clearness of the atmosphere, as not 
 being requisite for the production of dew, misled, I believe, partly 
 by theory, and partly by observing on misty mornings copious dews, 
 which had been produced during preceding clear nights. Respect- 
 ing this point I can aver, after much experience, that I never knew 
 dew to be abundant, except in serene weather.' In regard to the 
 
 * Meteor. Jjib. I, c. x. et De Mundo. c. iii. 
 f Nat. Phil. T. ii. De Rore. 
 
THE PHENOMENA OF DEW. 7 
 
 necessity of the air being still, I know of no person who rejects it, 
 except M. Prieur,* a late French author of little consideration, and 
 he affirms, in opposition to the most common observation, that a 
 fresh wind is requisite for the production of dew. 
 
 The remark of Aristotle, however, is not to be received in its 
 strictest sense, as I have frequently found a small quantity of dew 
 on grass, both on windy nights, if the sky was clear, or nearly so, 
 and on cloudy nights, if there was no wind. If, indeed, the clouds 
 were high, and the weather calm, I have sometimes seen on grass, 
 though the sky was entirely hidden, no very inconsiderable quantity 
 of dew. Again ; according to my observation, entire stillness of 
 the atmosphere is so far from being necessary for the formation of 
 this fluid, that its quantity has seemed to me to be increased, by a 
 very gentle motion in the air. Dew, however, has never been seen 
 by me, on nights both cloudy and windy. 
 
 If, in the course of the night, the weather, from being calm and 
 serene, should become windy and cloudy, not only will dew cease 
 to form, but that which has formed will either disappear or di- 
 minish considerably. 
 
 In calm weather, if the sky be partially covered with clouds, more 
 dew will appear than if it were entirely covered, but less than if it 
 were entirely clear. 
 
 Dew probably begins, in this country, to appear upon grass, in 
 places shaded from the sun, during clear and calm weather, soon 
 after the heat of the atmosphere has declined. My opportunities, 
 however, for making such observations have not been numerous, 
 since, while pursuing this subject, I seldom went into the country 
 till late in the afternoon ; but I have frequently felt grass moist, in 
 dry weather, several hours before sunset. On the other hand, I have 
 scarcely ever known dew to be present in such quantity upon grass, 
 as to exhibit visible drops before the sun was very near the horizon, 
 or to be very copious till some time after sunset. It also continues 
 to form, in shaded places, after sunrise ; but the interval between 
 sunrise, and its ceasing to form, is, according to my observation, 
 which, upon this point, has not been extensive, considerably shorter 
 than that between its first appearance in the afternoon and sunset. 
 Contrary, however, to what happens at sunset, if the weather be 
 favourable, more dew forms a little before, and, in shaded places, a 
 little after sunrise, than at any other time. Musschenbroek, there- 
 fore, errs greatly when he says, that dew does not form after the sun 
 has risen. The preceding observations, on the early appearance of 
 dew in the afternoon, are to be restricted to what happens to grass, 
 or other substances highly attractive of dew placed on the ground ; 
 for it occurs much later on similar substances which are elevated a 
 few feet above the ground, though upon these it continues to form 
 as long after the rising of the sun as upon the others, if they be 
 equally sheltered from the rays of that body. 
 
 * Journal de PEcole Polytechnique, Tom. ii. 409. 
 

 8 ESSAY ON DEW. 
 
 The formation of dew, after it has once commenced, continues 
 during the whole night, if the weather remain still and serene. M. 
 Prieur, indeed, of whom I have already spoken, asserts, that dew 
 forms only in the evening and morning, and that any which occurs 
 in the former season always disappears in the course of the night. 
 I can affirm, however, from long experience, that grass, after having 
 been dewed in the evening, is never found dry until after sunrise, 
 unless the weather has, in the mean time, changed. Upon one 
 serene and still night, I placed fresh parcels of wool upon grass every 
 hour, and by weighing each of them, after exposure for an hour, 
 found that they had all attracted dew. 
 
 When dew forms upon a smooth dense body as glass, and it is 
 only by means of such a body that the process can be accurately 
 observed, the appearances are altogether similar to those which oc- 
 cur on a like body when exposed to the steam of water a little 
 warmer than itself. The exposed surface has first its lustre dimi- 
 nished by a slight damp uniformly spread over it. As the moisture 
 increases, it gathers into irregularly shaped flat drops, which are, at 
 first, very small, but afterwards enlarge and run into one another, 
 forming streamlets, by means of which a great part escapes from the 
 body which had received it. 
 
 During nights that are equally clear and calm, dew often appears 
 in very unequal quantities, even after allowance has been made for 
 any difference in their lengths. One great source of these differ- 
 ences is very obvious. For, it being manifest, whatever theory be 
 adopted concerning the immediate cause of dew, that the more 
 replete the atmosphere is with moisture previously to the operation 
 of that cause, the more copious will the precipitation of water be 
 after this operation has commenced, all the circumstances which 
 tend to increase the quantity of moisture in the atmosphere, must 
 likewise tend to increase the production of dew. Thus dew, in 
 equally calm and clear nights, is more abundant shortly after rain, 
 than during a long tract of dry weather. It is more abundant, also, 
 throughout Europe, with perhaps a few exceptions, and in some 
 parts of Asia and Africa, during southerly and westerly winds, than 
 during those which blow from the north and the east. Aristotle* 
 says, that Pontus is the only country in which dew is more copious 
 during a northerly than during a southerly wind. But a similar 
 fact occurs in Egypt ; for dew is scarcely ever observed there, ex- 
 cept while the Etesian winds prevail. Both cases, however, though 
 contrary to the letter, are consonant with the spirit of the rule ; 
 since the north wind in one country proceeds from the Euxine sea, 
 and in the other from the Mediterranean. Another circumstance, 
 of the same kind with the blowing of wind from the south and the 
 west, as showing that the air contains much moisture, is the lessen- 
 ing of the weight of the atmosphere. My experience on this point 
 has not, indeed, been great, as the falling of the mercury in the 
 
 * Meteor. Lib. i. c. x. 
 
THE PHENOMENA OF DEW. 9 
 
 barometer is very commonly attended with wind or clouds, both 
 unfavourable to the production of dew ; but still the greatest dew, I 
 have ever witnessed, occurred while the barometer was sinking. A 
 corresponding observation is made by M. de Luc, who says, that 
 rain may be foretold, when dew is uncommonly abundant in relation 
 to the climate and season.* 
 
 To the greater or less quantity of moisture in the atmosphere, at 
 the time of the action of the immediate cause of dew, are likewise 
 to be referred several other facts respecting its copiousness, the 
 explanation of which is, perhaps, not so apparent as in the preced- 
 ing examples. 
 
 In the first place ; dew is commonly more plentiful in spring and 
 autumn than in summer ; the reason is, that a greater difference is 
 generally found between the temperatures of the day and the night, 
 in the former seasons of the year than in the latter. In spring this 
 circumstance is prevented often from having a considerable effect, 
 by the opposite influence of northerly and easterly winds ; but, during 
 still and serene nights in autumn, dew is almost always highly 
 abundant. 
 
 In the second place ; dew is always very copious on those clear 
 and calm nights which are followed by misty or foggy mornings ; 
 the turbidness ot the air in the morning showing, that it must have 
 contained, during the preceding night, a considerable quantity of 
 moisture. 
 
 Thirdly ; I have observed dew to be unusually plentiful on a 
 clear morning which had succeeded a cloudy night. For the air, 
 having in the course of the night lost little or no moisture, was in 
 in the morning more charged with watery vapour, than it would 
 have been if the night had also been clear. 
 
 Fourthly ; heat of the atmosphere, if other circumstances are 
 favourable, which according to my experience they seldom are in 
 this country, occasions a great formation of dew. For, as the power 
 of the air to retain watery vapour in a pellucid state increases con- 
 siderably faster while its temperature is rising, than in proportion 
 to the heat acquired, a decrease of its heat, in any small given 
 quantity during the night, must bring it, if the temperature be 
 high, much nearer to the point of repletion, before it be acted upon 
 by the immediate cause of dew, than if the temperature were low, 
 We read, accordingly, in the writings of those who have travelled 
 into hot climates, of a copiousness of dew frequently observed by 
 them there, which very much exceeds what occurs any time in this 
 country. But even here, dew, though for the most part scanty in 
 our hottest season, is sometimes very abundant during it, an example 
 of which occurred to me on the night common to the 29th and 30th 
 of July 1813 ; for on that night, notwithstanding its shortness, more 
 dew appeared than has ever been observed by me on any other. 
 
 In the last place ; I always found, when the clearness and 
 
 * Rech. sur les Mod. de PAtm. 725, 
 2* 
 
10 ESSAY ON DEW. 
 
 ness of the atmosphere were the same, that more dew was formed 
 between midnight and sunrise, than between sunset and midnight, 
 though the positive quantity of moisture in the air must have been 
 less in the former than in the latter time, in consequence of a pre- 
 vious precipitation of part of it. The reason, no doubt, is the cold 
 of the atmosphere being greater in the latter, than in the prior part 
 of the night. 
 
 But there are many circumstances influencing the quantity of 
 dew which, though much more open to accurate observation than 
 those hitherto mentioned, are yet much less easy to be understood. 
 
 In my first attempts to compare the quantities of dew formed 
 during different times, or in different situations, I attended only to 
 the appearance which it made on bodies having smooth surfaces. 
 But quickly seeing this method to be very imperfect, I next employed 
 wool to collect dew from the atmosphere, and found it well adapted 
 for my purpose, as it readily admits amongst its fibres the moisture 
 which forms on its outer parts, and retains what it receives so firmly, 
 that I never but once had occasion to suspect that it suffered any 
 portion of what it had thus acquired to pass entirely through it. 
 The wool which I used was white, moderately fine, and already 
 imbued with a little moisture, from having been long exposed to 
 the air of a room in which no fire was kept. I divided it into 
 parcels of 10 grains each, and, immediately before exposure, pulled 
 the fibres of every parcel somewhat asunder, so as to give it the 
 form of a flattened sphere, the greatest diameter of which was about 
 2 inches. As in doing this I went by the judgment of my sight 
 alone, some inequality, in point of size, must have existed among 
 different parcels, but none, I think, sufficient to affect the accuracy 
 of my conclusions from the experiments in which they were em- 
 ployed, more especially as my conclusions scarcely ever rested upon 
 single trials. 
 
 Previously to mentioning the results of any of my experiments 
 with these parcels of wool, I think it right to describe the place 
 where by far the greater part of my observations on dew were 
 made. This was a garden in Surrey, distant, by the public road, 
 about three miles from the bridge over the Thames at Blackfriars, but 
 not more than a mile and a quarter from a densely built part of the 
 suburbs on the south side of that river. The form of the garden 
 was oblong, its extent nearly half an acre, and its surface level. 
 At one end was a dwelling-house of moderate size, at the other a 
 range of low buildings ; on one side a row of high trees, on the 
 other a low fence, dividing it from another garden. If this 
 fence had been absent, the garden would have been on the latter 
 side entirely open. Within it were some fruit trees, but, as it had 
 not been long made, their size was small. Towards one end there 
 was a grassplat, in length 62 feet, and nearly 16 broad, the herbage 
 of which was kept short by frequent mowing. The rest of the 
 garden was employed for the production of culinary vegetables. 
 All of these circumstances, however trifling they may appear, had 
 influence on my experiments, and most of them, as will hereafter be 
 
THE PHENOMENA OF DEW. 11 
 
 seen, must have rendered the results less remarkable than they 
 would have been if they had occurred on a wide open plain, con- 
 siderably distant from a large city. 
 
 I now proceed to relate the influence which several differences 
 in the situation, mechanical state, and real nature of bodies, have 
 upon the production of dew. 
 
 I. One general fact relative to situation is, that whatever dimin- 
 ishes the view of the sky, as seen from the exposed body, occasions 
 the quantity of dew, which is formed upon it, to be less than would 
 have occurred if the exposure to the sky had been complete. 
 
 I placed on several clear and still nights, 10 grains of wool upon 
 the middle of a painted board, 4i feet long, 2 feet wide, and 1 inch 
 thick, elevated 4 feet above the grassplat, by means of 4 slender 
 wooden props of equal height ; and, at the same time, attached, 
 loosely, 10 grains of wool to the middle of its underside. The 
 two parcels were consequently only an inch asunder, and were 
 equally exposed to the action of the air. Upon one night, however, 
 I found that the upper parcel had gained 14 grains in weight, but 
 the lower only 4. On a second night, the quantities of moisture, 
 acquired by like parcels of wool, in the same situations as in the 
 first experiment, were 19 and 6 grains ; on a third, 11 and 2 ; on 
 a fourth, 20 and 4 ; the smaller quantity being always that which 
 was gained by the wool attached to the lower side of the board. 
 
 I bent a sheet of pasteboard into the shape of a house-roof, mak- 
 ing the angle of flexure 90 degrees, and leaving both ends open. 
 This was placed one evening, with its ridge uppermost, upon the 
 same grassplat, in the direction of the wind, as well as this could 
 be ascertained. I then laid 10 grains of wool on the middle of that 
 part of the grass which was sheltered by the roof, and the same 
 quantity on another part of the grassplat fully exposed to the sky. 
 In the morning, the sheltered wool was found to have increased in 
 weight only 2 grains, but that which had been exposed to the sky 
 16 grains. 
 
 .In these experiments, the view of the sky was almost entirely 
 cut off from the situations in which little dew was formed. In 
 others, where it was less so, the quantity gained was greater. Thus, 
 10 grains of wool, placed upon the spot of the grassplat, which was 
 directly under the middle of the raised board, and which enjoyed, 
 therefore, a considerable oblique view of the sky, acquired during 
 one night 7, during a second 9, and during a third 12 grains of 
 moisture, while the quantities gained, during the same times, by 
 equal parcels of wool, laid upon another part of the grassplat which 
 was entirely exposed to the heavens, were 10, 16, and 20 grains. 
 
 As no moisture, falling like rain from the atmosphere, could on 
 a calm night have reached the wool in any of the situations where 
 little dew was formed, it may be thought that the substances under 
 which the wool was placed, prevented, mechanically, the access of 
 that fluid. But on this supposition it cannot be explained, why 
 some dew was always found in the most sheltered places, and why 
 a considerable quantity occurred upon the grass under the middle 
 
12 ESSAY ON DEW. 
 
 of the raised board. A still stronger proof of the want of justness 
 in this supposition is afforded by the following experiment. I 
 placed, upright, on the grassplat a hollow cylinder of baked clay, 
 the height of which was 2% feet, and diameter 1 foot. On the grass, 
 surrounded by the cylinder, were laid 10 grains of wool, which, in 
 this situation, as there was not the least wind, would have received 
 as much rain as a like quantity of wool fully exposed to the sky. 
 But the quantity of moisture obtained by the wool surrounded by 
 the cylinder was only a little more than 2 grains, while that ac- 
 quired by 10 grains of fully exposed wool was 16. This occurred 
 on the night during which the wool under the bent pasteboard 
 gained only 2 grains of moisture. 
 
 Dew, however, will-, in consequence of other varieties of situation, 
 form in very different quantities upon substances of the same kind, 
 although these should be similarly exposed to the sky. 
 
 In the first place ; it is requisite, for the most abundant formation 
 of dew, that the substance attracting it should rest on a stable hori- 
 zontal body of some extent. Thus, upon one night, while 10 grains 
 of wool, laid upon the raised board, increased 20 grains in weight, 
 an equal quantity, suspended in the open air, 63 feet above the 
 ground, increased only 11 grains, notwithstanding that it presented 
 a greater surface to the air than the other parcel. On another night, 
 10 grains of wool gained on the raised board 19 grains, but the same 
 quantity suspended in the air, on a level with the board, only 13 ; 
 and on a third, 10 grains of wool acquired on the same board 2i 
 grains of weight, during the time in which other 10 grains, hung in 
 the air at the same height, acquired only 5 a grain. 
 
 In the second place ; the quantities of dew attracted by equal 
 masses of wool, similarly exposed to the sky, and resting on equally 
 stable and extended bodies, oftentimes vary considerably, in conse- 
 quence of some difference in the other circumstances of these 
 bodies. 10 grains of wool, for instance, having been placed upon 
 the grassplat, on a dewy evening, 10 grains upon a gravel walk 
 which bounded the grassplat, and 10 grains upon a bed of bare 
 garden mould, immediately adjoining the gravel walk ; in the 
 morning, the wool on the grass was found to have increased 16 
 grains in weight, but that on the gravel walk only 9, and that on 
 the garden mould only 8. On another night, during the time that 
 10 grains of wool, laid upon grass, acquired 2% grains of moisture, 
 the same quantity gained only 5 a grain upon the bed of garden 
 mould, and a like quantity, placed upon the gravel walk, received 
 no accession of weight whatever. 
 
 Two objections will probably be made against the accuracy of these, 
 as well as my other experiments with wool. One is, that wool 
 placed on grass may, by a kind of capillary attraction, receive dew 
 previously formed on the grass, in addition to its own. To this I 
 answer, that wool in a china saucer, placed on the grass, acquired 
 very nearly as much weight as an equal parcel immediately touch- 
 ing the grass. The second objection is, that a part of the increased 
 weight in the wool might arise from its imbibing moisture, as a 
 
THE PHENOMENA OF DEW. 13 
 
 ^&&_ 
 
 hygroscopic substance. I do not deny, that some weight was given 
 to the wool in this way ; but it may be safely affirmed, that this 
 quantity must have been very small. For, on very cloudy nights, 
 apparently the best fitted to increase the weight of hygroscopic 
 substances, wool upon the raised board would, in the course of many 
 hours, acquire little or no weight ; and in London I have never 
 found 10 grains of wool, exposed to the air on the outside of one of 
 my chamber windows, to increase, during a whole night, more than 
 i a grain in weight. When this weight was gained, the weather 
 was clear and still ; if the weather was cloudy and windy, the 
 wool received either less or no weight. This window is so situa- 
 ted, as to be, in great measure, deprived of the aspect of the sky. 
 
 It being shown that wool, though highly attractive of dew, was 
 prevented, by the mere vicinity of a gravel walk, or a bed of garden 
 mould, for only a small part of it actually touched those bodies, 
 from acquiring nearly as much dew as an equal parcel laid upon 
 grass, it may be readily inferred that little was formed upon them- 
 selves. In confirmation of this conclusion, I shall mention, that I 
 never saw dew upon either of them. Another fact of the same 
 kind is, that, while returning to London from the scene of my 
 experiments about sunrise, I never observed, if the atmosphere was 
 clear, the public road, or any stone pavement on the side of it, to be 
 moistened with dew, though grass within a few feet of it, and 
 painted doors and windows of houses not far from it, were frequent- 
 ly very wet. If, indeed, there was a foggy morning, after a clear 
 and calm night, even the streets of London would sometimes be 
 moist, though they had been dry the day before, and no rain had in 
 the meanwhile fallen. This entire, or almost entire, freedom of 
 certain situations from dew depends, however, much more upon 
 extraneous circumstances, than upon the nature of the substances 
 found there ; for river sand, though of the same nature with gravel, 
 when placed upon the raised board, or upon grass, attracted dew 
 copiously. 
 
 A third difference from situation, in the quantity of dew collec- 
 ted by similar bodies, similarly exposed to the sky, depends upon 
 their position with respect to the ground. Thus, a substance placed 
 several feet above the ground, though in this situation later dewed 
 than if it touched the earth, would, notwithstanding, if it Jay upon 
 a stable body of some extent, such as the raised board lately men- 
 tioned, acquire more dew during a very still night, than a similar 
 substance lying on grass. 
 
 A fourth difference of this kind occurred among bodies placed 
 on different parts of the raised board. For one that was placed at 
 the leeward end of it, generally acquired more dew than a similar 
 body at the windward extremity. 
 
 II. Difference in the mechanical state of bodies, though all other 
 circumstances be similar, haslikewisean effect on the quantity of dew 
 which they attract. Thus, more dew is formed upon fine shavings 
 of wood, than upon a thick piece of the same substance. It is 
 
14 ESSAY ON DEW. 
 
 chiefly for a similar reason, I believe, that fine raw silk, fine unwrought 
 cotton and flax, were found by me to attract somewhat more dew 
 than the wool I employed, the fibres of which were thicker than 
 those of the other substances just mentioned. 
 
 III. Bright metals, in consequence of some circumstance in their 
 constitution, attract dew much less powerfully than other bodies, 
 all of which, after allowance has been made for any difference 
 which may exist in their mechanical states, seem to attract dew in 
 quantities not very unequal, if they be similarly situated. 
 
 Musschenbroek was the first who distinctly remarked this 
 peculiarity of metals ; but Dufay,* I believe, published it before 
 him, referring, at the same time, the discovery to its proper author. 
 Both Musschenbroek and Dufay, however, made too large an infer- 
 ence from their experiments; for they asserted, that dew never 
 appears on the upper surface of bright metals, whereas the contrary 
 has since been observed by many persons, and I have myself known 
 dew to form on gold, silver, copper, tin, platina, iron, steel, zinc, 
 and lead. Dew, however, when it does form upon metals, com- 
 monly sullies on the lustre of their surface ; and even when it is 
 sufficiently abundant to gather into drops, these are almost always 
 small and distinct. Two other facts of the same kind are ; first, 
 that the dew, which has formed upon a metal, will often disappear, 
 while other substances in their neighbourhood remain wet ; and 
 secondly, that a metal, which has been purposely moistened, will 
 often become dry, though similarly exposed with bodies which are 
 attracting dew. This inaptitude to attract dew in metals is commu- 
 nicated to bodies of a very different nature which touch or are hear 
 to them. For I have found, that wool laid upon a metal will 
 acquire much less dew than an equal quantity laid upon grass in 
 the immediate vicinity. 
 
 A large metallic plate, lying on grass, resists the formation of dew 
 more powerfully than a very small one similarly situated. I con- 
 clude from various collateral facts, that a considerable difference in 
 the thickness of two pieces of metal, exposing equal surfaces to the 
 sky, will be attended with a similar consequence wherever they be 
 placed, though I have no observation which proves this directly. 
 If, however, a large and a very small plate be suspended horizon- 
 tally at the same height in the air, the small plate will resist the 
 formation of dew more powerfully than the large. 
 
 If a metal be closely attached to a substance of some thickness 
 which attracts dew powerfully, the attraction of the metal itself for 
 dew, instead of being increased from this circumstance becomes 
 diminished, provided the metal cover the whole of the upper surface 
 of the other body. If only a part of this body be covered, the pro- 
 duction of dew on the metal is forwarded by the conjunction, and 
 this somewhat in proportion to the quantity of surface in the lower 
 body left uncovered. The justness of the first of these observations 
 
 * Mem. de 1'Acad. Fran. 1736. 
 
THE PHENOMENA OF DEW. 15 
 
 is proved by the following experiment. I joined, in the form of a 
 cross, two pieces of very light wood, each 4 inches long, a third of 
 an inch in breadth, and 1 line in thickness. To one side of this cross I 
 fastened, by means of mucilage, a square piece of gilt paper, and 
 then exposed the instrument to the sky, with its metallic side upper- 
 most, on a dewy night, by suspending it, in a horizontal position, 
 about 6 inches above the ground. A few hours after, the unattached 
 parts of the metalled paper were found covered with minute drops 
 of dew, while those which adhered to the cross were dry. 
 
 A large metallic plate, laid upon grass, was dewed with more 
 difficulty on its upper surface, than a similar plate elevated a few 
 inches above the grass by means of slender props, which allowed 
 the air to pass freely under the metal. But the case with respect to 
 small pieces was the reverse ; for I have often seen covered with 
 dew the metallic sheath of a small thermometer lying upon grass, 
 while the similar sheath of another thermometer suspended in the 
 air remained dry. 
 
 Removing a metal several times in the course of the night from 
 one part of the grassplat to another facilitated its being dewed. 
 The same effect was produced on gilt and silvered paper, by first 
 exposing them to the sky, for some time, with the bare side upper- 
 most, and then turning them. 
 
 If a piece of glass, covered on one side with a metal, be placed 
 upon the ground, with this side downwards, the upper surface will 
 attract dew precisely as if no metal were attached to the lower surface. 
 
 The upper surfaces of metals are most readily, and most copiously 
 dewed on those nights, and in those parts of the night, during which 
 other substances are the most readily and the most copiously dewed. 
 
 If a metallic plate had been laid upon grass before dew began to 
 form anywhere, its lower side, notwithstanding, always became 
 moist in the course of the night ; and the same effect was almost 
 always observed, if the plate had been placed horizontally in the air 
 a few inches above the grass. While the undersides were thus 
 moist, the upper surfaces were very often dry. If, however, the 
 plate was elevated several feet in the air, the condition of both sides 
 was always the same, whether this was dry or moist. 
 
 The remarks hitherto made on the relation of metals to dew, 
 apply to the class generally ; but it is now to be mentioned, that 
 they do not all resist the formation of that fluid with the same force. 
 
 I saw, for example, platina one night distinctly dewed, while gold, 
 silver, copper, and tin, though similarly situated, were entirely dry; 
 and I have also several times seen these four metals free from dew, 
 while iron, steel, zinc, and lead were covered with it. 
 
 1 once supposed, in consequence of the difficulty with which 
 metals are dewed, that they might in all circumstances resist, in a 
 greater degree than other bodies, the condensation of watery vapour 
 upon their surface ; and I afterwards found, that Le Roi* asserts this 
 
 * Mem. de 1'Acad. Fran. 1751. 
 
16 ESSAY ON DEW. 
 
 to be the case. But having exposed at the same time, to the steam 
 of warm water, pieces of glass and of metal, I did not see that 
 moisture formed in the least more readily upon the former than 
 upon the latter. I have since learned, that Saussure* once enter- 
 tained a similar suspicion, which was also proved by an experiment 
 to be groundless. 
 
 All my experiments, hitherto spoken of, were made in the country. 
 But Le Roi having said, that dew is never deposited by the air of 
 cities, I determined to ascertain if his assertion was just. With 
 this view, I frequently exposed, at night, 10 grains of wool upon a 
 slight wooden frame, placed in such a manner, between two ridges 
 of the top of my house, which is situated in one of the most crowded 
 districts of London, as to be 3 feet distant from the nearest part of 
 the roof. The event was, that, upon clear and calm nights, dew was 
 always acquired by the wool, though never in any considerable 
 quantity ; probably, however, more from the wooden frame being 
 nearly surrounded by buildings much more elevated than itself, 
 than from any particular condition of the air in cities. The forma- 
 tion of dew, in this situation, proceeded much less regularly than in 
 the country. For, upon one evening ten grains of wool gained in 
 it 3 grains of moisture in 1 hour and 18 minutes, though I scarcely 
 ever knew a greater quantity to be collected by a similar parcel of 
 wool, in the same place, during a whole night. These experiments 
 will no doubt seem to many superfluous, since dew may be observed 
 every fine evening upon grass in London. But as dew upon grass 
 is said by Le Roi to proceed from the ground, and not from the 
 atmosphere, the argument derived from its appearance there, in 
 cities, against his assertion is thus eluded by him. 
 
 The last subject, which I shall here touch upon, is that of hoar- 
 frost. 
 
 This substance has, I believe, from the time of Aristotle,t been 
 uniformly, and according to my observations justly, considered as 
 frozen dew. I shall, therefore, frequently refer hereafter to the 
 experiments of the late Mr. Patrick Wilson of Glasgow respecting 
 it, as if they had been actually made upon that fluid. Indeed, 
 several of my experiments upon dew were only imitations of some 
 which had been previously made upon hoarfrost by that ingenious 
 and most worthy man. 
 
 SECTION II. 
 
 OF THE COLD CONNECTED WITH THE FORMATION OF DEW. 
 
 DEW is often spoken of as being cold by popular writers. Thus 
 Cicero and Virgil apply to it the epithet of " gelidus," Milton that 
 
 * Hygronometrie, page 329. f Meteor. Lib. I. c. x. 
 

 THE PHENOMENA OF DEW. 17 
 
 of " chill," and Collins that of " cold." Of the same import is a 
 passage in Herodotus, in which it is said, that in Egypt the croco- 
 dile passes a great part of the day on dry land, but the whole of the 
 night in the Nile, this being warmer than the atmosphere, and the 
 dew. Among philosophers, however, Mr. Wilson was the first, I 
 believe, who ever suspected the existence of such a conjunction. 
 
 In my experiments on the temperature of bodies moistened with 
 dew, small thermometers were employed, (the largest being only 8 
 inches long) having globular bulbs, which, in most of them, were 
 not more than from 2 to 2% lines in diameter. Their scales, which 
 were marked in the manner of Fahrenheit, were of ivory or wood, 
 and were furnished, almost all of them, with hinges. They were 
 always employed naked, except I wished to know the effect of cover- 
 ing them with any particular substance. 
 
 By means of these instruments I have very many times, during 
 serene and still nights, examined the temperature of dewed grass, 
 and have constantly observed it to be less than that of the air, any- 
 where between 1 inch and 9 feet above the ground, the latter 
 being the greatest height at which I ever marked the heat of the 
 atmosphere in these experiments. I generally, however, compared 
 the temperature of dewed grass with that of the air 4 feet above the 
 ground; and on nights that were calm and clear, very frequently 
 found the grass, at the ordinary place of my observations, 7, 8, or 9 
 degrees colder than the air at that height. Several times it was 10 
 and 11 colder than the air, and once 12. These differences are 
 not so great as those related in Mr. Six's posthumous work. But 
 in his experiments the temperature of grass was compared with that 
 of the air 7 feet above the ground, which, in clear and calm nights, 
 may be regarded as 5 a degree warmer than the air at the height of 
 4 feet. Besides ; the most considerable differences, mentioned by 
 Mr. Six, occurred in winter, when he says a greater degree of cold 
 is occasioned by dew, than at any other time ; whereas very few of 
 my experiments on the temperature of grass were instituted in that 
 season. In the last place ; my experiments were almost always 
 made on very short grass, while Mr. Six's thermometers were laid 
 upon long grass bent, by strong pressure, towards the earth ; in 
 which state they marked a temperature 1, 2, and 3 degrees lower, 
 than that shown by similar thermometers placed upon grass less 
 than an inch in height. Had it not been for these circumstances, 
 and the unfitness, in various respects, besides the shortness of the 
 grass for the production of a great cold, of the common scene of my 
 operations, I believe that, in consequence of my thermometers being 
 much better adapted to mark a superficial, or transitory cold, than 
 those of Mr. Six, I should at some time have seen a difference 
 several degrees greater than the greatest ever seen by that gentle- 
 man, which was one of 13. In confirmation of this opinion, I 
 shall mention, that having, during a short visit to a more distant part 
 of the country, exposed in the evening a thermometer upon the 
 surface of an open grass field, I found it soon after, although 
 
 OCT. 1838. R 3 
 
18 ESSAY ON DEW. 
 
 the grass was short, and the weather warm, 14 lower than a similar 
 thermometer, suspended in the air, 4 feet above the grass. If to 
 this quantity be added a degree, on account of the difference in 
 elevation between our suspended thermometers, the cold, connected 
 with dew, observed by me this night on grass, will exceed the 
 greatest ever observed by Mr. Six by 1 degree. 
 
 According to a few observations made by me, the greater cold- 
 ness of grass than that of the air begins to appear, in clear and 
 calm weather, in places sheltered in the afternoon from the sun, 
 but still open to a considerable portion of the sky, soon after the 
 heat of the atmosphere has declined. A similar coldness continues 
 upon grass in still and serene mornings for some time after 
 the rising of the sun, in places shaded from its direct light, but 
 otherwise open to the sky. My experiments on this point have 
 also not been many, and none of them were made in winter ; which 
 I presume are the reasons that I never observed a cold, from this 
 cause, later in the morning than an hour after sunrise. The surface 
 of snow, however, was once in the depth of winter observed by 
 Mr. Wilson of Glasgow to be considerably colder than the air, till 
 a little after mid-day.* 
 
 In cloudy nights, particularly if there was wind, the grass was 
 never much colder than the air. On such nights the temperatures 
 of both were sometimes the same ; at other times that of the grass 
 was the higher of the two, even when the grass was wet from pre- 
 ceding rain, and when, consequently, it must have been in some 
 measure cooled by evaporation. On one such night the grass was 
 found to be 4 colder than the earth an inch beneath the surface of 
 the plat, which afforded a sufficient reason for the grass itself being 
 warmer than the air. In windy weather, however, if the sky was 
 clear, some degree of cold, in addition to that of the air, was always 
 observed upon the grass ; and in calm weather, very high clouds, 
 though sufficiently extensive and dense to conceal the sky com- 
 pletely, would yet frequently allow of the grass being several 
 degrees colder than the air. I once observed, upon a night of this 
 kind, a difference of 5 between the temperature of those bodies. 
 
 If the night became cloudy, after having been very clear, though 
 there might be no change with respect to calmness, a considerable 
 alteration in the temperature of the grass always ensued ; and this 
 sometimes very suddenly. Upon one such night the grass, after 
 having been 12 colder than the air, became only 2 colder than it, 
 the temperature of the air being the same at both observations. On 
 a second night, grass became 9 warmer in the space of an hour and 
 a half. On a third night, in less than 45 minutes, for the whole 
 change occurred while I was absent 45 minutes, the temperature 
 of the grass rose 15, while that of the neighbouring air increased 
 3i. During a fourth night, the temperature of the grass at half 
 past 9 o'clock was 32. In 20 minutes afterwards it was found to 
 
 * Paper in Phil. Trans. 1781. 
 
THE PHENOMENA OF DEW. 19 
 
 be 39, the sky having in the mean time become cloudy. At the 
 end of 20 minutes more, the sky being clear, the temperature of the 
 grass was again 32. These were the most remarkable of my 
 observations on this subject ; but I may add to them, that I have 
 frequently seen, during nights that were generally clear, a ther- 
 mometer lying on the grassplat rise several degrees, upon the 
 zenith being occupied only a few minutes by a cloud. On the 
 other hand, upon two nights I observed a very great degree of cold 
 to occur on the ground, in addition to that of the atmosphere, during 
 short intervals of clearness of sky, between very cloudy states of it. 
 I did not speak in the preceding section of another obscure state 
 of the atmosphere, that occasioned by fog, or mist, as the moisture 
 deposited in it attaches to all bodies indiscriminately ; on which 
 account I was unable to determine whether or not dew forms during 
 its continuance. But, with respect to the connexion of this condi- 
 tion of the atmosphere with cold, I have to remark, that I have 
 several times on its appearance betwixt day break and sunrise, found 
 the difference between thermometers on grass and in the air, which 
 had been considerable during the night, to diminish greatly. I 
 never indeed observed it to vanish, but this I used to impute to the 
 air being not very much obscured. I have now, however, reason to 
 doubt the justness of this conclusion ; for on the evening of the first 
 of January in the present year, 1814, I found, during a dense fog, 
 while the weather was very calm, a thermometer lying on grass, 
 thickly covered with hoarfrost, 9 lower than another suspended in 
 the air, 4 feet above the former. On the following evening, when 
 the air was equally calm, but the fog sufficiently attenuated to allow 
 me to see that the sky was almost entirely covered with clouds, 
 the difference between two thermometers, similarly placed with the 
 former, was only 1. On comparing the observations of these two 
 evenings, I conclude, that on the first few or no clouds existed above 
 the fog, and consequently that fog, if there be no clouds above it, 
 may, in a very calm air, admit of the appearance of a considerable 
 degree of cold at night upon the surface of the earth, in addition to 
 that of the atmosphere. Mr. Six, indeed, says, while speaking of 
 the cold connected with dew, in his paper in the Philosophical 
 Transactions for 1788, "fogs did not, as far as I could perceive, at 
 all impede, but rather increase, the refrigeration." But this was a 
 mistake ; which in all probability arose from his ascribing the effect 
 of a clear night to an ensuing foggy morning, as he examined his 
 thermometers only in the daytime. He afterwards discovered his 
 error ; for, in his posthumous work, thick fogs are ranked among 
 the circumstances which always impede, and sometimes prevent 
 altogether, the appearance of a cold upon the surface of the earth 
 greater than that of the atmosphere. During a very dense fog, 
 Mr. Wilson found no difference, at night, between a thermometer 
 laid upon snow, and another suspended in the air.* 
 
 *Edin. Phil. Trans. I. 170. 
 
20 ESSAY ON DEW. 
 
 When, during a clear and still night, different thermometers were 
 examined, at the same time, which had been placed in different 
 situations, those which were situated where most dew was formed 
 were always found to be the lowest. Thus, upon one such night, I 
 found a thermometer placed upon a little wool, lying upon the middle 
 of the upper side of the raised board, to be 9 lower than another 
 thermometer, in contact with an equal quantity of wool attached to the 
 middle of the underside of the board. On two other nights the dif- 
 ference between two thermometers in the same situations was 8. I 
 found also, on two other serene and calm nights, a spot of grass co- 
 vered by the pasteboard roof, and another spot surrounded by the 
 earthen cylinder, to be both 10 warmer than neighbouring grass fully 
 exposed to the sky. Thinking it possiblethat the cylinder, which had 
 been exposed to the sun the preceding clay, might still possess some 
 of the heat which it had then imbibed, 1 placed near to it, on another 
 night, a cylinder made of very thin pasteboard ; but this was 
 equally efficacious with the earthen one in preventing cold from 
 occurring on grass. When the exposure was greater than in the 
 preceding examples, and more dew was in consequence formed, the 
 cold was also greater, but still less than where the exposure was 
 complete. For instance, upon the night during which 10 grains of 
 wool placed upon the middle of the grass, which was sheltered by 
 the raised board, had gained 7 grains, and the same quantity on 
 grass fully exposed to the sky had gained 10 grains, the difference 
 between the temperatures of the two portions of grass was only 25. 
 The same correspondence was observed, when the differences in 
 the quantity of dew did not depend, as in the preceding instances, 
 upon any diversity of exposure to the sky. Thus, the mercury in 
 a thermometer placed upon wool, lying on the raised board, was 
 found to be at the 44th degree, while that in another, pendent in 
 the air, at the same height from the ground, and wrapped in wool, 
 was at the 48th. Wool, also, on the raised board,* was commonly 
 a little colder than the same substance on grass, when the night was 
 very still ; and the leeward end of that board was generally colder 
 than the windward extremity. 
 
 But the most remarkable examples of this kind were exhibited 
 by the gravel walk and the bare garden mould. In still and serene 
 nights the surfaces of these bodies were always warmer than the 
 neighbouring grass, and frequently warmer than the air. On one 
 night of this description, I observed 2^ hours after sunset, the sur- 
 face of the gravel walk to be 16^, and that of the garden mould to 
 be 12i, warmer than grass very near to them, and similarly exposed 
 
 * The greater cold of the raised board in ray experiments, most probably de- 
 pended" on the grass being very short; since Mr. Wilson found, that snow on the 
 ground was colder than the same body on a raised board. If 1,2, or w 3 degrees 
 were added to the cold of the grass at my place of observation, agreeably to the 
 difference found by Mr. Six, between the temperatures of long and short grass in 
 dewy nights, the cold on my raised board would, upon such nights, have been 
 always less than that of the grassplat. 
 
THE PHENOMENA OF DEW. 21 
 
 to the heavens.- As the night proceeded, clouds formed and accu- 
 mulated ; in consequence of which the difference at sunrise, between 
 the temperatures of the grass and the gravel walk, was only 6, and 
 between those of the grass and the mould only 4, the temperature 
 of the grass having in the mean time increased considerably, while 
 that of the other bodies had decreased a little. At another time, 
 shortly before sunrise, a very clear morning having succeeded a 
 cloudy night, I found the gravel walk to be 10 and the garden bed to 
 be 9 warmer than neighbouring grass, which was 8 colder than the 
 air. Both of these examples occurred in summer, and I believe 
 that such considerable differences will occur in that season only. 
 It was on the first of these two nights, that 10 grains of wool gained 
 only $ a grain of moisture on the mould, and that the same quantity 
 gained no weight on the gravel walk. That the unfitness of the 
 gravel walk, however, to become cold, like its unfitness to attract 
 dew, arose from its situation, and not from the nature of the sub- 
 stance of which it was made, is proved by this circumstance, that 
 river sand, placed on the raised board, was on four different nights, 
 none of them highly favourable for the production of cold 7, 7, 8 
 and 8^ degrees colder than the air at the same height. 
 
 It may be added here, that I have always found, on dewy nights, 
 the temperature of the earth, \ an inch or an inch beneath its 
 surface, much warmer than the grass upon it. On five such nights 
 the differences were from 12 to 16 degrees. The earth, at the above 
 mentioned depth, was also almost constantly warmer on dewy 
 nights than the air; sometimes it was considerably so, for I once 
 observed it to be 10 warmer, at another time 9, and at a third 7i. 
 An exception will no doubt occur if very mild weather should 
 follow a long frost ; but of this I have had no experience. 
 
 In the experiments upon my housetop in London, I always found, 
 during clear and calm nights, wool lying on the wooden frame to be 
 colder than the air, at the same height ; but the difference was 
 seldom more than 3. On the evening, however, during which 
 dew formed there more copiously than usual, the difference was 5. 
 That the smallness of these differences was not wholly occasioned 
 by any thing special in the air of cities was afterwards proved, by 
 my finding others much greater, in a garden nearly in the middle 
 of London, from which almost the whole of the sky was visible. 
 
 Metals, likewise, furnish proofs of the connexion of dew with a 
 cold in the substance on which it forms, superior to that of the 
 neighbouring atmosphere. My observations, however, on the 
 temperature of metals, when exposed to the sky on dewy nights 
 were less numerous than those on several other subjects treated in 
 this Essay, by reason of the less frequent opportunity I enjoyed of 
 making them ; and many of those which I did make were after- 
 wards found by me to have been improperly conducted. I thought, 
 for instance, for some time, that the temperature of a metal, on a 
 dewy night, might easily be learned in the way in which I had 
 been accustomed to ascertain the temperature of dewed grass. But 
 
 3* 
 
33 ESSAY ON DEW. 
 
 observing dew one night on the glass tube of a thermometer, which 
 was lying on a metal placed upon grass, while the metal itself was 
 free from moisture, I conceived it probable, that the cold then indi- 
 cated by the thermometer was not the real temperature of the body 
 to which it was applied. To determine the point, I placed on the 
 same metal a second thermometer, covered with gilt paper, upon 
 which this was found at three observations to be 6^, 7, and 7 
 higher than the other. In this experiment, the bulb of the naked 
 thermometer from being very small did not project as far as the 
 outer surface of the scale, and, consequently, did not come in contact 
 with the metal. But even when the ball of a thermometer was 
 applied directly to a metal, on a clear and calm night, a temperature 
 was marked by it, commonly 2 and 3, and sometimes more degrees 
 less than that marked by a similar thermometer, inclosed in gilt 
 paper, and similarly placed. I found it likewise necessary, in this 
 inquiry, to correct the temperature of the air as given by a naked 
 thermometer. For, on still and serene nights, a thermometer in- 
 closed in a case of gilt or silvered paper, and suspended in the air 
 4 feet above the grassplat, was usually observed to be l^ or 2 
 higher than a bare thermometer of the same construction suspend- 
 ed near to it. The difference of two such thermometers, thus 
 placed, was once observed by me to be 2^, and once 85. It may be 
 thought, perhaps, that these differences were caused by the metalled 
 case obstructing the transmission of the temperature of the air to 
 the inclosed instrument. But that this was not the reason is shown 
 by my observing, that on cloudy nights there existed no difference 
 between the two thermometers ; that even on clear nights, a ther- 
 mometer contained in a case of white paper somewhat thicker than 
 the metalled was always nearly of the same temperature with a 
 naked one which was suspended close to it ; and that when a 
 difference did exist between the two latter, the thermometer in the 
 white paper case was commonly lower than the other. 
 
 The estimation of the heat, both of air and of metals, on a dewy 
 night, is liable to errors from other causes. As these, however, are 
 trifling, I shall not mention them, but proceed to state the results of 
 my observations, upon the temperature of metals exposed to the sky 
 at night, though unable to vouch for their entire accuracy. 
 
 Thin bright metallic plates, the least having a surface of 25 
 square inches, and some of them a surface of more that 100 such 
 inches, were several times observed, while lying on grass which 
 was attracting dew, to be 1 and 2, and once 3 degrees warmer than 
 the air 4 feet above them. At other times their temperature 
 was the same with that of the air. In both of these cases their 
 upper surfaces were always free from dew. Metals thus situated 
 were, consequently, often much warmer than the grass which sur- 
 rounded them. I made no experiments on this point, during the 
 nights on which occurred the greatest instances of cold on grass, 
 relatively to the temperature of the air; but I found, notwithstand- 
 ing, during one night, a metal on grass to be 10 warmer than the 
 
THE PHENOMENA OF DEW. 23 
 
 exposed grass near to it. On two other nights the differences were 
 9 and 8. The superiority of the heat of metals on grass over that 
 of the air, when it did exist, was evidently connected with the 
 temperature of the grass which they covered, and this again with 
 that of the earth under the same portion of grass; for this portion 
 was always a little warmer than the metal, but not so warm as the 
 earth. 
 
 On the other hand, metals, on which dew was forming while they 
 lay upon grass, were always colder than the air. In like manner, if 
 one metal upon the grassplat were dewed, while another similarly 
 situated remained dry, the former was always colder than the latter. 
 
 When a metal lying on the grassplat became dewed, the grass 
 under it was always colder than that under another metal, which 
 was undewed. 
 
 A metal, while receiving dew in consequence of being elevated 
 in the air, was always colder than a similar metal which remained 
 undewed on the grass. 
 
 The greatest instances of cold observed by me on metals, occurred 
 at times when other bodies near to them had become considerably 
 colder than the atmosphere. 
 
 The cold, however, contracted by metals from exposure to the 
 sky in a clear and still night, was always less than that of other 
 bodies similarly situated ; the greatest excess of cold ever observed 
 by me, in the larger metallic plates, from this cause, over that of the 
 air, being not more than 3 or 4 degrees. If much smaller pieces 
 were placed upon grass, the result was different. For I have found 
 a small thermometer placed in this situation, while inclosed in a 
 sheath of gilt paper, to be only 3 less cold than the surrounding 
 grass, during a night favourable to the production of cold on the 
 surface of the earth. 
 
 I collected only a few facts respecting the comparative tempera- 
 tures of different metals, when they were exposed together to the 
 sky on dewy nights ; but such as I did collect tend to prove, that 
 the most readily dewed metals become colder than the air, sooner 
 than those which receive dew with greater difficulty. 
 
 Many of the experiments which have been mentioned in this 
 section show, that when bodies which had been equally exposed to 
 the night air were examined at the same time, those which were 
 most dewed were also the coldest. No such correspondence, how- 
 ever, was found in the experiments of different nights, or even of 
 different parts of the same night. Thus, during two nights, on which 
 grass was 12 and 14 colder than the air, there was little dew ; 
 while on the night which afforded the most copious dew ever ob- 
 served by me, the cold possessed by the grass, beyond that of the 
 air, was for the most part only 3 and 4 ; and I have always seen 
 less dew about sunset than about sunrise, when the weather has been 
 <;alm and clear at both times, though there is commonly, in this 
 country at least, a greater difference between the temperature of 
 grass and of air in the evening than in the morning. I had early 
 
24 ESSAY ON DEW. 
 
 observed, also, bodies exposed to the sky, on a cloudy but calm 
 night, to be sometimes 2 or 3 colder than the air, without having 
 any appearance of dew; and when two metals possessing different 
 relations to dew were exposed together, I have seen the one, which 
 was the fitter to attract that fluid, colder than the other, though both 
 were dry. 
 
 I shall conclude this part of my Essay, with relating the results 
 of some experiments which were made for the purpose of ascertain- 
 ing the tendencies of various bodies to become cold upon exposure 
 to the sky at night. Unfortunately the weather was not always 
 favourable to my views ; but what occurred appears to me, notwith- 
 standing, worthy of being related. 
 
 In the observations hitherto given by me on the cold connected 
 with dew, the temperature of grass has been chiefly considered, 
 partly because my first experiments had been made upon it, and 
 partly from a wish, which arose afterwards, to compare my own 
 experiments with those of Mr. Six, which had been confined to that 
 substance. I found it, however, very unfit to furnish the means of 
 comparing the degrees of cold produced at night on the surface of 
 the earth at different times and places ; as its state on different 
 nights, on the same parts of the plat I commonly made use of, and 
 in different parts of the plat on the same nights, was often very 
 unequal, in point of height, thickness and fineness, all of which 
 circumstances influenced the degree of cold produced by it. I 
 observed, in consequence, a much greater uniformity in the results 
 of experiments made with various other bodies, whose condition 
 when first exposed to the air was always the same. Of these, the 
 most productive of cold were the filamentous and downy, as wool of 
 moderate fineness, very fine raw silk, very fine unspun cotton, fine 
 flax, and swandown, all of which were not only more steadily cold, 
 upon clear and calm nights, than grass, but also gave rise to a greater 
 degree of cold than was almost at any time observed upon it, even 
 in its best state. Among the bodies of this class, wool produced the 
 least cold, and I formerly mentioned that it attracted less dew than 
 silk, cotton, and flax. The last mentioned substances, and swan- 
 down, were found equal, or nearly so, in their tendency to become 
 cold. Swandown, however, exhibited the greatest cold rather more 
 frequently than any of the rest ; on which account, and from its 
 being more easily managed, as it was used while adhering to the 
 skin of the bird, I at length scarcely ever employed any other body 
 of the same class. On the night during which grass was observed 
 to be 14 colder than the air, swandown, lying upon a neighbouring 
 piece of grass, was still one degree lower. The difference of 15, 
 between the temperature at night of a body on the surface of the 
 earth, and that of the air a few feet above the earth is the greatest 
 which I have hitherto seen. 
 
 Fresh, unbroken straw, and shreds of white paper, though not 
 properly to be ranked among filamentous substances, were also found 
 to be a little more productive of cold than the wool which I used. 
 
THE PHENOMENA OF DEW. 25 
 
 The next class consisted of bodies in the state of a powder, more 
 or less fine. These were clean river sand, glass, chalk, charcoal, 
 lampblack, and a brown calx of iron. Chalk produced the least, and 
 the three last substances the greatest cold. They were all, however, 
 inferior in this respect to bodies of the first class. 
 
 Solid bodies, having a surface exposed to the sky of at least 25 
 inches square, formed a third class on which such experiments were 
 made. The particular substances of this description subjected to 
 trial, were glass, brick, cork, oak-wood, and wax ; all of which were, 
 likewise, found inferior to the filamentous substances. From these 
 last experiments it follows, that when a glass bulb of a thermometer 
 is applied at night to a body exposed to a clear sky, the temperature 
 exhibited by the instrument will not be accurately that of the body 
 in question, except the disposition of the latter to become cold in 
 such a situation, be the same as that of glass. An example of this 
 fact has been given in this Essay.* 
 
 My principal experiments, however, of this kind were made with 
 snow. 
 
 On the 25th of January, 181 3, the ground being then covered with 
 snow about an inch deep, I went to my usual place of experiment in 
 the country ; but, during 8 hours that I attended to my thermome- 
 ters, the whole sky was constantly overcast with clouds. The 
 atmosphere was, for the greater part of that time, very still, and a 
 thermometer on the snow was generally about 2 lower than another 
 in the air. That this difference was not owing to evaporation was 
 proved by the thermometer on the snow always rising from a half 
 to a whole degree whenever the air was a little moved, and falling 
 the same quantity as soon as a great stillness again took place. 
 
 I had no opportunity of renewing my observations upon snow 
 before the beginning of the present year, 1814. The state of my 
 health rendering it improper that I should incur much fatigue, or 
 be long exposed to night air, I restricted myself to the making a few 
 experiments in the large garden in Lincoln's-Inn Fields. I went 
 thither, for the first time, on the evening of the 4th of January, im- 
 mediately after a considerable snowfall had ceased, wishing to begin 
 my observations before any cold should arise on the snow's surface 
 from exposure to the sky. This was desirable on another account ; 
 for Mr. Kirwan, in direct opposition to indisputable facts most clearly 
 stated by Mr. Wilson, has said, that the great cold, observed by that 
 gentleman on snow, was occasioned by this substance having retained 
 the temperature of the high region from which it had fallen. t The 
 result of my inquiry was, that the surface of the snow and the air 
 4 feet above it had precisely the same heat. The depth of the snow 
 was 4 inches. 
 
 My next experiment took place on the evening of the 6th, the 
 intervening day having been snowy. The sky was clear, but the 
 air had a considerable motion. The heat of the atmosphere, at the 
 
 * Page 164. f On Temperatures, p. 30. 
 
26 ESSAY ON DEW. 
 
 height of 4 feet, was at 9h. 26 ; while that of the surface of the 
 snow, and of swandown lying upon it, was 22. The depth of the 
 snow was now about 5 inches. 
 
 On the 7th, a little after sunset the heat of the air in the garden 
 was 23, that of the surface of snow 19, but that of swandown lying 
 upon the snow only 15. There was then a gentle breeze ; some 
 parts of the sky were covered with clouds, and the lower atmo- 
 sphere was a little obscure. While the exposed surface of the snow 
 was 19, a part of its surface, which had been covered, about 20 
 minutes with a piece of pasteboard, was 22. Grass at the bottom 
 of the snow was 31, and the earth an inch beneath the grass 32. 
 
 After this there was no fit time for observation until the 13th. 
 The thermometers were exposed at 8h. On the evening of that 
 day, the sky being then without clouds ; but the stars were not 
 bright, and there was a perceptible motion in the air. At 8h. 
 the temperature of the air was 22^, that of the surface of the snow 
 13, and that of swandown lying on the snow 8. At 9h. the air 
 was 23^, snow 17, and swandown 15. The sky being now, in 
 great measure, covered with high thin clouds, my experiments 
 ceased. At lO^h.^the sky was very bright, and the atmosphere 
 very calm ; but it was not then convenient to me to renew my ob- 
 servations. Had I repeated them at that time I should probably 
 have found a difference, between the temperature of the swandown 
 and air, several degrees more considerable than the one of 145, 
 which had already occurred on this evening, and consequently 
 greater than the greatest observed by Mr. Wilson, between the 
 temperatures of snow and of the atmosphere, which was one of 16. 
 
 The next favourable evening was that of the 21st Much snow 
 having in the meanwhile fallen, its depth was now more than afoot. 
 The thermometers were observed 5 times between 4h. 15m. and 
 4h. 55m. At 4 of those times the swandown was 13, and at one 
 of them 13^, colder than the air, the heat of which at the 4 first 
 observations was 26, and at the last 25^. The temperature of the 
 surface of the snow, during the whole period of observation, was 17, 
 and consequently 4 times it was 4, and once 5, less cold than that 
 of the swandown. The atmosphere was altogether free from clouds, 
 and nearly quite calm, but a good deal hazy. 
 
 Before another proper evening arrived, my health became so 
 infirm that I was obliged to relinquish this pursuit. I conclude 
 therefore my account of it with two remarks. 1. If Mr. Wilson 
 had been accustomed to examine the temperature of swandown, or 
 any similar substance placed upon snow, he would probably have 
 observed a cold on the surface of the earth exceeding that of the 
 atmosphere by 20 or more, on the night of his actually observing 
 an excess of 16. 2. Since upon one evening when the atmosphere 
 was neither very clear nor very still, a difference of 14^ was found 
 by me between the temperatures of air and of swandown, which is 
 only ^ a degree less than the greatest difference I have ever observed 
 between the same substances on the stillest and clearest nights in 
 
THE THEORY OF DEW. 27 
 
 summer, a corroboration is hence derived of a conclusion, made by 
 Mr. Six from his experiments, that the greatest differences at night, 
 in point of temperature, between bodies on the surface of the earth 
 and the atmosphere near to it, are those which take place in very 
 cold weather. 
 
 PART II. 
 
 OF THE THEORY OF DEW. 
 
 DEW, according to Aristotle,* is a species of rain, formed in the 
 lower atmosphere, in consequence of its moisture being condensed 
 by the cold of the night into minute drops. Opinions of this kind, 
 respecting the cause of dew, are still entertained by many persons, 
 among whom is the very ingenious Mr. Leslie, of Edinburgh. t A 
 fact, however, first taken notice of by Gersten, who published his 
 treatise on dew, in 1733, proves them to be erroneous; for he found 
 that bodies a little elevated in the air often become moist with dew, 
 while similar bodies lying on the ground remain dry, though, 
 necessarily from their position, as liable to be wetted by whatever 
 falls from the heavens as the former. 
 
 Shortly after the appearance of Gersten's treatise, Musschenbroek 
 made the remark, already mentioned in this Essay, that metals will 
 be free from dew while other bodies attract it copiously. This 
 philosopher contented himself with publishing his discovery ; but 
 his friend Dufay concluded from it, that dew is an electric pheno- 
 menon, since it leaves untouched the bodies which conductelectricity, 
 while it appears upon those which cannot transmit that influence. 
 If dew, however, were to form on the latter only, its quantity 
 would never be sufficiently great to admit its being distinctly seen ; 
 for the nonconductors, as soon as they became in the least moist, 
 would be changed into conductors. Charcoal, too, it is now known, 
 though the best solid conductor of electricity after the metals, 
 attracts dew very powerfully ; and in the last place, contrary to the 
 assertion of Dufay, dew frequently forms upon metals themselves. 
 
 Other authors have ascribed the production of dew to electricity, 
 for reasons different from that of Dufay. But there are several 
 considerations, which seem to me to prove that no such opinion 
 can be just. 1. When dew is produced in a clear atmosphere, the 
 portion of air by which it is deposited must necessarily be unable, 
 at that moment to retain in a state of pellucid vapour, all the mois- 
 ture which it had immediately before held in that form. But I 
 know of no experiment, which shows that air, by becoming posi- 
 
 * Meteor. Lib. 1. c. x. et De Mundo. c. iii. 
 
 f Relations of Heat and Moisture, p. 37, and 132. 
 
1 $ 
 
 28 ESSAY ON DEW. 
 
 tively electrical, which is said to be its condition on the evenings 
 during which dew is most abundant, is rendered less able than 
 it had previously been to contain watery vapour in a state of 
 transparency. 2. Bodies in similar circumstances, as far as elec- 
 tricity is concerned, acquire very different quantities of dew. 
 Wool placed on the raised board, for example, attracted very much 
 more dew than wool attached to the lower side of the same board, 
 and even considerably more than the same substance freely suspend- 
 ed in the air, and entirely exposed to the sky. 3. Dew forms in 
 different parts of the night, in quantities no way proportioned to 
 the degrees of electricity found in the atmosphere at the same times. 
 Thus it is commonly more copious in the morning than in the 
 evening, notwithstanding that the air is observed to be in the latter 
 season more highly electrical than in the former. 4. I have several 
 nights held a glass bottle upon which dew was forming close to the 
 top of a Bennett's electrometer, which had been previously kept in 
 a dry place ; but I never saw the slips of gold leaf to move in con- 
 sequence. It is very probable, however, that more refined experi- 
 ments will show that electrical appearances attend the production 
 of dew. These, perhaps, accompany every change in the chemical 
 form of bodies. But the facts which have been stated, seem suffi- 
 cient to establish that any such appearances which may be hereafter 
 remarked during the formation of dew, must be considered as 
 effects, and not as the cause, of the conversion of the watery 
 vapour of a clear atmosphere into a fluid. 
 
 A remaining argument applies equally to all the theories, which 
 have hitherto been made public on the cause of dew. This is, that 
 none of them include the important fact, that its production is 
 attended with cold ; since no explanation of a natural appearance 
 can be well founded, which has been built without the knowledge 
 of one of its principal circumstances. It may seem strange to 
 many, that neither Mr. Wilson nor Mr. Six applied this fact to the 
 improvement of the theory of dew. But according to their view 
 of the subject no such use could have been made of it by them, as 
 they held the formation of that fluid to be the cause of the cold 
 observed with it. I had many years, as was formerly mentioned, 
 held the same opinion ; but I began to see reason, not long after my 
 regular course of experiments commenced, to doubt its truth, as I 
 found that bodies would sometimes become colder than the air 
 without being dewed ; and that, when dew was formed, if different 
 times were compared, its quantity, and the degree of cold which 
 appeared with it, were very far from being always in the same 
 proportion to each other. The frequent recurrence of such obser- 
 vations, at length converted the doubt of the justness of my ancient 
 opinion into a conviction of its error, and at the same time occa- 
 sioned me to conclude, that dew is the production of a preceding 
 cold in the substances upon which it appears. Wishing, however, 
 to obtain proofs, more striking in degree, of the validity of these 
 inferences, than such as had been afforded to me by casual observa- 
 
THE THEORY OF DEW. 29 
 
 tion while attending to other parts of my subject, I instituted the 
 experiments which will be next related. 
 
 I had frequently remarked, early in the evening, a considerable 
 degree of cold on substances exposed in calm weather to a clear sky, 
 and I had also sometimes seen, early in the evening, the raised board 
 altogether dry, while the grass was much moistened. I therefore 
 determined to make the experiments in view on the raised board, and 
 to commence them as soon as the sun should cease to shine upon it. 
 The first day I went to the country for this purpose, the 19th of 
 August, 1813, almost every circumstance was favourable to its 
 completion. There had been no rain for three weeks ; the wind 
 was northerly ; and the barometer was rising ; all which indicated, 
 that the atmosphere contained little moisture. The air too was 
 extremely still. The only appearance in the least unfavourable was, 
 that the sky was not entirely free from clouds ; but these were few, 
 of small extent, thin, and high. 
 
 At 6h. 25m. immediately after the sun had ceased to shine upon 
 the spot, where my experiments were to be carried on, though the 
 time of its setting was still 47 minutes distant, I placed upon the 
 raised board 10 grains of woo), and a small bag, made of the skin 
 of a swan's breast with the down adhering, and stuffed with wool, the 
 whole weighing nearly 5 drachms. On each of these substances the 
 naked bulb of a small and delicate thermometer was laid. A simi- 
 lar thermometer, with its bulb also naked, was suspended in the air 
 over the grassplat, at the same height with the board. Two ther- 
 mometers were placed in other situations, as will be seen in the 
 annexed table. After an exposure of 20 minutes, the wool was 7 
 colder than the air, but the swandown bag only 6, no doubt in 
 consequence of its comparatively great quantity of matter. Neither, 
 however, had gained the least weight, according to the scales 
 employed by me, which were sensibly moved by the 16th of a 
 grain. These observations were repeated several times during the 
 following hour, as will be seen by the table, at none of which, 
 except the last, was either the wool or swandown found in the least 
 heavier, than when first placed on the board. At this last observa- 
 tion, the wool, though 9| colder than the air, was still without any 
 increase in weight ; but the swandown which was 1 colder than 
 the wool, had gained i a grain. My experiments now properly 
 ceased; but having suffered the thermometers, which had been 
 placed on the wool and swandown, and in the air, to remain in those 
 situations, .1 examined them again at Sh. 45m., that is, 2h. 20m. 
 after they had been first exposed. The wool, which was still 9 
 colder than the air, had gained somewhat less than I a grain ; and 
 the swandown, which was now 11^ colder than the air, had gained 
 2 grains, including the 5 grain already mentioned. When these 
 last observations were made, the sky was entirely cloudless, and 
 the atmosphere very calm. 
 
 OCT. 1838.- S 4 
 
30 
 
 ESSAY ON DEW. 
 
 TABULAR VIEW OP OBSERVATIONS ON THE EVENING OF 
 AUGUST 19, 1813. 
 
 
 6h. 45m. 
 
 7h. 
 
 7h. 20m. 
 
 7h. 40m. 
 
 8h. 45m. 
 
 Heat of air 4 feet above the grass, 
 
 60io 
 
 60 
 
 59 
 
 58 
 
 54 
 
 wool on the raised board, 
 
 53 
 
 54 
 
 51$ 
 
 48i 
 
 44$ 
 
 swandown on the same, 
 
 54 
 
 53 
 
 51 
 
 47$ 
 
 42^ 
 
 surface of the raised board, 
 
 58 
 
 57 
 
 55 
 
 
 
 grassplat,* - 
 
 53 
 
 51 
 
 49 
 
 49 
 
 42 
 
 Similar experiments made at the same place, on the evenings of 
 the 25th of August and 17th of September, in the same year, had 
 results, which were also similar but less in degree ; the greatest 
 difference between the temperature of wool or swandown, while 
 they were without any increase of weight, and the temperature of 
 the air having been, on the first of those evenings, only 4, and on 
 the second only 5. The reasons were, in great measure, if not 
 wholly, that a considerable part of the sky was covered with clouds, 
 and that the air was commonly in that state of motion which is 
 denominated a gentle breeze. 
 
 On the evening of my first experiments, I had omitted to measure 
 the heat of the raised board, before the thermometers were placed 
 upon it. This was attended to on the two latter evenings, on the 
 first of which its upper surface was found, at the commencement of 
 the experiments, 4 warmer than the air ; on the second, both it 
 and the air were of the same temperature. Again ; on the first of 
 the latter evenings, 10 grains of wool, to which three grains of 
 water had been added, having been laid on the raised board, near 
 the thermometers ; at the end of 45 minutes the parcel was found 
 to have lost 2% grains of moisture by evaporation during the time 
 that dry wool had become several degrees colder than the air. 
 
 A fourth experiment of this kind was made by me on the 7th of 
 January, 1814, in the garden of Lincoln's-Inn-Fields, by placing 
 10 grains of wool on a sheet of pasteboard, which lay upon the 
 snow. At the end of 35 minutes the wool was 5 colder than the 
 air, without possessing any additional weight. 
 
 I took advantage of being in the country, at the distance of a few 
 miles from London , on the 2 1 st of the present month, the last day but 
 one of an unusually long' tract of dry weather, to expose to the sky, 28 
 minutes before t sunset, weighed parcels of wool and swandown, upon 
 a smooth, unpainted, and perfectly dry fir-table, 5 feet long, 3 broad, 
 and nearly 3 in height, which had been placed an hour before in the 
 sunshine, in a large level grass-field. At this time, and throughout 
 my experiments, the air was very still, and the sky very serene. 
 
 * In these experiments, contrary to what usually happens, the grass was 
 almost constantly colder than the filamentous substances, although they were 
 placed upon the raised board. 
 
, THE THEORY OF DEW. 31 
 
 The atmosphere, too, in all probability, contained but little moisture 
 in consequence of the long absence of rain ; and the surface of the 
 ground apparently contained none. The wool, 12 minutes after 
 sunset, was found to be 14 colder than the air, the temperature of the 
 latter being measured by a naked thermometer suspended 4 feet above 
 the ground, and to have acquired no weight. The swandown, the 
 quantity of which was much greater than that of the wool, was at 
 the same time 13 colder than the air, and was also without any 
 additional weight. In 20 minutes more, the swandown was 14^ 
 colder than the neighbouring air, and was still without any increase 
 of its weight. My experiments now ceased from a failure of daylight. 
 
 In my former experiments of this kind, the greatest cold observed 
 by me from radiation, without the appearance of dew, was only 9. 
 
 While making the experiments on wool and swandown, I attended 
 frequently to the temperature of the grass, and found it at one time 
 15 colder than that of the air 4 feet above the ground. This differ- 
 ence is 1 greater than any I had ever before seen between the 
 temperatures of the same substances, and is equal to the greatest 
 which I had ever known to occur, between those of the atmosphere 
 and of swandown lying upon grass. I had this evening placed no 
 swandown upon grass. 
 
 Having thus shown the justness of my former conclusion, that 
 the cold observed with dew is the previous occurrence, and conse- 
 quently, that the formation of this fluid has precisely the same im- 
 mediate cause, as the presence of moisture upon the outside of a 
 glass or metallic vessel, when a liquid considerably colder than the 
 air has been poured into it shortly before ; I shall next apply this 
 fact to the explanation of several atmospherical appearances. 
 
 I. The variety in the quantities of dew, which were found by 
 me upon bodies of the same kind, exposed to the air during the 
 same time of the night, but in different situations, is now seen to 
 have been occasioned by the diversity of temperature which existed 
 among them. 
 
 II. Agreeably to the opinion of Mr. Wilson and Mr. Six, the 
 cold connected with dew ought always to be proportional to the 
 quantity of that fluid ; but this is contradicted by experience. On 
 the other hand if it be granted, that dew is water precipitated from 
 the atmosphere, by the cold of the body on which it appears, the 
 same degree of cold, in the precipitating body, may be attended with 
 much, with little, or with no dew, according to the existing state 
 of the air in regard to moisture ; all of which circumstances are 
 found actually to take place. 
 
 III. The formation of dew, indeed, not only does not produce 
 cold, but like every other precipitation of water from the atmosphere 
 produces heat. I infer this, partly because very little dew appeared 
 upon the two nights of the greatest cold I have ever observed on 
 the surface of the earth, relatively to the temperature of the air, 
 both of them having occurred after a long tract of dry weather ; and 
 partly from the most dewy night, which I have ever seen, having 
 
32 ESSAY ON DEW. 
 
 been attended during the greater part of it with no considerable 
 degree of cold. On this night the difference between the tempera- 
 tures of grass and of air was at first 7$, the dew being then not 
 very abundant. But after the dew had become very abundant, the 
 difference of those temperatures never exceeded 4, and was fre- 
 quently only 3. 
 
 With the view of obtaining, though indirectly, some knowledge 
 of the quantity of cold, which had been prevented, by the formation 
 of dew, from appearing on the surface of the earth in the night just 
 spoken of, I made the following experiment. To 10 grains of wool 
 having the same form and extension as the parcels employed for 
 the collection of that fluid, were added 21 grains of water, this 
 being the quantity of moisture, which had been attracted by 10 
 grains of wool, lying on the grassplat, in the space of 8 hours on 
 that night. The wet wool having been then placed in a china 
 saucer, laid on a feather-bed in the room, the door and windows of 
 which were shut, its heat during the following 8 hours was, at fre- 
 quent examinations, uniformly found to be about 4 less than that 
 of a dry china saucer on the same bed ; the temperature of the air 
 in the room not having altered more than a degree in the course 
 of the experiment. At the end of the 8 hours, the wool still re- 
 tained 2^ grains of moisture. If this quantity had also evaporated 
 the cold uniformly produced during 8 hours would, in all probability, 
 have been about 4^ From this experiment, therefore, 1 think it 
 may be inferred, that the mean quantity of cold, which was pre- 
 vented, by the formation of dew, from appearing on the ground, 
 during the night which has been mentioned, was also about 4^. 
 But as the production of dew, during some parts of the night, was 
 at at a greater rate than that of 21 grains for eight hours, 1 or 2 
 degrees may be added for those times, which will raise the effect of 
 the dew in diminishing the appearance of cold during them to about 
 6, on the supposition, which cannot be far from the truth, that dew 
 had been attracted as copiously by the grass, as by wool which lay 
 upon it. 
 
 The less difference commonly observed between the temperatures 
 of grass and of air in the morning than what occurs in the evening, 
 is likewise to be, in part, attributed to a greater quantity of dew 
 appearing in the former than in the latter season. 
 
 A more remarkable fact, deriving an explanation from the same 
 source, is the greater difference which takes place in very cold 
 weather, if it be calm and clear, between the temperatures of the air 
 and of bodies on the earth, at night, than in equally clear and calm 
 weather in summer ; since, in very cold weather, any diminution 
 of the temperature of a portion of air, in contact with a cold body, 
 will be attended, in consequence of the well known relations of the 
 atmosphere to moisture, with a much less formation of water than 
 an equal diminution would be in summer, supposing the air, before 
 it touches the cold body, to be at both times equally near to its 
 point of repletion with moisture. 
 
THE THEORY OF DEW. 33 
 
 IV. In very calm nights, a portion of air which comes in contact 
 with cold grass, will not, when the surface is level, immediately 
 quit it, more especially as this air has become specifically heavier 
 than the higher, from a diminution of its heat, but will proceed hori- 
 zontally, and be applied successively to different parts of the same 
 surface. The air, therefore, which makes this progress, must at 
 length have no moisture to be precipitated, unless the cold of the 
 grass which it touches should increase. Hence in great measure is 
 to be explained, why on such nights as have been just mentioned, 
 more dew was acquired by substances placed on the raised board, 
 than by others of the same kind on the grass, though it began to 
 form much sooner in the latter than in the former situation, those 
 on the raised board having received air, which had previously de- 
 posited less of its moisture. 
 
 A reason is now also afforded, why a slight agitation of the atmo- 
 sphere, when very pregnant with'jnoisture, should increase the quan- 
 tity of dew ; since fresh parcels of air will hence be more frequently 
 brought into contact with the cold surface of the earth than if the 
 atmosphere were entirely calm. 
 
 V. Dew, in agreement with the immediate cause which has been 
 assigned by me for its production, can never be formed, in temperate 
 climates, upon the naked parts of a living and healthy human body 
 during the night; since their heat is never less in this season, in 
 such climates, than that of the atmosphere. I have, in fact, never 
 perceived dew on any naked part of my own body at night, though 
 my attention was much occupied, for three years, with every thing 
 relative to this fluid, and though I had been during that period, 
 much exposed to the night air. On the other hand, in very hot 
 countries the uncovered parts of a human body may sometimes, 
 from being considerably colder than the air, condense the watery 
 vapour of the atmosphere, and hence be covered with a real dew, 
 even in the day-time. 
 
 VI. Hygrometers formed of animal or vegetable substances, when 
 exposed to a clear sky at night, will become colder than the atmo- 
 sphere ; and hence by attracting dew, or according to an observation 
 of Saussure,* by merely cooling the air contiguous to them, mark a 
 degree of moisture beyond what the atmosphere actually contains. 
 This serves to explain an observation made by Mr. De Luc,t that 
 in serene and calm weather, the humidity of the air, as determined 
 by an hygrometer, increases about, and after sunset with a greater 
 rapidity, than can be attributed to a diminution of the general heat 
 of the atmosphere. 
 
 -These examples are sufficient to show the value of the fact, that 
 bodies become colder than the neighbouring air, before they are 
 dewed, in explaining many atmospherical appearances. To this 
 point, the investigation of the cause of dew might have been carried 
 at any time since the invention of thermometers ; but its complete 
 
 * Hygronometrie, p. 25. f Introduction a la Physique Terrestre, II. 491. 
 
 4* 
 

 34 ESSAY ON DEW. 
 
 theory could not possibly, in my opinion, have been attained before 
 the discoveries on heat were made, which are contained in the 
 works of Mr. Leslie and Count Rumford. 
 
 The experience of most persons, respecting the communication 
 of heat among bodies in the open air, is confined to what happens 
 during the day ; at which time, those that are situated near to one 
 another are always found to possess the same temperature, unless 
 some very evident reason for the contrary should exist. To many, 
 therefore, it may appear incredible that a perfectly dry body, placed 
 in contact on all sides, with other bodies of the same temperature 
 with itself, shall afterwards without undergoing any chemical change, 
 become much colder than they are, and shall remain so for many 
 hours; yet these circumstances are found to occur in substances at- 
 tractive of dew, when laid on the surface of the earth, in a still and 
 serene night, and are in perfect agreement with the doctrine of heat, 
 now universally admitted to be just. 
 
 To render this more easy of apprehension, let a small body which 
 radiates heat freely, and possesses a temperature in common with 
 the atmosphere, higher than 32, be placed while the air is clear and 
 still, on a slow conductor of heat lying on the surface of a large open 
 plain, and let a firmament of ice be supposed to exist at any height 
 in the atmosphere; the consequence must be, that the small body 
 will from its situation quickly become colder than the neighbouring 
 air. For, while it radiates its own heat upwards, it cannot receive 
 a sufficient quantity from the ice to compensate this loss; little also 
 can be conveyed to it from the earth, as a bad conductor is interposed 
 between them ; and there is no solid, or fluid except the air, to com- 
 municate it laterally either by radiation or conduction. This small 
 body, therefore, unless it shall receive from the air nearly as much 
 heat as it has emitted, which, considering the little that can be com- 
 municated from one part of the atmosphere to another in its present 
 calm state, must be regarded as impossible, will become colder than 
 the air, and condense the watery vapour of the contiguous parts of 
 it, if they should contain a sufficient quantity to admit of this effect. 
 But events similar to these occur when dew appears in an open and 
 level grass field during a still and serene night. The upper parts of 
 the grass radiate their heat into regions of empty space, which con- 
 sequently send back no heat in return ; its lower parts from the 
 smallness of their conducting power, transmit little of the earth's 
 heat to the upper parts, which at the same time receiving only a 
 small quantity from the atmosphere, and none from any other lateral 
 body, must remain colder than the air, and condense into dew its 
 watery vapour, if this be sufficiently abundant, in respect to the de- 
 creased temperature of the grass.* 
 
 * I have adopted in this explanation the hypothesis of Mr. Prevost of Geneva, 
 on the constant radiation of heat by bodies in contact with the atmosphere, even 
 at the time .that they are exposed to the influence of bodies warmer than them- 
 selves; as it appears to agree perfectly with all the phenomena of the communi- 
 cation of heat, which do not depend upon conduction. I shall hereafter make 
 frequent use of this hypothesis. 
 
THE THEORY OF DEW. 35 
 
 / ' *, "' 
 
 This subject may be further illustrated by a reference to what 
 happens in the experiment, which has been used to prove the reflec- 
 tion of cold. 
 
 In the simplest form of this experiment, a small body, the bulb of 
 a thermometer, possessing the temperature of the atmosphere, is 
 placed before a larger cold body, rendered equal in effect to one still 
 larger, by means of a concave metallic mirror. In this situation, 
 the small body radiates heat to the larger, without receiving an 
 equivalent from it, and, in consequence, becomes colder than the air 
 through which its heat is sent, notwithstanding that it is continually 
 gaining some heat, both from the air which surrounds it, and from 
 the walls and contents of the apartment in which the experiment 
 is made. Dew, therefore, would as readily form upon the thermo- 
 meter in this experiment, as it would upon one suspended in the 
 open air at night, under a clear sky, provided that the two instru- 
 ments were equally colder than the atmosphere, and that this was 
 in both cases equally near to being replete with moisture.* 
 
 Regarding now as established, that bodies situated on or near to 
 the surface of the earth become, under certain circumstances, colder 
 than the neighbouring air, by radiating more heat to the heavens 
 than they receive in every way,t I shall in the first place offer a 
 few remarks on the extent and use of this occurrence, and shall 
 afterwards apply the knowledge of it to the explanation of several 
 more of the appearances described in the former part of this Essay, 
 and of some others which have not hitherto been mentioned by me. 
 
 Radiation of heat by the earth to the heavens must exist at all 
 times ; but, if the sun be at some height above the horizon, the 
 degree of which is hitherto undetermined, and probably varies 
 according to season, and several other circumstances, the heat emit- 
 ted by it to the earth will overbalance, even in places shaded from 
 
 * The invention of this experiment having been ascribed a few years ago to Mr. 
 Pictet of Geneva, various English writers have shown, that it occurs in several 
 much older foreign authors. But I have not seen any mention made of its having 
 been also long since known in this country. That it was so appears from the 
 following extract of a letter, written by Mr. Oldenburgh to Mr. Boyle in 1665. 
 " I met the other day in the Astrological Discourse of Sir Christopher Heydon, 
 with an experiment, which he affirms to have tried himself, importing, that cold 
 accompanies reflected light, by employing burning spherical concaves, or para- 
 bolical sections, which, he saith, will as sensibly reflect the actual cold of snow 
 or ice, as they will the heat of the sun." Boyle's Works, folio, vol. v. p. 345. 
 
 | Count Rumford offered the following conjecture, in a paper printed in the 
 Philosophical Transactions for 1804. " The excessive cold which is known to 
 reign, in all seasons on the tops of very high mountains, and in the higher regions 
 of the atmosphere, and the frosts at night, which so frequently take place on the 
 surface of the plains below, in very clear and still weather, in spring and autumn, 
 seem to indicate, that frigorific rays arrive continually at the surface of the earth, 
 from every part of the heavens." But he gave no experiments to prove, that 
 such a communication actually exists between the heavens and the earth at night. 
 Neither does it appear from any of his writings which I have seen, that he ever 
 supposed, that the surface of the earth is more cooled by these frigorific rays, 
 than the air through which they pass, or that some solid bodies are more cooled 
 by them than others. 
 
36 ESSAY ON DEW. 
 
 ,. 
 
 its direct beams, that which the earth radiates upwards. I sus- 
 pended at midday, on the 24th of July, 1813, in the open air over 
 a grassplat, while the sky was wholly covered with very dense 
 clouds, and the weather calm, two delicate thermometers, one of 
 which was naked, but the other cased in gold paper. At two ob- 
 servations, having an interval of 10 minutes between them, the ther- 
 mometer in the gilt case was 2 lower than that which was naked. 
 A white paper case was then drawn over the gilt one, upon which, 
 after 5 minutes, the covered instrument was observed to be at the 
 same height with the naked. The outer white case having, in the 
 next place, been taken from the covered thermometer, but that which 
 was gilt suffered to remain, the two instruments were in a few mi- 
 nutes found again to differ 2. A thermometer on the grassplat was, 
 during these experiments, higher than the naked instrument in the 
 air by 2, and than that in the gilt case by 4. It is evident, therefore, 
 that heat radiated by the sun must, on this day, have been transmit- 
 ted in considerably quantity through the thickest clouds ; since not 
 only was the earth's surface warmer than the air, but a small body, 
 covered with a substance not readily admitting the entrance of 
 radiant heat was colder than a similar body which was uncovered. 
 In like manner, I observed at noon, on the 2d of January, 1814, 
 during the prevalence of a dense fog, a thermometer placed upon 
 swandown, which was lying upon grass thickly incrusted with 
 hoarfrost, to be 2 warmer than the air, and 1 warmer than the 
 grass.* 
 
 In a calm and serene night, however, when consequently little 
 impediment exists to the escape, by radiation, of the earth's heat to 
 the heavens, and when no heat can be radiated by the sun to the 
 place of observation, an immense degree of cold would occur on the 
 ground, if the following circumstances did not combine to lessen it. 
 
 1. The incapacity of all bodies to prevent, entirely, the passing of 
 heat, by conduction, from the earth to substances placed upon them. 
 
 2. The heat radiated to these substances by lateral objects. 3. The 
 heat communicated to the same substances by the air. 4. The heat 
 which is evolved, during the condensation of the watery vapour of 
 the atmosphere into dew. 
 
 The extent of the effect of all these checks upon the production 
 of cold, by the nightly radiation of heat from bodies on the surface 
 of the earth, cannot, in the present state of our knowledge, be pro- 
 perly estimated ; but facts show that, notwithstanding theiroperation, 
 the cold originating in this source must be often very considerable. 
 
 1. Mr. Wilson once observed a difference of 16, from this cause, 
 between the temperatures of snow and of air. In taking the latter 
 temperature, however, he employed a naked thermometer, on which 
 account, in consequence of what has already been mentioned by me, 
 
 * Another fact of the same kind, which occurred at the same time, is that, 
 although the temperature of the air was 30, the hoarfrost on trees rapidly de- 
 creased, the solid matter of the trees intercepting radiant heat, which had pene- 
 trated through the fog from the sun, and converting it into heat of temperature. 
 

 THE THEORY OF DEW. 37 
 
 about 2 are to be added to the 16 noted by him, in order to obtain 
 the real difference between the heat of the snow and the air at that 
 time.* 
 
 2. If Mr. Wilson, as was formerly said, had laid a thermometer 
 on any downy substance in contact with the snow, he would, in all 
 probability, have found a cold indicated by it at least 20 greater 
 than that of the air, as marked by a naked instrument, and conse- 
 quently at least 22 greater than the real cold of the surrounding 
 atmosphere. 
 
 3. Mr. Wilson's place of observation was not very favourable to 
 the occurrence of a great cold, from radiation of heat at night, it 
 being near to a large smoky city, in the immediate vicinity also, as 
 appears to me from what he says of it, of one or more considerable 
 buildings, and in a climate abounding in moisture. 
 
 4. None of Mr. Wilson's experiments, in which a very great 
 degree of cold occurred, were made within an hour or two after 
 sunset, during which time, according to my observation, the most 
 considerable differences between the temperatures of the air, and of 
 bodies on the surface of the earth, commonly happen. 
 
 If, then, such experiments should be made in an atmosphere still 
 colder than that in which Mr. Wilson made his, on a large plain 
 remote from any city, and free from objects of every kind that are 
 elevated above the ground, and in a country remarkable for the 
 dryness of its air, all which circumstances may be found in Russia 
 during the winter ; a difference of at least 30 would probably ap- 
 pear, on some still and serene night, between a small thermometer 
 placed with its bulb naked,t on the middle, or leeward side of a 
 stratum of a downy substance, occupying a space upon a grass field, 
 or bed of snow, one or two square yards in extent, and a similar 
 thermometer inclosed in a case of gilt paper, and suspended in the 
 air a few feet above the other. Two thermometers, thus placed, 
 would, I think, be sometimes found even in this country to differ 
 not much less than 30. I have myself never made any such ex- 
 periments with a downy substance, which had a surface of more 
 than a few square inches, or in a very cold night, when the atmo- 
 sphere was clear and calm, and the scene of observation remote from 
 large masses of building. 
 
 But even a cold of 30 appears not to be the greatest that can be 
 thought to occur from the radiation of heat to the heavens, at night, 
 by substances on the surface of the earth. For experiments by Mr. 
 Pictet, J Mr. Six, and I may add by myself, establish that, in ex- 
 
 * As bright metals, when suspended in the air, and exposed to a clear sky on 
 a calm night, become colder than the surrounding atmosphere, a thermometer 
 covered with metalled paper, and placed in the circumstances which have been 
 just mentioned, will mark a temperature less than that of the air near to it. But, 
 as the difference must be small, and as I know of no way to estimate it accurately, 
 I have hitherto always neglected to consider it. 
 
 f The effect would, perhaps, be a little increased, by covering the bulb with a 
 very thin layer of lamp-black. 
 
 | Essai sur le Feu, c. x. Phil. Trans. 1784, and 1788. 
 
38 ESSAY ON DEW. 
 
 ception to the common rule, the heat of the atmosphere in clear and 
 calm nights increases with the distance from the earth. Agreeably 
 to Mr. Six's experiments, the atmosphere at the height of 220 feet 
 is often, upon such nights, 10 warmer than what it is 7 feet above 
 the ground. If, therefore, I am able to show, as I expect I shall be 
 in the course of a few pages, that the air at the smaller height be- 
 comes colder than that of the greater, from its vicinity to the surface 
 of the earth, previously rendered cold by radiating its heat to the 
 heavens, it will follow, that these 10 must be added to the quantity 
 of cold already mentioned ; and, consequently, that a body on the 
 ground may become at night, at least 40 colder than the air two or 
 three hundred feet above it, by the radiation of its heat to a clear 
 sky, 
 
 I shall add, with the greatest diffidence, a few words upon a final 
 cause of the radiation of heat from the earth, at night, and upon 
 some of the circumstances which modify its action, though fully 
 conscious of the danger of error which is always incurred in the 
 attempt to appreciate the works of our Creator. 
 
 The heat which is radiated by the sun to the earth, if suffered to 
 accumulate, would quickly destroy the present constitution of our 
 globe.* This evil is prevented by the radiation of heat by the 
 earth to the heavens, during the night, when it receives from them 
 little or no heat in return. But through the wise economy of 
 means, which is witnessed in all the operations of nature, the pre- 
 vention of this evil is made the source of great positive good. For 
 the surface of the earth, having thus become colder than the 
 neighbouring air, condenses a part of the watery vapour of the 
 atmosphere into dew, the utility of which is too manifest to require 
 my speaking of it. I may remark, however, that this fluid appears 
 chiefly where it is most wanted, on herbage and low plants, avoid- 
 ing, in great measure, rocks, bare earth, and considerable masses of 
 water.t Its production, too, by another wise arrangement, tends to 
 prevent the injury that might arise from its own cause ; since the 
 precipitation of water upon the tender parts of plants, must lessen 
 the cold in them which ocsasions it. I shall observe in the last 
 
 * Count Rumford says ; " May it not be by the action of these (frigorific) 
 rays, that our planet is cooled continually, and enabled to preserve the same 
 mean temperature for ages, notwithstanding the immense quantities of heat that 
 are generated at its surface, by the continual action of the solar rays 1" Phil. 
 Trans. 1804, p. 181. 
 
 f I have no direct observations for the foundation of this assertion concerning 
 considerable masses of water. But, I hold it, notwithstanding, to be just; 
 because, as soon as the surface of the water is in the least cooled by radiation, 
 the particleSy composing it must fall downwards, from their increased gravity, 
 and be replaced by others that are warmer. The whole mass, therefore, can 
 never, in the course of a single night, be sufficiently cool to condense into dew 
 any great quantity of the watery vapour of the atmosphere. Besides; I 
 have found, that even a small mass of water, as will be more particularly men- 
 tioned in the last part of this essay, sometimes acquires no weight from the 
 reception of dew, in the space of a whole night favourable to the formation of 
 that fluid. 
 
THE THEORY OF DEW. 39 
 
 place, that the appearance of dew is not confined to any one part of 
 the night, but occurs during its whole course, from means the most 
 simple and efficacious. For after one part of the air has deposited 
 its moisture on the colder surface of the earth, it is removed, in 
 consequence of that agitation in the atmosphere which exists during 
 its stillest states, and gives place to another having its quantity of 
 water undiminished ; and again, as the night proceeds, a portion of 
 air which had before deposited all the moisture, which circumstan- 
 ces at that time permitted, is rendered fit, by the general increase of 
 the cold of the atmosphere, to give out a fresh parcel, when it comes 
 anew into contact with the ground. 
 
 I. The first fact which I shall here attempt to explain, is the 
 prevention, either wholly or in part, of cold from radiation, in sub- 
 stances on the ground, by the interposition of any solid body be- 
 tween them and the sky. This evidently appears to arise in the 
 following manner. The lower body radiates its heat upwards, as if 
 no other intervened between it and the sky ; but the loss, which it 
 hence suffers, is more or less compensated by what is radiated to it 
 from the body above, the under surface of which possesses always 
 the same, or very nearly the same temperature as the air. In this 
 way, therefore, is to be accounted for the warmth of the substances, 
 which were sheltered from the sky by the raised board, the paste- 
 board roof, and the hollow cylinders of earth and pasteboard. In 
 these examples, the interposed substances cannot be supposed to 
 have remitted more heat than they received. But in situations 
 where large masses of bare solid matter exist, which are warmer 
 than the atmosphere, from the heat of the preceding day or other 
 causes, a greater heat will be received by the exposed body than 
 what is radiated by itself. For example, it seems certain to me, 
 that the houses surrounding Lincoln's-Inn Fields had an influence 
 upon my thermometers, during my experiment there at night, 
 beyond what arose from their merely returning a quantity of heat, 
 equivalent to that which they received from the surface of the 
 garden. It is not, however, absolutely requisite that a body should 
 be itself exposed to the sky On a clear and calm night, in order to 
 become colder than the atmosphere ; exposure to the influence of 
 another body, so situated, is sufficient for the production of a slight 
 degree of this effect. Thus I have always found wool attached to 
 the underside of my raised board, on such a night, to be a little colder 
 than the air ; and it has appeared to me a sufficient reason for the 
 fact, that the wool in this situation was in some degree exposed to 
 the influence of grass, which had become considerably colder than 
 the atmosphere by radiating its heat to the sky, 
 
 II. No direct experiments can be made to ascertain the manner 
 in which clouds prevent, or occasion to be small, the appearance of 
 a cold at night, upon the surface of the earth, greater than that of the 
 atmosphere ; but it may, I think, be firmly concluded, from what 
 has been said in the preceding article, that they produce this effect, 
 almost entirely, by radiating heat to the earth, in return for that 
 

 40 ESSAY ON DEW, 
 
 which they intercept in its progress from the earth towards the 
 heavens. For although, upon the sky becoming suddenly cloudy 
 during a calm night, a naked thermometer, suspended in the air, 
 commonly rises 2 or 3 degrees, little of this rise is to be attributed 
 to the heat evolved by the condensation of watery vapour in the 
 atmosphere, as was supposed by Mr. Wilson ;* since, in consequence 
 of the ceasing of that part of the cold indicated by the thermometer, 
 which was owing to its own radiation to a clear sky, the tempera- 
 ture of the atmosphere may seem to increase 2, or more, notwith- 
 standing that it has received no real addition. Besides ; the heat 
 which is extricated by the condensation of vapour, during the for- 
 mation of a cloud, must soon be dissipated ; whereas the effect of 
 greatly lessening, or preventing altogether, the appearance of a supe- 
 rior cold on the earth to that of the air, will be produced by a cloudy 
 sky, during the whole of a long night. 
 
 Dense clouds, near the earth, must possess the same heat as the 
 lower atmosphere, and will therefore send to the earth, as much, or 
 nearly as much heat as they receive from it by radiation. But 
 similarly dense clouds, if very high, though they equally incercept 
 the communication of the earth with the sky, yet being, from their 
 elevated situation, colder than the earth, will radiate to it less heat 
 than they receive from it, and may, consequently, admit of bodies 
 on its surface becoming several degrees colder than the air. In the 
 first part of this Essay, an example was given of a body on the 
 ground becoming at night 5 colder than the air, though the whole 
 sky was thickly covered with high clouds, t 
 
 Islands, and parts of continents close to the sea, being, by their 
 situation, subject to a cloudy sky, will, from the smaller quantity of 
 heat lost by them through radiation to the heavens at night, in addi- 
 tion to the reasons commonly assigned, be less cold in winter than 
 countries considerably distant from any ocean. 
 
 III. Fogs, like clouds, will arrest heat, which is radiated upwards 
 
 *Edin. Phil. Trans. I. 157. 
 
 { Mr. Prevost of Geneva, in his work on Radiant Heat, p. 382, has already in 
 this way, conjecturally accounted for the effect of clouds, in diminishing at night 
 the cold of the atmosphere, and of the surface of the earth ; but he seems not to 
 have known, that their effect on the temperature of the latter is much greater 
 than that which they produce upon the air. My explanation of this influence of 
 clouds, on the temperature of the surface of the earth, during the night, is a di- 
 rect consequence from the facts, which I had observed respecting the prevention 
 of cold on the ground from radiation, by the interposition of solid bodies between 
 it and the heavens, and occurred to me in 1812. Mr. Prevost's work, indeed, 
 was published in 1809 ; but I did not see it before the summer of 1813 ; when it 
 was lent to me by his relation Dr. Marcet of London, who at the same time said, 
 that he believed there was no other copy of it in Great Britain, except one, which 
 had been sent by himself to Edinburgh. 
 
 Note to second edition.] I did not know, until after the first edition of this 
 Essay was printed, that Mr. Prevost had published his opinion on the effect of 
 clouds in preventing the occurrence of cold at night in the atmosphere, and upon 
 the surface of the earth, as early as 1792, in a work entitled Recherches sur la 
 Chaleur.' 
 
THE THEORY OF DEW. 41 
 
 by the earth, and, if they be very dense, and of considerable per- 
 pendicular extent, may remit to it as much as they receive. Ac- 
 cordingly, Mr. Wilson found no difference at night, in very foggy 
 weather, between the temperature of the surface of snow, and that 
 of the air. Several observations by myself tend to confirm that of 
 Mr. Wilson. An instance, however, as was formerly said, occurred 
 to me of a difference at night of 9 between the temperatures of 
 grass crusted over with hoarfrost, and of air, during a very dense 
 fog. A fact, remarked by Mr. Leslie, respecting fogs, serves to 
 explain this apparent anomaly. For it was found by that philoso- 
 pher,* from experiments made with his photometer, that in mists 
 and low fogs the diminution of the sun's heat is small, when com- 
 pared with what occurs when the sky is obscured by a dense body 
 of clouds ; and it will, I presume, be readily granted, that the same 
 state of the atmosphere, which allows the heat of the sun to pass 
 copiously, will also give a ready transit to heat radiated by the earth. 
 Now there are several reasons for believing, that the fog during 
 which grass was 9 colder than the air, did not ascend far above the 
 ground. 1. The barometer had been falling for some days before, 
 and it is a matter of common observation, that great fogs seldom 
 occur, except it be high. 2. On the day preceding the observation, 
 the air, after having been extremely foggy for nearly a week, had 
 become clear enough to allow the sun's being distinctly seen during 
 the whole of the afternoon, though there was still a sufficient ob- 
 scurity in the lowermost parts of the atmosphere, to obstruct consid- 
 erably the view of objects on the ground and very near to it. 3. On 
 the day following the observation, the fog was again much less ; 
 on the next it disappeared, and was succeeded by snow. It is to be 
 mentioned likewise, that on the evening in question the state of the 
 grass, which was the subject of experiment, was unusually favoura- 
 ble to the production of cold ; since, contrary to general experience, 
 it was as cold as swandown. If, then, the latter substance, from the 
 much greater regularity of the appearances exhibited by it, be taken 
 as the standard, by which the occurrences of different nights are to 
 be compared together, it will follow that the fog of which I am 
 speaking, though it did not prevent, must have lessened the produc- 
 tion of cold from radiation. For, on the preceding evening, when 
 there was little fog, the atmosphere being equally still on both, the 
 difference between swandown and the air was 12; and on another, 
 a fortnight after, the difference at the same place of observation 
 between thermometers in the same situations, was 14^, the air 
 being now free from fog. If the atmosphere had been as still on 
 this as on the former evenings, a greater difference would doubtless 
 have been seen. I conclude, therefore, that fogs do not in any in- 
 stance furnish a real exception to the general rule, that whatever 
 exists in the atmosphere, capable of stopping or impeding the pas- 
 sage of radiant heat, will prevent or lessen the appearance at night 
 
 * On Heat and Moisture, p 57. 
 OCT. 1838. T 5 
 
42 ESSAY ON DEW. 
 
 of a cold on the surface of the earth greater than that of the neigh- 
 bouring air. 
 
 It follows also, from what has been said in this article, that the 
 water deposited upon the earth, during a fog at night, may some- 
 times be derived from two different sources, one of which is a pre- 
 cipitation of moisture from a considerable part of the atmosphere, 
 in consequence of its general cold ; the other, a real formation of 
 dew from the condensation, by means of the superficial cold of the 
 ground, of the moisture of that portion of the air, which comes in 
 contact with it. In such a state of things, all bodies will become 
 moist, but those especially which most readily attract dew in clear 
 weather.* I have had no opportunity, however, of trying this 
 conclusion by the test of observation since it occurred to me. 
 
 IV. When bodies become cold from radiation, the degree of effect 
 observed must depend not only on their radiating power, but in 
 part also on the greater or less ease with which they can derive 
 heat, by conduction, from warmer substances in contact with them. 
 Thus grass on a clear and still night was constantly colder, some- 
 times very much colder than the gravel walk, though a small 
 quantity of sand placed upon grass was always nearly as cold as 
 this substance. In this case, the difference in temperature between 
 the gravel walk and sand, evidently depended on the different 
 quantities of heat which they received from the parts beneath. A 
 like reason is to be given for dew appearing in greater quantity on 
 shavings of wood than on the same substance in a more dense and 
 compact form ; and for filamentous and downy substances becoming 
 colder than all others, even than lampblack, which is placed by Mr. 
 Leslie, at the head of the best solid radiators of heat. For the 
 lampblack exposed by me being about 2 lines in depth, possessed, in 
 consequence, a fund of internal heat, which would more readily pass 
 to its cold surface, than the heat of the lower parts of the downy 
 substances would to their upper surface. 
 
 This subject is illustrated by the following experiment. On a 
 dewy evening I depressed into a soft garden mould a drinking glass 
 having a thick flat bottom, until its brim was upon a level with the 
 surrounding earth, and at the same time placed a similar vessel with 
 its cavity also towards the sky, on the surface of the mould. In the 
 morning the inside of the depressed glass was entirely dry, while 
 that of the other was dewed. I then applied the bulb of a small 
 thermometer to the inside of the bottom of each vessel, on which I 
 found the heat of that part of the depressed one to be 56, but of the 
 same part of that which stood on the mould only 49i. At this 
 time the temperature of the air was 53. The cause, therefore, was 
 evident, both of the witness of the first vessel and of the dryness of 
 the second. 
 
 From this source also is to be derived the reason, why the promi- 
 
 * The moisture observed at night by Musschenbroek in Holland, and called 
 by him dew, appears to me to have been of this kind. See this Essay, p. 6. 
 
THE THEORY OF DEW. 43 
 
 nent parts of various bodies were observed by Mr. Wilson to be 
 crusted with hoarfrost, while their more retired and massy parts 
 were free from it.* 
 
 V. Bodies exposed in a clear night to the sky, must radiate as 
 much heat to it during the prevalence of wind as they do if the air 
 were altogether still. But in the former case little or no cold will 
 be observed upon them above that of the atmosphere, as the frequent 
 application of warm air must quickly return a heat equal, or nearly 
 so, to that which they had lost by radiation. A slight agitation of 
 the air is sufficient to produce some effect of this kind ; though, as 
 has already been said, such an agitation when the air is very preg- 
 nant with moisture, will render greater the quantity of dew ; one 
 requisite for a considerable production of this fluid being more in- 
 creased by it, than another is diminished. 
 
 VI. A small body, as a thermometer, suspended in the air, will 
 even in the calmest night exhibit but little cold from radiation, since 
 it is continually exposed to the application of fresh parcels of warmer 
 air, both from the progressive motion of this fluid and from the 
 downward motion produced in it by the superior gravity of such 
 portions as have been cooled by contact with the suspended body. 
 On the other hand, a thermometer upon a board, raised above the 
 earth and possessing a surface of several square yards, will have its 
 cold from radiation much less diminished than the former, as it is 
 exposed to no loss from a downward motion of the air, and as the air 
 which approaches it horizontally must almost always have had its 
 temperature previously lowered by passing over another part of the 
 board. The reason then of the lee side of the raised board being 
 often colder than the windward is obvious. 
 
 VII. There is a remark by Theophrastus.t which has been con- 
 firmed by other writers, that the hurtful effects of cold occur chiefly 
 in hollow places. If this be restricted to what happens on serene 
 and calm nights, and it does not, I believe, hold true in any other 
 circumstances, two reasons from different sources are to be assigned 
 for it. The first is, that the air being stiller in such a situation than 
 in any other, the cold from radiation in the bodies which it contains 
 will be less diminished by renewed applications of warmer air; the 
 second, that from the longer continuance of the same air in contact 
 with the ground, in depressed places than in others, less dew will be 
 deposited, and therefore less heat extricated during its formation. 
 It will be seen in the last part of this Essay, that in the East Indies 
 depressions in the earth are artificially made, for the purpose of in- 
 creasing the cold, which appears in serene nights. On this Subject, 
 however, it is to be observed, that if the depressed or hollow places 
 be deep, in proportion to their horizontal extent, a contrary effect 
 must follow ; as a case will occur more or less similar to that which 
 existed in some experiments formerly related by me, in which a 
 small portion of grass was surrounded by a hollow cylinder. 
 
 * Paper in Phil. Trans. 1780. f Lib - v c - xvi - 
 
44 ESSAY ON DEW. 
 
 VIII. An observation closely connected with the preceding, 
 namely, that in clear and still nights frosts are less severe upon 
 hills than in neighbouring plains,* has excited more attention, 
 chiefly from its contradicting what is commonly regarded an esta- 
 blished -fact, that the cold of the atmosphere always increases 
 with the distance from the earth. This inferior cold of hills is 
 evidently a circumstance of the same kind with that ascertained by 
 Mr. Pictet and Mr. Six respecting the increasing warmth, in clear 
 and calm nights at all seasons of the year, of the different strata of 
 the atmosphere, in proportion as these are more elevated above the 
 earth. As the greater cold of the lower air is the less complicated 
 fact, I shall attempt to explain it in the first place. Mr. Pictet, 
 indeed, furnishes an explanation himself, by ascribing it to the evap- 
 oration of moisture from the ground. But to show that this is not 
 just, it need only be mentioned, that the appearance never occurs 
 in any considerable degree, except upon such nights as are attended 
 with some dew, and that its great degrees are commonly attended 
 with a copious formation of that fluid ; since it cannot be thought 
 that the same stratum of air will deposit moisture on the ground, 
 from an insufficiency of heat, at the very time it is receiving moisture 
 from the ground, in the state of pellucid vapour, as this presupposes 
 that it is not yet replete with water. 
 
 Our atmosphere has been very generally regarded, as incapable 
 of being heated directly by the rays of the sun, principally because 
 these give no heat to any particular portion of it, in which they 
 are brought to a focus. I do not know, whether this experiment 
 was ever made with all the accuracy of which it is susceptible ; but 
 granting that it has been thus made, my opinion is, notwithstanding, 
 that no reliance can be placed in it. For as air, if heated at all by 
 concentrated sunbeams, must be heated by them in a very slight 
 degree during the time that their focus may be looked upon as 
 stationary, otherwise the present question would not have arisen, it 
 is necessary for conducting the experiment properly, that during 
 the whole of it, the same individual small portion of air shall con- 
 stantly receive that focus ; but this, for various manifest reasons, 
 cannot possibly happen. Viewing therefore the argument founded 
 upon this experiment as without force, I shall now offer several 
 considerations, which seem to prove, that air is actually heated by 
 the sunbeams which enter it. 
 
 1. Air both reflects and refracts light, and all other bodies, as far 
 as I know, acquire heat, while they act thus on the light of the sun. 
 
 2. Air suffocates or absorbs the sun's light, which it cannot be 
 supposed to do without increasing in temperature. 
 
 3. If air, considered as an uniform fluid, were even incapable of 
 gaining heat directly from the sun ? s rays, heat would be communi- 
 cated by them to it, through the intervention of the innumerable 
 
 * Theophrastus also remarks, that it freezes less on [hills than on plains, but 
 without mentioning, that this happens only on calm and serene nights. Lib. v. 
 
 C. XX. 
 
THE THEORY OF DEW. 45 
 
 particles of solid matter, which the trivial experiment of receiving 
 a sunbeam into a darkened room shows to be present in the atmo- 
 sphere. Should it be said, that this appearance may occur only in 
 the neighbourhood of the earth, from the accidental admixture of 
 solid matter raised from its surface by winds, or in any other 
 way, the answer is that, as my inquiry is concerning the existence 
 of a certain condition of the atmosphere, it matters not how this 
 originates. Nothing more can be demanded than that it should 
 always be found, which I believe to be the case ; since, if I can trust 
 my memory with respect to what took place many years ago, I 
 should say that such particles are to be seen, by means of the sun's 
 light in the air, over the middle of the Atlantic ocean. These par- 
 ticles then,' must receive heat from the sunbeams, which impinge 
 upon them, and this they will communicate to the contiguous pel- 
 lucid air. 
 
 4. Unless it be admitted, that the atmosphere is capable of inter- 
 cepting part of the heat which is radiated into it by the sun, and of 
 converting this into heat of temperature, I deem it impossible to 
 find a sufficient reason for the great warmth which exists after a 
 long calm, in air incumbent upon the Atlantic and Pacific oceans, at 
 the distance of a thousand miles or more from any considerable body 
 of land. It cannot be derived from the neighbouring water, since 
 this is colder than the lower atmosphere ; and no one will suppose 
 it to be the same heat which the air had acquired from the last 
 continent it had passed over many days before. But if even this 
 were supposed, another difficulty would remain to be removed, 
 which is, that during the whole of the calm the air is cooled every 
 night, and again becomes warm in the day.* 
 
 Should what has .been said be thought sufficient to establish, that 
 the air arrests part of the sun's heat, which is radiated into it bound 
 up with light, two consequences must also be allowed. The first is, 
 that air will exert a greater power of the same kind upon heat ra- 
 diated into it without light, since the sun's heatpasses instantaneously 
 through many bodies, which refuse a similar way to heat radiated 
 by terrestrial substances ; the other, that air must be as capable of 
 becoming cold by radiating its own heat,t as of becoming warm 
 from heat radiated into it, as these two properties are uniformly 
 observed to exist together, and to be proportional to each other. 
 The truth of the latter conclusion may also be inferred from this 
 fact, that in still and calm weather the heat of the air, a few feet 
 above the earth, will sometimes decrease, even in this country, 
 18 or 20 degrees between sunset and sunrise, though no change 
 of wind has in the meantime occurred ; for the inconsiderable 
 conducting power, which air is now known to possess, will per- 
 
 * One reason is hence apparent for the great coldness of the high regions of 
 the atmosphere ; since the air in them must be less fit than that of the lower 
 strata to arrest heat which is radiated into it. 
 
 | M. Prevost says : "On peut supposer queles molecules de 1'airrayonnent." 
 Du Calorique Rayonnant, p. 24. 
 
46 ESSAY ON DEW. 
 
 mit only a small part of this diminution to arise from heat passing, 
 by means of that power, from the atmosphere to the colder earth. 
 Mr. Leslie,* indeed, ascribes this effect to the descent of cold air 
 from the higher regions of the atmosphere ; but if this were just, 
 a less cold ought to be found, on a clear and still night, in the 
 lower than in the higher strata, which is contrary to the uniform 
 results of numerous experiments by Mr. Pictet and Mr. Six. Winds, 
 too, which produce such a mixture, always lessen the nocturnal 
 decrease of temperature in the lowermost part of the atmosphere. 
 
 Having thus shown, that air is capable, both of absorbing heat 
 which is radiated into it, and of radiating heat which had before 
 formed a part of its temperature, I proceed to apply the knowledge 
 of these facts, to the explanation of the phenomenon observed by 
 Mr. Pictet and Mr. Six. 
 
 This phenomenon occurs on those nights only, which permit 
 bodies, on the surface of the earth, to become cold by radiating their 
 heat to the heavens. On other nights, when bodies, thus situated, 
 were not colder than the air, I have observed the atmosphere, within 
 the limits of 9 feet from the ground, the boundary of my own ex- 
 periments, to decrease a little in temperature as the distance from 
 the earth increased. Mr. Six likewise found, that, on cloudy nights, 
 the air was sometimes colder 220 feet above the ground than at the 
 distance of 9 feet from it. When, therefore, the earth has become 
 colder, from radiation, than the neighbouring air, in consequence 
 of the latter having, by reason of its small radiating power, emitted 
 a less proportion of its heat to the heavens, the warmer air must 
 radiate a part of its heat to the earth without receiving a full com- 
 pensation, and will therefore become colder than it otherwise 
 would have been. In proportion too as the air is nearer to the 
 earth must the cold of the former from this cause be the greater. 
 My own conception of this matter is facilitated,t by contemplating 
 the occurrence of an opposite effect, when the earth is warmer than 
 the air. Let it be supposed then, that while the earth, in this state, 
 radiates upwards a quantity of heat, a foot in depth of the incumbent 
 air is capable of stopping a 1000th of what it hence receives, and of 
 converting it into heat of temperature. The consequence must be 
 that the next foot, from receiving only 999 parts of what has been 
 emitted by the earth, will not be so much heated as the first foot, 
 though it should absorb the same proportional quantity of what 
 enters it. In this way, every successive foot will acquire a less 
 quantity of heat than the preceding, and a state of the atmosphere 
 be produced, like to that which is actually observed in a calm and 
 sunny day. In the day, however, the phenomena, from the heating 
 of air by rays from the earth, are somewhat confused by the warmed 
 portions rising upwards, and mixing with what is colder ; whereas, 
 at night, the air, which has been cooled by radiating heat to the 
 
 * On Heat and Moisture, p. 11, and 132. 
 
 | The same facility is afforded by considering cold as a body. 
 
THE THEORY OF DEW. 47 
 
 earth, is rendered, by an increase of gravity, the more fit to retain 
 its low position. I have here, for the sake of simplifying the argu- 
 ment, taken no notice of the cooling of any considerable mass of the 
 air, in consequence of the actual contact of its lowermost stratum 
 with the earth, or by the conduction of the temperature of one por- 
 tion of it to another. But, in a calm state of the atmosphere these 
 effects must be inconsiderable, though it appears to me impossible, 
 in the present state of our knowledge to determine them with any 
 precision. 
 
 According to the view which has been given by me of this subject, 
 the heat of the air, in a clear and calm night, ought to increase, 
 within the limits of the phenomenon, in some decreasing geometrical 
 ratio, as the atmosphere ascends ; and this conclusion is so far con- 
 firmed by the observations of Mr. Pictet and Mr. Six taken together, 
 that the increase of temperature is found to be greater in a given 
 space very near to the earth than in an equal space more remote 
 from it. 
 
 To return to the immediate object of this article, the fact is cer- 
 tain, whatever may be thought of my explanation of it, that in 
 every clear and still night the air near to the earth is colder than 
 that which is more distant from it, to the height at least of 220 feet, 
 this being the greatest to which Mr. Six's experiments relate. If 
 then a hill be supposed to rise from a plain, to the height of 220 
 feet, having upon its summit a small flat surface covered with grass; 
 and if the atmosphere, during a calm and serene night, be admitted 
 to be 10 warmer there than it is near the surface of the low ground, 
 which is a less difference, according to the observations of Mr. Six, 
 than what sometimes occurs in such circumstances, it is manifest 
 that, should both the grass upon the hill and that upon the plain 
 acquire a cold of 10 degrees by radiation, the former will, notwith- 
 standing, be 10 warmer than the latter. 
 
 But the equality here supposed to be in the cold acquired by 
 grass, in two such situations, can seldom exist. For according to 
 an observation made by Aristotle,* and since frequently repeated, 
 the air of high places is much more agitated than that upon low 
 ground. The frequent renewal, therefore, from this cause, of the 
 air in contact with the grass on the hill, will prevent it from ever 
 becoming much colder than the general mass of the atmosphere at 
 the same height. Consequently, any diminution in this way of the 
 10 of cold formerly supposed to occur there from radiation, must 
 be added to the difference of temperature in the grass in the two 
 situations. 
 
 What has hitherto been said refers only to the occurrences on the 
 very summit of the hill. With respect to its sides, these can be 
 only a little colder than the atmosphere upon a level with them, 
 even in its calmest state. For, in the first place, they do not enjoy 
 the full aspect of the sky j and, in the second, the air which is 
 
 * Meteor, lib. l.c, x. 
 
48 ESSAY ON DEW. 
 
 cooled by contact with them, will, from its increased gravity, slide 
 down their declivity, and thus make room for the application of new 
 and warm parcels to the same surface. The motion, too, thus 
 excited in the air, near to the sides of the hill, must occasion a 
 motion in that upon the summit, which may, in some measure, 
 account for the last mentioned observation of Aristotle, as far as 
 relates to what happens in a clear night. 
 
 The height of the hill in this example has been supposed to be 
 small, to make it accord with that of the stations whose tempera- 
 tures were compared by Mr. Six with the heat of the air near the 
 ground. But observations of the same kind will apply to hills of 
 much greater elevation. For granting, first, that the air at the height 
 of 220 feet is never more than 10 colder than that near to the earth, 
 which is not probable, and is indeed contradicted by some of Mr. 
 Six's observations ; and again, that the increase of the air's heat in 
 a calm and serene night, ceases precisely at the greatest height to 
 which Mr. Six carried his observations, which is also improbable ; 
 still a reduction to the extent of 10 in the temperature of the air 
 near to the earth, will render the cold of this low portion of the 
 atmosphere greater than that of any other portion, which is not 
 more than 2500 or 3000 feet above the former, if the estimate be 
 just, which makes a declension in the heat of the atmosphere of 1 
 for every 250 or 300 feet of its height, when no counteracting cause 
 exists. 
 
 The remarks, however, which have been offered on the greater 
 warmth of hills at night, in a certain state of weather, are strictly 
 applicable to those only which are insulated, and of inconsider- 
 able lateral extent ; and it is upon such chiefly, if not solely, that 
 this phenomenon has been observed. The superiority of the cold 
 of a low plain, from radiation, over that of a wide expanse of hilly 
 ground, will, for obvious reasons, be less ; and no superiority of 
 this kind will probably exist in the former situation, when the high 
 ground is not only extensive but flat on the top, forming what is 
 called a table land ; unless, indeed, which seems to be actually the 
 case, the air of such an elevated country should be commonly 
 more agitated than that of lower places equally level. 
 
 An explanation may be now easily given of an observation by 
 Mr. Jefferson of Virginia,* which, however, had also been made by 
 Aristotle,t and Plutarch, j that dew is much less copious on hills 
 than it is upon plains. For allowing, at first, the surface of the 
 ground to be in both situations equally colder than the air which is 
 near to it ; still, as the production of dew must be in proportion to 
 the whole depression of the temperature of the air which furnishes 
 it, below what its heat had been in the preceding day, and as one 
 part of this depression, the general cooling of the atmosphere, is 
 much more considerable on the plain than on the hill, moisture 
 must necessarily be deposited more copiously in the former than in 
 
 * Notes on Virginia, p, 132. f Meteor. Lib. 1. c. x. De Primo Frigido. 
 
THE THEORY OF DEW. 49 
 
 the latter place. If the greater agitation of the atmosphere, and 
 the less quantity of moisture during clear weather, in its higher 
 region than in the lower, be added, it may readily be inferred that 
 dew shall sometimes be altogether wanting on a hill, though abundant 
 on a plain at its foot, agreeably to what has been actually observed 
 by Mr. Jefferson. 
 
 IX. The leaves of trees often remain dry throughout the night, 
 while those of grass are covered with dew. As this is a similar 
 fact to the smallness of dew on hills, I shall in accounting for it do 
 little more than enumerate the circumstances on which it depends. 
 
 1. The atmosphere is several degrees warmer near the upper parts 
 of trees on dewy nights than close to the ground. 2. The air in 
 the higher situation is more agitated than that in the lower. 
 
 3. The air at a little distance from the ground, from being nearer to 
 one of its sources of moisture, will on a calm evening contain more 
 of it than that which surrounds the leaves of elevated trees. 
 
 4. Only the leaves of the very tops of trees are fully exposed to the 
 sky. 5. The declension of the leaves from a horizontal position 
 will occasion the air, which has been cooled by them, to slide 
 quickly away, and be succeeded be warmer parcels. 6. The length 
 of the branches of the trees, the tenderness of their twigs, and the 
 pliancy of the footstalks of their leaves, will cause in the leaves an 
 almost perpetual motion, even in states of air that may be denomi- 
 nated calm. I have hence frequently heard, during the stillness of 
 night, a rustling noise in the trees, which formed one of the bounda- 
 ries of the ordinary place of my observations, while the air below 
 seemed without motion. 
 
 Nearly in the same manner is to be explained why shrubs and 
 bushes also receive dew more readily than lofty trees. 
 
 X. Bright metals, exposed to a clear sky in a calm night, will be 
 less dewed on their upper surface than other solid bodies ; since of 
 all bodies they will, in such a situation, lose the smallest quantity 
 of heat by radiation to the heavens, at the same time that they are 
 capable of receiving, by conduction, at least as much heat as any 
 others from the atmosphere, and more than any others from the 
 warmer solid substances which they happen to touch. 
 
 If the exposed pieces of metal be not very small, another reason 
 will contribute somewhat to their being later and less dewed than 
 other solid substances. For in consequence of their great con- 
 ducting power, dew cannot form upon them, unless their whole 
 mass be sufficiently cold to condense the watery vapour of the 
 atmosphere ; while the same fluid will appear on a bad conductor of 
 heat, though the parts a very little beneath the surface are warmer 
 than the air.* 
 
 * I hence think it probable, that dew will sometimes form on the bulb of a 
 thermometer, before the mercury in it is cooled below the temperature of the air. 
 It seems certain to me, also, that dew may appear upon substances, which, from 
 the thinness of the layer of matter their cold is confined to, will produce little or 
 no sensible effect upon a thermometer that is applied to them. 
 

 -'''. ' :'' '''' 
 
 50 ESSAY ON DEW. 
 
 From the same ready passage of heat from one part of a metal to 
 another, a metallic plate suspended, horizontally, in the air several 
 feet above the ground, will be found dewed on its lower side, if the 
 upper has become so ; while the lower surface of other bodies, more 
 attractive of dew, but worse conductors of heat, are without dew in 
 a similar situation. 
 
 A metal placed at night in the air, near to the ground, is, for the 
 most part, sufficiently cold to condense, on its underside, the vapour 
 which arises from the warmer earth ; though its upper surface may 
 be dry, from possessing the same, or almost the same temperature, 
 as the atmosphere near to it. 
 
 As the temperature of metals is never much below that of the 
 neighbouring air, a slight diminution of their cold from radiation 
 will often occasion them to evaporate the dew, which they had 
 previously acquired, though other substances, which had been more 
 cooled by radiation, are still attracting dew. For a like reason, a 
 metal, which has been purposely wetted, will often become dry at 
 night, while other substances are becoming moist. 
 
 A substance highly attractive of dew, such as wool, if laid upon 
 a metal, will derive heat from it, and will therefore acquire less 
 dew than an equal portion of the same substance laid upon grass. 
 
 A large metallic plate will be less readily dewed while lying on 
 grass, than if it were placed in the air, though only a few inches 
 above the grass ; because, in the former situation, it receives freely, 
 by means of its great conducting power, heat from the earth ; where- 
 as, when placed in the air, it powerfully resists by another property, 
 possessed in a great degree by bright metals, the entrance of heat 
 radiated towards it by the grass beneath. Besides ; the grass under 
 the metal possesses now less heat than when this substance was in 
 contact with it, partly from having a small oblique aspect of the 
 sky, and partly from receiving air, which has been cooled by pass- 
 ing over other grass fully exposed to the heavens. 
 
 When a piece of metal, having closely applied to its under surface 
 a substance of some thickness, which attracts dew powerfully, and 
 therefore imbibes readily heat that is radiated to it, is exposed to 
 the sky at night, the heat supplied by the attached substance, both 
 from its own original store, and from what it has acquired through 
 the radiation of the ground to it during the exposure, will enable 
 this piece to resist longer than a bare piece the formation of dew, 
 or even than another piece which has only a thin coat of matter con- 
 siderably attractive of dew attached to its underside. The experi- 
 ment with the wooden cross, covered with gilt paper, affords an 
 example of the latter fact. 
 
 A very small metallic plate, suspended in the air, is less readily 
 dewed than a large one similarly situated, as it receives, in propor- 
 tion to its size, more heat from the atmosphere. On the other 
 hand, a very small plate laid upon grass, rendered cold by radiation, 
 will be sooner dewed than a larger one in the same situation, from 
 presenting a greater proportional circumference to the surrounding 
 
THE THEORY OF DEW. 51 
 
 grass, and therefore losing more quickly its heat by conduction. 
 It will he also sooner dewed than another very small plate suspended 
 in the air; since the latter, like other small bodies similarly placed, 
 must be continually acquiring more heat than the former, in the 
 manner described above in this Essay.* 
 
 A piece of metal, applied to different portions of cold grass in 
 succession, will sooner become cold itself, than another piece which 
 is suffered to remain constantly upon one portion of the same grass, 
 and will in consequence be sooner dewed. 
 
 If the bare side of a piece of metalled paper be exposed to a clear 
 and calm sky at night, it will become cold, by radiation, and receive, 
 by conduction, the heat of the inferior metallic surface ; whence, if 
 this surface be afterwards made the upper one, it will sooner acquire 
 dew than a similar metallic surface which has been exposed to the 
 sky during the whole of the experiment. 
 
 When a metal covers, in part only, the upper surface of a piece 
 of glass, the uncovered portion of the glass quickly becomes cold by 
 radiation, on exposure to a serene sky in a still night, and then, by 
 deriving to itself a part of the heat of the metal, occasions this body 
 to be more readily dewed than if the -whole of the exposed surface 
 had been metallic. In this experiment, the outer edge of the metallic 
 surface, from being nearest to the colder glass, will be the first and 
 the most dewed, while the parts of the uncovered glass, which are 
 contiguous to the warmer mettl, will be the last and the least dewed, 
 of their respective substances. 
 
 A piece of glass, covered on one side with a metal, being placed 
 on grass, with this side down, its upper surface attracts dew as 
 readily as if no metal were attached to it ; since the metal in this 
 situation has no power to lessen the radiation of heat from the upper 
 surface of the glass. I conclude, however, from general principles, 
 for I have not made the trial, that if the same piece of glass, having 
 its metallic side still undermost, were raised in the air a little above 
 the grass, it would be more readily dewed on its upper surface than 
 if it had been without a metallic coating on the lower, as this coating 
 must resist; the introduction of heat radiated by the warmer grass, 
 and thus preserve nearly undiminished the cold acquired from radi- 
 ation of heat to the sky, by the bare upper surface. 
 
 The preceding remarks apply to the whole class of metals ; but 
 the discoveries of Mr. Leslie, respecting the difference in the capa- 
 cities of these bodies to radiate heat, furnish an explanation of a 
 diversity among themselves, in regard to attraction for dew, which 
 was noted in the/oregoing part of this Essay. Gold, silver, copper, 
 and tin, are there said to resist the formation of dew more strongly 
 than other substances of the same class ; but these metals according 
 to Mr. Leslie radiate heat the most sparingly. On the other hand, 
 lead, iron, and steel, which according to the same author radiate heat 
 more copiously than the former metals, were found by me to acquire 
 
 *Page 43. 
 
52 ESSAY ON DEW. 
 
 dew more readily. I do not know if the radiating power of platina 
 has been ascertained by direct experiments ; but as its conducting 
 power is small its radiation must be great, since these qualities exist 
 always in opposite degrees in the same substance ; and I have ac- 
 cordingly observed it to be dewed, while the four first-mentioned 
 metals were dry. I am ignorant both of the radiating and the con- 
 ducting power of zinc, as determined by ordinary experiments ; but 
 I infer from its being more easily dewed than gold or silver, that it 
 radiates heat more copiously than they do ; unless indeed, the pieces 
 which I used, from having had their surfaces roughened by friction 
 with sand, which was employed to brighten them, had acquired a 
 radiating power greater than that possessed by polished pieces, 
 agreeably to the results of some of Mr. Leslie's experiments.* 
 
 XI. Thinking it probable, that black bodies might radiate more 
 heat to the sky at night than white, I placed upon grass, on five 
 different evenings, equal parcels of black and white wool. On four 
 of the succeeding mornings the black wool was found to have ac- 
 quired a little more dew than the white; whence I inferred that it 
 had, in consequence of its colour, radiated a little more heat. But I 
 afterwards remarked that the white wool was somewhat coarser than 
 the black; which circumstance alone was sufficient to occasion a 
 difference in their quantities of moisture. Another night I laid on 
 the raised board a piece of pasteboard covered with white paper, 
 and close to this a second piece similar to the former in every re- 
 spect, except that it was covered with paper blackened with ink. 
 At daylight I saw hoarfrost upon both pieces ; but the black seemed 
 to have a greater quantity than the white. A doubt, however, after- 
 wards arose upon the accuracy of this experiment likewise ; for as 
 the light was faint when I viewed the two surfaces, the quantity of 
 hoarfrost, though equal on both, might have appeared greater on the 
 black than on the white, from the contrast of its colour with that of 
 the former surface. But trials of this kind, as Mr. Lesliet has ob- 
 served, never afford firm conclusions ; since a black body must 
 always differ from a white in one or more chemical properties, and 
 
 * I once intended to subjoin here an explanation of some very curious observa- 
 tions by Mr. Benedict Prevost on dew, which were published, first in the 44th 
 volume of the French Annals of Chemistry, and afterwards by Mr. Peter Prevost 
 of Geneva, in his Essay on Radiant Heat ; but fearing to be very tedious, I have 
 since given up the design. I will say, however, that if to what is now generally 
 known on the different modes, in which heat is communicated from one body to 
 another, be added the two following circumstances ; that substances become 
 colder by radiation than the air, before they attract dew ; and that bright metals 
 when exposed to a clear sky at night, become colder than the air much less 
 readily than other bodies ; the whole of the appearances observed by Mr. Prevost 
 may be easily accounted for. 
 
 Note to second edition."] I found, shortly after the publication of the former 
 edition of this Essay, that the learned Dr. Young had several years before, in 
 his great work on Natural Philosophy, employed the principle of the radiation 
 of heat to account for several of the facts observed by Mr. B. Prevost. On the 
 subject of Dr. Young's explanation, I have spoken somewhat fully in the 28th 
 number of Dr. Thomson's Annals of Philosophy. 
 
 t On Heat, p. 95. 
 
THE THEORY OF DEW. 53 
 
 this difference may of itself be competent to produce a diversity in 
 their powers to radiate heat. 
 
 With the view to render the subject less complicated, I have 
 hitherto treated of dew as if it were altogether derived from watery 
 vapour previously diffused through the atmosphere; this appearing 
 to me to be by far its most considerable source, and none of my con- 
 clusions of any importance being liable to be affected, even by the 
 establishment of a contrary opinion. Other writers, however, have 
 regarded dew as being entirely the product of vapour emitted during 
 the night, by the earth and plants upon it. According to this 
 theory dew is said to rise. 
 
 The first trace, which I have found of the opinion that dew rises 
 from the earth at night, occurs in the History of the Academy of 
 Sciences for 1697. It is mentioned there briefly and obscurely, 
 and was, probably, shortly forgotten ; for Gersten, who advanced 
 it anew in 1733, held himself to be its author. Musschenbroek and 
 Dufay embraced it immediately after Gersten ; but the former soon 
 admitted, that dew sometimes falls. As far as I have learned, no 
 writer upon dew has since ascribed its total production to vapour, 
 emitted by the earth at night, except Mr. Webster of New Eng- 
 land.* But this opinion is frequently advanced in conversation by 
 persons not much accustomed to philosophical pursuits, chiefly, I 
 think, because it contradicts a popular belief. 
 
 The only argument used by the French academicians, in support 
 of their opinion, is, if I understand it rightly, that as much dew is 
 observed under an inverted glass-bell as in any other situation. 
 But admitting, for a moment, this to be true, they would not thus 
 prove that the ground is the only source of that fluid. 
 
 Gersten was led to think that dew rises from the earth, by often 
 finding grass, and low shrubs, moistened- with it, while trees were 
 dry. Respecting this fact, I shall add nothing to what I have lately 
 said upon it. But his chief argument is derived from another fact 
 related in the first part of this Essay, which is, that a plate of metal, 
 laid upon bare earth on a dewy night, will remain dry on its upper 
 surface, while it becomes moist on the lower. This also is easily 
 explicable by what has already been mentioned by me. For the 
 lower side of the metal, in consequence of the upper being in con- 
 tact with the air and being exposed to a clear sky, is colder than the 
 earth a little below the surface, and therefore condenses the vapour, 
 which strikes against its bottom ; while the upper side, from being 
 frequently warmer, and never more than a little colder than the air, 
 is for the most part unable to condense the watery vapour of the 
 atmosphere. t Gersten, moreover, describes several appearances 
 
 * Mem. of American Acad. vol. iii. 
 
 f I have, in like manner, observed, on a cloudy night, a piece of glass, laid 
 over an earthen pan containing water and placed upon the ground, to be wet on 
 its lower side, while the upper was dry ; the glass being, in this situation, suffi- 
 ciently cold to condense the vapour of water heated by the earth, but not enough 
 so to condense the watery vapour of the atmosphere. 
 
 OCT. 1838.- U 6 
 
54 ESSAY ON DEW. 
 
 himself, which refute his opinion. He mentions, for example, that 
 the higher parts of shrubs are more dewed than the lower ; that 
 metallic plates, placed horizontally in the air, are as much dewed 
 on their superior as on their inferior surfaces ; and that convex and 
 cylindrical bodies, suspended in the air, the latter having a position 
 parallel to the horizon, are dewed only on their upper parts. 
 
 The principal reason given by Dufay for the rising of dew is, 
 that it appears more early on bodies near to the earth than on those 
 which are at a greater height. But this fact readily admits of an 
 explanation on other grounds that have already been mentioned. 
 1. The lower air, on a clear and calm evening, is colder than the 
 upper, and will, therefore, be sooner in a condition to deposit a 
 part of its moisture. 2. It is less liable to agitation than the upper. 
 3. It contains more moisture than the upper, from receiving the 
 last which has risen from the earth, in addition to what it had pre- 
 viously possessed, in common with other parts of the atmosphere. 
 Dufay attempted to strengthen his argument, by exposing, on three 
 dewy nights, similar substances at different heights from the ground, 
 expecting that the lower would always acquire more moisture than 
 the upper ; but, upon all the nights, some one of the lower substan- 
 ces acquired less moisture than some one of the higher. 
 
 Mr. Webster has advanced no new fact in favour of the opinion 
 of which I am speaking. 
 
 Enough having be said to prove, that dew is not entirely the 
 product of vapour rising from the earth at night, I shall next show, 
 that it often occurs when this cause can have little or no operation. 
 
 1. It appears from Hasselquist and Bruce, that in Egypt, shortly 
 before the rising of the Nile, and consequently when the ground 
 there is in its dryest state, dew becomes exceedingly plentiful, 
 though little or none had formed before, while the earth was some- 
 what less dry. The cause evidently, is, as was formerly mentioned, 
 the moist air brought from the Mediterranean by the north wind, 
 which then prevails. 
 
 2. Mr. Webster, speaking of hoar-frost, which he properly re- 
 gards as frozen dew, candidly says, though it overthrows his 
 opinion : " This frost appears when the surface of the earth is 
 sealed with frost, and of course the vapour of which it is formed, 
 cannot at the time, perspire from the earth." 
 
 3. I have myself, at all seasons of the year, frequently observed 
 wool, upon the middle of the raised board, and therefore out of the 
 way of vapour rising from the ground, to acquire more dew than 
 wool laid upon the grassplat. 
 
 4. The bodies that condense the rising vapour must necessarily 
 be colder than it ; but, as they are likewise, according to the opinion 
 under view, of the same temperature with the air surrounding them, 
 this also should condense the rising vapour. Dew, therefore, should 
 never appear in any considerable quantity, without being accompanied 
 with fog or mist. Now I can assert, after much attention to this 
 point, that the formation of the most abundant dew is consistent 
 
THE THEORY OF DEW. 55 
 
 with a pellucid state of the atmosphere. Hasselquist makes a similar 
 observation with regard to Egypt ; where, during the season re- 
 markable for the most profuse dews, "the nights," he says, "are as 
 resplendent with stars, in the midst of summer, as the lightest and 
 clearest winter nights in the north." 
 
 But, although these facts prove that copious dews may occur 
 with little or no contribution by vapour immediately rising from 
 the earth, it must yet be admitted, that some of the moisture, which 
 forms during clear and still weather, on bodies situated upon or near 
 its surface, is in most cases to be attributed to this source ; since, in 
 my experiments, substances on the raised board became much later 
 moist than others on the ground, though equally cold with them. 
 The quantity from this cause, however, can never be great. . For 
 in the first place, until the air be cooled by the substance attractive 
 of dew, with which it comes in contact, below its point of repletion 
 with moisture, it will be always in a condition to take up that which 
 has been deposited upon grass, or other low bodies, by warm vapour 
 emitted by the earth ; just as the moisture formed upon a mirror by 
 our breath is, in temperate weather, almost immediately carried 
 away by the surrounding air. Accordingly ; I have sometimes, in 
 serene and still weather, observed dew to appear sparingly upon 
 grass in the shade, several hours before sunset, and to continue in 
 nearly the same quantity till about sunset, when it would increase 
 considerably, at the time that the same fluid began to show itself on 
 the raised board. In the second place ; though bodies situated on 
 the ground, after they have been made sufficiently cold, by radiation, 
 to condense the vapour of the atmosphere, will be able to retain the 
 moisture which they acquire by condensing the vapour of the 
 earth ; yet, before this happens, the rising vapour must have been 
 greatly diminished, by the surface of the ground having become 
 much colder. These considerations, added to the fact, that sub- 
 stances on the raised board attracted rather more dew, throughout 
 the night, than similar substances lying on the grass, warrant me to 
 conclude, that on nights favourable to the production of dew, only 
 a very small part of what occurs is owing to vapour rising from 
 the earth ; though I am acquainted with no means of determining 
 the proportion of this part to the whole. On the other hand, how- 
 ever, in a cloudy night, all the dew that appears upon grass may 
 sometimes be attributed to a condensation of the earth's vapour ; 
 since I have several times, in such nights, remarked the raised board 
 to be dry, while the grass was moist. These nights were calm, and 
 evaporation from the grass consequently not copious. When evapora- 
 tion on cloudy nights was assisted by wind, dew has never, as was 
 mentioned in the first part of this Essay, been any where observed 
 by me.* 
 
 * The interval between the first appearance of dew in the afternoon on grass in 
 shaded places, and sunset, was formerly said by me, on the authority, however, 
 of only a few observations, to be considerably greater than that between sunrise 
 and the ceasing of the formation of dew upon grass in the morning. These ob* 
 

 56 ESSAY ON DEW. 
 
 Agreeably to another opinion, the dew found upon growing 
 vegetables is the condensed vapour of the very plants on which it 
 appears. But this also seems to me erroneous for several reasons. 
 1. Dew forms as copiously upon dead as upon living vegetable 
 substances. 2. The transpired humour of plants will be carried 
 away by the air which passes over them when they are not suf- 
 ficiently cold to condense the watery vapour contained in it ; unless, 
 which is almost never the case if mist does not already exist, the 
 general mass of the atmosphere be incapable of receiving moisture 
 in a pellucid form. Accordingly, on cloudy nights, when the air, 
 consequently, can never be cooled more than a little below the 
 point of repletion with moisture, by bodies in contact with it, dew is 
 never observed upon any plants that are elevated a few feet above 
 the ground. 3. If a plant has become, by radiating its heat to the 
 heavens, so cold, as to be enabled to bring the air in contact with 
 it below the point of repletion with moisture, that which forms 
 upon it, from its own transpiration, will not then indeed evaporate. 
 But other moisture will, at the same time, be communicated to it 
 by the atmosphere ; and when the difference in the copiousness of 
 these two sources is considered, it may, I think, be safely concluded, 
 that almost the whole of the dew which will afterwards form on 
 the plant must be derived from the air ; more especially when the 
 coldness of a clear night, and the general inactivity of plants in the 
 absence of light, both lessening their transpiration, are taken into 
 account. 
 
 An experiment however has been appealed to in proof, that the 
 dew of plants actually does originate from fluid transpired by them ; 
 that, namely, in which a plant shut up in an air-tight case becomes 
 covered with moisture. But this experiment, if attentively exam- 
 ined, will be found to have little weight. First ; the inclosed plant 
 being exempt from the cold which its own radiation would have 
 produced in its natural situation, on a dewy night, will transpire 
 a greater quantity of fluid than a similar plant exposed at the same 
 time to the open air. Again ; the small quantity of air contained 
 in the case must soon be replete with moisture, after which, the 
 whole of what is further emitted by the plant will necessarily 
 assume the form of a fluid, whatever may be the condition of the 
 external atmosphere ; whereas, during even the clearest night, only a 
 part of the smaller quantity of moisture, emitted by the exposed 
 plant, will be condensed on its surface. In the last place ; notwith- 
 
 servations were made on spots exposed during the greater part of the day to the 
 sun. In such places, the heat acquired, from the sun, by the uppermost layer 
 of earth, will be longer retained than that acquired by the grass, which will, 
 therefore, be sufficiently cool, soon after the heat of the day has declined, to con- 
 dense a part of the vapour then copiously rising from the earth ; whereas in the 
 morning, both less vapour will rise, the surface of the earth having now lost a 
 great part of its heat, and a less proportion of that which does rise will be con- 
 densed by the grass as the temperature of this body now more nearly approaches 
 that of the ground, from first receiving the heat of the sun reflected from the 
 atmosphere and other substances. 
 
APPEARANCES CONNECTED WITH DEW. 57 
 
 standing the circumstances which favour the appearance of moisture 
 upon inclosed plants from their own transpiration, still the quantity 
 observed on them is said to be, for I have made no experiment 
 myself respecting this matter, much less considerable than what is 
 seen upon plants of the same kind, exposed to the air for the same 
 time, during a calm and serene night. 
 
 PART III. 
 
 OF SEVERAL APPEARANCES CONNECTED WITH DEW. 
 
 THERE are various occurrences in nature which seem to me 
 strictly allied to dew, though their relation to it be not always at 
 first sight perceivable. The statement and explanation of several 
 of these will form the concluding part of the present Essay. 
 
 I. I observed one morning in winter, that the insidesof the panes 
 of glass in the windows of my bedchamber were all of them moist, 
 but that those, which had been covered by an inside shutter during 
 the night, were much more so than ? others which had been uncovered. 
 Supposing that this diversity of appearance depended upon a differ- 
 ence of temperature, I applied the naked bulbs of two delicate ther- 
 mometers to a covered and uncovered pane ; on which I found, that 
 the former was 3 colder than the latter. The air of the chamber, 
 though no fire was kept in it, was at this time ll warmer than 
 that without. Similar experiments were made on many other 
 mornings, the results of which were ; that when the warmth of the 
 internal air exceeded that of the external, from 8 to 18, the tem- 
 perature of the covered panes would be from 1 to 5 less than that of 
 the uncovered ; that the covered were sometimes dewed, while the 
 uncovered were dry ; that at other times both were free from mois- 
 ture : that the outsides of the covered and uncovered panes had 
 similar differences with respect to heat, though not so great as those 
 of the inner surfaces ; and that no variation in the quantity of these 
 differences was occasioned by the weather's being cloudy or fair, 
 provided the heat of the internal air exceeded that of the external 
 equally in both of those states of the atmosphere. 
 
 The remote reason of these differences did not immediately pre- 
 sent itself. I soon, however, saw, that the closed shutter shielded 
 the glass which it covered from the heat that was radiated to the 
 windows by the walls and furniture of the room, and thus kept it 
 nearer to the temperature of the external air, than those parts could 
 be, which from being uncovered received the heat emitted to them 
 by the bodies just mentioned. 
 
 In making these experiments I seldom observed the inside of 
 any pane to be more than a little damped, though it might be from 
 S to 12 colder than the general mass of the air in the room ; while* 
 
58 ESSAY ON DEW. 
 
 in the open air I had often found a great dew'to form on substances 
 only 3 or 4 colder than the atmosphere. This at first surprised 
 me : but the cause now seems plain. The air of the chamber had 
 once been a portion of the external atmosphere, and had afterwards 
 been heated, when it could receive little accession to its original 
 moisture. It consequently required being cooled considerably, 
 before it was even brought back to its former nearness to repletion 
 with water ; whereas the whole external air is commonly, at night, 
 nearly replete with moisture, and therefore readily precipitates dew 
 on bodies only a little colder than itself. 
 
 When the air of a room is warmer than the external atmosphere, 
 the effect of an outside shutter, on the temperature of the glass of 
 the window, will be directly opposite to what has been just stated ; 
 since it must prevent the radiation into the atmosphere of the heat 
 of the chamber transmitted through the glass. 
 
 II. Count Rumford* appears to have rightly conjectured, that the 
 inhabitants of certain hot countries, who sleep at nights on the tops 
 of their houses, are cold during this exposure, by the radiation of 
 their heat to the sky ; or, according to his manner of expression, 
 by receiving frigorific rays from the heavens. Another fact of this 
 kind seems to be the greater chill, which we often experience upon 
 passing, at night, from the cover of a house into the open air, than 
 might have been expected from the cold of the external atmosphere. 
 The cause, indeed, is said to be the quickness of transition from one 
 situation to another. But, if this were the whole reason, an equal 
 chill would be felt in the day, when the difference, in point of heat, 
 between the internal and external air was the same as at night, 
 which is not the case. Besides ; if I can trust my own observation, 
 the feeling of cold from this cause is more remarkable in a clear 
 than in a cloudy night, and in the country than in towns. The fol- 
 lowing appears to be the manner in which these things are chiefly 
 to be explained. 
 
 During the day, our bodies while in the open air, although not 
 immediately exposed to the sun's rays, are yet constantly deriving 
 heat from them, by means of the reflection of the atmosphere. 
 This heat, though it produces little change on the temperature of the 
 air which it traverses, affords us some compensation for what we 
 radiate to the heavens. At night also, if the sky be overcast, some 
 compensation will be made to us, both in towns and in the country, 
 though in a less degree than during the day, as the clouds will remit 
 towards the earth no inconsiderable quantity of heat. But on a clear 
 night, in an open part of the country, nothing almost can be returned 
 to us from above, in place of the heat which we radiate upwards. 
 In towns, however, some compensation will be afforded, even on 
 the clearest nights, for the heat which we lose in the open air, by 
 that which is radiated to us by the surrounding buildings. 
 
 To our loss of heat by radiation at times that we derive little 
 
 * Phil. Trans. 1804. p. 182. 
 
APPEARANCES CONNECTED WITH DEW. 59 
 
 compensation from the radiation of other bodies, is probably to be 
 attributed a great part of the hurtful effects of the night air. Des- 
 cartes* says that these are not owing to dew, as was the common 
 opinion of his contemporaries, but to the descent of Certain noxious 
 vapours, which having been exhaled from the earth during the heat 
 of the day, are afterwards condensed by the cold of a serene night. 
 The effects in question certainly cannot be occasioned by dew, since 
 that fluid does not form upon a healthy human body, in temperate 
 climates ; but they may, notwithstanding, arise from the same 
 cause that produces dew on those substances, which do not, like 
 the human body, possess the power of generating heat for the supply 
 of what they lose by radiation or any other means. 
 
 III. I had often, in the pride of half knowledge, smiled at the 
 means frequently employed by gardeners, to protect tender plants 
 from cold, as it appeared to me impossible that a thin mat. or any 
 such flimsy substance, could prevent them from attaining the tem- 
 perature of the atmosphere, by which alone I thought them liable 
 to be injured. But, when I had learned that bodies on the surface 
 of the earth become, during a still and serene night, colder than the 
 atmosphere, by radiating their heat to the heavens, I perceived im- 
 mediately a just reason for the practice which I had before deemed 
 useless. Being desirous, however, of acquiring some precise infor- 
 mation on this subject, I fixed perpendicularly in the earth of a grass- 
 plat, 4 small sticks, and over their upper extremities, which were 6 
 inches above the grass, and formed the corners of a square, the sides 
 of which were 2 feet long, drew tightly a very thin cambric hand- 
 kerchief. In this disposition of things, therefore, nothing existed 
 to prevent the free passage of air from the exposed grass, to that 
 which was sheltered, except the 4 small sticks, and there was no 
 substance to radiate heat downwards to the latter grass, except the 
 cambric handkerchief. The temperature of the grass, which was thus 
 shielded from the sky, was upon many nights afterwards examined 
 by me, and was always found higher than that of neighbouring 
 grass which was uncovered, if this was colder than the air. When 
 the difference in temperature, between the air several feet above the 
 ground and the unsheltered grass, did not exceed 5, the sheltered 
 grass was about as warm as the air. If that difference, however, 
 exceeded 5, the air was found to be somewhat warmer than the 
 sheltered grass. Thus, upon one night, when fully exposed grass 
 was 11 colder than the air, the latter was 3 warmer than the shel- 
 tered grass ; and the same difference existed on another night, when 
 the air was 14 warmer than the exposed grass. One reason for this 
 difference, no doubt, was that the air, which passed from the exposed 
 grass, by which it had been very much cooled, to that under the 
 handkerchief, had deprived the latter of part of its heat ; another, 
 that the handkerchief, from being made colder than the atmosphere 
 by the radiation of its upper surface to the heavens, would remit 
 
 Meteorolog. c. vi. 
 
60 ESSAY ON DEW. 
 
 somewhat less heat to the grass beneath than what it received 
 from that substance. But still, as the sheltered grass, notwithstand- 
 ing these drawbacks, was upon one night, as may be collected from 
 the preceding relation, 8, and upon another 11, warmer than grass 
 fully exposed to the sky, a sufficient reason was now obtained for 
 the utility of a very slight shelter to plants, in averting or les- 
 sening injury from cold, on a still and serene night. 
 
 In the next place ; in order to learn whether any difference would 
 arise from placing the sheltering substance at a much greater distance 
 from the ground, I had 4 slender posts driven perpendicularly into 
 the soil of a grass field, and had them so disposed in other respects 
 that their upper ends were 6 feet above the surface, and formed the 
 angular points of a square having sides 8 feet in length. Lastly ; 
 over the tops of the posts was thrown an old ship flag of a very 
 loose texture. Concerning the experiments made by means of 
 this arrangement of things, I shall only say, that they led to the 
 conclusion, as far as the events of different nights could rightly be 
 compared, that the higher shelter had the same efficacy with the 
 lovver a in preventing the occurrence of a cold upon the ground, in 
 a clear night, greater than that of the atmosphere, provided the 
 oblique aspect of the sky was equally excluded from the spots on 
 which my thermometers were laid. 
 
 On the other hand ; a difference in temperature, of some magni- 
 tude, was always observed on still and serene nights, between 
 bodies sheltered from the sky by substances touching them, and 
 similar bodies, which were sheltered by a substance a little above 
 them. I found, for example, upon one night, that the warmth of 
 grass, sheltered by a cambric handkerchief raised a few inches in 
 the air, was 3 greater than that of a neighbouring piece of grass, 
 which was sheltered by a similar handkerchief actually in contact 
 with it. On another night, the difference between the temperatures 
 of two portions of grass, shielded in the same manner as the two 
 above-mentioned from the influence of the sky, was 4. Possibly, 
 experience has long ago taught gardeners the superior advantage of 
 defending tender vegetables, from the cold of clear and calm nights, 
 by means of substances not directly touching them ; though I do not 
 recollect ever having seen any contrivance for keeping mats, or 
 such like bodies, at a distance from the plants which they were 
 meant to protect. 
 
 Walls, I believe, as far as warmth is concerned, are regarded as 
 useful during a cold night, to the plants which touch them, or are 
 near to them, only in two ways ; first, by the mechanical shelter 
 wh'ich they afford against cold winds, and secondly, by giving out 
 the heat which they had acquired during the day. It appearing to 
 me, however, that, on clear and calm nights, those on which plants 
 frequently receive much injury from cold, walls must be beneficial 
 in a third way, namely, by preventing, in part, the loss of heat, 
 which they would sustain from radiation, if they were fully exposed 
 
APPEARANCES CONNECTED WITH DEW. 61 
 
 to the sky, the following experiment was made for the purpose of 
 determining the justness of this opinion. 
 
 A cambric handkerchief having been placed, by means of two 
 upright sticks, perpendicularly to a grassplat, and at right angles to 
 the course of the air, a thermometer was laid upon the grass close to 
 the lower edge of the handkerchief, on its windward side. The 
 thermometer thus situated was several nights compared with 
 another lying on the same grassplat, but on a part of it fully ex- 
 posed to the sky. On two of these nights, the air being clear and 
 calm, the grass close to the handkerchief was found to be 4 warmer 
 than the fully exposed grass. On a third, the difference was 6. 
 An analogous fact is mentioned by Gersten, who says, that a hori- 
 zontal surface is more abundantly dewed than one which is per- 
 pendicular to the ground. 
 
 IV. The covering of snow, which countries in high latitudes 
 enjoy during the winter, has been very commonly thought to be 
 beneficial to vegetable substances on the surface of the earth, as far 
 as their temperature is concerned, solely by protecting them from 
 the cold of the atmosphere. But were this supposition just, the 
 advantage of the covering would be greatly circumscribed ; since 
 the upper parts of trees and of tall shrubs are still exposed to the 
 influence of the air. Another reason, however, is furnished for its 
 usefulness, by what has been said in this Essay ; which is, that it 
 prevents the occurrence of the cold which bodies on the earth ac- 
 quire, in addition to that of the atmosphere, by the radiation of their 
 heat to the heavens during still and clear nights. The cause, indeed, 
 of this additional cold, does not constantly operate : but its presence, 
 during only a few hours, might effectually destroy plants which now 
 pass unhurt through the winter. Again ; as things are, while low 
 vegetable productions are prevented, by their covering of snow, 
 from becoming colder than the atmosphere in consequence of their 
 own radiation, the parts of trees and tall shrubs, which rise above 
 the snow, are little affected by cold from this cause. For their 
 outermost twigs, now that they are destitute of leaves, are much 
 smaller than the thermometers suspended by me in the air, which 
 in this situation very seldom became more than 2 colder than the 
 atmosphere. The larger branches too, which, if fully exposed to 
 the sky, would become colder than the extreme parts, are, in a 
 great degree, sheltered by them ; and, in the last place, the trunks 
 are sheltered both by the smaller and the larger parts, not to mention 
 that the trunks must derive heat, by conduction through the roots, 
 from the earth kept warm by the snow.* 
 
 In a similar way is partly to be explained the manner in which 
 
 * It may be remarked here, however, that a thick covering of snow, while 
 it renders the surface of the earth warmer than it would otherwise be, must 
 occasion the lower atmosphere to be colder, by preventing the passage of the 
 heat of the ground to the air, either by radiation or conduction. 
 
62 ESSAY ON DEW. 
 
 
 a layer of earth or straw preserves vegetable matters in our own 
 fields from the injurious effects of cold in winter. 
 
 V. The bare mention of the subject of this article will be apt to 
 excite ridicule, it being an attempt to show in what way the expo- 
 sure of animal substances to the moon's light promotes their putre- 
 faction. I have no certain knowledge that such an opinion pre- 
 vails any where, at present, except in the West Indies ; but I con- 
 clude, from various circumstances, that it exists also in Africa, and 
 that it was carried thence by negro slaves to America. It was en- 
 tertained, however, by persons of considerable rank and intelligence 
 among the ancients ; for Pliny* affirms it to be true, and Plutarch, 
 after making it a subject of discussion in one of his Symposia,t 
 admits it to be well founded. 
 
 As moonbeams communicate no sensible heat to the bodies on 
 which they fall, it seems impossible that they can directly promote 
 putrefaction. But still a reason for ascribing such a power to them, 
 may be derived from their being received by animal substances, at 
 the very time that a real, but generally unnoticed cause of putrefac- 
 tion in warm climates, (and it is in these alone the opinion I am 
 treating of has ever prevailed) is taking place, which ceases to act 
 as soon as the moon's light is excluded. 
 
 The nights on which a steady moonshine occurs must necessarily 
 be clear; and nights which are clear are almost always calm.J A 
 moonshiny night, therefore, is one on which dew forms plentifully ; 
 hence the expressions " roscida" and " rorifera luna" employed by 
 Virgil and Statius ; and hence also an opinion held, as appears from 
 Plutarch, even by philosophers among the ancients, that the moon 
 communicates moisture to the bodies which are exposed to its light. 
 
 Animal substances are among those which acquire dew in the 
 greatest quantity. To do this, indeed, they must previously become 
 colder than the atmosphere; but having acquired the moisture of 
 dew in addition to their own, they will on the following day be in 
 that condition which is known by experience to favour putrefac- 
 tion most powerfully in hot climates. 
 
 The immediate cause assigned here for the quick putrefaction of 
 animal substances, which have been exposed to the moon's rays in a 
 hot country, is the same as that given by Pliny and Plutarch ; but 
 they attributed the origin of this immediate cause, the additional 
 moisture, to the peculiar humefying quality which they supposed 
 that luminary to possess. This false theory has probably contri- 
 
 * Lib. ii. civ. t ^ib. iii. Prob. x. 
 
 $ Mr. De Luc has remarked, that clouds frequently disappear soon after sunset. 
 Idees sur la Meteorologie, ii. 98. I have often observed this myself, and at;the 
 same time another fact of which he takes no notice; namely, that the atmosphere 
 is then calmer than it had been before sunset. This calmness of the air very 
 commonly, if not always, precedes the dissipation of the clouds. 
 
 A kin to this opinion of the ancients respecting the humefying quality of the 
 moon, is one, which has been held by modern writers as well as ancients, upon 
 that planet's being a cause of cold to the bodies which receive its rays ; though 
 I know of no author who has taken notice of this affinity. 
 
APPEARANCES CONNECTED WITH DEW. 63 
 
 buted to discredit, with the moderns, the circumstance which it was 
 employed to explain. * 
 
 VI. The last fact of which I shall treat in this Essay, is the forma- 
 tion of ice, during the night in Bengal, while the temperature of the 
 air is above 32. 
 
 I have seen only two original descriptions of this process, both of 
 which are contained in the Philosophical Transactions ; the first by 
 Sir Robert Barker, in the 65th volume; the other in the 83d, by 
 Mr. Williams. 
 
 According to the method followed by Sir R. Barker's ice-maker, 
 square excavations 2 feet deep, and 30 wide, having been formed in 
 a large open plain, their bottoms are covered with sugar-cane, or 
 stems of Indian corn, dried to the thickness of 8 inches or 1 foot. 
 On this layer, are afterwards placed in rows, near to each other, 
 small unglazed earthen pans, \ of an inch thick, and 1 inch and 
 deep, filled with boiled soft water. The pans are sufficiently porous 
 to allow their outer surface to appear moist, after water has been 
 poured into them. Sir. R. Barker adds ; that the nights the most fa- 
 vourable for the production of ice, are those which are the calmest 
 and most serene, and on which very little dew appears after mid- 
 night; that clouds and frequent changes of wind are certain pre- 
 ventives of its formation ; and that, although ice is thus very readily 
 procured by art in Bengal, during the winter, it scarcely ever occurs 
 there naturally. 
 
 The process described by Mr. Williams must, from its extent, 
 300 persons being employed in it, have been carried on for profit, 
 and would, consequently be conducted in the most economical man- 
 ner. A piece of ground nearly level, containing about 4 acres, was 
 divided into square plats, from 4 to 5 feet wide, which were sur- 
 rounded by little mounds of earth 4 inches high. In these inclosures, 
 previously filled with dry straw, or sugar-cane haum, were placed 
 as many broad, shallow, unglazed earthen pans, containing unboiled 
 pump water, as they could hold. The air was generally very still 
 when much ice was formed ; wind prevented its formation alto- 
 gether. In the morning, between 5 and 6h., at which time alone 
 Mr. Williams made his observations, a thermometer, with its bulb 
 naked, placed on the straw, amidst the freezing vessels, was never 
 found by him lower than 35; and he has observed ice, when a 
 thermometer so placed was 42. Another thermometer, suspended 
 5^ feet above the ground, was commonly 4 higher than that among 
 the pans. It is possible, therefore, that Mr. Williams may have 
 seen ice a little before sunrise, when the temperature of the air was 
 46. But granting this were the fact, it would not hence follow, 
 that the ice was formed while the air possessed that heat. For 
 although the air is generally held to be in all countries colder about 
 sunrise than at any other time, I know from my own observations, 
 that this is not always the case in England ; and similar exceptions 
 may dccur in Bengal. Sir H. Davy has said, in his Elements of 
 Chemistry, that ice will form in Bengal when the temperature of 
 
64 ESSAY ON DEW. 
 
 the air is not below 50 ; but he has given no authority for this 
 assertion. 
 
 The formation of ice, in the circumstances which have been 
 just mentioned, was attributed by Sir R. Barker altogether, and by 
 Mr. Williams in great measure, to cold produced by evaporation. 
 Sir R. Barker's opinion has since been adopted by some of our 
 most distinguished writers on Natural Philosophy, as Watson, 
 Thompson, Young, Davy, and Leslie, apparently, however, without 
 their having fully considered it, as I shall now attempt to show. 
 
 1. It is necessary for the complete success of the process, that 
 the air should be very still ; wind, which so greatly promotes 
 evaporation, prevents the freezing altogether. Sir R. Barker admits, 
 that the excavations in the earth are made to increase the stillness 
 of the air in contact with the water in the pans ; but, with the view to 
 explain the utility of this stillness he supposes, in opposition to all 
 experience, that water kept very quiet freezes more readily, when 
 other circumstances are the same, than if it were a little agitated. 
 
 2. No proof is given, that evaporation from the pans actually 
 does occur at the times which are the most favourable for the ap- 
 pearance of ice. At any rate it cannot be considerable ; since 
 agreeably to what is mentioned by Sir R. Barker, dew forms in a 
 greater or less degree during the whole of the nights the most 
 productive of ice; and it is not to bethought, as was said upon a for- 
 mer occasion, that one portion of air will be depositing moisture, 
 from possessing a superabundance of it, while another in the imme- 
 diate vicinity is receiving moisture in great quantity, in the state of 
 pellucid vapour ; as the latter fact can exist only when the air is 
 far removed from a state of repletion with water. 
 
 3. If evaporation produced the cold under consideration, the 
 wetting of the straw or other matter, upon which the pans are 
 placed would tend to increase it; and, accordingly, Sir H.Davy 
 affirms this to be the case. But Mr. Williams, who must here be 
 regarded as the better authority says, that it is necessary to the 
 success of the process that the straw be dry ; in proof of which he 
 mentions, that when the straw becomes wet by accident, it is re- 
 placed ; and that when he purposely wetted it. in some of the 
 inclosures, the formation of ice there was always prevented. The 
 reasons are clear. The water, by softening the straw, renders it 
 easily compressible by the weight of the pans, and at the same time 
 fills up what would otherwise be vacant spaces among its parts. 
 The straw, therefore, in this condensed state, must afford a ready 
 passage to heat from the earth to the pans, the hindrance of which 
 is allowed by every person to be the use of it in this process when 
 dry. Again ; the moisture which passes through the straw to the 
 earth it covers, will rise afterwards in the form of vapour, having 
 the same temperature with the warm ground, and will communicate 
 heat to the pans. In the last place; a part of this vapour will be 
 condensed into water by the pans, in consequence of which heat 
 must be extricated. 
 
APPEARANCES CONNECTED WITH DEW. 65 
 
 4. It is mentioned both by Sir. R. Barker and Mr. Wjlliams, in, 
 support of their opinions, that the pans, when new, are so porous, 
 that they readily permit water to transude them ; and that old pans, 
 which permit this in a less degree are less fit for the making of ice. 
 But the argument, which is hence derived by them, is completely 
 refuted by a fact related by Mr. Williams himself; for he says that 
 the pans are greased before they are used, to prevent the adhesion 
 of the ice to their sides; since, if this purpose be answered, the 
 water can never be in contact with the pans, and therefore can 
 never pass through them. 
 
 The real reason of the less fitness of old pans for the making of 
 ice is perhaps the following. The production of the cold, which 
 occurs in this process must take place in the water; since neither 
 the straw upon which the pans are placed, nor the air above them, 
 was ever found by Mr. Williams of so low a temperature as 32. 
 Whatever, therefore, obstructs the passage of heat from the straw 
 to the water, must favour the freezing of the latter. But this will 
 be less effectually done by an old than by a new pan, as the density 
 of the former is greater, from the grease forced into it by rubbing, 
 and from the slime and sand that will enter with the water into its 
 pores, when these are not entirely closed by the grease : which 
 must often happen as the smearing is performed only once in three 
 our four days. The difference, however, in effect betwixt old and 
 new pans must be very small ; as it does not appear that the old are 
 ever laid aside on account of their unfitness. 
 
 In a like way may be explained, without the aid of cold pro- 
 duced by the evaporation of moisture from the outsides of the pans, 
 another fact mentioned by Mr. Williams, that ice was often found 
 by him in those vessels, while water contained in a china plate, sur- 
 rounded by them had none ; since the thin and dense substance of 
 the plate must have transmitted more readily, than the thick and 
 rare substance of the pans, the heat of the straw to the water. 
 
 5. In accounting for the making of ice in Bengal, it is requisite 
 to show, not only how the first film is produced, but also in what 
 way the thickness of this film is afterwards increased. If evapora- 
 tion be the cause of this increase, it follows that a plate of ice in the 
 night-time, and in the stillest air, both unfavourable to that process, 
 must yet emit as much moisture, as is necessary for the production 
 of a cold, according to Mr. Williams, of at least 14, and according 
 to Sir H. Davy of at least 18 ; a conclusion, as it appears to me, of 
 itself sufficient to destroy the credit of the theory from which it is 
 drawn. 
 
 While attending to this subject, I became desirous of acquiring 
 some knowledge of the degree of cold, which might be produced 
 by evaporation from water contained in a shallow vessel. With 
 this view I placed on a feather bed, situated between the door and 
 window of a room in my house in London, two china plates, into 
 one of which as much water was poured as covered its bottom to 
 the depth of $ of an inch. The other plate was kept dry. The 
 OCT. 1838. X 7 
 
66 ESSAY ON DEW. 
 
 bulb of a small thermometer being then applied to the inside of the 
 bottom of each plate, I observed upon many days in various seasons 
 of the year, the difference between these instruments while the 
 door and window were open. I found in consequence, that when 
 the temperature of the air in the room was 75, the highest at which 
 any experiment was made, the thermometer in the plate, containing 
 water, was between 6 and 7 degrees lower than the one in the dry 
 plate ; that the difference between these thermometers diminished 
 gradually as the air became colder ; and that when the temperature 
 of the air was 40, the lowest for which I have any observation, the 
 difference was only l^ . At 32, therefore it would have been very 
 small, and at a few degrees below 32 it would probably have van- 
 ished. This supposition agrees with an observation made by Mr. 
 Wilson of Glasgow, who found'that no cold was produced by evapo- 
 ration from snow possessing a temperature of 27, though the air in 
 the immediate neighbourhood was purposely much agitated by him. 
 
 The conclusions here given by me, respecting the cold produced 
 by the evaporation of water, were drawn from experiments made 
 in the day, while the sky was clear, the air very calm, and the 
 temperature of the atmosphere stationary. At night and during a 
 cloudy day, the differences were less. On the other hand, if there 
 was any perceptible motion in the air, they were greater. They 
 were also greater if the heat of the atmosphere was increasing ; 
 but less if this^was decreasing. 
 
 Having thus, I think, placed beyond doubt, that the formation of 
 ice in Bengal is not occasioned by evaporation, I shall now state 
 several reasons, which have induced me to believe, that it depends 
 upon the radiation of heat to the heavens. 
 
 1. This cause not only exists, but exists in a degree sufficient for 
 the production of the effect which I attribute to it. For Mr. 
 Wilson found the surface of snow, during a clear and calm night, to be 
 16 colder than air 2 feet above it, the temperature of the latter being 
 taken by a naked thermometer ; whereas the greatest heat of the 
 atmosphere ever observed by Mr. Williams, at the distance of 5i 
 feet from the ground, during the time that he supposed ice to be 
 forming, was only 14 higher than the freezing point of water. I 
 need say nothing of the difference of 18 related by Sir H. Davy, 
 as he does not speak from his own observation, and as he gives no 
 authority for what he advances ; though even this difference is con- 
 siderably less than what I have attempted to show must sometimes 
 occur from the radiation of heat at night, between the temperature 
 of air a few feet above the earth and that of bodies placed on its 
 surface. 
 
 It is to be mentioned here also, that, according to Mr. Leslie,* 
 the power of water to radiate heat exceeds, perhaps, that of all other 
 substances. 
 
 2. Ice is chiefly formed in Bengal during the clearest and calmest 
 
 * On Heat, p. 80. 
 
APPEARANCES CONNECTED WITH DEW. 67 
 
 nights : and it is on such nights that the greatest cold, from radia- 
 tion, is observed on the surface of the earth. In Sir R. Barker's 
 more refined mode of conducting the process, an unusual stillness 
 of the air, in contact with the water to be frozen is procured, by 
 placing the pans containing it a little below the level of the ground, 
 in which situation, it was formerly shown, bodies must grow colder 
 from radiation to the heavens at night than in any other. 
 
 3. The cold, by means of which ice is produced in Bengal, ap- 
 pears, as I think may be inferred from what is said by Sir R. Barker, 
 in its greatest degree, like cold from radiation in other substances, 
 on those still and serene nights, during which little dew is deposited 
 by the atmosphere. 
 
 4. Clouds and wind prevent the formation of ice in Bengal ; and 
 the same states of the atmosphere either prevent, or considerably 
 diminish the occurrence of cold from the radiation of heat at night 
 by bodies on the ground. 
 
 I shall close this subject, by giving some account of a few attempts 
 to procure the freezing of water at night in this country, by ex-' 
 posing it to air of a temperature higher than that of 32. These 
 were made by me in 1812, at my usual place of experiment, which 
 was formerly stated to be not well adapted for the appearance of a 
 great cold from radiation, and on nights not among the most favour- 
 able to such an undertaking, even of those which occur in this 
 country. It is proper also to mention, that I was then less able to 
 conduct such experiments, and to make use of them, than I after- 
 wards became from a longer attention to similar objects. 
 
 EXPERIMENT I. 
 
 With a view to imitate the method of making ice described by 
 Sir R. Barker, I had a pit dug, on the evening of the 3d of May, 
 in the middle of the garden so often spoken of, 4i feet long, 3 wide, 
 and 2 deep. It consequently had the same depth as the excavations 
 mentioned by that gentleman, but was considerably less in its other 
 dimensions. Clean dry straw was then strewed, to the height of a 
 foot, over the bottom of the pit. On the straw were next laid a 
 number of small shallow earthen pans, a part of which were glazed, 
 and a part unglazed. In the last place ; all the pans were filled with 
 soft water, which had been boiled on the same evening. Contrary to 
 my expectation, the unglazed pans remained as dry on the outside, 
 after water had been poured into them, as those which were glazed. 
 I conclude, therefore, that the former were more dense in their 
 substance, than the unglazed pans used in India ; and that their 
 density was probably the reason, why ice did not afterwards form 
 in them sooner than in the glazed pans which were employed by 
 me. 
 
 Two pans, containing boiled water, were set upon the grassplat, 
 
68 ESSAY ON DEW. 
 
 at a little distance from the pit. A watch-glass filled with boiled 
 water was also placed upon the grassplat, and another was laid upon 
 the raised board, which had been thinly covered with sand. All 
 these arrangements were not completed before lOh. at night. 
 
 At Ih. in the morning, ice appeared in the watch-glasses on the 
 grassplat and raised board ; the heat of the air, as measured by a 
 naked thermometer, being then, at 4 feet above the ground, 39, 
 and at 7 feet, 40i. At 2h. ice was observed in the pans in the pit, 
 while a thermometer in the air, 2 feet above the ground, was 36^. 
 Shortly afterwards, ice began also to form in the pans upon the 
 grassplat. The temperature of grass fully exposed to the sky, was 
 at the same time 30, while that of the earth an inch below the 
 bottom of the grass was 45. During the time of these observations 
 dew formed copiously. 
 
 EXPERIMENT II. 
 
 My next attempt was in the manner mentioned by Mr. Williams. 
 
 On the evening of the 22d of May, I encompassed a square piece 
 of level ground, the sides of which were 3 feet long, with a border 
 of earth 4 inches high, and filled the area with dry straw. On this 
 were placed several of the earthen pans, which had been formerly 
 used, and a few smaller vessels, all containing unboiled water. After 
 an exposure of little more than an hour, water in a watch-glass 
 upon the straw was found frozen, the temperature of the air, 2 feet 
 above the straw, being then 37. In half an hour more, ice began 
 to appear in the earthen pans, while a thermometer 5i feet above 
 them, this being the height at which Mr. Williams used to suspend 
 his instrument, was 36. The air soon after became colder ; but 
 its temperature was never less than 33, though taken by a naked 
 thermometer, which, as was before said, upon a clear and calm 
 night, occasions the air to seem about 2 colder than it really is. 
 
 It might be inferred, from what is mentioned by Mr. Williams, 
 that the temperature of the straw beds, on which the ice-pans were 
 set at Benares, was always found by him above the freezing point, 
 for this reason, that the straw, from containing no moisture, could 
 not, like the water, grow cold by evaporation. I had, therefore, 
 been surprised, during the first experiment, for I had then but 
 little acquaintance with the phenomena of cold observed with dew, 
 that a thermometer, laid upon an exposed part, of the straw, was 
 always below the freezing point, after ice had begun to form in the 
 pans. On reading, however, his account of the process a second 
 time, with increased attention, my wonder ceased. For, as the 
 pans he speaks of were large., and touched one another, and as all the 
 pans employed in India, for the making of ice, widen as they rise 
 from the bottom, like our milk-pans, the thermometer placed by 
 him on the straw must have been secluded from all view of the 
 
APPEARANCES CONNECTED WITH DEW. 59 
 
 sky, and would therefore mark a temperature much higher than if 
 it had been laid, as in my experiment, upon straw fully exposed to 
 the heavens. On this, the second night, therefore, I placed a 
 thermometer under the edge of one of the pans lying on the straw 
 bed, and found it some time afterwards 6 higher than a similar in- 
 strument upon a part of the straw bed which was uncovered. Gener- 
 ally, however, the difference was not so great. If my pans had 
 been large, like those of Mr. Williams, I should, no doubt, have 
 observed more considerable differences ; for, in consequence of their 
 smallness, I could not lay a thermometer on the straw bed, so as to 
 be fully screened from the sky by the edge of any of them, with- 
 out its being almost in contact with the vessel, every part of which 
 was always colder than the sheltered straw. 
 
 Much dew formed in the course of this night. The greatest dif- 
 ference marked by me, during it, between the temperatures of grass 
 and of air, was 6, and between those of air and a fully exposed part 
 of the straw bed 9. 
 
 EXPERIMENT III. 
 
 This was begun on the evening of the 16th of October, and was 
 likewise made agreeably to the method related by Mr. Williams. 
 
 Ice appeared in the pans, when the temperature of the air, at 
 the height of 5i feet, was, according to the naked thermometer, 37. 
 
 On this night, I placed upon the straw bed a dry earthen pan, 
 among those which contained water, and found the inside of its 
 bottom to be as much colder than the air as the water was in the 
 other pans, before ice appeared in them. After the water had begun 
 to freeze, no proper comparison could be made between its tempe- 
 rature and that of the empty pan. This pan, in the course of the 
 night, attracted moisture, which was afterwards converted into a 
 film of ice. 
 
 But the chief fact established by the present experiment was, that 
 water may freeze at night, in air of a temperature higher than 32, 
 not only without any loss of weight from evaporation, but with a 
 gain of weight from an opposite process. 
 
 I had observed that water, exposed early in the evening in the 
 open air to the sky, lost a little weight, in the course of a clear night. 
 This I imputed to evaporation taking place before the water had 
 been cooled enough to condense the vapour of the atmosphere, and 
 to the weight gained afterwards being insufficient to compensate 
 the previous loss. I exposed, therefore, on this night, water to the 
 influence of the sky, until it was cooled to 34. Of this I put 2 
 ounces into each of two china saucers, which had also been exposed 
 to the air, and then placed the saucers upon the straw bed. In the 
 morning a thin cake of ice was found in both saucers, one of which 
 had gained 2, and the other 3 grains, in weight. Dew was also 
 
70 ESSAY ON DEW. 
 
 copious on this night. At one time, grass was 9^, and the exposed 
 part of the straw bed 12, colder than the air.* 
 
 It must be evident to every person, that the formation of ice, in 
 the three preceding experiments, was the effect of a natural opera- 
 tion, similar to that by which the same substance is produced in 
 Bengal. These two facts must, therefore, have a common cause, 
 and this has been shown, by the last experiment, independently of 
 what was said before in this Essay, not to be evaporation. It is also 
 clear, that the cold induced on the water in those experiments, had 
 a common cause with that observed at the same time upon the grass 
 and the straw ; which latter cold must, in consequence of proofs 
 formerly given, be admitted to have arisen from the radiation of 
 the heat of those substances to the heavens. A necessary inference, 
 therefore appears to be, that the formation of ice, in Bengal, in the 
 circumstances described by Sir R. Barker and Mr. Williams, must 
 likewise be attributed in by far the greater measure, if not altogether 
 to a loss of heat, which the water suffers by its own radiation, while 
 situated in such a manner, that it can receive little heat from other 
 bodies, either by radiation or conduction.! 
 
 CONCLUSION. 
 
 The experiments which were made by me on dew, and other 
 subjects treated of in the preceding Essay were unavoidably attend- 
 ed with many inconveniences, which were the more felt, as my 
 health had long been feeble, and as my professional duties obliged 
 me often to return to London in the morning, without having pre- 
 viously taken rest, after the whole of a night having been spent in 
 attending to the objects of my pursuit. The inconveniences here 
 
 * The greater cold, observed in this and the preceding experiment, upon straw 
 than upon grass, is to be referred to the shortness of the latter, by reason of which 
 heat was readily communicated to its upper parts by the earth. 
 
 j- On the evenings preceding the nights during which ice is produced in Ben- 
 gal, the temperature of the water exposed in the pans is, probably, often 60 or 
 more. But water of the heat of 60, if exposed in a shallow earthen vessel to 
 air of the same temperature, during the day, while the weather is calm and clear, 
 will loose about 3 of heat by evaporation. A cold from this cause may, there- 
 fore, concur with that from radiation, and, consequently, may, in Bengal, ac- 
 celerate somewhat the formation of ice. The influence, however, of evaporation 
 there, in this respect, should the state of the air with regard to moisture still per- 
 mit it, which must often not be the case while dew is forming, will, as the night 
 proceeds, gradually diminish, and at length almost disappear, before the freezing 
 of the water commences ; since I have lately shown, that evaporation from water 
 of 32 produces very little cold, even in the day-time. Indeed, it seems to me 
 much more probable, that on a clear and calm night, though in a dry winter of 
 Bengal, water, at the temperature of 32 will acquire warmth from the formation 
 of dew upon it, than that it will become cold from evaporation. 
 
APPEARANCES CONNECTED WITH DEW. 71 
 
 alluded to were indeed so great, that I was twice or thrice obliged 
 to intermit my labours for several months together, and at length 
 found it necessary to cease from them entirely, before I had nearly 
 completed the plan which I had formed. I take the liberty of 
 mentioning these things, to excuse in part the imperfections which 
 will be observed in what I have written, as some of them would, 
 no doubt, have been removed by a further interrogation of Nature. 
 
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