Post 8vo. 10. 6rf.
OBSERVATIONS IN METEOROLOGY.
By the Rev. LEONARD JENYNS, M.A., F.L.S., &c. ;
LATE VICAR OF SWAFFHAM BULBECK.
" For thirty years, Mr. Jenyns informs us, he has kept
a meteorological register, and for nineteen years his
observations have been made with the best instruments,
and with all the precautions which modern science could
furnish. A thoroughly practised observer, therefore,
writes this book, which, as a guide to the climate in
Cambridgeshire, is of the utmost interest, and as a record
of meteorological phenomena is of high scientific value.
The subjects which have especially claimed the attention
of the author are the following : The Thermometer and
Temperature, leading on to a careful consideration of the
Phenomena of the Winds. The Barometer and Atmo-
spheric Pressure. The Aqueous Phenomena of the At-
mosphere forms a well-considered section of the work ;
and the facts brought together in relation to Dew,
Clouds, Rain, Hail and Snow are especially important,
while the deductions are made with the caution and
exactness of a well-trained philosophic mind. The chap-
ters on Thunder Storms and on ' General Observations
on the Weather ' are each good. The latter, especially,
explains in a satisfactory manner the causes leading, in
this climate, to the sudden changes for which these
islands are remarkable ; and it treats of many of the
popular errors respecting the influences of the moon, &c.,
in a clear and logical manner. The chapter on the
Climate of Cambridgeshire may be thought to be of
merely local interest j but, although it bears especially
upon meteorological phenomena observed in that county,
it will be found to have a much wider application. The
1 Observations on Meteorology ' may fairly take its place
beside Howard's 'Climate of London' and White's
' Natural History of Selborne.' " Athenceum. April 17,
1858.
"For nineteen years Mr. Jenyns, an acute observer
and eminent naturalist, resided at Swaffham Bulbeck, a
little place in Cambridgeshire, diligently noting down
during all that time the variations in the weather, and
drawing conclusions therefrom, when anything could be
concluded. The result of his long observations is now
before the public in the form of a very well-written,
well-arranged, well-considered, well-condensed, and well-
indexed volume, which will we trust become the table
companion of all who wish to know something of the
true nature of this our variable climate." Gardener's
Chronicle, May 22, 1858.
" If the example of the author were followed by other
clergymen through the country, within a few years a
mass of meteorological information might be collected
which would cease to be of merely local value, and
become of the highest scientific importance. As an ex-
ample of the mode in which such a work should be
undertaken, we regard Mr. Jenyns' book as a most use-
ful one, and we hope that it may be the precursor of
many similar contributions to our knowledge of climate."
Natural History Review, January, 1860.
" The book has the merit of its author's other writings,
clearness of statement, sound judgment, and accuracy
of observation." Westminster Review, Oct., 1858.
" The ' Observations in Meteorology ' is written with
the painstaking care which characterizes all the works of
that gentleman; whatever he states as a fact may be
relied on. It contains a great deal of original observa-
tion, and many hints which will assist the meteorologist."
Edinburgh New Philosophical Journal, July, 1858.
"Chapter XXX. is full of minute and little-known
particulars about clouds and fogs and mists He has
endeavoured, not unsuccessfully, to do for the sky what
Gilbert White did for the fields." Guardian, June 30,
1858.
JOHN VAN VOOEST, 1 PATEENOSTEE EOW.
If i e
Jfy.3.
'
M.
Jta.
/
Lonjdon: JoTxn. Van Voorst, 1855 .
PRACTICAL
METEOROLOGY
BY
JOHN DREW, PH.D., F.R.A.S.
H
SECOND EDITION,
EDITED BY
FREDERIC DREW.
LONDON :
JOHN VAN VOORST, PATERNOSTER ROW.
MDCCCLX.
/I
PRINTED BY TAYLOR AND FRANCIS,
RED LION COURT, FLEET STREET.
ALEEE ft FLAMMAM.
J}7
PEEP ACE.
IN the year 1847, the author, having resolved to
undertake a series of observations on the climate
of Southampton, made inquiries for some treatise
on the subject of Meteorology which should assist
him in the choice and use of instruments ; such a
work was not to be found, and the required infor-
mation was obtained from intercourse with some
of the most eminent meteorologists at home and
abroad, together with an attentive study of the
Greenwich Observations, and of the records of
various scientific Societies.
Three years later the author was invited to
(5372415
VI PREFACE.
assist at the formation of the British Meteorolo-
gical Society ; and being anxious, as a Member of
the Council, to promote its object, he published in
a scientific periodical, a series of papers " On the
Instruments used in Meteorology, and on the
Deductions from the Observations," which were
extensively circulated by the Secretary among the
Members, of whom several, whose position added
weight to their representations, urged the author
to republish the series in a more permanent form ;
the result of seven years' experience as an observer
and student is the present volume, which professes
to teach " how to observe " meteorological phe-
nomena.
The author apprehends that every possessor of
a Barometer, Thermometer, or Hygrometer, will
here find ample directions for its advantageous
use ; while the general reader will learn from this
work the progress which has been made and the
PREFACE. Vli
appliances which have been brought to bear in
the pursuit of the laws which regulate atmospheric
changes.
The work consists of three Parts :
Part I. Introductory : on the Laws of Heat as
affecting Atmospheric Changes.
Part II. Instruments of Observation described.
Deductions from Observations on the Thermo-
metric, Hygrometric, Barometric, and Electric
Condition of the Air.
Part III. The present state of Practical Meteo-
rology in this Country. Description of the
Photographic Registration of Phenomena at
the Royal Observatory, Greenwich.
The various Tables which are scattered through-
out the work will be found to contain every
requisite for the reduction of observations. For
viii PREFACE.
facility of reference the principal of these are here
enumerated :
Comparison of Therm ometric Scales, p. 285.
Diurnal Range of Temperature, p. 63.
Diurnal Range of Evaporation, p. 168.
Tension of Aqueous Vapour, p. 292.
Degree of Humidity from Readings of the
Dry- and Wet-bulb Thermometers, p. 296.
For measuring Heights with the Barometer,
p. 215.
Weight of Vapour in a Cubic Foot of Air, p. 293.
Weight of a Cubic Foot of Air, p. 295.
Correction of the Barometric Reading for Tem-
perature, p. 297.
Surbiton, Kingston-on-Thames,
March, 1855.
PREFACE TO THE SECOND EDITION.
IN this edition of "Practical Meteorology," I
have endeavoured to give an account of every
useful instrument that has been invented since
the first was published, and, by a slight change
in the arrangement of the sections of the book,
to put more clearly the information it is in-
tended to convey ; I have, in fact, tried to make
it bear the same relation to Meteorological
Science as it is now, that, from the way in
which the first edition was received, it may well
be supposed the book had five years ago.
F. D.
Museum of Practical Geology,
Feb. 2, 1860.
CONTENTS,
PABT I. INTRODUCTION.
Page
Definition. General view of the relation of the
atmosphere to the earth. Composition of the
atmosphere. Proportion of the constituents.
Air at high elevations. Ozone. How the ele-
ments are combined. Mechanical properties of
air. Weight. Elasticity. Aqueous vapour
combined with the atmosphere. Dalton's
theory of the composition of the air. Baro-
meter explained. Ratio of decrease of atmo-
spheric pressure. Mariotte's law. Experi-
mental proof. Ratio between height and pres-
sure explained. Proof of the same. Limit of
the atmosphere. Heat a powerful agent in
atmospheric changes. Laws of heat : expan-
sion. Exception to the law. Amount of ex-
pansion. Expansion of gases by heat. Con-
duction. Illustrative experiment. Good and
bad conductors. Convection. Radiation and
absorption. Reflexion. Latent heat. Evapo-
ration. Specific heat. Coldness of the upper
regions of the air. Colour of the air 1-35
Xll CONTENTS.
PART II. OBSERVATION AND DEDUCTIONS.
Page
Arrangement of the subject 36
1. The Thermometric Condition of the Air.
The Thermometer. Construction of a standard
thermometer. The zero-point. The boiling-
point. The Scale : Fahrenheit's division.
Divisions of the Scale according to Celsius
and Reaumur. Comparison of thermometers.
Maximum and Minimum Thermometers.
Sixe's Register Thermometer. Rutherford's
Register Thermometer. Negretti and Zambra's
Maximum and Minimum Thermometers.
Hick's Maximum and Minimum Thermometer.
Observations to determine temperature.
Diurnal range. Projection of the same. Hours
in the day when the thermometer shows the mean
monthly temperatures at Greenwich. Proper
times of observation. Glaisher's factors.
Example of their application. Monthly mean
temperature from maxima and minima. Ex-
ample. Explanations. Projection of six years'
observations. Care necessary in observation.
Position of instruments. The Author's ther-
mometer stand. Greenwich thermometer stand.
Mean yearly temperature at Greenwich.
Comparison with that of other places. Mean
temperature fails in describing climate. Mean
daily temperature at Greenwich. Table show-
ing how the heat increases through the year.
Dove's Isothermal Charts. Deductions from
them. Continental and insular climate com-
CONTENTS. Xlll
Page
pared : Example. Temperature affected by the
neighbourhood of water: Example. Decrease of
temperature as height from the sea-level in-
creases. Effect of the sun's heat below the sur-
face of the soil. Observations at Greenwich.
Stratum of invariable temperature. Radiating
thermometers. Actinometer. Wind. Land
and sea breezes. Trade- winds. Circular
storms. Anemometers : Osier's. WhewelTs.
Robinson's. Lind's Wind-gauge. Graphic
delineation of results of wind observations.
Rules for observing 36-122
2. The Hygrometric Condition of the Air.
General considerations. Dew. Hoar-frost.
Rain. Hail. Condensation of the vapour in
the air. Saussure's Hygrometer. Tension of
aqueous vapour. Important conclusion. The
dew-point. Degree of humidity. Illustra-
tions. Daniell's Hygrometer. Regnault's Hy-
grometer. Council's Hygrometer. Dry- and
wet-bulb thermometers, or Mason's Hygrometer,
Dr. Babington's Evaporation-gauge. Ap-
john's formula. Example of its use. Other for-
mulae. Glaisher's factors. Other deductions.
Weight of a cubic foot of dry air. Weight of
a cubic foot of vapour. Amount required for
saturation. Weight of a cubic foot of moist air.
Welsh's Sliding-rule. Glaisher's Tables.
Deductions worked out in full. Diurnal range
of temperature of evaporation and the dew-
point. Observations in the higher regions of
the atmosphere. General remarks. Clouds.
XIV COJsiTENTS.
Page
Their appearance. Height. Classification.
The Cirrus. The Cumulus. The Stratus.
Registration of amount of cloud. Rain : 'Snow :
Hail. : Rain-gauge. Position of rain-gauge.
Weight of rain-water. Example. Rule.
Relative amount of rain. Deductions. Snow.
Hail 122-196
3. Tlie Barometric Condition of the Air.
Fluctuations of the barometer. Atmospheric
waves. Diurnal variation of the barometer.
Maxima and Minima. Explanation. Charac-
ter of instruments. Corrections of the baro-
metric reading. Capillarity. Temperature.
Capacity. Mountain Barometer. Measure-
ment of heights. Reduction to the sea-level.
Adie's Sympiesometer. The Aneroid Baro-
meter. Heights determined by the boiling-
point of water. Example. Barometric indi-
cations 196-223
4. The Electric Condition of the Air.
Nature of atmospheric electricity. Quetelet's
deductions. Electrical Apparatus at the Kew
Observatory. Peltier's Electrometer. Ozone.
Nature of Ozone. To procure Ozone. Test for
Ozone. Schonbein's Ozonometer. Properties.
Ozone observations. Moffatt's Ozonometer 224-241
PART III. PRESENT STATE OF METEOROLO-
GICAL SCIENCE IN ENGLAND.
British Meteorological Society. Registrar-Gene-
ral's Reports. Conference at Brussels. Re-
CONTENTS. XV
Page
commendation adopted by Government. In-
struments supplied. The Royal Observatory,
Greenwich. On Photographic Registration of
Meteorological Phenomena. Anemometers.
Concluding remarks 242-284
APPENDIX.
Table I. Correspondence of the different Thermo-
metrical Scales 285-292
Table II. Tension, or Elastic Force, of aqueous
vapour in inches of mercury, for every degree of
temperature from to 95. From the Green-
wich Meteorological Observations 292
Table III. The weight (in grains Troy) of vapour
in a cubic foot of saturated air; at all tem-
peratures between 10 and 89 293
Table IV. Factors to be multiplied into the quan-
tities in Table III., when the air-temperature
and dew-point temperature differ by the num-
ber of degrees in the first column 294
Table V. Showing the weight in grains Troy of a
cubic foot of air saturated with moisture, under
the pressure of 30 inches of mercury, at any
temperature between 31 and 90 ; and the ex-
cess of the weight of a cubic foot of dry air,
under the same pressure, over that of a cubic
foot of saturated air, also in grains, throughout
the same range of temperature 295
Table VI. Showing the degree of humidity of tfre
atmosphere, deduced by Apjohn's formula from
the readings of the dry-bulb and wet-bulb ther-
mometers, for the usual range occurring in
England ; complete saturation being 1 296
XVI CONTENTS.
Page
Table VII. Corrections to be subtracted from the
readings of barometers, with brass scales ex-
tending from the cistern to the top of the mer-
curial column, to reduce the observations to
32Fahr. ..297-299
PRACTICAL METEOROLOGY.
PART I.
INTRODUCTION.
1. Definition. A knowledge of the causes which
produce, and of the laws which regulate atmo-
spheric changes, is the object of the science of
Meteorology.
2. General view of the relation of the atmosphere
to the earth. To realize the extent and the pro-
portion which the atmospheric envelope bears to
the earth, let us imagine her to be viewed from a
neighbouring globe. Before the spectator would
float a massive sphere, nearly 8000 miles in dia-
meter ; the whole of its illumined portion would
never be free from clouds and vapours, which
would draw an impenetrable veil over certain
parts of the continents and oceans, so that the
outline of these formations would only be seen
occasionally; in some parts, however, more di-
stinctly than in others. The presence of an
atmosphere would be indicated when the earth
2 PRACTICAL METEOROLOGY.
occulted a star, that is, passed over it as we ob-
serve the moon does in traversing her orbit : the
light of the star would fade by degrees before
the body of the earth reached it, the diminution
commencing when the star entered the atmo-
sphere ; but whether it would be affected when it
first approached its limits is questionable. The
utmost extent to which the air reaches is -^
of the earth's diameter, but probably the effect
would be imperceptible till the star had advanced
towards the earth's disc within -^WJf of her dia-
meter. The observer, if he had the means of
ascertaining, would see that as the earth is not a
sphere, but a spheroid, the equatorial diameter
exceeding the polar by 26^ miles, .so the atmo-
sphere would be in form a spheroid, but not ex-
actly similar to that of the earth which it sur-
rounds ; since, as the force of gravity is at its maxi-
mum at the poles, there the extent of the atmo-
sphere would be less, or it would be more com-
pressed or more dense, seeing that air, like all
other matter, would be under the influence of
gravitating attraction.
The shadows of the mountains would not be
absolutely without light like those on the moon,
but would be partially enlightened by the refrac-
tive and reflective powers of the atmosphere ; the
boundary of light and shade between the bright
INTRODUCTION. 3
and dark portions of the earth's surface would
show a blending of one into the other ; in other
words, day and night would be separated by an
extensive twilight, arising from the dispersion of
the sun's rays by the upper strata of the atmo-
sphere, on which his light continues to fall when
he has set to the surface immediately below them.
It is indeed from the reflective and dispersive
properties of the atmosphere as regards light,
that the shadows of objects on the earth are not
a deep black ; every particle has this power of
scattering the rays of the sun and affording a
secondary illumination.
Such would be the appearance of that " aerial
ocean/' on the bottom or shoals of which man
and all other animated beings find the means of
their subsistence : general views would be attained
from such a point of observation as that we have
supposed; for more particular investigation we
must return to our position on the surface of the
earth itself.
3. Composition of the atmosphere. The at-
mosphere is found, when analysed, to consist for
the most part of two gases, oxygen and nitrogen ;
not chemically combined, but mechanically united.
Oxygen supports combustion energetically, and
in it the vital functions are carried on, but far too
rapidly to consist with the well-being of living
B2
4 PRACTICAL METEOROLOGY.
creatures. Nitrogen extinguishes flame and de-
stroys animal life. If a piece of phosphorus be
burnt in a closed jar of air over water, it will
combine with the oxygen and form phosphoric
acid, which, being taken up by the water, will
leave the nitrogen in the glass ; into this insert a
lighted taper and it will be extinguished a small
animal, and it will expire.
In addition to these two gases, which form the
bulk of the atmosphere, a small portion of car-
bonic acid gas is always present ; it may be de-
tected by exposing to the air a saucer of lime-
water, which will soon become turbid from the
formation of insoluble carbonate of lime.
The air, at all times, has diffused through it a
certain amount of water in the form of vapour,
the quantity of which is variable.
4. Proportion of the constituents. From the
recent very careful analyses of air by M. Dumas,
the following proportions have resulted :
Air by weight. Air by volume.
Oxygen 23*10 20*90
Nitrogen 76*90 79* 10
lOO'OO IOO*OO
The proportion of the constituents of air freed
from moisture, including those of which traces
only are indicated, are as follows in 10,000
volumes :
INTRODUCTION. 5
Composition of dry air by volume.
Nitrogen 7,9 1 2
Oxygen 2,080
Carbonic acid 4
Carburetted hydrogen . 4
Ammonia trace.
10,000
5. Air at high elevations. It is found that in
air, whether taken from near the level of the sea
or at the greatest elevations above it, to which, in
the pursuit of science, we have attained, the pro-
portions of oxygen and nitrogen are the same.
The latest experiments by which this conclusion
has been confirmed, were performed by Dr. Miller,
of King's College, London, on the air brought
down from various heights in one of the balloon
ascents, projected by the Council of the British
Association for the purpose of scientific inves-
tigation.
Dr. Miller thus describes the method and the
results of his analysis, on samples of air brought
down on August 26, 1851 *.
The recipients for the air were wide glass tubes
about 5 cubic inches in capacity ; they were filled
with the specimens to be examined by simply
opening and then closing a stopcock, the alti-
tude being determined by an observation of the
* Philosophical Transactions, 1853.
6 PRACTICAL METEOROLOGY.
barometer at the moment ; the tubes were, within
twenty hours after the air had been collected,
hermetically sealed, and the proportions of oxygen
and nitrogen determined with great care by de-
tonation with hydrogen in " Regnault's Eudio-
meter/' The volumes of oxygen found in the air
collected at different altitudes are given in the
following Table :
Altitude. Vol. of oxygen.
Air collected at King's"!
College / 20 '9 20
Tube 2 1 3,460 ft. ... 20-888
Tube 3 i8,oooft. ... 20747
Tube 1 18,630 ft. ... 20-888
Hence it is found that the composition of the
atmosphere, as Gay-Lussac had announced as the
result of his experiments at a time when the
methods of gaseous analysis were less perfect than
at present, exhibits no sensible difference, whether
taken from near the surface, or at the greatest
heights accessible to man.
The presence of carbonic acid was distinctly
shown by a film of carbonate of lead upon a solu-
tion of the subacetate, which was introduced to a
portion of the air confined over mercury.
6. Ozone. Under certain electric conditions of
the atmosphere, a principle would seem to be de-
veloped, to which the discoverer, Dr. Schonbein
INTRODUCTION. 7
of Bale, has given the name of " ozone;" to this
reference will be made hereafter.
7. How the elements are combined. That the
air is not a compound of oxygen and nitrogen,
but only a mixture of these two gases, has been
argued by a laborious investigator (M. Regnault)
on the following grounds.
a. By the law of chemical composition, gases
combining chemically in volumes are found to be
in certain definite proportions ; the following is
the nearest approach to the proportionate quan-
tities of each gas in atmospheric air :
j- of oxygen or oxygen 20*00
^ of nitrogen or nitrogen 80*00
100*00
These numbers deviate more from the results of
direct analysis than can be due to errors in the
experiments, which are all consistent in giving
20-9 and 79*1 as the per-centage of oxygen and
nitrogen.
/3. When gases combine, heat is always given
out ; but there is no appreciable change of tem-
perature when oxygen and nitrogen gas are mixed
experimentally in the proportions constituting
common air, although the union produces a gas
which is identical with the atmosphere.
y. Water which has remained for a long time
8 PRACTICAL METEOROLOGY.
in contact with air, always holds a certain amount
of it in solution. If the air were a chemical com-
pound, that derived by very nice analysis from
water ought to exhibit precisely the same pro-
perties as atmospheric air, and the proportions of
the gases ought to be the same. If, on the con-
trary, the gases are only mechanically combined,
seeing that water will dissolve -046 of a volume
of oxygen, and only '025 of a volume of nitrogen,
the one gas will be in excess in air derived from
water. The careful analysis of such air has clearly
indicated this fact. Hence we conclude that oxygen
and nitrogen are intermingled, and not combined,
in the compound gas which constitutes the air we
breathe.
8. Mechanical properties of air. Of the mecha-
nical properties of air we shall mention weight
and elasticity.
Weight. Its weight becomes evident when a
receiver, over the top of which has been strained
a piece of bladder, is exhausted by means of an
air-pump : the bladder will be found to curve
downward as the exhaustion proceeds ; and, as the
air underneath is pumped out, it will at last burst
with a loud report.
M. Regnault has lately most carefully deter-
mined the weight of air deprived of carbonic acid
and aqueous vapour. With a pressure equal to
INTRODUCTION. 9
that of 30 inches of mercury, and at a tempera-
ture of 60, 100 cubic inches of air have been
found to weigh 30-82926 grs.
The mean pressure of a vertical column of air
at the sea-level is usually estimated as equal to
30 inches of mercury, or as 15 Ibs. to every square
inch of surface. If, in the experiment mentioned
above, the mouth of the receiver was 6 square
inches in area, on the supposition of a perfect
vacuum being formed underneath, the pressure
on it would be equal to 15 x 6=90 Ibs.
Elasticity. The property of elasticity by
which contraction of bulk occurs on applying pres-
sure, and expansion on its removal is well illus-
trated by the air-gun. A large quantity of air
is forced into a small space ; on its being allowed
to escape, it urges a bullet before it with consider-
able force, in attempting to resume its original
volume.
9. Aqueous vapour exists in combination with
the atmosphere. The vital air the lumen spira-
bile coeli does not consist only of the admixture
of oxygen and nitrogen which has been described ;
co-existing with it is another gaseous fluid, which
is in fact the vapour of water. We may with
truth consider the globe surrounded with two
atmospheres, the one of air, the other of aqueous
vapour, not chemically combined, but commingled
10 PRACTICAL METEOROLOGY.
or mechanically united ; each being, as it were,
diffused through the pores of the other: at all
times and in all places this union exists, though
the amount of the aqueous vapour is variable to
an extreme degreej depending indeed on the tem-
perature of the air, and on the neighbourhood of
large quantities of water. The exact quantity
which exists in the air at any particular instant
may be determined most accurately. The changes
which we observe in climate or in weather, may in
general be traced to the preponderance or defi-
ciency of the vapour of water in the air, and to
the interference of the laws to which it is subject,
with those which would obtain in an atmosphere
of perfect dry ness.
10. Dalton's theory of the composition of the air.
The theory of Dalton with respect to the con-
stitution of the compound atmosphere was, that,
of the various elastic fluids which compose it, the
particles of one have neither an attractive nor a
repulsive power towards those of another, and
that the weight or pressure of any one particle of
any gaseous mixture of this sort arises solely
from the particles of its own kind. According to
this view, it is possible that oxygen, nitrogen, and
aqueous vapour may exist together under any
pressure, while each of them is diffused through-
out the whole space allotted for all. Whether or
INTRODUCTION. 11
not this is absolutely correct, we shall show here-
after that it is possible to appropriate to the air,
and to the water in it, the portion of the entire
pressure of the two united gaseous fluids which is
due to each.
On the supposition of Dalton, Dr. Henry cal-
culated the pressure due to each constituent of
the atmosphere, in a mean state as regards
moisture, to be in such proportion as the follow-
ing:
inches of mercury.
The nitrogen gas exerts a pressure! . ^
equivalent to J
The oxygen gas 6 - i8
The aqueous vapour 0-44
The carbonic acid gas 0*02
3<D'OO
11. Barometer explained. The instrument by
which we arrive at a knowledge of the amount of
atmospheric pressure is the Barometer, the mode
of action of which must be explained at this stage
of our progress. The details of construction will
be omitted till we enter upon the consideration of
the barometric condition of the air more fully in
Part II.
Fill a glass tube 33 or 34 inches in length, and
closed at one end, with mercury, invert it in a
vessel of the same liquid, and a barometer will be
12 PRACTICAL METEOROLOGY.
constructed. The column of mercury d c (Plate I.
fig. 1) will descend until its weight exactly balances
the pressure of the atmosphere on the surface a c
which is open to its influence. A scale measuring
the height of the top of the column above the sur-
face of the liquid in the vessel, which height will
be the same whatever the diameter of the tube,
will show, from time to time, the variation in the
pressure of the air vertical to the place of obser-
vation. The mercurial column will descend as the
pressure or weight of the air decreases, and will
rise as the pressure becomes greater, or as the air
becomes more dense.
12. Ratio of decrease of atmospheric pressure.
It will be readily granted that air near the level
of the sea, having to bear the weight of the entire
mass above it, is denser than any superior stra-
tum. The illustration of Pascal who compared
the atmosphere to a mass of wool, the lower parts
of which having to bear the superincumbent
mass will be compressed into a smaller space
than the upper, will serve to render the fact clear
to every comprehension. To demonstrate the
ratio of decrease as we ascend above the sea-level,
will require the aid of mathematical formula.
13. Mariotte's law. Before entering on this
investigation, we must fully apprehend the bear-
ing of Mariotte's law, namely, that the density
INTRODUCTION. 13
of a gas, other things being equal, is directly as
the pressure that confines it, as is also its elastic
force; while the volume occupied by a certain
mass is inversely proportional to the pressure;
and therefore the density and the elastic force of
a given mass are inversely as the volume. Thus,
if a portion of air which fills two cubic feet of
space be compressed into the space of one cubic
foot, its density will be doubled, and its elastic
force made double of what it was originally ; or,
again, if we take a certain volume of air under a
given pressure, that volume will become twice,
three times, or ten times less, if the pressure
become two, three, or ten times greater.
Mariotte demonstrated this law in 1650, and
late experiments have shown that it holds good,
as regards the air, when it has been expanded to
300 volumes, and also when compressed to -i-th
of its primary value : to this extent have the ex-
periments ranged.
14. Experimental proof. That the density of
the air is proportional to the force which com-
presses it may be thus shown : Let a siphon tube
be taken, CBAD, Plate I. fig. 2, of which the
longer end, not less than 90 or 100 inches, is
open, and the short end supplied with a stopcock
D, which may be opened or closed at pleasure ;
by pouring in a small quantity of mercury, the
14 PRACTICAL METEOROLOGY.
communication between the two ends will be cut
off. Let A a be the level of the mercury in the
two tubes ; close the stopcock D, and it is plain
that the air contained in a D will be of the same
density as the external air. Observe the height
of the mercury in a barometer adjacent at the
time of the experiment, and pour in at C as much
mercury as, when measured from the level of b,
the point to which it will have risen in the short
arm, shall equal the height of the mercury in the
barometer ; we shall now have the column B Z>',
equal to the pressure of another atmosphere, tend-
ing to compress the air in the tube, which will be
found to occupy the space b D, just half of the
original space a D ; that is to say, its density is
doubled ; in other words, the air at first was of
such a density as resulted from its having to bear
the pressure of the atmospheric column above it ;
now, the additional weight of a column of mer-
cury equal to such pressure, has halved its volume
and doubled its density. By adding mercury to
the same amount as before, so that the height
c / C = 2&'B, the air will become subject to the
pressure of three atmospheres, and will occupy
c D, one-third of the original space, and therefore
will be of three times the original density. Hence
we find that in all cases the density is directly
proportional to the compressing force, and the
INTRODUCTION. 15
volume will vary inversely as that force. Now,
to apply this law to the elucidation of the varia-
tion in the pressure of the air on ascending to
different heights above the level of the sea,
15. Ratio between height and pressure explained.
Let us suppose an ascent to have taken place
from the level of the sea, at which the barometer
indicated a pressure of 30 inches of mercury, the
temperature shown by the thermometer being 60
throughout; the decreasing pressures at each
mile of direct ascent would be as follows :
Pressure in inches. Height in miles.
30*000 = a o
24-817 = b i
2O'53O = G 2
16-983 = d 3
14-049 = e 4
Here the heights are evidently a series of num-
bers in arithmetical progression, whose first term
is 0, and common difference 1.
The pressures may be shown to be in geome-
trical progression, for a : b : : b : c : : c : d : : d : e ; or
calling r the ratio between a and Z>, b and c, &c.,
a b c
7 =- = -&c. = r.
bed
The first term in the above series is 30, and
the ratio '827 : here 30 x *827 = 24-817, and
24-817 x -827 = 20-53, and so on.
16 PRACTICAL METEOROLOGY.
To exhibit the same law of decrease in pres-
sure, or increase in volume, of the air in another
point of view, let 1 represent a volume of air at
the sea-level ; then we shall have
Pressure in inches. Height in miles. Volume.
30 o i
IS 2705 2
71 S'4 1 4
31 8-115 8
if- 10-82 1 6
i| '3*525 3*
I| 16-23 64
Here the heights form an arithmetical series, of
which the first term is 0, the common difference
2*705 ; the volumes a geometrical series, the first
term 1, and the ratio 2 ; the pressures (inversely
as the volumes) another geometrical series, the
first term of which is 30, and the ratio ^.
16. Proof of the same. The following is per-
haps the simplest proof that can be given of the
law of the decrease of pressure :
Let B T, Plate I. fig. 3, represent a cylindrical
column of the atmosphere, pressing vertically on
the earth's surface at B ; let it be supposed to be
divided into a number of layers so thin that the
density may be considered uniform throughout
the extent of each. Let BC, CD, DE, EF
be four of these layers, equal in thickness. Now,
INTRODUCTION. 17
the weights of these equal volumes, BC, CD,
DE, EF, are directly as their densities, and the
density of each is directly as the pressure upon
it ; therefore the weights, which let us call
a, b, c, d, are directly proportional to the weight
of air at C, D, E, F, respectively ; that is, putting
t for the weight of air above F, a, b, c, d are as
b+c+d+t, c+d+t, d+t, and t,
a _ b c _d
c+d~+t ~ ~d+i ~ t'
that is, there is a fixed ratio between the weight
of a stratum of air, of a certain thickness, but at
any level, and the weight of air incumbent on it.
Now if we call this ratio -, the pressures at the
successive stations will be
atF t;
atE t+*-,o
at D t
at C t\
(=?
which quantities are a geometrical series with -
for the ratio. Therefore, when the heights as-
cended are, as in this case, in arithmetical pro-
c
18 PRACTICAL METEOROLOGY.
gression, the pressures are in geometrical pro-
gression. Q. E. D.
17. Limit of the atmosphere. From the prin-
ciple now established it may be considered that
although the density of the atmosphere may be
continuously decreasing, yet that it never amounts
to 0, and therefore that the atmosphere is un-
limited in extent, or extending through space in
infinite divisibility. But we must be careful how
we thus reason ; laws regulating matter so atte-
nuated may be yet unknown ; it may be that the
repulsive power of the particles of the atmosphere,
which in all gases is great, is diminished, as the
rarity increases, to such an extent that the weight
or gravitation of the particles at last balances it,
and prevents further divergence ; and thus at last
it may have an upper surface like a liquid. We
are led to conclude, moreover, from the pheno-
mena of refraction, that about 45 miles is the
extent to which the atmosphere reaches above the
surface of the earth.
The exact ratio between the heights ascended
and the pressure will hold good only on the sup-
position that the temperature throughout remains
the same, which is in fact never the case.
18. Heatapowerful agentinatmosphericchanges.
Most of the changes which we observe in climate
or in the weather arise immediately from heat,
INTRODUCTION. 19
which, among its other effects, produces a constant
variation in the amount of aqueous vapour in the
air; whence arises the interference of the laws
regulating a mixed atmosphere, with those to which
dry air would be subject. We shall find, more-
over, that various meteorological phenomena are
brought about by heat absorbed or radiated by the
earth ; and as in the construction of instruments
of observation the effect of heat on the materials of
which they are made must be taken into account,
we shall facilitate our progress by explaining the
laws of heat, at this stage, for future reference.
19. Laws of heat : Expansion. Experiments
show that bodies, with few exceptions, increase in
volume by the application of heat, the increase
taking place regularly on the application of equal
increments of heat, and recover their original size
when reduced to the initial temperature : thus if
the temperature of an iron bar be raised 5, and
thereby increased in length *1 of an inch, by
elevating the temperature 5 more, it will become
2 of an inch longer than at first.
Liquids are amenable to the law of expansion :
insert a tube of glass through a cork into a Flo-
rence-flask filled with water ; apply heat, and the
water will rise in the tube the higher as it be-
comes warmer.
The thermometer, the instrument by which we
c2
20 PRACTICAL METEOROLOGY.
measure any degree of heat below that of a fire or
a furnace, is constructed on the principle of mer-
cury expanding, or rather dilating, in equal
amounts for equal increments of heat.
20. Exception to the law. The exception to
the law of expansion by heat, and contraction by
the loss of it, is water, which obeys it however
till cooled down to 39* 5 ; if the process of
cooling is continued below that point, it begins
to expand, and, in a state of perfect stillness, it
will continue to do so, without freezing, for 20
below the freezing-point : when frozen it is still
lighter than when liquid, hence ice floats on the
surface and is readily exposed to thaw on the
return of warmth to the air : had water followed
the usual law, the ice and coldest water would
have accumulated, year after year, at the bottom
of rivers and lakes, which the heat of the summer
sun would never have reached.
21. Amount of expansion.
Table of the linear expansion of certain solids by heat.
Dimensions which a bar takes at 212, whose
length, at 32, is rooooooo :
Cast brass .... i '0018750 i part in 533
Zinc .......... 1^002942 i part in 340
Glass ........ 1*0008613 i part in 1248
INTRODUCTION. 21
Expansion in volume by heat
of Mercury from 32 to 212 0*0 1 8099 or i part in 5 5
Water from 39 to 212 0*043 3 20 or i part in 23
22. Expansion of gases by heat. Gases expand
by heat ; and air, which is a gaseous compound
as we have seen, does the same. The subject of
the expansion of gases has been investigated by
Dalton and Gay-Lussac, and more recently by
Regnault, who has given ^~ or ^ as the amount
of the expansion of a volume of air in passing
from the freezing to the boiling-point of water ;
that is to say, a quantity of air equal in volume
to 1000 cubic inches at 32, will expand to
1366'5 cubic inches at 212, or by an increase of
180 of heat ; hence, as it has been proved that
the expansion is the same for each additional
degree of heat, the expansion will be equal to
0-002036, or ^ for each degree ; in other words,
491 cubic feet of air at a temperature of 32 will
become
o
492 at 33
493 at 34 & c -
490 at 31
489 at 30
488 at 29 &c.
23. Laws of heat : Conduction. Bodies lose or
22 PRACTICAL METEOROLOGY.
acquire heat by conduction ; the particles nearest
the source acquire heat and transmit it to the
nearest to them; these to the next, and so on
till the whole mass has attained the heat of the
surrounding medium. If the temperature of
this medium, suppose the air, be reduced, the
surface of the heated body radiates forth its heat,
and, as it cools, borrows heat from the interior
till equilibrium is restored.
According to the greater or less rapidity with
which heat is diffused throughout a substance, so
is it said to be a good or a bad conductor of
heat ; metals are good conductors of heat, gases,
fluids and earthy matters scarcely conduct heat
at all.
24. Illustrative experiment. The different
conducting powers of solid bodies may be shown
experimentally thus : into a rectangular tin ves-
sel insert, through apertures in the side, small
cylindrical bars, equal in size, of various substances,
which have been previously dipped in melted wax,
from which they will have acquired a coating of
that material of equal thickness throughout. Fill
the vessel with boiling water, and the wax will
melt soonest on the bar of that material which is
the best conductor of heat. By making the bars
of iron, lead, porcelain, &c., and noting the time
the wax takes to melt on each, the following Table
INTRODUCTION. 23
of the conducting powers of different substances
has been formed :
Gold 100 Tin 30-38
Silver 97-3 Lead 1 7'9&
Copper 89*82 Marble 2*34
Iron 37'4 J Porcelain... i'22
Zinc 36*37 BrickEarth 1*13
25. Good and bad conductors. Glass and wood
sre bad conductors of heat ; thus we may burn a
pece of wood in the flame of a lamp, and not feel
ii the hand any inconvenience from the heat,
even at a short distance from the flame. The
extremity of a glass tube, held with the hand at
me inch from the flame, may be fused with a
llowpipe and no unpleasant heat will be ex-
perienced; whereas an iron wire in the same
flame will be with difficulty heated to redness,
from the heat passing through its whole length
by conduction very rapidly, and certainly it could
not be held in the hand. Air is a bad conductor,
and hence the double windows in cold climates,
which include a layer of air, prevent the conduc-
tion of heat from the rooms to the outside, whence
it would be dispersed.
In the vegetable kingdom the barks of trees
exhibit a structure with difficulty penetrated by
heat ; the bark is porous, and, in addition to its
non-conducting quality, these pores are filled with
24 PRACTICAL METEOROLOGY.
air ; hence the stem of the tree is little affected
either by summer's heat or winter's cold.
Ice is a bad conductor, snow still worse; a
coating of ice prevents the cooling of the water
underneath, while snow protects the delicate roots
of plants from the effect of severe cold.
The boiler of a locomotive steam-engine is
covered with wood and felt, to prevent the heat
being conducted and radiated away : ice is wrapped,
in flannel when brought from the ice-house,
prevent the external heat from reaching it.
The crust of the earth is composed of bad con
ductors of heat ; hence although the interior df
the earth is well known to be of a much highei
temperature than the superficial layers, it com-
municates little heat to the surface, which is sup<
plied chiefly from the sun. Nor does the heat of
summer, still less that of mid-day, affect the tem-
perature of mines or wells even a few feet deep ;
a thermometer at no great depth below the soil
would be beyond the influences which affect the
surface. At the Observatory of Paris one has
been stationed, for the last seventy years, 91 feet
below the foundation of the building, which has
indicated an invariable temperature of 53, two
degrees higher than the mean temperature of the
air above.
Liquids and gases appear to possess little or no
INTRODUCTION. 25
conductive power ; if a tube be filled with water
and held slantingly over the flame of a spirit-
lamp, the superficial layer of water may be made
to boil while the temperature of that in the lower
portion of the tube remains unchanged ; a piece
of ice placed there will not be melted.
26. Convection. The method by which heat is
communicated throughout liquids is called con-
vection. Heat must be applied to the lower part
of the mass ; the lowest particles will expand,
and, becoming specifically lighter, will rise, while
the colder particles descending from above will
supply their place. If this process is remarked
when water is boiled in a glass tube in which
some powder has been thrown whose specific
gravity is the same as that of water, a series of
upward and downward currents will be exhibited,
which will not cease till the source of heat is
removed.
Gases, air of course included, become heated in
the same way, though the currents have more of
a wavy or cloudy form, unless, as is seldom the
case, the air is perfectly still, when smoke will
ascend perpendicularly, and fogs and vapours will
rise in the same manner.
In the ocean the phenomena of convection are
exhibited on a grand scale. In the torrid zone
the water of the sea weighs less, quantity by quan-
26 PRACTICAL METEOROLOGY.
tity, than at the poles, or near them ; therefore
the cold water of high latitudes has a tendency
to flow towards the equator, and form under-cur-
rents throughout the ocean ; to supply the equi-
librium, the warm waters of the tropical regions,
forming the upper current, are carried as far as
the arctic oceans, warming the air in their passage,
and tempering the rigour of the climates of places
situated on the sea-coast. It is invariably found
that maritime localities, exposed to the influence
of these currents, have a less variable tempera-
ture than those inland at the same distance from
the equator. This difference is partly owing to
the oceanic currents, whereby there is kept up a
constant circulation of immense masses of water
from the tropical regions to the higher latitudes ;
thus we find the Gulf-stream flowing from the
Gulf of Mexico, with an initial temperature of 84,
along the coasts of North America, and across the
Atlantic to the shores of Europe. Water has,
as will be shown hereafter, a great capacity for
heat, and parts with it slowly ; so that the Gulf-
stream continues of a temperature far above that
of the air, after travelling many degrees north :
the heating and cooling of the water proceeds
very deliberately, and the air partakes in a mea-
sure of the uniformity of temperature which be-
longs to the surface of the ocean ; thus the ex-
INTRODUCTION. 27
tremes of temperature are much less at places near
the ocean than at those at a distance from it.
An illustration taken from two places in the
same latitude, but one on the sea-coast and the
other inland, Penzance, and Barnaul at the foot
of the Altai in Siberia, will tend to impress the
mind with this truth :
Winter Summer T>iffi>ren<^
temperature. temperature.
Penzance 44*6 60*4 15*8
Barnaul 6*6 61*9 55-3
27. Laws of heat : Radiation and Absorption.
Besides the motion of heat from particle to par-
ticle of a body, there is another mode in which
diffusion of it takes place ; that is by radiation.
Any body heated higher than the surrounding
medium sends forth rays of heat from its surface.
It is thus that we receive heat from the sun, and
not by conduction through the air else it would
be as hot, or nearly so, in the shade as in the
sun.
The radiating power of a body depends greatly
on the character of its surface ; smooth or polished
radiate more slowly than rough surfaces; dark
surfaces more quickly than light.
Absorption of heat radiated from another body
takes place in different degrees with different kinds
of matter, the power of absorption being directly
proportioned to that of radiation.
28 PRACTICAL METEOROLOGY.
28. Laws of heat : Reflexion. Heat is reflected
from surfaces,, obeying laws similar to those of
reflected light ; bright polished metals are power-
ful reflectors of heat ; a red-hot ball in the focus
of a parabolic mirror will radiate heat which will
be reflected from the curved surface in parallel
lines ; these being received on the surface of
another parabolic mirror will be reflected to its
focus, and a powerful heat sufficient to inflame
gunpowder or phosphorus will be felt at that
point.
A thermometer hung in the sunshine against
a wall, or where the rays of the sun are reflected
on it, will show a very much higher temperature
than one in the sun away from all reflected heat.
Such a thermometer not only receives the direct
rays, but heat from those which are reflected ;
and thus the temperature it shows is the result of
the combination of direct and reflected heat, and
therefore not to be trusted as a fair record of the
heating power of the solar rays.
Those bodies reflect heat most perfectly that
absorb it least.
29. Latent heat. A very important condition
of heat is that called latent. Latent heat is so
named because it does not affect the sense of touch
or the thermometer ; it may be thus illustrated.
If a vessel be filled with ice in a melting state
INTRODUCTION. 29
and subjected to the application of heat, a ther-
mometer placed in it will not show a higher tem-
perature than 32 till the whole of the ice has
melted ; not till then will the temperature of the
water begin to rise. The heat thus absorbed by
the ice in passing from a solid to a liquid state
has therefore become latent. Heat thus becoming
latent on liquefaction will account for the chilling
effect of the air during a thaw, for the ice and
snow absorb a considerable quantity of heat in the
process of melting, which is borrowed from the
surrounding air, so that the latter is continually
kept down to near the freezing-point of water.
The temperature of cold water, after it has been
placed over a fire, will continue to rise until it has
attained to 212, when ebullition takes place and
the water passes with violent action into steam :
the effect of a continuance or increase of heat will
not be that of raising the temperature, which in
an open vessel is impossible, but the additional
heat will be employed in converting the liquid
water into steam.
When matter passes from a liquid to a gaseous
state, heat is absorbed and becomes latent: in
the process of freezing carbonic acid gas, the gas
is first liquefied by pressure ; on being allowed to
rush out of the vessel into the air it immediately
recovers its gaseous state, but for this purpose the
30 PRACTICAL METEOROLOGY.
portion first liberated demands and obtains from
that which remains so much heat, that what is
left behind is solidified, the solid carbonic acid
presenting an appearance like flakes of snow.
On the contrary, when a vapour becomes liquid
the latent heat is given forth and becomes sen-
sible. It requires a much greater amount of cold
water to condense a given weight of steam at a
temperature of 212, into water whose tempera-
ture shall be 100, than to cool down a similar
weight of boiling water to that temperature ; hence,
in condensing engines, an immense quantity of
cold water is requisite to preserve a sufficient
vacuum in the cylinder. It is usual in some
manufactories to raise a large quantity of cold
water to the boiling-point in a few minutes by
passing steam into it ; on condensation the steam
gives out its latent heat, which the water absorbs.
The three conditions in which matter exists, of
solid, liquid and aeriform, differ apparently only
in this, that in the one case there is sufficient heat
to sustain a liquid or gaseous form, which a solid
fails to possess. Water becomes solid at 32;
and even mercury, usually met with as a fluid,
will solidify when the temperature falls to 38 0< 2.
30. Evaporation. For liquids to assume the
gaseous form ebullition is not necessary : this is
a violent process of evaporation, and takes place
INTRODUCTION. 31
only when the elastic force of the vapour of the
liquid, which has been heated up to a certain fixed
temperature, in the case of water 212, at which
point steam is rapidly generated balances the
pressure of the atmosphere. Evaporation from
the surface of water proceeds at all temperatures,
and goes on gradually and insensibly ; the par-
ticles of water rise in the air and are mixed with
it, and, unless they exist in large amount, are
invisible; they do, however, always exist there,
and, when condensed, fall in refreshing showers
or in storms of rain and hail. Whenever eva-
poration takes place, heat must be borrowed from
contiguous substances to supply the amount which
becomes latent in the conversion of the liquid into
vapour; hence it is always a cooling process.
From the same cause, ether, which evaporates
rapidly at a low temperature, applied to the sur-
face of the skin, produces a sensation of cold ;
and the evaporation of water from the surface of
a porous jar cools its contents, as is beneficially
experienced in hot climates ; in this case the water
in the vessel exudes through the pores and forms
on the outside a kind of dew, which is taken up
by the hot air very rapidly.
The insensible evaporation of water, and the
combining of its vapour with air, may be proved
by exposing a shallow vessel out of doors full of
32 PRACTICAL METEOROLOGY.
water ; in a few days it will be found emptied of
its contents. If the surface of the vessel be of a
given area, say one square foot, and the water
be weighed from time to time, the loss of weight
will give the rate of evaporation, for the tempera-
ture and locality, due to one square foot of surface.
The process of evaporation is always going on on
a large scale over the surface of oceans, seas,
rivers, and lakes ; and the amount of water thus
taken up into the air is enormous.
31. Specific heat. Some substances have a
greater amount of absolute heat than others,
although their temperature may be expressed by
the same degree of the thermometer : if two glass
tubes, in every way alike, one containing water
and the other mercury, be subjected to the same
degree of heat by being plunged into a vessel of
hot water, the mercury will attain the heat of the
water in half the time which the water will take ;
and on removing the tubes and allowing them to
cool, the mercury will only take half the time to
recover the initial temperature which the water
will : this effect arises from the mercury absorbing
less heat than the water does in being raised to a
similar temperature ; as the fact is usually ex-
pressed, the water, compared with mercury, has
twice the capacity for heat.
In comparing the capacity for heat of different
INTRODUCTION. 33
bodies, it is usual to take equal weights of each,
rather than equal volumes : a pound of distilled
water takes a certain amount of heat to raise it a
certain number of degrees ; to this standard other
substances are reduced; if a pound of mercury
requires *033 of the same amount of heat to arrive
at the same temperature^ then the specific heat of
water is said to be to that of mercury as 1000 to
33. M. Regnault has conducted a series of very
careful experiments to determine the specific heat
of various substances, of which the following are
some of the results :
Subst
Water ......... 1000
Ice ............ 513
Iron ............ 113*8
Copper ...... 95* ! 5
Zinc ............ 95-55
Glass ......... 198
Mercury ...... 33'3 2
Lead ......... 31-4
Air ............ 3705
It will appear from this Table that the capacity of
water for heat is very great ; hence, as it forms so
large a portion of the envelope of the globe, it
becomes a great reservoir of heat, and performs an
important office in equalizing the temperature of
the earth, as we have before seen.
D
34 PRACTICAL METEOROLOGY.
Mercury has little capacity for heat, and there-
fore is very readily operated upon by the slightest
change of temperature. Were there no other
reason why it should be adopted for thermometers,
this would be sufficient, as no other liquid would
exhibit such sensibility.
The air, it will be seen, has a very considerable
capacity for heat, varying, however, with its density.
Great difficulties and some uncertainty still attend
the determination of the specific heat of all gases ;
it would appear, however, that air cools by expan-
sion to the amount of 40 or 50 in doubling its
volume; and that, on compression into half its
volume, its heat is raised to that amount. The
match syringe is a small cylinder of brass with a
tightly fitting piston ; on suddenly driving this
down to the bottom of the cylinder and compress-
ing the air within into a very small space, sufficient
heat is evolved to light an inflammable substance,
such as amadou or German tinder. We shall, in
the progress of this work, have occasion to refer to
the decrease of temperature in the higher regions
of the air, as dependent on the circumstance of
the capacity for heat increasing with the rarefac-
tion of the upper strata of the atmosphere.
32. Coldness of the upper regions of the air.
Near the surface of the earth the temperature of
the atmosphere is observed to diminish 1 in every
INTRODUCTION. 35
352 feet of ascent, but there is reason to suppose
that in the higher regions this ratio does not exist :
the observations at great altitudes have not been
conducted in sufficient amount to enable us to
arrive at anything like certainty on this point.
33. Colour of the air. The colour of the atmo-
sphere is, according to Sir David Brewster, due to
light which has suffered polarization ; the red and
yellow rays are absorbed, and hence the beautiful
azure of the vault of heaven, which deepens the
higher we ascend. The colours which accompany
the setting sun are due to the vapours through
which his rays reach us when he is near the
horizon; the aqueous vapour absorbs the blue
rays, and admits the passage of the red and yel-
low, the blending of which two colours produces
the varied tints which surround him.
36
PART II.
OBSEKYATION AND DEDUCTIONS.
34. Arrangement of the subject. Having
cleared our way by establishing general principles,
we are prepared to enter upon the explanation
of the construction of meteorological instruments
the mode of observation, and the deductions which
may be drawn from the observations themselves.
The subjects may be treated in the following order:
1. The Thermometric |
2. The Hygrometric
3. The Barometric ( condition ^ the air.
4. The Electric
Under each head will be laid down the principles
of construction and the use of meteorological in-
struments, the cautions to be applied in making
observations with them, and the atmospheric laws
which may be fairly deduced.
1. The Thermometric Condition of the Air.
35. The Thermometer. The importance of
having at hand, at all times, the means of deter-
THERMOMETRIC CONDITION OF THE AIR. 37
mining the temperature of the air, on which all
nations shall be agreed, is self-evident; without
this there could be no comparison of results ; the
mercurial thermometer is the instrument adopted
for the purpose. A small portion of mercury is
enclosed in a glass tube with a narrow bore, one
end of which is blown into a small bulb, and then
hermetically sealed : its principle of construction
is sufficiently simple ; for since mercury expands
to a great extent by heat, the amount of this ex-
pansion over that of the glass tube, which is slight,
affords us the means of estimating the degree of
heat due to the surrounding medium. The ap-
parent dilatation of mercury in a glass tube
between 32 and 212 (the freezing and the boil-
ing points of water) is ^ of its volume ; and its
true dilatation within the same limits is -g^. The
construction of a thermometer whose indications
may be relied upon with certainty in small varia-
tions of temperature, is attended with practical
difficulties, as will be at once apprehended from
the following description of the method of con-
structing a sc standard thermometer ;" that is, an
instrument whose scale has been divided indepen-
dently, or without comparison with others, and
whose readings ought to be correct throughout
the whole extent, within a very small fraction of a
degree.
38 PRACTICAL METEOROLOGY.
36. Construction of a standard thermometer.
The construction of such a measurer of tempe-
rature will embrace three operations :
a. The choice of a suitable tube.
/3. The filling and hermetically sealing of it.
<y. The adaptation of the scale.
a. The tube adapted for a standard thermo-
meter must have either a cylindrical bore, or one
whose section is an ellipse, the longer diameter of
which must be presented to the eye in reading off,
the shorter must be almost evanescent; in this
latter construction, the exact point at which the
mercury intercepts the scale is very distinct ; more
so than in a tube of greater capacity whose section
is a circle. As the glass, as well as the mercury,
expands by heat, the rise of the mercury will be
the difference of the expansions of the two in the
line of the scale. Mr. Sheepshanks, who gave
great attention to the construction of perfect ther-
mometers, found that those with round bores were
far more nearly true as regarded uniformity than
those with flat bores ; but his method of con-
structing the scale rendered him altogether in-
dependent of the workman's skill beyond what
may be ordinarily attained.
If the section of the bore of the tube be exactly
the same throughout its whole length, the tube
is so far perfect ; it may be thus tested. Let a
THERMOMETRIC CONDITION OF THE AIR. 39
small portion of mercury be admitted into the
tube, which shall occupy a space (a few degrees)
throughout which no error will be likely to arise
from any deviation from uniformity if it exists ;
measure accurately the length of the mercury in
several parts of the tube ; if the length be inva-
riable, the bore of the tube is the same through-
out ; if the differences in the measures are va-
riable and very evident, the tube must be rejected
altogether, or very nice measurements and calcula-
tions entered upon, which need not be described
in this place. The subject of the calibration
of the tube has been fully discussed by Mr.
Welsh, in a report presented to the Royal Society,
May 1852, " On the general process adopted in
graduating and comparing the Standard Meteoro-
logical Instruments for the Kew Observatory/'
The bulbs should be neither very small nor very
large ; if too small, the amount of mercury will be
so diminished that the space allotted to a degree
will be too much reduced in size for ver^ near
readings ; if too large, the mercury will take some
considerable time to be heated throughout, and
will not indicate sudden changes of temperature.
Mr. Sheepshanks approves of bulbs 3 or 4 tenths
of an inch in diameter. The bulb is never blown
by the breath, lest moisture should find its way
into the tube ; but air is urged in by means of
40 PRACTICAL METEOROLOGY.
an elastic ball, which is compressed at the instant
that the closed end of the tube is heated to the
point of fusion.
/3. The mercury, which should have been pre-
viously boiled to separate it from moisture and
air, is introduced into the bulb as follows :
The extremity of the tube opposite to the bulb
is expanded into the shape of a funnel, a, Plate I.
fig. 4, for the purpose of receiving a quantity of
mercury (some workmen only form a temporary
funnel of paper) ; the air in the bulb and tube is
driven out by the heat of a spirit-lamp, and the
mercury takes its place when the lamp is removed
and the tube cools ; the mercury is then gently
boiled in the tube for some time. In sealing the
tube, the mercury, by expansion, is made to fill it
entirely ; and, at the instant of its arrival at the
upper extremity of the tube, the flame of a blow-
pipe is made to melt the glass and hermetically
close it ; so delicately is this process performed,
that not a particle of air is allowed to remain
within. The test of the absence of air is a simple
one. After the tube has cooled, incline it, and
the mercury will flow out of the bulb down to the
other extremity if no air be present ; but if any
remain, it will form a cushion against which the
mercury will impinge and never reach the end.
An experienced workman will readily judge how
THERMOMETRIC CONDITION OF THE AIR. 41
much mercury is necessary to be introduced into
a tube of a particular range ; he may do so by
plunging the tube into water of different tem-
peratures, and, observing the length of the column
between the two temperatures, he will manage to
insert the necessary quantity.
7. To adapt a correct scale to the thermometer
is a matter of the utmost consequence. The
civilized world has agreed on fixing two points on
the scale, viz. that to which the mercury rises when
the instrument is plunged in boiling water, and
that to which it descends when placed in melting
ice. The space between these points is not how-
ever divided by all nations into the same number
of equal parts, and hence, as we shall see, the
degrees of one scale must be converted into the
corresponding number of another, before tempe-
ratures registered under one method can be com-
pared with those of a different one.
37. The zero-point. The first point to be deter-
mined is the freezing-point of water, which is the
same under all temperatures and every variety of
barometric pressure, provided the water is free
from salts of every kind. The thermometer tube
not the bulb only is plunged into melting ice,
and a mark is made across the glass of the tube
at the point to which the mercury sinks, or with-
draws, towards the bulb ; this is the zero of every
42 PRACTICAL METEOROLOGY.
scale except Fahrenheit's, the one in general use
in England.
It is a remarkable fact that this zero-point is
not permanent ; when thermometers have been
made for some time, a very perceptible difference
is found between the point to which the mercury
descends when plunged in melting ice, and the
original zero of the scale ; hence the necessity of
determining it anew from time to time, and apply-
ing the difference as an index correction. A tube
intended for a standard thermometer is allowed to
remain, after it has been filled, for at least six
months before the freezing-point is marked on it,
in order that the glass, which has been heated to
the boiling-point of mercury, may recover its nor-
mal state. If the zero were marked at an earlier
period of its existence, there would be a rise
during the first year of considerable amount.
Certain peculiarities have been remarked in
the construction of thermometers by Mr. Welsh,
which he has thus described : "If a thermometer,
after having been for some weeks exposed to the
ordinary temperature of the air, is placed in melt-
ing ice, its freezing-point may be, for example,
32 -2 ; if the bulb be then put for two or three
minutes into boiling water and soon afterwards
placed in ice, the reading will have fallen to 32 :
if in a day or two it is again placed in ice, the
THERMOMETRIC CONDITION OF THE AIR. 43
freezing-point will have risen a little about 0*1 ;
and if tried again after two or three weeks, the
freezing-point will be found to have acquired
exactly the original position of 32'2." It would
appear that the glass, after a considerable change
of temperature, requires a long time to return to
its normal dimensions.
Mr. Sheepshanks, who did not desist from his
labours in the structure of the thermometers used
in connexion with the experiments to determine
the length of the National Standard Yard till he
felt assured of being able to reach the second
decimal of a degree, used ice found in tubs of
rain-water, in which the thermometer was placed
horizontally, and the intersection of the mercurial
column with the tube was read off with a vertical
telescope carrying a wire moved by a micrometer-
screw; he obtained identical results when he
made use of newly-fallen snow ; and he recom-
mends the determination of the zero to be per-
formed in winter, when the air around is but
little above the freezing-point, and the ice or snow
melts but slowly.
38. The boiling-point. The other constant
the boiling-point is not so easily ascertained,
seeing that the temperature of boiling water will
differ with the pressure of the atmosphere.
In the division of the scale adopted by the
44 PRACTICAL METEOROLOGY.
Commissioners appointed by Government to con-
struct Standard Weights and Measures, and also
by tbe Kew Committee of the British Association,
the boiling-point, 212, is made to represent the
temperature of steam under Laplace's Standard
Atmospheric Pressure, which corresponds to the
following number of inches in the barometric
reading, reduced to 32 Fahrenheit,
29*921 8 + 0*0766 x cosine (2Lat.) + (0*000001 79 x
height in feet above the sea-level) .
The boiling-point, therefore, will be fixed at
212, neglecting the small correction for height,
when the barometer reading, reduced to 32 Fahr.,
amounts, at
London, to 29*905 inches.
Dublin, to 29-900
Edinburgh, to 29*893
If the reading of the barometer differ from this
quantity, and the temperature, at the time of the
experiment, be not 32, an allowance must be
made whereby the distance on the scale between
the boiling-point and the freezing-point must be
increased or diminished.
According to Wollaston, 1 of Fahrenheit will
correspond to a difference of 0*589 inch of baro-
metric pressure. Hence if the barometric reading
when reduced is one inch below the standard
THERMOMETRIC CONDITION OF THE AIR. 45
pressure, water will boil at 210*3 F. ; and when
the reading is one inch above, the temperature of
the boiling-point will be 213'7.
Mr. Sheepshanks thus describes the boiler he
made use of in his experiments :
"My boiler is made of sheet copper, square
above and cylindrical below ; the dimensions are,
length 24 inches and depth about 6 inches, with
flat ends and top. In one end there is a round
hole filled with a large cork ; through the centre
of the cork a small pipe of copper pierces, large
enough however to allow the thermometer tube
to pass : a bar, stretching across the inside of the
boiler at the same height nearly as the centre of
the cork, supports the thermometer near the bulb,
when the thermometer, cork and all, is inserted
pretty tightly in its hole. As much of the tube
of the thermometer is exposed as will show the
division below the boiling-point, and the joint be-
tween the tube and the pipe, which projects a
little, is made good with a binder of very thin
vulcanized India-rubber. Distilled water is poured
into the boiler, but not so much as to touch the
thermometer, which is thus boiled in steam.
There are some round holes in the flat top which
can be closed sufficiently by flat pieces of brass.
When the steam rises strongly the flat pieces on
the top of the boiler begin to chatter, and it is
40 PRACTICAL METEOROLOGY.
certain that the necessary heat, at least, is attained.
By removing one, two or three of the flat pieces, it
will be found that, in a little time, the position of
the mercury becomes steady, and is not affected
by closing one of the holes or unclosing another.
The steadiness of the boiling-point, whether the
steam issues languidly or with considerable vehe-
mence, is rather a puzzle to me, but the fact is
quite certain/'
In Plate I. fig. 5, is represented the cylindrical
boiler : a is the cork through which the thermo-
meter tube is inserted into the steam without
touching the water ; b is the escape tube ; i the
spirit-lamp ; w the water used, which must be
pure rain or distilled water, for the admixture of
any salt would raise the boiling-point.
39. The Scale. Fahrenheit's division. We have
now on the scale two fixed points, and unless we
aim at extreme nicety, we have only to divide the
space between them into a certain number of
equal parts, extending the divisions above the
boiling-point and below the freezing, and the
scale of the thermometer will be complete. In
the thermometer in general use in England, this
space is divided into 180 portions called degrees;
it derives its name from Gabriel Fahrenheit, a
native of Dantzic, who fixed his zero at the point
of the lowest cold observed in Iceland, which was
THERMOMETRIC CONDITION OF THE AIR. 47
supposed to be as low a temperature as was likely
to become the subject of philosophical investiga-
tion : this zero is 32 below the freezing-point,
and 212 below the boiling-point of water. The
advantages of this scale, which it possesses above
others, are, that the observer, especially in me-
teorology, is very seldom troubled with negative
degrees, which do not commence till the extreme
cold of 0, or 32 below freezing, has been reached ;
as the divisions moreover are more numerous than
in other methods of division, we do not so fre-
quently find it necessary, in ordinary operations,
to use fractions of degrees.
40. Divisions of the Scale according to Celsius
and Reaumur. Celsius, a Swedish professor at
Upsal in 1742, proposed to divide the space between
the two standard points into 100, the zero being
the freezing-point ; temperatures above this are
positive quantities, and those below, the scale being
continued as far as may be desirable, are negative.
A thermometer thus divided has a " Centigrade
scale " and is in very general use in France. The
principal objection to this division is that, in a
record of degrees of natural heat, one column may
be embarrassed by + and degrees.
Reaumur's thermometer has the same space
divided into 80, and also necessitates the use of
+ and to mark what are sometimes inaccu-
48 PRACTICAL METEOROLOGY.
rately called degrees of heat and degrees of cold ;
it is in extensive use in Germany and Switzerland,
and in Spain.
As meteorological records are kept under all
these three systems, it is of some considerable im-
portance to have a formula for readily converting
the degrees of one into those of another. Water
freezes at 32 of Fahrenheit's scale, and at of
the Centigrade and Reaumur's ; while the boiling-
points are respectively 180, 100, and 80 above
that point: hence the number of degrees of
Fahrenheit in a given range of temperature are to
those of Celsius as 180 : 100, i.e. as 18 : 10 or
as 9 : 5 ; and to those of Reaumur as 180 : 80,
t . e. as 9 : 4 ; to convert, therefore, Centigrade
degrees into those of Fahrenheit, we must multiply
them by 9, divide by 5, and add 32, which is the
number of degrees marked in Fahrenheit's scale
at the freezing-point, or zero of the others j con-
versely, to convert degrees Fahr. into degrees
Cent., subtract 32, multiply by 5, and divide by
9. By substituting 4 for 5, we obtain the same
results on Reaumur's scale.
The following formulse will meet every case, if
the negative sign is used when the reading is
below zero :
THERMOMETRIC CONDITION OF THE AIR. 49
4. E=
6. E=iC.
5
Since, however, in reading or writing, these re-
ductions perpetually occur, it is very useful to
have at hand such a table as Table I. (Appendix),
by which we are spared the labour of reduction,
and obtain the corresponding degrees immediately
by inspection.
It is seldom that a thermometer used in this
country for meteorological observations reads
higher than 120 ; in such cases, for the thermo-
meter to be valuable, the freezing-point is deter-
mined in the same way as for a standard, and every
precaution is taken to ensure a tube with an
equable bore : an upper reading is fixed on, by
plunging it, with a standard, into water of a
certain temperature, and the space between the
freezing-point and the height to which the mer-
cury rises, the degree of which temperature is
shown by the standard, is divided into as many
50 PRACTICAL METEOROLOGY.
degrees as the temperature is above 32; the
divisions being carried above and below as far as
may be required to complete the scale.
The accuracy with which such thermometers
are constructed by good workmen is shown by
Mr. Glaisher in his "Essay on Radiation"; he
says that he received from Messrs. Watkins and
Hill upwards of fifty instruments, whose extreme
difference of reading from the standard with which
they were compared was, in one thermometer
a constant quantity of half a degree, in three
others a constant quantity of 0'2 or 0> 3, the
remainder being absolutely without error.
41. Comparison of thermometers. If a thermo-
meter has to be compared with a standard for the
purpose of ascertaining the differences of its read-
ings throughout the whole range of the scale, the
best method is to put them both into water heated
up to the highest degree the thermometers read,
and record the degree of temperature shown by
both every quarter of an hour, taking care to agitate
the water from time to time : this record, if the
readings differ, will supply a number of index
corrections to be applied to the thermometer to
bring it up to that of the standard ; and it may
be presumed, if that be carefully done, that an
inferior thermometer may be rendered nearly as
useful as a standard, though, it must be confessed,
THERMOMETRIC CONDITION OF THE AIR. 51
at some considerable expense of labour. At the
present time, however, there is no excuse for using
an inferior instrument. The Kew Committee of the
British Association have caused to be constructed
by Mr. Welsh, at the Kew Observatory, a large
number of standard thermometers which are sold
to the public at a moderate price ; they have all
been subjected to the most accurate scrutiny, and
the scales are engraved on the tubes, which are of
considerable length so that parts of degrees may
be estimated with very close precision.
42. Maximum and Minimum Thermometers.
To obtain a record of the greatest heat attained
during the day, and of the lowest reading of the
thermometer during the night, is a matter of much
importance in observations on climate, and various
means have been adopted to ensure such registra-
tion. The thermometers used in this country are
Sixe's and Rutherford's, as well as a new maxi-
mum and a new minimum thermometer patented
by Messrs. Negretti and Zambra which have
received the approbation of many practical me-
teorologists.
43. Sixe's Register Thermometer. Mr. Sixe
described his register thermometer originally in
the Philosophical Transactions, vol. Ixxii. It is,
in fact, a spirit of wine thermometer, with a long
cylindrical bulb, and a tube bent in the form of a
E2
52 PRACTICAL METEOROLOGY.
siphon with parallel legs, and terminating upwards
in a small cavity. A portion of the two legs of the
siphon tube, from a to b (Plate II. fig. 1), is filled
with mercury, the bulb and the whole of the rest
of the tube with spirits of wine ; the double
column of mercury gives motion to the two in-
dices c and d y each of which is a piece of iron
wire capped with enamel at each end ; they would
move freely in the tube and rest on the mercury
were it not for springs, made of a thread of glass
or a hair, which, surrounding them, press against
the side of the glass with sufficient power to keep
either index stationary in the spot where it is left
by the retreat of the mercury. The action of the
instrument is as follows : when the increase of
temperature expands the spirit in the lengthened
bulb G, the mercury in the leg of the siphon, ,
is depressed, and a corresponding rise takes place
in the leg b ; in its rise* the mercury urges before
it the index d, which, being retained at its highest
point by the spring, does not follow the retreat of
the mercury, as the temperature decreases and the
spirit in G contracts ; this shows then the maxi-
mum heat in any determined period of time.
When the spirit contracts, the mercury descends
in the tube b on account of the elasticity of some
compressed air in the small bulb above, and pro-
portionately rises in a, urging before it the index
THERMOMETRIC CONDITION OF THE AIR. 53
c ; leaving, on the increase of heat, its lower ex-
tremity exactly at the highest point to which the
column (the degrees decreasing upward) had risen
on that side, pointing out in fact the minimum
degree of temperature attained.
To prepare the instrument for future observa-
tions, the indices are brought down, by a magnet,
to touch the mercurial column. If this instru-
ment be read every twenty-four hours, it will
evidently give the greatest and least temperature
during the day.
Unfortunatelythiselegant instrument can hardly
be trusted for very nice observations, and is very
liable to get out of order. The use of two liquids,
both expanding in different degrees, is a defect ;
and although it may, in part, be remedied by very
nice dividing, and comparing the scale with a
standard for every 5 or 6, yet this process would
increase the price considerably. The other defect
arises from the liability of the springs to get out
of order a glass one by breakage, and the hair
by losing its elasticity after long immersion in the
spirit ; while, from the perpendicular position of
the tube, agitation from the wind is very likely to
cause the indices to slide down the tube, and thus
the observations would be lost. For these rea-
sons Rutherford's maximum and minimum ther-
mometers are preferred for registering the greatest
54 PRACTICAL METEOROLOGY.
and least degrees of heat ; these we shall proceed
to describe.
44. Rutherford's Register Thermometer (Plate
II. tig. 2). A represents a spirit thermometer, B
a mercurial, each furnished with a scale and
fixed horizontally on the same plate of box-wood
or metal ; B contains within it a steel index, c,
which is urged forward as the mercury expands
by heat, and is left to indicate the highest tem-
perature attained when the metal again contracts.
The spirit thermometer, A, contains a glass
index, n, half an inch long, with a small knob at
each end ; it lies in the tube and allows the spirit
freely to pass it as it expands ; when contracted
by cold, in consequence of the capillary attraction
between the spirit and the glass index, the last
film of the column of spirit is sufficient to over-
come the slight friction of the index on the inside
of the tube and to carry it backwards towards the
bulb ; it will rest, on the spirit again expanding,
at the lowest degree of temperature attained within
a given period. After reading off, and to prepare
the instruments for future observations, both in-
dices are brought to the extremities the one of
the column of spirit, the other of that of mercury
by gently inclining the plate on which the
thermometers are fixed, downward from the hori-
zontal position. This must be done with some
THERMOMETRIC CONDITION OP THE AIR. 55
care, or the indices will get entangled with the
liquids, from which they will be with difficulty ex-
tricated. For three years the author has used a
register thermometer of this construction without
any mishap, though he ruined many before he
discovered the careful treatment they required.
45. Negretti and Zambra's Maximum and Mini-
mum Thermometers. The maximum thermometer
of Messrs. Negretti and Zambra is strongly recom-
mended: Fig. 3. Plate II. will explain its con-
struction.
The tube is originally straight throughout ; in
this state a small piece of enamel, a, is introduced
down it to within a short distance from the bulb ;
by means of a spirit-lamp the tube is then bent
just at the point where the enamel rests, and the
heat required for this purpose is sufficient to
cause its adhesion to the glass. The enamel does
not fill the tube, but allows the mercury to pass
freely above it ; on the decrease of heat, all that
part of the mercurial column which has passed
the enamel is left in the tube, while that portion
nearer the bulb is separated and withdraws from
it, the maximum heat will therefore be shown by
the extremity of the detached mercurial column ;
after this is registered, by depressing the bulb
the detached column may be made to reunite with
the rest of the mercury, and thus the instrument
56 PRACTICAL METEOROLOGY.
is prepared for another observation. The advan-
tage of the instrument seems to be, that there is
no index to get out of order; the disadvantage,
that there is some considerable trouble in accu-
rately determining the corrections to bring its
readings in unison with a standard thermometer.
The following is the construction of the minimum
thermometer of Messrs. Negretti and Zambra
(Plate XI. fig. 1) : a b is a long bulb containing
much more mercury than is usual in a thermo-
meter, this enables the tube c d to be made of a
large bore without diminishing the length of a
degree on the scale ; the tube is prolonged and
blown at the end into a bulb, d e ; a steel needle
or index (/), pointed at the lower end, lies loose
in the tube. The instrument is kept upright, and
yet able to be moved to a horizontal position by
being hung from a single support ; the tempera-
ture is reckoned from the upper end of the index
when it just floats on the surface of the mercury
(as it will do on account of the capillary repulsion) ;
as the mercury sinks in the tube on loss of heat,
the needle falls with it, but when it expands it
passes along the side of the needle without raising
it at all, jambing it indeed against the glass : so
the higher end of the index will always denote
the lowest temperature reached.
To free the index from the mercury so as to set
THERMOA4ETRIC CONDITION OF THE AIR. 57
the instrument for another observation, the lower
part must be raised till the mercury that is in the
tube runs down into the small bulb at the end,
leaving the index free ; this is then led down to
the same place by a magnet, and held there while
the thermometer is being restored to its former
position, on which the mercury will fall back to
its proper place, and the index may then be
gradually lowered till it just touches the surface.
Great care must be taken not to let the needle fall
on to the mercury from a height, else it will
plunge in and cause the reading of the thermo-
meter to be too low. If before an observation is
recorded the mercury has risen above the top of
the index, and the degree this points to cannot
well be observed, the magnet should be applied so
as to hold the needle in its place (for the general
position of it is told by the slight displacement of
the mercury), and the bulb elevated as before di-
rected till the index is free from quicksilver, when
the exact reading can be taken, and the instrument
readjusted as usual.
A great advantage of this form of thermometer
is that it is not at all likely to get out of order,
from the absence of air and from the large bore of
the tube. A disadvantage is, that its reading may
not always be correct to a decimal of a degree,
58 PRACTICAL METEOROLOGY.
because the index, unless carefully managed, may
sink too low in the mercury.
46. Hick's Maximum and Minimum Thermo-
meter. With this newly-invented thermometer,
which is made by Mr. Casella of Hatton Garden,
the extremes of heat and cold are registered by one
instrument, and by the expansion and contraction
of one fluid, mercury, though spirit is used for
carrying one of the indices. Its construction will
be understood from the drawing of it in Plate XI.
(fig. 3.). It is a thermometer with its tube abc
bent at right angles and ending in a small bulb at
c ; the mercury of the bulb extends some way
into the upright part of the tube and moves an
index d, like those of Sixe's thermometer, which
is left at the highest point reached by the mercury,
and therefore registers the maximum heat, read off
from the scale fg ; from the top of the mercury,
as far as e, the tube contains spirits of wine, which,
on the mercury contracting, also recedes (being
pressed back by the air at c), carrying with it the
index e, which will be left to mark the lowest read-
ing, the scale h i denoting it. This upper scale is
graduated in such a manner as to compensate for
the expansion and contraction of the spirit, which
however affect the spaces for the degrees to a very
small extent.
THEEMOMETRIC CONDITION OF THE AIR. 59
In a maximum thermometer devised by Pro-
fessor Phillips, a portion of the mercurial column
was separated from the rest by the intervention
of a small bubble of air ; this portion, as in the
case of Negretti's, remains in the tube on the
contraction of the mercury in the bulb, and thus
serves to mark the highest reading attained ; by
the inclination of the tube it is then brought to
its original position preparatory to another obser-
vation. This thermometer does not seem to have
received the attention it deserves ; its construction
is simple and its indications sure, as those can
testify who have used it.
47. Observations to determine temperature.
Diurnal Range. Provided with a good thermo-
meter for general purposes, and with others for
recording the highest and lowest temperatures
occurring within a definite period, we are in a
position to take observations on the temperature
of any place which shall be of some value in a
scientific point of view.
One of the first desiderata will be the mean
temperature of the place.
We cannot arrive at this without a series of ob-
servations extending over at least three years ; we
might, if time and means allowed, advantageously
begin with twelve observations in the course of
the day, taken at the even hours, throughout the
60 PRACTICAL METEOROLOGY.
year; from which we could deduce the mean
temperature of each day by dividing the sum of
the temperatures recorded by 12 ; of the month,
by dividing the sum of the mean daily temperatures
by the number of days it contained; and of the
year, by dividing the sum of the mean monthly
temperatures by 12, the number of months in a
year. As however few observers are in circum-
stances favourable to the undertaking of so elabo-
rate a series, we shall, for general advantage, show
that we have the power of eliciting the mean tem-
perature with far less expenditure of time and
labour.
For many years, at the Royal Observatory
Greenwich, the thermometer (as well as other
meteorological instruments) was registered every
two hours throughout the 24, under Mr. Glaisher,
the Superintendent of the Meteorological depart-
ment, and the results he has deduced from the
series of observations, extending from 1841 to
1845 inclusive, are published in the ' Philosophical
Transactions/ Part 1, for 1848. Mr. Glaisher has
since been able to extend his investigations by
reducing observations antecedent to the elaborate
work performed at Greenwich, and thus he has
determined the average mean temperature of each
day throughout 38 years, and from this has ob-
tained much valuable knowledge of the laws of the
THERMOMETRIC CONDITION OF THE AIR. 61
daily increase and decrease of temperature. Some
of the results of his labours we shall now place
before the reader.
The monthly mean of the readings of a ther-
mometer at a particular hour differs from the
monthly mean temperature by a quantity which
is constant for each particular month ; by apply-
ing, therefore, this quantity as a correction, we get
a true monthly mean from our single daily obser-
vation. The numbers to be applied as corrections,
for the several months, were thus obtained. Two-
hourly observations at Greenwich, commencing at
noon, from January 1,1841 to December 31, 1845,
were the data from which was calculated the
mean temperature of every day in the year. The
mean of the daily temperatures for the month
gave the mean temperature for each month. Com-
bining then the whole series of observations taken
at any particular hour say at noon during each
similar month of the five years, the difference
between the temperature due to that hour (which
was found by dividing the sum of all the noon
observations for the month, during the five years,
by their number) and the mean temperature for
the month was noted. This process was repeated
for every alternate hour in the day ; and thus the
excess of the mean value at each even hour in the
day for every month, above the mean value for
62 PRACTICAL METEOROLOGY.
the month, or else the amount by which it fell
short of it, was arrived at.
The accordance in the results of observations
taken at the same hour in the same month in the
different years, was found to be very close and
satisfactory ^-the small discrepancies disappeared
when the whole series was combined.
48. Projection of the same. To illustrate the
meaning of what we have just stated, we must
refer to Plate IV. fig. 2; along the horizontal
line are set off the hours for one day ; at each
hour for the hottest and coldest months of the
year, July and January, ordinates are taken equal
to the differences between the mean of the obser-
vations due to that hour, and the mean tempera-
ture for the month : a curve which shall pass
through the extremities of these ordinates will
show the rise and fall of the temperature through-
out the period of twenty-four hours for either
month this will be the normal diurnal curve
for the month ; and although from various causes
the heat at each hour may, on any day, be greater
or less than that indicated by the point where this
curve cuts an ordinate, yet, in the run of a month,
the irregularities will in general be neutralized ;
those above the mean being compensated by those
below.
These remarks will explain the object of the
THERMOMETRIC CONDITION OF THE AIR. 63
following Table, which shows the " corrections to
be applied to the monthly mean reading of a ther-
mometer, placed at the height of 4 feet above the
soil, with its bulb freely exposed to the air, but in
other respects protected from the influence of ra-
diation and rain, at any hour, to deduce the true
mean temperature of the air for the month from
the observations taken at that hour."
Local
mean time.
Jan.
Feb.
March.
April.
May.
June.
h
o
o
o
o
o
12 Midn.
4-0-9
+ r6
4-2-9
4-4-8
45-4
4-6-2
I A.M.
+ ro
+ r8
4-3'o
4-5-2
4-6-0
4-7-1
2
4-1-2
+2-0
+ 3'3
4-57
4-6-4
4-8-0
3
+ i'3
+2'I
+ 3'6
4-6-2
467
4-8-7
4
+ 1-6
+2-3
4-3-9
4-6-6
4-67
49'3
5
+ x-8
+ 2-2
4-4-0
4-6-7
4-6-3
4-8-8
6
+ r 9
4-*-3
4-3-9
4-6-o
4-4-8
4-6-4
7
+ i'9
+2-i
4-3-6
4-4-3
4-2-6
4-3'
8
+ i-5
+ r6
4-2-5
4-2-0
4-0-5
O'O
9
4-ro
+07
4-0-2
-0-9
2'0
-2-5
10
+ 0-2
-o*S
-1-9
-3-2
- 4 -0
-4'5
II A.M.
-i'3
2-1
-3'5
-5'3
-5"5
-5-8
12 Noon.
~ 2 *3
-3'2
-5-0
-6-8
-67
-7'3
I P.M.
-2-9
~3"9
-5-8
-7'9
-7'5
-8-1
2
-3-0
-3'9
-5-8
-8-2
-7'7
-8-6
3
-2-5
-3-6
-5'5
-7'7
-7*3
-8-4
4
-1-9
-2-8
-4'5
-6-7
-6'!
-7'4
5
ri
-1-6
-3'3
-5'4
-4-8
-6'!
6
-0-6
-0-6
-r8
-3'5
-3-0
-4'5
7
-0-3
+0-3
-0-4
i*i
ro
-2-4
8
+ 0-1
4-0-6
4-o'9
4-0-7
4-0-9
O'O
9
+0-4
+ ro
4-1-7
4-2-0
42-3
4-1-8
10
4-0-6
+ r3
4-2-3
4-3-2
4-3-5
43-6
II P.M.
+07
+ i'5
4-2-6
44-i
4-4-5
45-o
PRACTICAL METEOROLOGY.
Table continued.
Local
mean time.
July.
Aug.
Sept.
Oct.
Nov.
Dec.
h
12 Midn.
I A.M.
o
4-5"0
+ 5'5
o
+5'5
o
o
+2-9
+ 3'o
o
+ 1-7
o
4-0-9
4~i'o
2
-j-6*o
+6-0
4"5'5
4~3'4
+2*0
4-1*2
3
+6-4
+6-3
+6-4
4-2-0
+ 1 '3
4
+6-6
46-5
4-6-6
4-3'8
4-2-1
+ r 4
5
+ 6-2
4-6'S
+6-2
+ 3*8
-j-2'0
+ r 4
6
+4'5
-f~5*5
+ 5*3
_L,.f
4 1*9
-+-1*4.
7
+*'5
4-3*3
+ 2'8
4-i'7
+ x "5
8
O'O
+0-9
4-2'I
4-1*6
4-1-0
_j_j.j
9
2*0
1-6
-0-4
O'O
-j-o'4
4-0-9
10
-4'0
-3*5
-3-0
2*0
-0-6
O'O
II A.M.
-5*4
-5*4
3'8
2-0
1-3
12 Noon.
-6-4
-6-5
-6-4
-5' 1
3-1
2*1
I P.M.
-67
-7'5
-7-1
-5'5
-3*5
2*4
2
-6-7
-7'7
-7-1
-4-9
2-3
3
-6-5
-7'o
-6-6
-37
-3-0
4
-5-8
-5'5
-5*5
-2-8
2-1
1*3
S
-4-9
-3-6
-4-2
-17
I"2
-0*8
6
-3*5
2*0
-2-5
-0-8
0-4
0*4
7
1*5
0'5
-0-6
O'O
4-0-1
O'l
8
4-0-3
4-1-0
-f-i'o
+0-7
4-0-6
4-0-2
9
4-i'9
-j-2'4
-f-i*8
-j-i'o
-t-0'4
10
II P.M.
40
+ 3|3
+2-7
-f-i'9
-H'4
+?s
4-o*5
4-o-8
By comparing this Table with the projection,
Plate IV. fig. 2, its use will be immediately
apprehended. Let us suppose that observations
have been taken at the noon of every day in July,
and that the mean of these is 69 ; from the pro-
jection, we see that the normal curve at noon
THERMOMETRIC CONDITION OF THE AIR. 65
rises between 6 and 7 above the mean tempera-
ture, and the table gives us the exact quantity,
namely 6'4, which must be applied with a nega-
tive sign, to obtain the mean temperature for
that month ; therefore we subtract this amount,
and the remainder will be the element sought,
viz. 696-4, or 62-6.
49. Hours in the day when the thermometer shows
the mean monthly temperatures at Greenwich.
The January curve will be observed to cross the
line of mean temperature twice in the twenty-
four hours, viz. at 10 A.M. and 8 P.M.
The July curve crosses its line of mean tem-
perature at 8 A.M. and at 8 h 5 m P.M.
In like manner it was found that at certain
times of the day throughout every separate month,
the temperature of the air was at its mean value ;
these times are as follows in the several months :
h m h m
In January ...at 10 o A.M. and again at 8 o P.M.
,, 6 40 P.M.
7 20 P.M.
70 P.M.
7 30 P.M.
80 P.M.
85 P.M.
7 20 P.M.
7 20 P.M.
70 P.M.
6 45 P.M.
7 20 P.M.
F
In February
9 30 A.M.
In March
9 10 A.M.
In April
8 40 A.M.
In May
8 25 A.M.
In June ,,
8 A.M.
In July
8 A.M.
In August
8 20 A.M.
In September
8 55 A.M.
In October
9 A.M.
In November
9 2 5 A.M.
In December
10 A.M.
66 PRACTICAL METEOROLOGY.
In a very elaborate treatise ' Sur le Climat de
la Belgique/ by M. Quetelet, founded on obser-
vations taken at Brussels during the ten years
1833-1842, may be found discussions of a similar
character to those of the Greenwich observations.
The following Table of the times when the mean
temperature is reached in each month at Brussels
is extracted from it :
h m h m
In January.. .at 9 30 A.M. and again at 6 40 P.M.
In February 9 42 A.M. 7 12 P.M.
In March 9 24 A.M. 80 P.M.
In April 8 40 A.M. 80 P.M.
In May 8 30 A.M. 7 58 P.M.
In June 8 24 A.M. 85 P.M.
In July 8 12 A.M. 86 P.M.
In August 8 42 A.M. 7 57 P.M.
In September,, 848A.M. 7 35P.M.
In October 96 A.M. 79 P.M.
In November 915 A.M. 7 12 P.M.
In December 9 36 A.M. 80 P.M.
It is generally found that the greatest heat
of the day is attained about 2 P.M., and the
least a short time before sunrise in every
month of the year, at Greenwich : on this
point M. Quetelet says, referring to Brussels,
"Ces deux termes critiques varient, quand on
considere separement les differents mois. La
plus haute temperature, pendant les jours de
Janvier, tombe a l h 34 m apres midi, et s'eloigne
THERMOMETRIC CONDITION OF THE AIR. 67
(Tautant plus de ce point qu'on se rapproche
davantage de 1'etd. Dans cette saison, c'est trois
heures qui est Pepoque la plus chaude de la
journee. La temperature la plus froide du jour
arrive vers 6 heures du matin en hiver; et, en
ete, vers 3 heures et \"
As it is instructive to compare phenomena
observed under different circumstances and in
various localities, the corrections for diurnal range
for January and July at Brussels are here given ;
they are deduced from observations by M. Quete-
let, extending through 1841-1844, Centigrade
degrees being expressed in those of Fahrenheit.
These may be projected and compared with the
normal diurnal curve at Greenwich (Plate IV.
fig. 2).
Hours.
Jan.
July.
Hours.
Jan.
July.
Noon.
-r8
-4'5
Midnight.
+ 1-6
+4'9
P.M. 2
2'2
-5*
A.M. 2
+ r8
+ 3'3
4
-i'3
-S'6
4
+ r8
+6-5
6
+ '2
-4'5
6
+ r8
+4'3
8
+ '7
0*
8
+ r6
+ -5
9
+ '9
+ r8
9
+ '9
-i'3
10
+ !
+ 3'i
10
*2
-27
50. Proper times of observation. To determine
the mean temperature of the air, it might be
thought advisable to take an observation at one
of the two periods of the day when the mean
temperature is reached -, but it must be borne in
F 2
68 PRACTICAL METEOROLOGY.
mind, that at these times the changes of tempera-
ture are rapid, as may be seen by the sudden
ascent and descent of the branches of the curve
(Plate IV. fig. 2), and, consequently, if the ob-
servation is made a little too soon or a little too
late, considerable errors might be committed;
observations at these times, therefore, unless they
are made very accurately with regard to time, are
not worthy of implicit confidence.
The general hours of observation adopted by
those whose time is occupied by other than scien-
tific matters are either 9 A.M. only, or 9 A.M.,
3 P.M., and 9 P.M. : observers commencing a
series would do well to follow plans already in
use, whereby their registers will be more readily
comparable with those in progress or existence.
51. Grlaisher's factors. From inspection of the
projected curve of mean diurnal range for the
month, it will be seen to assume the form of a
single progression, having one ascending branch
arid one descending : the maximum heat occurs
early in the afternoon, generally at 2 P.M., and
the minimum at about sunrise. For other loca-
lities differing in latitude from Greenwich, this
curve may not be of the same form ; the range of
temperature may be greater or less; if so, the
apex of the curve will be more or less pointed,
and the return from the lowest point more or less
THERMOMETRIC CONDITION OF THE AIR. 69
sudden; but, amidst all the varieties which may
occur, the maximum and minimum heat, or the
turning-points of the curve, will most probably
occur at nearly the same local time, if the lati-
tude be not very different. Hence it may be
concluded, that the amount of the correction ap-
plicable to observations taken at any hour at any
place, is the same part of the whole monthly mean
daily range at that place, as the correction at
Greenwich is of the monthly mean daily range at
Greenwich. Now this element is found by adding
up the readings of the maximum thermometer
daily and taking the mean ; doing the same with
the minimum ; then the mean of the maxima,
minus the mean of the minima, will be the mean
daily range. The following extracts from the
Tables already quoted, founded on the above as-
sumption, will give certain factors, which, multi-
plied into the mean daily range, will give the
correction to be applied to hourly observations to
obtain the mean temperature for the month, for
any other place beside Greenwich.
Factors to be multiplied into the mean daily range
of the thermometer to deduce the correction to
be applied to the monthly mean reading at the
hours 9 A.M., 3 P.M.,and 9 P.M., to determine the
true mean temperature of the air for the month :
70
PRACTICAL METEOROLOGY.
Hours.
Jan.
Feb.
March.
April.
May.
June.
9 A.M.
3 P.M.
9 P.M.
+'"3
-309
+049
+074
-'383
+106
+015
-414
+128
-053
-458
-P"9
'112
'410
+ 130
-128
-431
+092
Hours.
July.
August.
Sept.
Oct.
Nov.
Dec.
9 A.M.
3 P-M.
9 P.M.
-116
-376
+ 'IIO
-094
-410
+140
-025
408
+ 111,
0'
-'34
+ '120
+*43
-316
+106
+106
-224
+ 048
These factors are formed from the Table, 48,
by dividing the correction for the hour by the
mean daily range, and it is considered that just
such a portion as the hourly correction is of the
mean daily range at Greenwich, such portion will
the correction for any other place be, in which
the range of the thermometer during the twenty-
four hours may be greater or less, of the mean
daily range at that place.
52. Example of their application. The applica-
tion of them will be very evident from an example
which I shall extract from my own observations
during the month of January 1849, at South-
ampton ; taken at 9 A.M., 3 P.M., and 9 P.M.
Mean of the 9 A.M. readings from the month ... 407
44-1
9 P.M.
Mean of the maxima 45*3 ; of the minima 37*4.
Mean daily range 45*3 37'4^ 7'9
THERMOMETRIC CONDITION OF THE AIR. 71
Correction for 9 A.M.
7*9 X +'123 = +'97 4'7+*97 = 4 r6 7 mean tempera-
ture for the month from the 9 A.M. observations.
Correction for 3 P.M.
7-9 x '309= -2*44; 44*1 2'44 = 41*66 mean tempera-
ture from the 3 P.M. observations.
Correction for 9 P.M.
7' 9 x +'049 = -f'39 5 4 r 3+'39 = 4 r6 9 mean tempera-
ture from the 9 P.M. observations.
Referring to the Table in 48, let us deduce
the mean temperature by the application of the
Greenwich corrections :
g A.M. 3 P.M. 9 P.M.
407 44-1 41-3
Cor. + r 2-5 -f -^
M.T. 417 41-6 417
The similarity in these results very satisfacto-
rily proves the extended utility of the Greenwich
factors, and shows that the mean daily range of
the thermometer at Southampton differs very
little from that at Greenwich, for this month at
least. Although the month was not chosen with
design, yet I am bound to say that seldom does
the mean temperature result so identical from
the three daily observations as in the example
above worked out.
53. Monthly mean temperature from maxima
and minima. The relation between the tempera-
ture taken at any hour of the day throughout the
72 PRACTICAL METEOROLOGY.
month and the mean temperature for the month
having been thus satisfactorily established, it was
found that the method of determining this latter
element, which had been generally in use, viz. by
deriving the mean temperature from the arith-
metical mean of the maxima and minima, was to
some extent erroneous, and that the mean of the
maxima and minima needed a correction corre-
sponding to the time of the year, before the mean
temperature could be deduced ; that only in the
month of December was no correction requisite ;
that such a mean temperature would be too great
during every other month of the year by the fol-
lowing amounts, which must, therefore, be sub-
tracted from the means of the maxima and
minima readings before a true result could be
obtained :
January... 0*2
Julv... . i'o
February 0-4
March ... ro
April 1-5
May i'7
June i'8
August ... i -7
September .1-3
October ... 1*0
November . 0*4
December. . cro
These quantities, strictly speaking, are only
adapted to the middle day of each month ; if it be
required to obtain the temperature for a particular
day, and not the monthly mean, the following
plan may be adopted.
THERMOMETRIC CONDITION OF THE AIR. 73
Let the correction for any month, say June, be
represented by b \ for July by c ; the difference
between these two divided by 30, the number of
days between the middle days of the two months,
and multiplied by (a) the number of days between
June 15 and the particular date in question,
will give a number which must be added to the
June correction ; thus,
max.+mm._ { I . 8+ 2=2> 1 = the
2 I 30 J
mean temperature of the day.
Where the mean temperature of the day is only
required to the nearest half-degree, the monthly
correction, without alteration, may be applied to
every day's mean of the maximum and minimum ;
this will give the mean temperature of the day
sufficiently near to serve the purpose of tracing
connexion between change of temperature and the
progress of disease ; in the consideration of which
it enters as an important element.
Now, on the supposition that these quantities
are the same for the whole of Great Britain, we
have two independent methods of arriving at the
mean temperature of the month, which may serve
mutually to correct each other ; that they are of
this extensive application will not admit of proof
until we have more registers of temperature taken
frequently in the twenty-four hours than exist at
74 PRACTICAL METEOROLOGY.
present ; if, however, we find that the mean tem-
peratures derived from observations at set hours
in the day agree in the main with those resulting
from the means of the maximum and minimum
observations at any place, we may fairly conclude
that these corrections apply as well to that locality
as to Greenwich.
54. Example. To test the application of these
corrections, we will take the maxima and minima
observed at Southampton in January 1849, the
month before quoted; I find the mean of the
maximum observations 45-3, of the minimum
37-4.
The mean temperature deduced from these,
with Glaisher's corrections applied, will be
which differs from the former result obtained in
51, by 0-4.
Now the mean temperature derived from the
maxima and minima results from two observations
daily; in determining therefore the mean tem-
perature from a combination of all our observa-
tions, we give this determination double weight ;
and thus the " adopted mean temperature" would
be that derived from the following formula, in
which are combined, with the maxima and mi-
THERMOMETRIC CONDITION OF THE AIR. 75
nima, the observations taken at 9 A.M., 3 P.M.,
and 9 P.M.
= . 5 M T
The results of the two methods of determining the
monthly mean temperature will be seen in the
accompanying Table ; they are from the author's
184
8.
184
9.
18.
iO.
S
12
J>
S'O
s*
STJ
I'll
*.u
3 ^
g'3 ^
*ls
z*
c ^r
2*
**"
^r
^ rt
40*1
7Q'8
7A"Q
*8
February
4.7*7
4.7-6
AA'6
.a
March
A7*6
III
A7*2
4.7*1
8*
April
47*7
AQ'I
44-* 4-
44*6
4.8'A
A8*6
May
58*8
5I*6
June
56-7
CQ'A
^
60'7
CQ'C
cn'2
July
61*7
62*2
6o'8
59'9
6r6
62-1
August
58*5
59*6
6r
60-5
59'5
6o*2
September
5 6 '9
57'2
56*3
57'
5 6 '3
55'5
October
51*9
5i'7
5 r
46-6
46-2
November
December
42*8
44*1
42*6
44'
45'5
39'9
44-8
38*7
47*1
42*8
467
42*0
Means for 1
the year J ' '
5^4
52*1
50-2
50*1
49*4
49*1
Mean temperature from three observations daily during
the three years 50*3
Mean temperature from the maxima and minima 50*4
three years' observations taken three times daily,
viz. 9 A.M., 3 P.M., and 9 P.M. The comparison will
show a sufficient accordance to give a confidence
in the use of the Table of Corrections, 48, which
76
PRACTICAL METEOROLOGY.
was originally calculated for the Royal Observatory
at Greenwich.
Feeling a confidence in the use of the Greenwich
corrections for diurnal range from these observa-
tions, I gave up the two later observations in the
day after the year 1850, and confined myself to
9 A.M. j the results for the following three years
are here given :
18.
,1.
18
52.
18
53.
Ns
jl<
jy
S 1
!
J-d
rt c
January
44"}
44-* I
42*
42*5
A-J'A
4-3 '1
February
March
40*6
40*8
41*2
4.2" I
42-3
4.2*1
35'
35-8
38*6
April
4.6-4.
4.1*
46*8
46*
4.7*6
May
11*6
CI'I
52*6
12*6
12*8
June
c6'8
17*
17*7
Julv
61*5
6i"?*
6c*o
6 1*7
1Q' I
August
62*2
6r
61*2
61-4
61*2
10*8
September
October
57'5
57-6
56-8
48*1
56*9
78*
55-9
55'9
10*4.
November
December
38-5
41*4
397
49'9
48-2
50*2
47*8
42*6
36*6
44' 3
377
Means for 1
the year J ' '
5*
497
5'9
$*'
48*5
48*6
Mean temperature from the 9 A.M. daily observations
during the three years 49*8
Mean temperature from the maxima and minima 49*8
Mean temperature of Southampton from the six years'
observations 50*0
Interpolated.
THERMOMETRIC CONDITION OF THE AIR. 77
55. Explanations. It may be useful,, in guiding
those who are about to commence a series of ob-
servations, to make some remarks on the above
observations. The instruments with which the
observations were taken, were good of their kind,
but not expensive ; the thermometer used at the
stated hours was compared with a standard by
immersion in warm water, frequent readings being
taken at intervals of ten minutes or a quarter of
an hour, as the water gradually cooled ; the sum
of the readings of each thermometer divided by
the number of readings gave a mean temperature,
which, if the readings of both thermometers had
been the same throughout, would have been the
same .quantity; the mean reading of my ther-
mometer was, however, 0*4 higher than the
standard of comparison, hence to all my readings
a correction of 0*4 has been applied to reduce
them to true readings. The maximum and mi-
nimum thermometers were, in like manner, com-
pared with the other corrected, and the index
correction applied throughout.
The thermometers are only divided to degrees ;
hence, in reading off, if the scale is cut by the
mercury between two degrees, we estimate how
many tenths of a degree it is above the next lowest,
and enter the reading accordingly.
It will be remarked, and the remark will apply
78 PRACTICAL METEOROLOGY.
to all other observations, that though the differ-
ences, in some individual months, between the
mean temperature derived from observations taken
at certain hours and corrected for diurnal range
by Glaisher's Tables, and that deduced from the
mean of the maxima + the mean of the minima
4- 2 the quantities given in 53, is consider-
able, these all vanish in a series of no very great
extent. Rejecting 1848, which is an incomplete
year, the difference between mean temperatures
of the year by the two methods will be found to
be:
50-2 50-1 or H-o'i difference for 1849
49*4 49' i or +0-3 difference for 1850
50' 49'7 or -fo'3 difference for 1851
50^9 51*0 or o'i difference for 1852
48^5 48^6 or o'i difference for 1853
56. Projection of six years' observations. An
excellent method of recording phenomena, so as
to present them to the eye at a glance, is that of
projecting them in curves, taking the time for
abscissa? and the variables for ordinates ; thus the
monthly temperatures recorded in the two prece-
ding Tables may be projected in the form shown in
Plate III., in which the monthly irregularities are
rendered visible. In addition to these, the same
elements for Greenwich are projected on the same
scale, for the purpose of ready comparison, as also
THERMOMETRIC CONDITION OF THE AIR. 79
the normal curve of monthly temperature at the
Royal Observatory, Greenwich, derived from
seventy-nine years of observation ; the data being
the sum of the temperatures for each month of
the same name, divided by the number of years
through which the observations extended.
From all the observations combined, the mean
temperature of each month at the Royal Obser-
vatory, Greenwich, is
jj i
' 3
of February . . .
38-2
of August
60-5
of March
40-9
of September ...
5^3
of April
457
of October
49*3
of May
52-6
of November . . .
42-4
of June
58-0
of December . . .
38-8
The mean of all the monthly results or mean tem-
perature for the year is 48'3.
Height of the Observatory above the mean level
of the sea, 160 feet.
The height of my observatory above the sea-
level was only 60 feet ; the difference between this
and the height of the Royal Observatory, is suf-
ficient to account for the slight excess of cold in
that position during the winter months; the
proximity of Southampton to the sea is indicated
in the extremes of temperature being somewhat
less than at Greenwich.
57. Care necessary in observation. It may be
80 PRACTICAL METEOROLOGY.
stated as an indisputable fact, that unless great
care be taken in choosing a position for the ther-
mometers, the whole of the observations will be
vitiated ; many sets of records have proved worth-
less from want of attention to this important
matter, and much valuable labour has been thrown
away, not only from this circumstance, but also
from the use of inferior instruments. It is true,
that, by a careful comparison with a standard
throughout the whole extent of the scale, quan-
tities might be obtained which, applied to the
observations made with a bad instrument, may
bring them near the truth ; but if the observations
have been taken in a place where reflected heat
has been recorded, or where the sun's rays may
have had an influence, no good results can possibly
be obtained.
58. Position of instruments. The corps of ob-
servers in England who combine their observa-
tions, as will be explained more fully in Part III.,
suspend their thermometers in positions and under
circumstances as much as possible similar : the
bulb of the dry thermometer is supposed to be
4 feet from the ground, and the temperature shown
by it is due to the stratum of air at that distance
from the soil ; it is sheltered in every way, not
only from the direct rays of the sun, but it is also
so far removed from walls and buildings as to be
THERMOMETRIC CONDITION OF THE AIR. 81
out of the way of the reflected heat, or that de-
rived from radiation; it must be protected from
rain, but the air must be allowed an uninterrupted
passage around it in every direction. The author
has endeavoured to ensure all these advantages
by a stand of the following construction.
59. The Author's thermometer stand. In it
are suspended the dry-bulb and wet-bulb thermo-
meters, and also the thermometers which register
the greatest and least degrees of heat. The stand
is doubly boarded on the east, west, and south
sides, the instruments facing the north without
any screen; hence when the sun shines on the
outer covering, a stratum of air intervening, the
inner case does not get heated by its rays ; holes
are bored in various parts of both the inner and
outer case, with especial care that they be not
opposite to each other, lest the sun's rays should
find admission to the thermometers ; the air thus
circulating freely is supposed to be of an equal
temperature with that in the shade in any position,
however sheltered from the sun : a pent-house, or
sloping roof, also double, throws off the rain, and
thus the thermometers are kept dry. The up-
right support of the stand is firmly fixed in the
ground, and braced to prevent agitation; the
whole should be at least 12 or 15 feet from lateral
walls, and 40 or 50 from walls directly north,
82
PRACTICAL METEOROLOGY.
which would reflect the sun's rays and influence
the instruments. The thermometers face the
north and are about 4 feet from the ground.
Without some such stand, it must be impressively
stated, any thermometric observations will be use-
less, as the instruments will not be protected from
local or celestial interferences which would en-
tirely vitiate the most careful records ; nor would
the register be comparable with observations taken
in different parts of the country.
60. Greenwich thermometer stand. The ther-
mometer stand after the form of that at Green-
wich, and, till lately, at Kew, differs from this,
as may be seen in Plate IV. fig. 1.
A is a wooden painted frame, 2 ft. 6 in. square,
a 1 a base board attached to the under part of A,
d z the underside of a strong piece attached to the
upper part of A, a 3 an interior portion of a pent-
house composed of very thin boards attached to
a 1 and a 2 , 4 a 5 strips of wood to which the ther-
mometers are attached.
B is a strong spar or post firmly secured to the
ground at Greenwich, to a stone balustrade at
Kew ; in its upper end is fixed a cylindrical pin
which freely enters a socket in the central part of
2 , and allows A, with all its adjuncts, to be re-
volved on the axis of B.
Now the great objection to this kind of stand
THERMOMETRIC CONDITION OF THE AIR. 83
for general use is that it requires, during the
summer time when the sun's azimuth is north of
east or west, to be turned bodily, so that the roof
be directed to his rays which would otherwise
impinge laterally on the thermometers. The
problem to be solved in the construction of a
stand is that the instruments be at all times pro-
tected from the direct influence of the sun and
the rain, at the same time that they are freely ex-
posed to unimpeded circulation of air. The stand
erected at Kew by Mr. Welsh is perhaps as near
an approach to the solution as has yet been at-
tained, having the recommendation of requiring
no adjustment after it has once been fixed in the
ground, being open to the movement of the air
all round, and shielded from the approach of the
sun's rays whatever may be his azimuth.
The thermometers are in a cubical case, the
sides being about 2 feet, open at the bottom ; it is
composed of louvre-boards, like Venetian shutters.
Another case of the same construction, but of twice
the dimensions every way, encloses the smaller.
The cases reach to within 4 feet of the ground, so
that there is nothing to obstruct the free passage
of the air horizontally, which moreover finds its
way readily through the open woodwork, while the
sun can only reach the external case and not that
in which the instruments are suspended.
G2
84 PRACTICAL METEOROLOGY.
61. Mean yearly temperature at Greenwich.
In the ' Philosophical Transactions/ Part IL,1849,
and Part II., 1850, will be found two most valuable
papers by Mr. Glaisher, in which he has reduced
observations made at the Royal Society's Apart-
ments Somerset House, at Epping, and at Lyndon
in Rutlandshire, to an agreement with those made
at the Royal Observatory, Greenwich ; this series
extends from the year 1771 to 1849, and from it
that gentleman has determined the mean tempera-
ture of every year, at Greenwich, through a period
of seventy-nine years. Now as Greenwich is ever
likely to be a point of reference in all observations
on climate, and as the difference in temperature
between it and any other place in England is
readily ascertained, we consider this series to be
one of great value ; it is probably the best deter-
mination of this element ever completed, and it
will assist in arriving at other valuable results ;
the number of observations treated of exceeds
200,000, spread nearly equally over the seventy-
nine years. Mr. Hugh Gordon, of the Ordnance
Map Office, Southampton, having been employed
in discussing these observations, determined to
equate the several yearly temperatures by . an
elliptic curve, starting from one lowest point to
the next. He has kindly supplied the author
with the projection, engraved in Plate V., whose
THERMOMETRIC CONDITION OF THE AIR. 85
elliptically equated curve shows, in a very striking
manner, the cycle of the greatly variable mean tem-
perature at the Royal Observatory. The numbers
below the lines show the mean temperature of
each year as deduced from observation, the equated
values may be ascertained by inspection ; it will
be found that the summation of the equated tem-
peratures in each cycle equals the summation of the
recorded mean temperatures within the same period.
An inspection of the form assumed by this
curve will exhibit certain remarkable phenomena ;
commencing with 1771, the years gradually be-
came warmer till 1779, when the temperature in
like manner declined, and a batch of cold years
occurs, of which 1784 was the coldest. The heat
then increased, but not in so great a degree, till
1794, when the extreme cold of that cycle, not so
severe as before, was reached gradually in five
years from that time. In periods varying from
nine to fifteen years throughout the whole series,
we find the cycle of hot and cold years repeated ;
and nothing can more distinctly present the re-
currence of these phenomena to the mind than
the graphic method here adopted. In this pro-
jection, it is indicated to the eye in the sequence
of the repeated elliptic curves.
62. Comparison with that of other places.
By examining closely and comparing the meteo-
86 PRACTICAL METEOROLOGY.
rological reports from various places in England,
Mr. Glaisher has endeavoured to arrive at some
empirical formula, by which, the mean yearly tem-
perature at Greenwich being known, that of any
other place may be obtained; he takes into ac-
count the law of decrease of heat with increasing
elevation, and the difference of latitude, and the
results, in nearly every case, have very closely
approximated to those of observation, which in
some instances they have tended to correct, since
a want of accordance between the two, has been
found, more than once, to be due to errors either
in the instruments employed or in the observations
made with them.
Let the mean temperature of the air at the level
of the sea at Greenwich, latitude 51 0< 5, be denoted
by T ; on the supposition that this value, at a
uniform level, becomes less by 0'9 for an increase
of latitude of one degree ; then
The mean temperature of any place in England
for that year may be calculated approximately from
the following formula :
T+(51-5 lat.) x 0-9- -00345 x height of the
place in feet above the level of the sea.
By substituting the mean temperature of Green-
wich for any year, the formula is of perpetual ap-
plication ; thus, from my observations I find the
mean temperature of Southampton for 1853 to be
THERMOMETRIC CONDITION OF THE AIR. 87
48-5, the latitude being 50'9, and the height
above the sea 60 feet ; the mean temperature of
Greenwich for the same year reduced to the sea-
level =48*2; by substitution,
48-2 + (51-5-50 -9)0-9--00345x60=48-5,
a result exactly coincident with that deduced from
the observations.
63. Mean temperature fails in describing cli-
mate. Mean temperature, whether of the year or
month, though an important element in investi-
gation, conveys but a very remote idea of the
climate of any locality. The average degree of
heat will remain uninfluenced by sudden tempo-
rary rises and corresponding depressions, so that
the result for a place subject to extreme variations
may be the same as for one whose temperature
is equable. The differences in this element in
various localities may arise from four different
causes : a. Distance from the equator, which is
the cause of the diurnal arc of the sun varying
with the season. /?. Height above the sea-level,
which will cause, at moderate elevations, a de-
crease in temperature of about 1 for every 300
feet. 7. The geological character of the neigh-
bourhood. $. Situation as regards the sea-coast.
Of these causes, the last produces the greatest
irregularities, and such as are most difficult to
reduce to system.
88 PRACTICAL METEOROLOGY.
64. Mean daily temperature at Greenwich.
The average daily temperature for Greenwich,
which probably varies according to a law that is
the same for a great part of England, has been de-
termined from all the thermometrical observations
taken at the Royal Observatory during 43 years,
ending with 1856. Tables of the adopted mean
temperature of every day in the year and of the data
from which this was computed, are to be found in a
paper by Mr. Glaisher in the Report of the Council
of the British Meteorological Society for 1857.
The Tables show that the temperature does not
regularly increase from its lowest in the middle of
January up to its greatest height in July, but that
there are variations of some days' duration in the
months of February, March, and May : its decline
goes on regularly from the end of July till the end
of November, when an increase takes place which
just lasts into December, when it again falls, though
it is somewhat irregular all through this month.
Another of the Tables gives factors for each
day of the year, which, when multiplied into
the mean temperature of each month, will give
the mean temperature of any particular day of the
month. It is by these numbers that the results
of the Greenwich observations are made applicable
to other places ; for it is likely that in many lo-
calities the average temperature of a day bears
THERMOMETRIC CONDITION OF THE AIR. 89
the same proportion to the mean monthly tempe-
rature as it does at Greenwich. Another Table
shows the distribution of heat over the year in the
following manner : the average amount of heat is
represented by 1 (which number occurs at the
29th of April and the 21st of October), and for
each day a decimal shows the ratio the number of
degrees representing its mean temperature bears
to that of the mean yearly temperature ; the lowest
fraction is (for January 15 and 16) '726, and
the highest (for the days from July 28 to Au-
gust 1) 1*279. It must not, however, be sup-
posed that this tells us the absolute proportion of
heat received on each day, for it is clear that the
numbers would be different if Reaumur's or the
Centigrade scale were used, from their having a
different zero, and therefore they depend on an
arrangement that is artificial. In order to show
the true proportion of the differences of tempera-
ture on the different days, the Editor has con-
structed the following Table, taking for data the
adopted mean temperature of every day of the
year, given in one of those mentioned above.
Here the lowest daily temperature is the zero, the
highest is unity, and the decimals tell what part
of the whole increase of daily temperature that
takes place during the year has occurred at the
date opposite each number.
90
PRACTICAL METEOROLOGY.
65. Table showing how the heat increases through
the year.
Days
of the
Jan.
Feb.
March,
April.
May.
June.
Month.
I
o
037
o
'063
o
170
o
3 00
o
'537
o
774
2
*33
'55
166
3 l8
*555
7 8l
3
33
066
162
'333
570
788
4
029
08 1
162
'34-4
585
800
5
022
107
166
'355
*59 6
800
6
018
130
170
366
613
803
7
on
i37
170
370
607
807
8
ooi
137
170
370
603
811
9
'000
130
174
366
596
814
10
007
118
177
'355
588
822
ii
003
in
185
"35 1
585
833
12
003
103
188
348
585
844
J 3
003
"100
203
"355
588
851
*4
003
092
214
370
603
862
15
'000
096
222
'377
618
870
16
ooo
096
22 9
-388
633
88 1
J 7
014
*1OO
'*33
39 6
640
885
18
029
103
233
403
651
888
J 9
037
107
233
407
659
892
20
048
III
237
414
670
900
21
059
III
237
425
677
903
22
066
114
237
'444
688
907
2 3
074
126
240
448
696
914
2 4
085
133
244
448
700
922
25
092
148
248
448
707
929
26
103
"'55
251
448
711
'937
27
103
'59
255
462
718
'944
28
096
166
262
481
729
*955
2 9
088
..
270
503
737
962
30
077
277
518
748
962
31
066
292
762
Means
040
"112
215
*397
643
868
THEEMOMETRIC CONDITION OF THE AIR. 91
Table continued.
Days
of the
July.
August.
Sept.
October.
Nov.
Dec.
Month.
I
o
962
rooo
o
862
666
o
*4 3
22 9
2
959
992
855
662
39 6
'*33
3
'959
988
851
6 5 I
'39*
229
4
962
985
848
648
385
'222
5
966
981
840
640
'377
218
6
970
981
833
'629
370
207
7
'974
981 ! -825
622
'355
I 9 2
8
970
981 ! '822
614
'344
188
9
962
977
818
-607
'333
185
10
962
974
814
600
'3*5
181
ii
966
.970
8 1 1
588
*3H
174
12
970
966
807
"574
*3"
166
'3
'974
962
803
'555
"296
151
*4
970
962
796
'54
288
i55
*5
970
- '959
785
529
277
166
16
970
'959
'777
522
262
177
*7
970
'955
770
'5'4
' 2 55
170
18
970
*944
'759
507
248
162
'9
'970
940
'755
'503
248
148
20
966
*937
'744
'503
248
129
21
962
929
740
496
240
107
22
962
922
'737
485
229
088
2 3
962
918
'733
'474
218
070
2 4
966
918
722
'459
203
'55
25
'974
914
718
'444
196
037
26
985
*93
711
440
200
033
27
992
896
703
'433
207
037
28
I '000
892
696
425
222
'55
29
rooo
885
685
418
225
066
3
rooo
874
677
411
225
070
3i
rooo
870
407
077
Means
972
*945
776
'534
286
141
92 PRACTICAL METEOROLOGY.
66. Dove's Isothermal Charts. Professor Dove
has given great attention to the subject of mean
temperatures, and his valuable Tables, published
in the Report of the British Association for 1847,
with his Isothermal Charts (described in that for
1851) for every month of the year, will well repay
consultation. In the formation of these, 900
places have been selected at which continuous
observations have been made ; these have all
been reduced to the level of the sea, and the
temperatures of other places have been deter-
mined by empirical formulae ; lines are then drawn
across a map of the world through those places
whose monthly mean temperature is the same.
From mere inspection of these we learn some
very important particulars, the discussion of which
in full would be foreign to the purpose of this
work, though we should not be justified in passing
over altogether certain conclusions at which Pro-
fessor Dove has arrived.
67. Deductions from them. a. He has shown
that the mean temperature of the western hemi-
sphere supposing the meridian of Faro to be the
separation exceeds that of the eastern for every
degree of latitude except 70, but that the differ-
ence is less as the places approach the equator,
which is accounted for by the preponderating
mass of water in the west. There is no point in
THERMOMETRIC CONDITION OF THE AIR. 93
the western hemisphere which approaches the
extreme cold of Yakutsk, whose January tempe-
rature is 40.
/3. The winters of the northern hemisphere
are colder than those of the southern ; on the
contrary, the summers are warmer. Taking the
mean heat of all the places on the globe in Janu-
ary and July, he calculates that of the entire
earth to be
{In the northern hemisphere 7*5 R. 48-9 Fahr.
In the southern 12*2 R. = 59-5 F.
Mean of the whole earth ... 9-9 R. = 54-3 F.
r In the northern hemisphere 17^3 R. = 71 F.
July < In the southern 9-6 R. = 53-3 F.
I Mean of the whole earth ... 13-5 R. = 62-4 F.
The temperature of the earth, therefore, in-
creases 8 from January to July.
The preponderance of water in the southern
hemisphere will account for -the less variable de-
gree of heat to which it is subject. The increase
of the temperature of the globe between the
winter and summer of the northern hemisphere,
is the result of the unequal distribution of land
and sea in the two hemispheres, and the difference
of the effect produced by the rays of the sun,
according as they impinge on a solid or a liquid
surface.
<y. The difference between the hottest and
94 PRACTICAL METEOROLOGY.
coldest months of the year is strikingly exhi-
bited in Dove's Maps ; the range of temperature
between the months of January and July, accord-
ing as it is small or great, produces those differ-
ences in atmospheric phenomena, to which may
be given the names of " continental" and " in-
sular" climates. As extreme examples we may
cite Commewine in Guiana, where the tempera-
ture in July differs from that in January only
2'2; and Yakutsk in Siberia, where the variation
in temperature of these months is 114'4.
68. Continental and insular climate compared :
Example. To illustrate the important distinction
of continental and insular climates, I may select
an example from a series of observations, with
which, for some years, I have been favoured by
W. Palk, Esq., M.D., of Union, Franklin Co.,
Missouri, United States, Lat. 38 36' N., Long.
91 10' W., in the very centre of North America.
They have been taken with great care three times
daily, viz. at 9 A.M., 3 P.M., and 9 P.M.; the
thermometer stands facing the north under a
portico, in such a position as will render safe the
comparison of his observations with my own.
I find the approximate mean temperature of
Union for January 1853 to be 36'2, which differs
but slightly from the normal mean temperature
of that month at Greenwich, and agrees re-
THERMOMETRIC CONDITION OF THE AIR. 95
markably with the January temperature given to its
position in North America on Dove's chart, which
is so far a proof of the accuracy of his empirical
formula.
When, however, we examine the daily observa-
tions, we shall find that the range of temperature
is so extraordinary that we experience no varia-
tions at all corresponding with them ; thus on
the 7th, 9th and 10th of the month, the thermo-
meter at 3 P.M. stood at 70 ; at 9 A.M. of the
3rd it showed 10; the monthly range was then
60; the corresponding monthly range at South-
ampton, which exhibits every symptom of an in-
sular climate, was 55 32 = 23. The fol-
lowing extracts from the register will show the
variable character of the continental climate :
9 A.M. 3 P.M. 9 P.M.
O
January 3 10 20 18
7 S 2 70 54
10 58 70 48
26 12 26 24
27 22 50 26
The 7th of January, at Union, must have been
like a pleasant summer's day, while on the 3rd
the cold was greater almost than we ever expe-
rience; on the 27th, the range of the thermo-
meter in six hours was 28 ; and in the following
six, the air cooled down 24.
96 PRACTICAL METEOROLOGY.
The mean heat of the month of July was about
76 ; the range of mean temperature between that
and January was 40; the same element at
Southampton was 16. These were the observa-
tions on some of the hottest days in the year
1853 :
9 A.M. 3 P.M. 9 P.M.
o o o
July i 82 100 80
4 84 96 76
7 76 9 6 74
8 86 102 80
14 86 100 86
We see, then, that a continental climate deals in
extremes, especially during the winter months ;
and it is clear that we learn little of its peculiar
features from the mean value of either yearly or
monthly temperatures ; we must descend further
into particulars, and mark the sudden transitions
from heat to cold and the reverse before we can
judge of the peculiarities due to any particular
locality.
These extreme changes of temperature are very
trying to the constitution, and diseases of a cer-
tain character are prevalent during extraordinary
transitions. " An old practitioner," adds Dr.
Palk, " can always tell what diseases will follow a
sudden change of weather, whether to hot or
cold."
THERMOMETRIC CONDITION OF THE AIR. 97
It is interesting to compare climates differing
greatly from each other ; we will therefore inspect
the register of temperatures kept at Alten in
Norway, latitude 70 north.
1846. 1847. 1848.
00 O
Maximum ... 83*3 847 86-9
Minimum ... i/j/8 3*1 20*2
Bange 98-1 87-8 107-1
This place, although situated on the sea-coast,
has such an extreme range of temperature that
its climate may be designated as " continental,"
a word which, as well as " insular," is frequently
applied to climate by meteorologists, without
reference to the position of a place as regards the
sea-board. In this case it will be seen, by a re-
ference to the map of Europe, that Alten is far
removed from the influence of the Gulf-stream,
which modifies the climate of all those places
which it reaches in its course.
69. Temperature affected by the neighbourhood
of water : Example. The fact of the neighbour-
hood of water tending to equalize temperature is
shown in localities where even a very limited ex-
tent of surface, as that of a river, is able to in-
fluence the super] acent air. In a comparison before
alluded to, undertaken in the course of the inves-
tigations about the degree of temperature due
to Epping, Lyndon, and Somerset House, it was
H
90 PRACTICAL METEOROLOGY.
found, after reduction to the sea-level, that the
thermometer at Somerset House indicated a higher
winter temperature than at Epping by about 1 0< 4,
and a lower summer temperature by about 0'9 ;
and it was concluded that this variation was due
to the vicinity of the river Thames. For many
years the temperature of the water has been re-
corded by a maximum and a minimum thermometer
2 feet below the surface of the water of the Thames,
suspended from the sides of the Dreadnought
Hospital Ship, in a perforated trunk. The results
of these observations, undertaken by Captain
Saunders, R.N., are distinctly shown in the fol-
lowing account by Mr. Glaisher : " By comparing
the mean monthly temperature of the water of
the Thames for the four years (1846, 1847,
1848, 1849) with the means of the readings
of the maximum and minimum thermometers in
air at the Royal Observatory, for the same months,
we find that the mean lowest readings of the water
were higher in the twelve months respectively by
3-9; 4-6; 7'7 ; 12-3; 11'5; 16-7 ; 12'4;
10-4; 10*7; 6-8; 6'3; and 4-9, than the
mean of the lowest readings of the air ; and it was
lower than the mean maximum readings of the air
by3-2; 4-6; 5-3; 9'2 ; 6; 7-5; 8-l; 6-7;
7-4; 6; 4*7; and 3, in the respective months
from January. These numbers are very large, and
THERMOMETRIC CONDITION OF THE AIR. 99
will fully account for the little higher temperature
possessed by places in the vicinity of the river ;
and these differences of temperature are probably
the fruitful source of the London fogs."
We can very directly trace the effect of the pre-
sence of water upon temperature by looking at
the readings of a thermometer in the air at the
Dreadnought itself, and comparing them with
those taken under similar circumstances at
Greenwich ; this is done in the following extract
from the same paper of Mr. Glaisher's : " The
temperature of the air 32 ft. above the water
(from observations at 6 A.M. and 6 P.M. in 1847
and 1848, and at 9 A.M. and 9 P.M. in 1849)
exceeds that at the Observatory (height 160 feet)
at 6 A.M., by 1'6; 1; 0'8; 0'3; 0'6; 07 ;
0-9 ; 0-8 ; 0'2; ; and 0'8, in the twelve months
respectively; and at 6 P.M. by l-2; 0-8; 1-0;
0'8 ; 0'7 ; 0'8 ; 0'6 ; 0'8 ; l-3 ; 1 ; l7 ; 0'9,
in the twelve months respectively; at 9 A.M. it
was in excess in January by 1 0> 3 ; February by
l-5 ; March by 0'6 ; April by 0'4 ; May by 2'2 ;
June by 0-4 ; and in October by 0-5 ; it was of a
lower temperature in July by 0'7; August by 0'5;
and in September by 0'l ; at 9 P.M. it was always
of a higher temperature : the excesses were 0'l ;
0'3; 0'7; 0'3 ; l-9; 2'9; 1'5 ; 3'2; l-2;
and 1'3 respectively/ 5
H2
100 PRACTICAL METEOROLOGY.
" From these numbers, it seems that during the
night hours, at all seasons of the year, the tem-
perature of the air at the Dreadnought Hospital
Ship is higher than at the Observatory, and that it
is below only during the mid- day hours."
" At times of extreme temperature, the effect of
the water upon the temperature of the air is very-
great. On Feb. 12, 1847, the temperature of the
air, at my house situated one mile and a half from
the river, was 6 ; the lowest reading 32 feet above
the water of the Thames was 16 ; the temperature
of the water was 33 ; its heating effect upon the
air in its immediate vicinity amounted to 10; at
the Observatory the reading was 10 0> 5, and the
heat of the water of the Thames seems to have
influenced the temperature of the air at the Ob-
servatory to the amount of 4."
70. Decrease of temperature as height from the
sea-level increases. The following Table, derived
from the Report of the Balloon ascent mentioned
in 5, will show the decrease of heat as the
aeronauts rose in the air, on Aug. 17, 1852, be-
tween 4 and 5 P.M. :
Temperatures.
ft. O
120 71*2
2,440 62-8
3,460 59'2
4,110 58-1
ft. o
5,880 57'8
6,800 54*0
7.53 5 r 4
8,550 49-0
THERMOMETRIC CONDITION OF THE AIR. 101
Heights above the
level of the sea.
Temperatures.
Heights above the
level of the sea.
Temperatures.
ft.
ft.
o
9>4?o
44*4
15,510
24*4
10,680
40-4
16,600
2O'6
11,620
37'2
17,440
19*6
12,250
34'9
18,490
15-0
13,480
30 8
,19,320
10-5
1 4>55
27-0
Mr. Welsh, who discussed the whole series of
observations, comes to the conclusion, " that the
hypothesis of a regular progression," in the de-
crease of temperature, " at all heights, can scarcely
be maintained." The results of the observa-
tions of decrease of temperature were discordant
in the four balloon ascents, and we must wait for
further observations in elevated regions, before
we can arrive at satisfactory conclusions as to
the thermometric state of the upper strata of
the air.
71. Effect of the sun's heat below the surface
of the soil. An important difference of tempera-
ture between places otherwise similarly situated
will arise from their geological formations ; those
on arid dry soils will have a greater range of tem-
perature than those situated on a clayey soil, or
any other which prevents the drainage or carrying
away of the water received in the form of rain.
To assist in arriving at distinct conclusions on this
head, and to judge of the depth to which the sun's
102
PRACTICAL METEOROLOGY.
rays areable to penetrate, and of the radiating power
of earthy materials, thermometers, constructed for
the purpose, should be sunk in the ground and
registered at stated hours. During the year 1850,
at Southampton, the author kept a daily record of
the readings of a thermometer near the surface of
the soil, which was garden mould, and of another
sunk one foot below. They were read simul-
taneously at 3 P.M.
Readings of a thermometer near the surface and
another one foot below, at Southampton.
1850.
Therm,
at the
surface.
Therm,
one foot
below the
surface.
1850.
Therm,
at the
surface.
Therm,
one foot
below the
surface.
January ...
February...
March
April
May
June
35-8
45'
45'8
54'4
587
68-8
35'4
40-8
39-2
47'
47*8
6i'5
July
August . . .
September
October ...
November .
December .
69-5
66-1
637
51-1
47*4
41-1
61-3
59-1
54*5
47' i
46-
40-8
M. Quetelet has discussed this subject very
fully in the " Memoir " referred to in 48 ; found-
ing his remarks on observations taken between
1834 and 1842 ; to illustrate it we will, however,
look nearer home.
72. Observations at Greenwich. Resorting to
the Greenwich observations for 1847, we find
THERMOMETRIC CONDITION OF THE AIR. 103
some very striking results with regard to the
range of heat at different distances below the
earth's surface, which are best exhibited in a
tabular form.
Table I. Mean monthly reading of a thermo-
meter whose bulb is placed on a level with the
scales of the deep sunk thermometers, from
daily observation in 1845.
Jan.
Feb.
Mar.
April.
May.
June.
Mean monthly reading
Mean daily range
36-4
6-9
36-4
8-4
42-
H'3
46-
I 3 -6
37'4
17-1
59'*
16*9
July.
Aug.
Sept.
Oct.
Nov.
Dec.
Mean monthly reading
Mean daily range
66-9
19-9
6 3*4
17-1
55'
i5'3
S3'*
11-4
47'3
9'4
42-9
7*4
Table II. Mean monthly reading of a thermome-
ter whose bulb is sunk 1 inch below the soil.
'Jan.
Feb.
Mar.
April.
May.
June.
Mean monthly reading
Mean daily range
37'9
4' i
38'
4*4
4*' 3
r
46-4
6-9
567
9*4
i:
July.
Aug.
Sept.
Oct.
Nov.
Dec.
Mean monthly reading
Mean daily range
66-8
10' I
63-9
97
56'
7*4
53*7
5*9
48-4
5'
44'
4*5
104
PRACTICAL METEOROLOGY.
Table III. Mean monthly reading of a thermome-
ter whose bulb is sunk 3'2 ft. ( = 3 French feet.)
Jan.
Feb.
Mar.
April.
May.
June.
Mean monthly reading
Mean daily range
39'3
0-4
39 <6
0-3
4 ri
o*3
44*4
'3
5 l-2
0-4
57'2
0-4
July.
Aug.
Sept.
Oct.
Nov.
Dec.
Mean monthly reading
Mean daily range
61-5
0-5
62-3
0-4
57'9
0-3
54*7
'3
50*6
o*3
46-4
0-3
Table IV. Mean monthly reading of a thermome-
ter whose bulb is sunk 6'4 ft. ( = 6 French feet.)
Jan.
Feb.
Mar.
April.
May.
June.
Mean monthly reading
Mean daily range
44'
0*07
4Z'8
O*I2
43'3
O'll
45' 1
0-13
48-6
O*22
53'9
0'2I
July.
Aug.
Sept.
Oct.
Nov.
Dec.
Mean monthly reading
Mean daily range
0-36
58-9
O'ly
57-8
0-14
55'3
0-15
5 2-8
0-14
49*5
O'H
Table V. Mean monthly reading of a thermome-
ter whose bulb issunk 12'8ft. ( = 12 French feet.)
Jan.
Feb.
Mar.
April.
May.
June.
Mean monthly reading
Mean daily range
49'
0'12
467
0*09
45-8
0*09
45-8
O'lO
46-8
O'll
49*5
0-14
July.
Aug.
Sept.
Oct.
Nov.
Dec.
Mean monthly reading
Mean daily range
51-9
0-15
54-2
0*16
55-11
0-13
54-6
0*10
53-6
0-08
5 r8
0-09
THERMOMETRTC CONDITION OF THE AIR. 105
Table VI. Mean monthly reading of a thermo-
meter whose bulb is sunk 25'6 ft. ( = 24 French
feet) below the surface of the soil.
Jan.
Feb.
Mar.
April.
May.
June.
Mean monthly reading
Mean daily range
5*'
0*05
51-2
o'04
50-2
0*05
49*3
0*05
48-8
0*05
497
0*04
July.
Aug.
Sept.
Oct.
Nov.
Dec.
Mean monthly reading
Mean daily range
49-0
0-06
49-8
0-07
50-6
00-6
5''3
O'OD
5 r 7
0*05
5 I-8
0*05
We see from these Tables that the yearly ex-
tremes occur in a later month the deeper the ther-
mometer is placed, until, in the last instance when
the thermometer bulb was 25^ feet below the
surface, the highest reading occurred in January
and the lowest in July.
Also we see distinctly that the deeper we descend
below the earth's surface, the more equable is the
temperature ; at every place we should at last
arrive at a depth where the action of the sun's
rays would not be felt at all, but a thermometer
would show the same degree of heat at all times
and seasons.
73. Stratum of invariable temperature. The
(< stratum of invariable temperature " will be at
different depths at different places.
From M. Quetelet we extract the following
106 PRACTICAL METEOROLOGY.
table of depths at which the annual variation of
the thermometer is reduced to 0'01 Centigrade :
the feet are French, 1 foot French being equal to
1-0658 English.
ft.
ft.
Leith CA'7
Strasburg ... 8r6
Heidelberg... 83-3
Schwetzingen 89-8
Bonn 72-6
Paris 69*4
Edinburgh... 55*5 Trap,
ib. 66-2 Sand,
ib. 96-6 Sandstone.
Upsal 62-6
Trevandrum 53-6
Below the " stratum of invariable temperature"
the heat of the earth would seem to increase in
different proportions, for the most part at the rate
of 1 for 72 feet of depth; but as this subject
does not bear directly upon meteorology, we shall
not carry the discussion of it further.
74. Radiating thermometers. A very import-
ant element, registered at Greenwich, is the
amount of " solar radiation," the measurer of
which is a self-registering mercurial thermometer
with a blackened bulb ; it is placed on the ground
in an open box, the sides of which are sufficiently
high to prevent lateral wind from striking the
bulb. The degrees are marked on the tube itself,
to prevent accumulation of heat, reflexion, and
radiation from a scale of wood or metal
An arrangement likely to give yet more accurate
results is to enclose the bulb of the thermometer
THERMOMETRIC CONDITION OF THE AIR. 107
in a glass globe, from which the air is exhausted ;
this will prevent the lowering of the temperature
by currents of air, which, there is reason to be-
lieve, have brought the radiation thermometer
down below its proper reading.
For the " sky-radiation " or terrestrial radiation,
that is, radiation from the earth upwards, a self-
registering minimum thermometer is used ; its
bulb is placed in the focus of a parabolic metallic
reflector, which is turned upwards towards the
clear aspect of the sky and screened from currents.
Even in the day-time a thermometer so placed and
turned towards the clear sky, but away from the
rays of the sun, will fall several degrees below the
temperature of the surrounding air.
It would be well, also, to observe a minimum
thermometer placed near it and exposed to the sky
too but without the mirror.
75. Actinometer. An instrument, termed the
Actinometer, devised by Sir John Herschel, has
been extensively used to ascertain the absolute
heating effect of the solar rays, in which time is
considered as one of the elements of observation.
The actinometer consists of a large cylindrical
thermometer-bulb, with a scale considerably
lengthened, so that minute changes in tempera-
ture may be easily appreciated. The bulb, which
is of transparent glass, is filled with a deep blue
108 PRACTICAL METEOROLOGY.
liquid, which is of course expanded when the rays
of the sun fall direct on the bulb. To take an
observation with the actinometer, it is exposed to
the sunshine for one minute and read off; it is
then withdrawn to the shade for a minute and its
indication recorded ; it is then again placed in the
sun and this alternation continued for any time,
care being taken to begin and end the series with
a sun observation ; the mean of the readings in
the shade, subtracted from the mean of those in
the sun, gives the actual amount of dilatation of
the liquid produced by the sun's rays in one
minute of time. The mode of registration is ex-
tensively explained in the Report of the Com-
mittee of the Royal Society on Physics and
Meteorology, to which the reader is referred for
further information.
Very interesting observations may be made by
two observers simultaneously with actinometers
previously compared, the one at the base, the
other at the summit of some great elevation.
We know very little of the different effect of the
sun's rays at heights not usually visited, and this
instrument is calculated to supply useful inform-
ation on that head. So nicely will it show a
decrease or increase of force in the solar heat, that
the altitude of the sun may be determined by
observations with it very approximately.
THERMOMETRIC CONDITION OF THE AIR. 109
WIND.
76. The phenomenon of wind, which is simply
air in motion, is produced by inequality of tem-
perature in the atmosphere at different points on
the surface of the earth, or in different regions of
the atmospheric envelope.
If the air at a particular place is heated it will
become specifically lighter ; heavier air will rush
towards the spot, and, occupying the lower place,
will force up the heated air to the higher regions,
where it will spread laterally and form an upper
stratum ; the horizontal currents of air are what
produces the effect of wind to an observer on the
surface of the earth. It is not always that wind
extends to a great distance : a storm of wind and
rain has been known to be raging on one side of
a mountain, while the air on the other was in a
state of calm.
As clouds are wafted by the wind, they will
serve to show the direction of the wind in elevated
portions of the air; frequently the lower clouds
may be observed to be moving in one direction,
and the higher in an opposite one.
77. Land and sea breezes. The land and sea
breezes of the tropics change their direction twice
in the day, and the cause is obvious. When the
sun shines, the surface of the land gets heated
110 PRACTICAL METEOROLOGY.
more rapidly than the sea ; nor is the heat con-
ducted far below the surface, seeing that the con-
ducting power of the superficial layers is usually
low. The air superjacent is heated from below by
its contact with the surface and rises, from the
cooler air over the sea flowing towards and under-
neath it, making the " sea breeze." After the sun
has set, the surface of the earth soon radiates the
heat it had received, and becomes cooler than the
surface of the sea, which retains its acquired heat
for a longer period. The air above the earth,
being thus gradually cooled, flows outward and
produces the " land breeze/'
78. Trade-winds. The " trade-winds " are the
effect of the different degrees of heat experienced
in regions within the tropics, and in those of
the temperate and frigid zones. The air super-
incumbent over the tropical regions, which are
subject to the heat of a vertical sun, will con-
stantly receive heat from beneath by conduction,
and will therefore ascend, being displaced by hori-
zontal currents from the polar across the tempe-
rate zones. Were the earth stationary, these hori-
zontal draughts would constitute northerly winds
in the northern hemisphere near the surface, while
the upper current would flow from the south, the
reverse occurring south of the equator. But as
the earth, accompanied by its atmosphere, revolves
THERMOMETRIC CONDITION OF THE AIR. Ill
from west to east in its diurnal motion, and as the
rate of motion of a point on its surface increases
in rapidity as the equator is neared, it is clear that
air from near the poles, or from high latitudes,
will bring with it less of the easterly tendency
than the surface of the earth within the tropics
possesses ; hence it will lag, as it were, or the
tropical districts will, so to speak, strike against
it or leave it behind j and by composition of the
two motions, the original northerly wind will
appear as coming from the north-east ; and wind
coming from the south polar regions will be
reckoned south-east : where these two currents
meet, not far from the equator, is a " region of
calms," across which ships can only reach by
catching every fitful breeze.
The cold air from regions north and south of
the tropics, by friction against the surface, par-
takes by degrees of the motion of the earth in its
passage towards the equator ; if it could be trans-
ferred suddenly from higher latitudes, its velocity
would be that of the most fearful hurricane ; as it
is, the effect of the inrush of colder air results in
the mild and constant " trade-winds/' which
blow constantly across those portions of the At-
lantic and Pacific oceans which lie between or not
far beyond the tropics,
79. Circular storms. The origin of the keen
112 PRACTICAL METEOROLOGY.
and biting east wind with us is in the region of
arctic cold ; the west and south-west winds, having
traversed the ocean, have imbibed a portion of its
warmth, hence they are usually genial. If two
opposite currents meet, they frequently form a
vortex and whirl over the ocean as cyclones or
typhoons, travelling forward at a rate slow in com-
parison with the velocity of rotation. Mr. Red-
field and Col. Reid have devoted much attention
to the theory of circular storms, and have laid
down principles to guide the navigator in escaping
the hurricane by sailing away from its sphere of
operation. According to those writers, hurricanes
are whirlwinds on an enormous scale, revolving
storms in which the air is carried with extraordi-
nary velocity round a calm centre or focus of
variable extent, in the neighbourhood of which
the force of the wind is greatest ; while the hurri-
cane is thus rushing round its focus, it advances
onward, at a variable rate, from the place of its
origin, along, in some districts where cyclones
occur, a course of remarkable regularity. The
cyclones vary in size from under fifty miles in
diameter to several hundred.
The direction in which the hurricane blows is
always against the course of the sun, or the hands
of a watch, in the northern hemisphere, but with
it in the southern ; from this has been deduced a
THERMOMETRIC CONDITION OF THE AIR. 113
rule to tell in what direction a ship should steer
to avoid the central part, near which, as has been
said, the force is greatest. In the northern hemi-
sphere, with your face to the wind, the centre will
be on your right ; in the southern, with your face
to the wind, it will be on your left. The value
of this rule has been attested by many hundred
narratives of escape from the danger to which,
without it, the mariner would have been ex-
posed.
These fearful visitations are confined to the
China sea, the ocean between Australia and
Southern Africa, and the west of the Atlantic
from the Gulf of Mexico to Newfoundland.
The revolving movement would appear to be
propagated from place to place, not by the bodily
transfer of the mass of air which constitutes the
hurricane, but by the transmission of the rotatory
motion from one portion of the air to another.
An idea of the movement may be obtained by
watching the water above a mill-dam when it is
allowed to escape by a small hole only ; a funnel-
shaped eddy will be noticed, which will keep its
position as long as the hole is open ; the moment
the hole is closed it will begin to wander, and will
continue so to do for a considerable time.
It is the opinion of some eminent meteorologists,
among whom is the Rev. Dr. Lloyd, that the cir-
i
114 PRACTICAL METEOROLOGY.
cular movement of the air is by no means confined
to violent hurricanes, but that it may be traced
even in the gentlest breeze. Under his superin-
tendence simultaneous observations have been
taken for some years at the Coast Guard Stations
in Ireland, and the discussion of these have led
him to this conclusion, which would appear to be
supported by the observations.
An arrangement was made a few years since,
which lasted for some length of time, whereby the
direction and force of the wind, and the state of
the weather at 9 A.M. every day in the year (Sun-
days excepted), were transmitted by electric tele-
graph to Mr. Glaisher, and published, on the fol-
lowing morning, in the Daily News ; the returns
were sent from England, Ireland and Belgium,
and the following remarks on one day's observa-
tions only, will illustrate the subject now under
discussion.
November 20th, 1850, is worthy of special
notice, as it is evident from the returns that the
air was moving in a circle on the morning of that
day ; the direction of the wind in Ireland being
N.W., north of France W., on the south-east
coast of England and Belgium S.W., and on the
south of Scotland S.E. On the outside of the
circle, of which Birmingham seemed the centre,
the force of the wind was very great, while in the
THEBMOMETRIC CONDITION OF THE AIR. 115
middle of England a calm was registered by the
observers ; a projection of the course and force of
the wind at the various points of observation, in-
serted in a map of England now before the author,
shows clearly that the air, at this period, was
describing concentric circles around a position
not far removed from the centre of England.
Such observations and registrations, taken exten-
sively over the earth's surface, would be of the
utmost value in making us acquainted with the
phenomena of air in motion, and with the laws
which regulate storms and hurricanes, which laws
have only lately begun to receive the attention
their importance demands.
80. Anemometers : Osier's. Meteorological ob-
servers are expected to register at certain periods
the force and direction of the wind; in a few
places in England, namely Greenwich, Plymouth,
the Royal Exchange, and Liverpool, this is done
most accurately by means of Osier's anemometer,
which will be described at length when we treat
of the instruments in the Royal Observatory at
Greenwich (see 169). A board of one foot square
is opposed to the wind from whatever quarter it
may blow, and, by means of pencils attached in a
peculiar manner, a register of the pressure per
square foot, and of the direction of the wind, is
traced on a sheet of paper divided into twenty-four
i 2
116 PRACTICAL METEOROLOGY.
spaces, each representing an hour, which is ad-
vanced by clock-work ; so that the moment of in-
crease, decrease, or change, can be seen at a glance.
81. Whewell's Anemometer. WhewelPs ane-
mometer described at full in 170 registers
the horizontal motion of the air during the same
period ; and the daily results, both of this register
and the preceding, are published in the B-egistrar-
GeneraPs weekly reports.
82. Robinson's Anemometer. This anemo-
meter, which seems likely to get into more general
use than either of the others, consists, as will be
seen on looking to Plate XI. fig. 1, of four hol-
low hemispheres or cups placed at the ends of
two horizontal bars crossing at right angles and
supported on a vertical axis which revolves freely.
Dr. Robinson, who invented this instrument,
found that the velocity of the cups when made to
revolve by the wind was one-third of that of the
air itself. The rate is measured either by a system
of index wheels set in motion by an endless screw
on the axis, as shown in the figure, or by more
complicated machinery for continuous registration
of the velocity, such as is in use at the Kew and
Oxford Observatories. In the former case the
dials are made to denote either the number of re-
volutions of the cups, or, better, the miles and
decimals of a mile passed over by the air since the
THERMOMETRIC CONDITION OF THE AIR. 117
last observation and setting of the index ; the
present velocity, if it is required, can be got by
noting the space passed over in a certain limited
time, as a minute or two. In the latter kinds of
this instrument an index or a pencil is moved from
one side to the other of a sheet of paper which is
carried along endways by clock-work, so that the
inclined line traced (by photography at Oxford,
by contact of a metal pencil at Kew) shows the
velocity of the wind at every moment of the day.
In the very ingenious arrangement at the Kew
Observatory, devised by Mr. R. Beckley, the pen-
cil is a strip of brass placed spirally round a cylin-
der made to revolve by connexion with the axis
of the anemometer in contact with another cylin-
der covered by a sheet of the registering paper
and set in motion on an axis parallel to that of
the last by clock-work ; as the first cylinder
goes round, different parts of the spiral pencil
touch the paper in succession till the mark reaches
the extreme end, when the pencil again comes into
play at its first point. This construction is fully
described, with plates, in the ' Report of the British
Association for 1 85 8/ while an account of the one at
the Radcliffe Observatory, Oxford, is given with the
meteorological observations taken there in 1856.
83. Liud's Wind-gauge. If such complete me-
thods were in more general use, we should no doubt
118 PRACTICAL METEOROLOGY.
be much assisted in arriving at a knowledge of the
laws regulating the movement of the air ; mean-
while, although we cannot accomplish as much as
we desire, it behoves us to contribute as much as
lies in our power ; a useful and available wind-
gauge for private observers is Land's ; and on its
indications are founded the reports of the force of
the horizontal movement of the air, which are re-
corded by those who are not provided with more
elaborate apparatus. Plate II. fig. 5.
Lindas wind-gauge consists of a glass tube, c,
about half an inch in diameter, of a siphon-like
form, one end, d, being again bent at right angles
to the general direction of the tube, so as to pre-
sent a horizontal opening to the wind. The tube
is half-filled with water, and the pressure of the
wind on that portion directed towards it will drive
the water up the other leg. A scale is attached,
by which the force of the wind is ascertained ; and
the whole turns freely on a vertical axis, P, so that
the mouth may always be towards the quarter from
whence the wind blows; or a vane may be fixed to it
above in the same plane as the tube,which will en-
sure the mouth being directing towards the wind.
A new form of this wind-gauge has been con-
structed by Sir W. Snow Harris, in which the
after tube is made of a smaller bore than the other,
and a plumb-line is made to hang within the frame
THERMOMETRIC CONDITION OF THE AIR. 119
of the instrument, protected by a plate of glass,
to show exactly when it is in a vertical position.
The Table below shows the pressure per square
foot for the indications of the scale.
Not having a convenient place to fix this in-
strument, for it should be far above the inter-
ference of buildings or trees, and screwed into a
firm support, I generally estimate the force of
the wind from the knowledge gained by its occa-
sional use. Many observers do so without any
reference to the wind-gauge at all ; and from fol-
lowing the directions in the Table subjoined, they
cannot be far out. A calm is universally repre-
sented by ; a hurricane or violent gale by 6.
Table showing the force of the wind on a square
foot for different heights of the column of
water in " Lind's Wind-gauge."
Inches.
Force in Ibs.
Common designation of
such a wind.
6
31*75
A hurricane.
5
26-04
A very great storm.
4
20-83
A great storm.
3
15*62
A storm.
2
10*42
A very high wind.
I
5'*i
A high wind.
'5
2*6
A brisk gale.
'I
"5
A fresh breeze.
05
26
A pleasant wind.
0*
0*
A calm.
84. Graphic delineation of results of wind ob-
120 PRACTICAL METEOROLOGY.
servations. The resultant of all the forces of the
winds, combined with their directions, may be
graphically shown for any definite period of time,
as one month. To illustrate the method, I refer
to my own observations, taken three times daily
during the month of October 1848; it will be
sufficient to take eight out of the thirty-two
points of the compass : in the following Table are
arranged the sum of the forces of the different
winds registered during the month, from which
figure 6, Plate II. , is constructed.
N. N.E E. S.E. S. S.W. W. N.W.
Force. .2 5 o i 10*7 29*7 3 3
Describe a circle, and divide it into eight equal
portions, which mark with the names of the
points of the compass used in the Table.
Towards the south, set off from a scale of equal
parts, two divisions, seeing that that amount of
direction southward is due to the northerly wind.
On the other lines of direction set off, from the
same scale, parts equal to the amounts recorded
above, but in opposite directions to those of the
winds ; join the points thus obtained, and the
general direction of the wind for the month will
be seen : in this case it is evident that the whole
mass of air was moved, during the month, hori-
zontally towards the north-east. If this be done
for every month of the year, the prevalent winds,
THERMOMETRIC CONDITION OF THE AIR. 121
both in amount and direction, can be compared
with little trouble, and far more readily than by
the inspection of the register ; care must be taken
that the same scale be used throughout, and if
great nicety is required, the whole thirty-two
points of the compass may be laid down. The same
plan may be adopted for much shorter periods, if
any remarkable changes take place which it may
be important to register.
85. Rules for observing. As observations on
the wind may be easily made by those who are not
provided with barometers or thermometers, and
even more advantageously than by students, if
their occupation leads them to be much in the
open air, the following directions are given in
order that such observations may become of
scientific value. It is most important to remark,
in addition to the intensity and direction of the
wind at certain fixed hours,
1. The time when it commences to blow from
a calm, or subsides into one from a breeze.
2. The time of any remarkable or sudden
change of direction.
3. The course it takes in varying, and the
quarter in which it ultimately settles.
4. The existence of cross currents in the higher
regions of the atmosphere, as indicated by the
course taken by the higher clouds.
122 PRACTICAL METEOROLOGY.
5. The times of setting-in of hot and cold
winds, and the quarters from which they come.
6. The connexion of rainy, cloudy, or fair
weather with the quarter from which the wind
blows, or has blown for some time previously.
Such series of observations, continued through
fixed periods in various parts of the country, would
be of the utmost value in meteorology ; and if the
stations were very numerous, would throw light
on the aerial movements, a branch of the science
in which we are much in want of well-arranged
and abundant registers.
2. The Hygrometric Condition of the Air.
86. General considerations. The atmosphere is
subject to agitations far more extensive than the
swell of the ocean ; waves as broad as the Atlantic
itself pass over us from time to time. Indepen-
dently of these atmospheric waves, hourly varia-
tions in the pressure of the air have been remarked
which seem to follow a definite law. The cause
of these fluctuations is variation of temperature
depending upon the sun's hour-angle ; and this
also affects the amount of aqueous vapour, which
combines with the air in raising or depressing the
mercury in the barometer. Such is the tendency
of aqueous vapour to rise in the air, that the atmo-
sphere may be said, in no case, to be found in a
HYGROMETRIC CONDITION OF THE AIR. 123
state of absolute dryness ; the supply is obtained
from rivers, oceans, and the surface of the soil.
It is possible to separate, experimentally, from a
given quantity of air, the aqueous and gaseous
atmosphere ; but more important for the purpose
of meteorological investigation are the means we
possess of ascertaining the amount of vapour of
water commingled with the air at any moment,
so that, from the amount of pressure shown by
the height of the barometric column, we can attri-
bute to each atmosphere, the aqueous and the
gaseous, exactly the proportion which is due to
it. The experiments of Dalton and others have
proved this remarkable fact, in a way which will
be pointed out hereafter, viz. that, under similar
circumstances as to temperature, the quantity of
watery vapour existing in air will be exactly equal
to what it would be in a vacuum of equal capacity ;
and that, if we have the means of computing the
tension or elastic force of vapour in vacuo, we
shall be able to determine with equal accuracy the
actual tension of moisture in the air ; in fact, that
in either case the tension or pressure or elasticity
of vapour will be the same.
87. Dew. Hoar-frost. The capacity of air for
moisture increases with the temperature; and
when the limit, varying with the temperature, is
attained, no more moisture will ascend, and the
124 PRACTICAL METEOROLOGY.
air is then in a state of complete saturation. If
from any cause a volume of air in this condition
should be suddenly cooled, a deposition of moist-
ure succeeds ; the air parts with aqueous vapour
in minute particles, and, especially if it be free
from agitation, these appear in the form of dew,
which is witnessed in perfection after the removal
of the heat of the sun on a still, autumnal night.
The effect of a temperature below the freezing-
point will be to convert the dew into hoar-frosty
as is visible when winter approaches.
88. Rain. Hail. Should the air part from its
moisture at a distance from the earth's surface,
the aqueous particles will descend at first but
slowly, or not at all. By the law of aggregation,
they will unite in globules ; and, when their
weight is sufficient to overcome atmospheric re-
sistance, or, perhaps, when the electric state of the
moisture or the air undergoes a change from some
unknown cause, they will descend by gravitation
towards the earth, and fall in the form of drops of
rain.
That electricity is concerned in the production
of rain is more than probable, as is indicated by
the copious and heavy showers which fall during
a thunder-storm. Its agency in the phenomenon
of hail is undoubted ; this is formed in the upper
regions of the air, where the temperature is below
HYGROMETRIC CONDITION OF THE AIR. 125
the freezing-point ; the height from which the
hail-stones descend is indicated by the force and
rapidity of their fall.
Evaporation and condensation of aqueous vapour
tend, in various ways, to diffuse heat more equally
throughout the globe. As the power of air to
imbibe moisture increases with the temperature,
evaporation goes on with most rapidity in warm
climates, and from heat being absorbed in the
process, has a cooling tendency ; in the less heated
regions the vapour is condensed, and its latent
heat is given forth to mitigate the severity of the
colder climate.
89. Condensation of the vapour in the air. If
two saturated volumes of air of unequal tem-
peratures, and therefore of varying capacities for
moisture, meet each other, their tendency will be
to unite and equalize both the temperature and
the moisture. The moisture, however, will always
be in excess, for the two processes will not pro-
ceed at the same rate. Thus, suppose two volumes
of air saturated with moisture, one of the tem-
perature of 40 and the other of 60, to unite, the
mean temperature of the mass will be 50. The
elastic force of vapour at 40 of temperature (as
will be explained more fully hereafter) is 0*247
measured as pressure in inches of mercury ; at
60 it is 0-518; but at 50, the mean of the two
126 PRACTICAL METEOROLOGY.
temperatures, the elastic force is 0'361, which is
less than 0*382, the mean of the others, by O021 ;
this represents the tension of that portion of
aqueous vapour which would be set free. If this
vapour, in its liberated state, meet with a stratum
of air not saturated with moisture, it will be re-
absorbed, either partially or entirely; if only
partially, the remaining portion, consisting of
aqueous vapour in an extremely minute state of
subdivision, may be arrested in its descent and
float in the atmosphere in the form of clouds.
From the preceding observations, it will appear
that the atmospheric pressure, as shown by the
barometer, is compounded of that of the air itself
and the co-existing vapour of water, from which it
is never entirely disunited. It will now be shown
in what manner these two forces may be separated
and the proper value assigned to each, according
to the views of many eminent meteorologists.
90. Saussure's Hygrometer. The plansadopted
by observers in the last century to determine the
hygrometric condition of the air were imperfect
and unsatisfactory ; the contraction and dilatation
of some animal substance indicated changes in
the amount of moisture in the air, and the register
of these formed the hygrometers of De Luc and
Saussure. At the present state of the science of
meteorology such- contrivances are only interesting
HYGROMETBIC CONDITION OF THE AIR. 127
historically, and on this ground simply we shall
describe Saussure's hygrometer, which is repre-
sented in Plate VI. fig. 3.
The action of this instrument depends upon the
longitudinal expansion of a human hair, which
has been freed from all unctuosity by boiling in
an alkaline solution. It is kept in equable dis-
tension by a small weight at one end, the other
being fixed ; the hair, passing round an axis which
carries an index, on its contraction causes the
index to revolve and point to certain divisions on
a circular plate. Saussure determined the point
of extreme dryness by placing the instrument
under a receiver, in which was inserted some
powerful desiccant ; here the hair attained its mini-
mum of length ; this was the zero of his scale.
He then introduced moistened pieces of linen which
had been previously weighed, and on their with-
drawal the diminished weight denoted the amount
of vapour in the receiver ; the temperature being
noted at the same time, and also the portion of the
arc moved over by the index, another point on the
divided arc was secured, and thus data were ob-
tained for arriving at an approximation rough,
it is true to the amount of moisture existing in
the air at any precise moment. The superiority
of the methods now adopted to arrive at this con-
clusion, will be easily apprehended from the de-
128 PRACTICAL METEOROLOGY.
scription of the hygrometers at present in use. We
must, however, preface the description with a con-
sideration of the laws to which the vapour of
water is subject.
91. Tension of aqueous vapour. The laws of
the formation of vapours, and the relations existing
between their elastic force and temperature, were
ascertained by Dalton experimentally, and the
results were published by him in 1802. The
following is an account of the apparatus employed
by him in his investigations.
In Plate IV. fig. 3, a, b is a barometer tube in-
verted in a vessel of mercury in the manner de-
scribed in 11. The graduated scales measures
the height of the mercurial column, its lower point
being brought to touch the surface of the mercury
in the vessel. Before the tube is inverted, its in-
side is moistened with the liquid whose vapour it
is intended to experimentalize upon, we shall
only consider the vapour of water which, being
lighter than mercury, will rise and float upon the
upper surface of the column, forming a thin film
over it. The mercury will then be found to sink
lower than it would do were the space above it, as
in the barometer, a vacuum ; and the amount of
depression measured on the scale is the elastic
force of the vapour of water due to the existing
temperature.
HYGROMETRIC CONDITION OF THE AIR. 129
Suppose the tube to be elevated further above
the reservoir of mercury, but not lifted out of it,
there will then be a larger space to be occupied by
the aqueous vapour ; as long as there is any liquid
above the mercury, evaporation will go on to fill
the increased space, and its elastic force (the tem-
perature, pressure of the air, and consequently the
density, being the same) will remain constant.
The amount of descent of the mercury in the tube
produced by the aqueous vapour, measures the
maximum density at the given temperature.
When the liquid has entirely evaporated, if the
space in which it exists be enlarged, its elastic
force will diminish ; if, again, it be contracted in
volume, its density will become greater, until it
attains the maximum density due to the given tem-
perature ; after this a further diminution of volume
will reduce some portion of it to a liquid state.
The upper part of the tube is surrounded by a
glass cylinder, c d ; this is closed at both ends
with a cork, admitting the tube through it. By
filling this cylinder with water at different tem-
peratures, we subject the vapour to the operation
of any degree of heat we wish ; the scale attached
enables us to measure the depression of the mer-
cury at each degree, and thus is formed a table of
the elastic force of vapour for a range of many
degrees. By using melting ice for the freezing-
130 PRACTICAL METEOROLOGY.
point, and freezing mixtures for temperatures still
lower, this table has been carried to a great extent ;
above the boiling-point hot. oil has been made use
of; but for the purposes of meteorology, the table
of the tension or elasticity of aqueous vapour may
be confined to much narrower limits.
If the space above the liquid in the barometric
tube is occupied by air (or indeed by any perma-
nent gas), the vapour of water will follow the
same laws as regulate it in vacuo, that is, the
same relation will be preserved between the
density, tension, and temperature ; they will, in
fact, be precisely the same as in vacuo, with this
difference only, that, while in vacuo the quantity
of vapour necessary to saturate the space above
the column is formed instantaneously, time is re-
quisite for the evaporation to go on in air.
The conclusions of Dalton have been confirmed
more recently by Dr. Ure, Regnault, and others ;
they have been the foundation of tables showing
the elastic force of vapour, as measured in inches
of mercury, for all degrees of temperature. Dr.
lire's table was constructed from experiments
performed by an apparatus of which the following
is a description (Plate IV. fig. 4).
D L E is a siphon barometer, the leg E being
closed, and the other open at D. On the ad-
mission of the mercury there will of course be an
HYGROMETRIC CONDITION OP THE AIR. 131
equilibrium when the column in the closed leg
balances the atmospheric pressure on the surface
of the mercury in the other, and the space above
G will be a vacuum; a glass vessel is adapted to
the outside of this portion of the tube, and rings
of platinum wire on the exterior of the tube serve
to mark the height of the mercury in each leg.
A drop of water is now introduced into the vacant
space above G, which is forthwith, in part, changed
to vapour, and the vessel A is filled with water,
the temperature of which, shown by the thermo-
meter inserted therein, will give that to which the
tension of the vapour is due. The elasticity of
the vapour will cause the mercury to descend in
the tube E, and rise in the tube D ; a portion of
the metal is now poured into the open tube, until
its weight counterbalances the tension of the
aqueous vapour, and brings the mercury to its
original level at G. Let L be the space occu-
pied by the additional quantity of mercury ; this
space, accurately ascertained, will be the measure,
in inches of mercury, of the elasticity of the
aqueous vapour at the temperature shown by the
immersed thermometer. By varying the heat of
the water, by using freezing mixtures for the low
temperatures, and boiling oil for the higher, Dr.
Ure obtained the tension of aqueous vapour for
degrees of heat ranging between 24 and 312,
K2
132 PRACTICAL METEOROLOGY.
the elastic force of the former being O17 inch,
and of the latter 167 inches. Table II. (Appen-
dix) is copied from the Greenwich observations ;
it shows the tension of aqueous vapour for every
degree of temperature from to 95.
92. Important conclusion. The point more
particularly to be borne in mind from the pre-
ceding paragraphs is, that, at a certain tempera-
ture, a definite amount of vapour will arise by
evaporation, and that amount will be constant,
whether the space into which it rises be a perfect
vacuum, or whether it be the open air, the only
difference being that in the latter case the pro-
cess will go on much more slowly.
93. The dew-point. If a volume of air of the
temperature of 35 be saturated with moisture, it
will be seen, from inspecting Table II. (Appendix),
that the tension of such moisture is equivalent to
0-222 inch of mercury, and it is concluded that
if the air were suddenly to part with its moisture,
the barometric column would fall to that extent.
Should the barometer reading be 29'886, the
pressure of dry air vertical to the place of obser-
vation would be 29-886- -222 = 29-664.
Complete saturation, however, is of compara-
tively rare occurrence ; only, indeed, as we shall
see, when the wet- and dry-bulb thermometer
readings are the same, which will occur when the
HYGROMETRIC CONDITION OF THE AIR. 133
tension of aqueous vapour is at a maximum for
the temperature of the air. In every other ease
the air is capable of holding in suspension a
greater quantity of aqueous vapour than it con-
tains at the instant of observation. The know-
ledge of the dew-point here comes to our assist-
ance. The dew-point is that temperature at which
the air would be saturated with the moisture it
contains; and the abstraction of heat to the
smallest amount below this point of saturation,
would cause an immediate deposit of water.
Dalton's method of ascertaining the dew-point
was a very simple one and easily practised ; he
poured water, cooled below the temperature of the
atmosphere by a freezing mixture, into a glass,
and marked the temperature by a thermometer
inserted in it when the dew which had been
deposited on the outside of the glass disappeared
in the open air.
The tension of vapour for each degree of the
thermometer differs, in a slight degree, as deter-
mined by different physicists ; the table published
in the Greenwich observations is derived from the
experiments of Dalton and Ure (compared, how-
ever, with other tables), whose methods have on
that account been explained.
134
PRACTICAL METEOROLOGY.
Tension of vapour, according to various authorities,
for 32, 95, and 122 of temperature.
Dalton.
Ure.
Roy. Soc.
Regnault.
Kaemtz.
3*
O*2OO
0-200
0-l86
0-18124
0-17999
95
1-59297
1-640
1*610
1-6438
l'S7789
122
3-500
3'S 1 *
3 '54*
3-619
94. Degree of humidity. With a table of the
tension of aqueous vapour and the dew-point, we
are immediately provided with data for deter-
mining another important element, namely the
degree of humidity of the air.
Suppose the temperature of the air to be 67,
and that of the dew-point to be 50. By inspect-
ing Table II. (Appendix), we find the tension of
aqueous vapour at 67 to be -659, at 50 it is -373;
if the air were saturated the tension would be '659;
but, in the instance supposed, the tension of
aqueous vapour is only '373 ; the proportion, there-
fore, between the amount of vapour in the air and
that which would exist in it were complete satura-
tion reached, is as 373 to 659 ; or considering the
amount of vapour at the point of saturation as
unity, 659 : 373 : : 1 : g or '566, which will re-
present the degree of humidity of the air when
its temperature is 67 and that of the dew-
point 59.
HYGROMETRIC CONDITION OF THE AIR. 135
As this element gives immediately the prepon-
derance or defect of aqueous vapour in the air,
which the dew-point does not, it is one of consi-
derable importance in the register of phenomena.
The dew-point may be the same at extremely
different degrees of air-temperature ; and the state
of the air, as regards dryness, will depend on the
greater or less depression of the dew-point tem-
perature below that of the air.
When the degrees of humidity are registered,
we see at once the variation in the hygrometric
state of the air, and comparisons are readily in-
stituted.
If from the tension of aqueous vapour at the
air-temperature, we subtract that at the dew-point
temperature, we shall obtain the force of evapora-
tion; thus, in the preceding example, '659 '373
= 286.
95. Illustrations. The phenomenon of the dew-
point wil receive illustration from the affairs of
common life.
A bottle of wine has been iced before its ap-
pearance in the dining-room the bottle will be
found covered with a coating of dew the moment
it enters the room ; the temperature of its contents
being far below the point of saturation of the air,
the watery vapour will be condensed on the surface;
if it be removed, more will settle, till the wine has
136 PRACTICAL METEOROLOGY.
acquired a heat above the temperature of the dew-
point.
On entering an observatory in the morning,
the temperature of the air having risen very
considerably above that of the preceding night
when the instruments were in use, nothing is
more usual than to find every instrument stream-
ing with moisture. The instruments, having been
cooled down to the night-temperature, and the
day chancing to become suddenly warm, have not
had time to get heated above the temperature of
the dew-point, and an abundant deposition of
moisture is the result.
On entering a detached room on a morning
which may happen to be warm after a series of
cold days and nights, the walls may be frequently
found covered with moisture, which will be due
to their not having acquired a temperature, since
the sudden change, above that of the dew-point ;
the moisture will disappear as soon as they have
attained a heat but just exceeding it.
96. Daniell's hygrometer. Instruments have
been invented for obtaining the dew-point by
direct observation, three of which we shall de-
scribe. The first, that of the late Professor
Daniell, known as Daniell's hygrometer, is a
very elegant and portable instrument.
It consists of two glass balls, communicating
HYGROMETRIC CONDITION OF THE AIR. 137
with each other through a bent glass tube. The
ball a, Plate VI. fig. 1, is of black glass, about
one and a quarter inch in diameter ; the ball d
is of the same size, but transparent. A small
mercurial thermometer is fixed in the limb a b,
with a pyriforrn bulb, which descends to the
centre of the blackened ball. A portion of sul-
phuric ether, sufficient to fill three-fourths of the
ball 0, is introduced, and the atmospheric air
having been expelled by boiling as completely as
possible, the whole is hermetically sealed at e.
The ball d is covered with muslin, and the ap-
paratus is supported on a brass stand, / g, to
which is fixed another delicate thermometer,
whose readings show the air-temperature. To
ascertain the temperature of the dew-point, the
ether is all brought into the ball , by inclining
the tube; the temperature of the air is then
registered, and ether is poured from a dropping
tube, which fits the mouth of a small phial, on
the muslin cover, d' y the cold produced by the
evaporation causes a condensation of the vapour
of ether which fills the connecting tube and the
ball, d e, and produces a rapid evaporation from
the ether in a-, as this is a cooling process, the
temperature shown by the enclosed thermometer
rapidly sinks. The instant that the ether within
the black ball is cooled down to the temperature
138 PRACTICAL METEOROLOGY.
of the dew-point, a film of condensed vapour from
the air surrounds the ball like a ring at the level
of the surface of the ether within, a i; if the
thermometer be read off at the instant this ring
of dew is formed, we obtain nearly the true tem-
perature of the dew-point, that of the air at the
time being shown by the exterior thermo-
meter,/^.
The ring of dew will gradually lessen as the
ether within the ball is recovering its original
temperature, and will finally disappear. At that
instant the enclosed thermometer should be again
read off, and this reading will give another ap-
proximation to the dew-point temperature; the
mean of this and the former determination may
be supposed to approach very near the truth.
The portability of this beautiful instrument is a
great recommendation; the whole apparatus is
packed in a box, which may be readily carried in
the pocket. An important deficiency is the small-
ness of the enclosed thermometer, which will not
admit of a reading by estimation of nearer than
half a degree ; other drawbacks to its use are the
difficulty of obtaining pure ether for the experi-
ment, and of catching the instant of the forma-
tion of the ring of dew, seeing that the thermo-
meter has to be carefully watched and read off at the
same moment that the dew appears or vanishes.
HYGROMETRIC CONDITION OF THE AIR. 139
97. Regnault's hygrometer. The dew-point
may also be obtained by direct observation from
Regnault's hygrometer, and it is presumed with a
nearer approximation to the truth than Darnell's
will supply.
Regnault's hygrometer (Plate VI. fig. 4) con-
sists of a glass tube over which is slipped a
thimble, a, b, c, made of silver, very thin and
highly polished, 1'8 inch in depth and 0'8 inch
in diameter ; it is fitted tightly on the glass tube,
c d, which is open at both ends.
The tube leads by a small aperture into the
hollow upright support, n m. The upper open-
ing of the tube is closed by a perforated cork
(sometimes of india-rubber) through which passes
a very sensitive thermometer, T, which, being much
longer than any that can be used with DanielPs
hygrometer, is more nearly and readily read off;
its bulb descends nearly to the bottom of the
silver thimble, and from the same depth rises
a hollow thin glass tube, g, which also passes
through the cork. Ether is poured into the tube
as high as p q -, the pipe, d, leads down the hollow
upright, which is in communication with the
flexible tube o, to an aspirator (not shown in the
drawing), that is, a jar containing something less
than a gallon of water ; this jar is closed, except
a small aperture, over which the flexible tube is
140 PRACTICAL METEOROLOGY.
made to fit air-tight ; at the bottom of the jar is
a stop-cock, on turning which the water will run
out. The aspirator-jar is near the observer, but
the instrument may be at any convenient di-
stance, so as to be removed from the heat of the
person.
When the dew-point is to be ascertained, water
is allowed to run from the aspirator-jar ; this will
produce a current of air through the fine tube,
whose mouth is at g, and the pipe of communica-
tion, m n ; to reach the space above the ether, the
air must pass, bubble by bubble, through it, which
will produce a uniform temperature throughout
the whole mass while it is subject to the agita-
tion produced by the rapid passage of air, and
the silver thimble will be of the same temperature
as the ether within, the degree of which will be
shown by the immersed thermometer, T. If very
great nicety is required, the thermometer may be
read off by a small telescope, but with care this is
seldom necessary.
At the moment that the ether is cooled down
to the dew-point temperature, the whole external
surface of the silver thimble will be covered with
a coating of moisture, and the degree shown by
the thermometer at that instant of time must be
marked. Let us suppose the first reading to be
46, it is probable that as a fraction of a second
HYGROMETRIC CONDITION OF THE AIR. 141
was lost in the eye glancing from the silver to
the thermometer, this reading is too low. In a
few moments the dew disappears and the thermo-
meter rises ; but now its reading is probably too
great, say 48. The stop-cock of the aspirator is
then gently opened, and a small stream of air-
bubbles rises through the ether, and, by nice
adjustment, the thermometer is observed steadily
to read 47' 3 at the instant that the dew is
formed ; repeated trials are sometimes necessary,
but five minutes will generally suffice for the
whole operation. The inventor found three or
four sufficient to determine the dew-point to the
tenth of a degree.
The second tube, a 1 b 1 , holds a second thermo-
meter, T ; , circumstanced in every way like that
used in determining the dew-point, and its read-
ing will give the temperature of the air at the
time of observation. This additional thermo-
meter need not necessarily be mounted in the
same manner as the dew-point thermometer, as
the temperature of the air may be learnt from
one suspended in the usual way.
The direct method by which the dew-point
may be ascertained by marking the clouding of
the polished silver surface, is so great a recom-
mendation, that nothing but cheapness is requisite
to bring Regnault's hygrometer into general
142 PRACTICAL METEOROLOGY.
use. On the author's representation, Messrs.
Negretti and Zambra have constructed the in-
strument in a simple form, and at a price not
greater than is usually paid for DanielFs.
The aspirator need not be a vessel of water ; at
Kew two circular boards are united by leathern
sides after the manner of a pair of bellows, to
the bottom board is attached a weight and in
the top board is a projection to which the india-
rubber tube is attached ; when an observation is
to be taken, the boards are made to approximate
by a pulley, and as the lower one is allowed to
descend, a draught of air is produced, the force of
which may be regulated at pleasure. This appa-
ratus does away with the trouble of constantly
supplying water to the aspirator-jar, as is neces-
sary on the usual construction, though some
aspirators are made double, so that they may be
reversed when the water has run out of one com-
partment into the other, and thus the same
quantity of water may be made serviceable for
any length of time.
Regnault's hygrometer has not yet been used
in the Greenwich observations; but, for many
years, a long series of experiments and compari-
sons was undertaken with DanielPs hygrometer
and the dry- and wet-bulb thermometers, which
has resulted in the rejection, for general registra-
HYGROMETRIC CONDITION OF THE AIR. 143
tion, of the use of DanielFs hygrometer, and in
assuming as the dew-point, a deduction from
simultaneous observations with the dry- and wet-
bulb thermometers.
98. Connell's hygrometer. Figure 5, Plate VI.
is a representation of a dew-point instrument, by
A. Connell, Esq., Professor of Chemistry in the
University of St. Andrews. A is a small bottle
made of highly-polished brass or silver ; it is par-
tially filled with ether, and the temperature is
lowered by means of an exhausting syringe, D E,
which causes the ether to evaporate rapidly; a
thermometer, t, whose bulb dips into the ether,
shows the temperature of the dew-point at the
instant that the polished exterior of the bottle
becomes clouded with moisture. G is a clamp to
fix the instrument to a steady support.
99. Dry- and wet-bulb thermometers, or Mason's
hygrometer. The dry- and wet-bulb thermo-
meters, known also as Mason's hygrometer in
England, and as August's psychrometer on
the continent, is the form of hygrometer in
general use; it consists simply of two thermo-
meters exactly alike, stationed side by side, which
are presumed, under the same circumstances, to
give similar indications. The dry -bulb thermo-
meter, of course, shows the temperature of the
air ; the wet-bulb thermometer has its bulb sur-
144 PRACTICAL METEOROLOGY.
rounded with muslin, and from it lead a few
inches of lamp-wick, or floss-silk, into a small
vessel filled with rain or distilled water.
Under general circumstances, or rather, when-
ever it is not saturated, the atmosphere will take
up the vapour of water ; the drier it is the more
rapidly will evaporation proceed, and the more
slowly as its condition approaches that of com-
plete saturation. When in that state, no more
moisture will rise in the air. Now, as evapora-
tion proceeds, heat is absorbed by the conversion
of the water around the wet-bulb into vapour, and
the mercury in the wet-bulb thermometer will
fall a greater or less number of degrees below the
air-temperature, according to the dryness of the
atmosphere. When the air is saturated, the read-
ings will be the same*.
Evaporation from the wet-bulb will proceed
even when the temperature is below freezing ; but
in this case the readings must be taken with
great care, and the differences will always be
small.
Plate VI. fig. 2, is a drawing of the arrange-
ment adopted by Negretti and Zambra. The
* The greatest difference in the readings of the dry- and wet-
bulb thermometers which the author registered during seven
years of observation at Southampton, occurred April 19, 1854,
at 3 P.M., when the dry-bulb thermometer reading was 69 and
the wet-bulb 53 : diff. 16.
HYGROMETRIC CONDITION OF THE AIR. 145
thermometers are very superior instruments, with
enamelled divisions on the stem, and they are
fixed to a frame of plate-glass. It will be re-
marked that they are not very near each other ;
and this is important, for it may be that the air
surrounding the dry-bulb would be, to a certain
extent, moistened by evaporation from the water
supplied to the wet-bulb. It is advised that the
cup of water should even be removed several
inches from the wet-bulb, so that there may be
no chance of incorrectness from this source. The
thermometers are kept in place by metal clips,
e, fj and may be used for other purposes, if re-
quired, being removeable at will.
100. Dr. Babington's Evaporation-gauge. The
evaporation-gauge invented by Dr. Babington
will be found convenient for estimating the ab-
solute amount of evaporation taking place over a
given area. It consists of a vessel containing
water in which floats a glass tube poised like a
hydrometer, and graduated to grains and fractions
of a grain ; on the top of this tube is fixed a
shallow pan whose area is known ; water is poured
into this pan until the tube sinks to the point
marked zero on the scale ; as evaporation goes on,
the tube rises, and the loss of weight is indicated
on the scale in grains and fractions of a grain.
101. Apjohn's formula. The reading of the
146 PRACTICAL METEOROLOGY.
wet-bulb thermometer gives the temperature of eva-
poration ; and an important problem to be solved
is to deduce the dew-point from this record com-
bined with the temperature of the air as indicated
by the dry-bulb. Dr. Apjohn of Dublin, in 1834
and 1835, read before the Royal Irish Academy a
very elaborate paper on the theory of the moist-bulb
hygrometer; and from observation, experiment,
and theoretical considerations, he was induced to
adopt the formula which has since gone by his
name.
Let / = tension of aqueous vapour at the dew-
point temperature which we desire to
know.
/'=the tension of vapour at the tempera-
ture of evaporation, as shown by the
wet-bulb thermometer.
a =the specific heat of air.
e =the latent heat of aqueous vapour.
(tf) ord =the difference between the reading of
the dry-bulb thermometer and that of
the wet.
p = the pressure of the air in inches: then
Apjohn' s formula is
'
, -tr) p-f .
~ j ~~ X ~~
or with the coefficient,
f=f --01147 (t-f )
HYGROMETRIC CONDITION OF THE AIR. 147
The following is the formula (derived from
Apjohn's), as given in the Greenwich observations,
which will be made use of in this work ; h being
the height of the barometer.
f__fi d_ b_
J J 88*30'
When the reading of the wet-bulb thermo-
meter is below 32, the formula becomes
96 30
102. Example of its use. Suppose the reading
of the dry-bulb thermometer to be 67 and of the
wet 59, it is required to determine the dew-point,
the barometer standing at 29-000 inches. By
Table II. (Appendix), we find /' the tension of
aqueous vapour due to 59 = '5; then, by sub-
stitution,
The tension corresponding to -412 in the Table is
due to 53'6, which is therefore the temperature
of the dew-point required.
103. Other formulae. Other physicists have
given separate formulae. One, practically equivalent
to Apjohn's and originally put forward by August,
has been modified by Regnault, thus :
L2
_
7 610-*'
148 PRACTICAL METEOROLOGY.
where n is the tension due to the dew-point, t the
temperature of the air, and t' that of evaporation
in degrees Centigrade, f 1 and h as before.
Take the same example, and changing 67 Fahr-
enheit into 19'44 Centigrade, and 59 F. into
15 C., we have, by substitution,
tension due to 53'3, the dew-point.
Burg, from observation, has given ( p being the
height of the barometer)
which will give by substitution,
5-'5 8 ='44 2
tension due to 55 0< 5, the dew-point ; and Bohnen-
berger, also from observation^
which, by substitution, gives
5 --05! = -449
tension due to 56, the dew-point.
By Glaisher's factors, hereafter to be explained,
67-(67-59)l'8 = 52-6, the dew-point; while
the mean of the other four results is 54'6.
By comparing these results, it will be seen that
there is a certain amount of discrepancy between
them ; nor is this surprising, when we consider that
the fundamental quantities which enter into the
HYGROMETRIC CONDITION OF THE AIR. 149
calculation, viz. the specific heat of air and the
latent heat of aqueous vapour, are not determined
with absolute certainty.
104. Glaisher's factors. This circumstance, and
other considerations, led Mr. Glaisher to under-
take his elaborate series of comparisons with
DanielPs hygrometer, and the dry- and wet-bulb
thermometers. In practice, he at times experienced
difficulties in the use of the former, and not un-
frequently found that the simultaneous results of
the dew-point, as found from DanielPs hygrometer
and the dry- and wet-bulb thermometers, were dis-
cordant, and on investigating the causes he con-
siders that the error rested solely with DanielPs
hygrometer. The times at which these discord-
ances existed were in those particular states of
the air when great dryness was prevalent, and the
depression of the temperature of the dew-point
below that of the air was great, and a long time
elapsed after the dropping of ether on the white
ball, before dew was deposited on the black ball.
Such would require the long continuance of the
observer near the instrument, which would neces-
sarily influence both the hygrometrical state and
the temperature of the air around the instru-
ment; and this would be especially the case if
-the observer were short-sighted, and obliged to
approach the instrument very nearly. He makes
150 PRACTICAL METEOROLOGY.
the following objections to the use of this hygro-
meter :
" Supposing the inclosed thermometer to be
one of extreme delicacy, which it is not, it would
then indicate the temperature of the portion of
ether only with which its bulb was in contact, and
which portion may be very different indeed from
that part of the outside of the glass upon which
the dew is deposited. And if the ether be dropped
very slowly upon the white bulb, so that evapora-
tion should proceed very slowly, the evil of long-
continued watching is required; and if more
quickly, then the different layers of the enclosed
ether will be of different temperatures. It must
also be recollected that the situation of the black
ball upon which the deposit of dew takes place, is
not very far from the white ball, and in cases
where large quantities of ether are necessary,
such must influence materially the hygrometric
state of the air in the space included betwee
both bulbs."
In consequence of these sources of error in the
use of DanielPs hygrometer, together with its ex-
pense in use and trouble of using, Mr. Glaisher
made many attempts, by different combinations
of the results derived from the observations of the
dry- and wet-bulb thermometers, to deduce the
temperature of the dew-point from them ; and at
HYGROMETRIC CONDITION OF THE AIR. 151
last he found that the difference between the tem-
peratures of the air and evaporation was constant
at the same temperature ; but that this value was
different with every different temperature.
He then collected all the simultaneous observa-
tions, amounting to many thousands, which had
been made at Greenwich from the year 1841 to
1854, and combined with them some observations
taken in India for the higher temperatures, and
some at low temperatures that were taken at
Toronto. From these he deduced the following
Table, the use of which is very simple. From the
reading of the dry-bulb subtract the reading of
the wet, multiply the remainder by the factor
standing in the Table opposite the reading of the
dry-bulb thermometer, subtract the product from
the dry-bulb reading, and the result will be the
temperature of the dew-point.
Thus, if the dry-bulb thermometer reads 68
and the wet-bulb 63, we take the factor 1*7 from
the Table (opposite to 68), and multiply it into
the difference of the two readings, (6863)
xl'7 = 8'5, and subtract the quantity from the
reading of the dry-bulb ; so that we get 59*5 for
the temperature of the dew-point when the read-
ings of the two thermometers are as above.
152
PRACTICAL METEOROLOGY.
Table of Factors.
Reading of
the Dry-bulb
Thermometer.
Factor.
Reading of
the Dry-bulb
Thermometer.
Factor.
Reading of
the Dry-bulb
Thermometer.
Factor.
o
o
20
8*1
44
2*1
68
17
21
7-8
45
2*1
69
17
22
7-6
46
2' I
70
17
23
7-2
47
2' I
71
17
24
6'9
48
2'I
72
17
25
6-5
49
2'0
73
17
26
6-0
5
2*0
74
17
2 7
5*6
51
2*0
75
17
28
5' 1
5 2
2'0
76
17
2 9
4-6
53
2'0
77
17
3
4' i
54
'9
78
r6
3 1
37
55
'9
79
r6
3'3
56
"9
80
1-6
33
57
'9
81
r6
34
2-7
58
'9
82
r6
11
2-6
2-5
59
60
8
8
! 3
84
r6
r6
37
2-4
61
8
85
r6
38
2-3
62
8
86
r6
39
63
8
87
r6
40
2'2
64
8
88
r6
4 1
2'2
65
8
89
r6
42
2'2
66
8
90
r6
43
2'2
67
8
r6
Many meteorologists, however, prefer to calcu-
late the dew-point by Apjohn's formula, and with
a little practice, aided by the table of the elasticity
of aqueous vapour, it may be done almost as readily
as by the factors given above. Had the series of
comparisons been instituted between the dry- and
wet-bulb thermometers and Regnault's dew-point
instrument, the result would have been far more
HYGROMETEIC CONDITION OF THE AIR. 153
satisfactory. Such a series has yet to be undertaken
for the advantage of observers in general, through
very considerable ranges of temperature, humidity,
and pressure, and it would either correct or con-
firm the truth of Mr. Glaisher's determinations.
105. Other deductions. Having ascertained the
dew-point, upon which every other deduction de-
pends, we are prepared to determine other very
important particulars respecting the condition of
the air ; we shall first explain the method of
ascertaining the weight of a cubic foot of air of
any density and temperature.
106. Weight of a cubic foot of dry air. It was
experimentally determined by M. Gay-Lussac that
air expands ~ih of its bulk for every addition of 1
of heat; inasmuch as it was found to expand
equally, with equal increments of heat, from the
freezing- to the boiling-point to the amount of-|ths
of its bulk.
The later investigations of M. Regnault have
shown that this expansion is somewhat in excess ;
his researches have led physicists to adopt ^, or
more nearly still j~^, as the rate of expansion for
each degree of Fahrenheit. (See 21.)
The whole of the calculations in the Greenwich
Tables are, however, founded on the earlier deter-
mination ; while the determination of the weight
154 PRACTICAL METEOROLOGY.
of a mass of dry air, which shall be one cubic foot
in volume under a pressure of 30 inches, and at a
temperature of 32, is derived from the experi-
ments of Sir George Shuckburgh, Biot and The-
nard, which are not in accordance with those of
Regnault.
Regnault gives the weight of 100 cubic inches
(the French measures being reduced to English)
of dry air deprived of carbonic acid gas =
32*58684 grains ; the barometric pressure being
29*92 inches, and the temperature the zero of the
Centigrade scale, or 32 Fahrenheit hence the
weight of a cubic foot of such air is 563'1 grains.
The weight therefore due to 30 inches of baro-
metric pressure will be
The weight of a cubic foot of dry air, at the
same pressure and temperature, adopted in the
Greenwich observations, is 563 grains.
Taking a cubic foot of dry air at a pressure of
30 inches and a temperature of 32 as unity, a
simple proportion will give the space it will occupy
at any given temperature say 44.
We have seen that the expansion of volume
is I_ for every degree of heat ; required the ex-
pansion for 4432 or 12 ; 1 : 12 : : Jj : ^ or
0*0244, so that a cubic foot of air increases to
HYGROMETRIC CONDITION OF THE AIR. 155
1-0244 feet between 32 and 44; but the weight
of the cubic foot of air originally was 564'6 grains;
hence, the weight varying inversely as the volume,,
1-0244 ft. : 1 ft. : : 564'6 grains : 551'1 grains, the
weight of the same volume after expansion.
In the " Table showing the weight in grains of a
cubic foot of dry air under the pressure of 30 inches
of mercury for every degree of temperature from
to 90," published in the Greenwich Meteoro-
logical Observations, the weight of a cubic foot of
air at 44 is given as 549'27.
Two other tables given in the Greenwich Me-
teorological Observations, as being the foundation,
in union with the former just explained, of valua-
ble calculations, may be alluded to. The first is
entitled " A table showing the enlargement which
a volume of dry air receives, when saturated with
vapour, under the pressure of 30 inches of mer-
cury," from to 90 of temperature.
If a cubic foot of dry air of known elasticity be
mixed with a cubic foot of vapour, also of known
elasticity, and if the mixture be compressed into
the space of one cubic foot, the elasticity of the
mixture will be the sum of the two elasticities of
the air and vapour ; or, if it be allowed to expand
till its elasticity is equal to that of the unmixed
air, it will occupy a larger volume in the pro-
portion of the sum of the two elasticities to the
156 PRACTICAL METEOROLOGY.
elasticity of the air alone. Now, from Table II.
(Appendix), we know the elastic force of vapour
for every degree of temperature ; let it be re-
quired to find the space occupied by a cubic foot
of dry air, and such an amount of aqueous vapour
as will ascend into it at the temperature of 44.
Tension of aqueous vapour at 44= 0-288 inches.
Tension of dry air 30-000
Tension of both combined 30*288
Then as the spaces occupied are inversely as the
tension or elasticity,
30 : 30-288 : : I cubic foot : 1-096 cubic feet,
the space occupied by the mixture of the aqueous
and gaseous fluids.
The Greenwich formula will lead to precisely
the same result.
Let^?=the atmospheric pressure, as measured by
the inches of mercury in the barometer.
E^=the elasticity of vapour at temperature t.
n=the bulk of a certain quantity of air when
dry, at the given temperature t, and
under the pressure p.
ri = the bulk of the same quantity of air when
saturated with vapour at the same tem-
perature t, and under the same press-
ure^?.
Then since the elasticity varies inversely as the
volume, the temperature remaining the same, that
HYGROMETRIC CONDITION OF THE AIR. 157
portion of the elastic force p which depends
on the air only which occupies the space ri is
and this, together with E^, must make up the
atmospheric pressure,
or
or
n
n
i
or n
=(-!>
P
By substituting the values from the above example
in this expression, we obtain
n'= = i -096 as before.
30
From calculations of this kind the quantities
tabulated have been obtained for each degree, but
as the Table is not absolutely necessary for hygro-
metrical deductions, it is not inserted in this work.
107. Weight of a cubic foot of vapour. The next
Table demanding explanation gives " The weight
in grains of a cubic foot of vapour under the
pressure of 30 inches of mercury for every degree
of temperature from to 90."
158 PRACTICAL METEOROLOGY.
As vapours expand by the increase of tempera-
ture by the same law as permanently elastic fluids,
and undergo a change of volume proportional to
the change of pressure, and as air expands f ths of
its bulk* from 32 to 212, its expansion being
uniform between these points ;
Therefore if the weight of a cubic foot of vapour
under the pressure equivalent to 30 inches of
mercury, and at a temperature of 212, be called
W, and the weight, expressed in the same deno-
mination of an equal volume of vapour at the
temperature t, be called W, and if E^ be the
elasticity of vapour at the temperature t y then
(the expansion of dry air from 32 to 212 being
0-375 or 0-002083 for each degree of temperature)
_ i'375
32)
Now Gay-Lussac has also determined that a
cubic inch of vapour at 212 weighs 0-149176
grains under the pressure of 29*922 inches of
mercury ; and consequently a cubic foot of vapour,
under the same circumstances, weighs 0' 149 176
xl728=257'776 grains; and under a pressure
of 30 inches it weighs
X 257776=258-448 grains.
* This is the ratio of expansion applied in the Greenwich
Tables ; see, however, 22.
HYGROMETRIC CONDITION OF THE AIR. 159
Therefore, substituting this weight for W, the
formula becomes
W '_ i'375 x 258-448 xE, .
30(1 +'002083 X if 32)
from a like formula but with Regnault's elements
is calculated Table III. (Appendix), taken from
Glaisher's Hygrometrical Tables, from which may
be obtained the weight of vapour in a cubic foot
of air saturated with moisture, at any temperature.
If the temperature /, denoting that of the dew-
point, be also that of the air, which will only be the
case when the dry-bulb and wet-bulb thermometers
read alike, this Table would give by inspection the
weight of vapour in a cubic foot of air, the argu-
ment being the temperature; but when, as is
usually the case, the dew-point temperature is
below that of the air, the vapour in the air will
have expanded, and its density will have diminished
in the same ratio as air. On this fact is founded
Table IV. (Appendix), which is thus used.
The dew-point temperature being known, enter
Table III. with it, and take out the corresponding
weight (in grains) of a cubic foot of air. Enter-
ing Table IV. with the difference between the air-
temperature and that of the dew-point, take out
the factor opposite the number of degrees ; and
this, multiplied into the quantity extracted from
Table III., will give the weight of vapour in a
160 PRACTICAL METEOROLOGY.
cubic foot of air at the temperature shown by the
dry-bulb thermometer.
Example : let the dry-bulb read 50 and the wet-
bulb 47 ; the barometer standing at 29 inches.
The tension of vapour at the dew-point, from
Apjohn's formula, will be
corresponding to dew-point 44. Enter Table III.
with 44 ; and opposite will be found 3*3 grains ;
with 6, the difference between the air-temperature
and the dew-point, enter Table IV. and take out
the factor -988; then 3'3x -988=3-26, which
will be the weight (in grains) of vapour in a cubic
foot of air at a temperature of 50 when the dew-
point temperature is 44.
Another formula of great simplicity will give a
near approximation : let / = elastic force of
vapour at the dew-point ; t the air-temperature ;
the weight (in grains) of moisture in a cubic foot
of air will be / nearly ; by substitution,
~E~JcO J" (,
from the above example 5656 'g x '288 = 3-27.
448 + 50
108. Amount required for saturation. Table III.
will enable us to ascertain how much more moisture
would be requisite completely to saturate the air.
Entering it with the air-temperature 50, we find
opposite to that number 4-1 grains, which would
HYGROMETRIC CONDITION OF THE AIR. 161
be the weight of vapour in the air if it were
saturated with moisture ; but we have found that,
with a dew-point at 44, it holds in suspension only
3*26 grains; therefore, in this case, 4*1 3*26,
or O84 grain additional, would be absorbed before
complete saturation would be attained.
109. Weight of a cubic foot of moist air. This
quantity we may obtain readily from Table V. ; if
the temperature of the air and the dew-point be
alike, the quantity ranging with the temperature
will be the weight of the air, seeing that in this
case it will be saturated with moisture. But if, as
is usually the case, the air- temperature be above
that of the dew-point, enter the Table with the air-
temperature and take out the quantity under the
heading " Excess : " this, added to the weight of a
cubic foot of saturated air, will give the weight of
a cubic foot of dry air ; the same quantity, multi-
plied by the degree of humidity supposed to have
been determined previously, must be subtracted
from the weight of a cubic foot of dry air, and the
result will be the weight of a cubic foot of air of
the given temperature and humidity; this last
i, !, v 1 1_ heiaht of barometer M1
result, multiplied by 2- - t will be
ou
the true weight of the air under the existing
pressure.
Taking the last example ; it is required to find
M
162 PRACTICAL METEOROLOGY.
the weight of a cubic foot of air under the circum-
stances there recorded.
.QQO
The degree of humidity will be =-8.
*ool
Entering Table V. with 50, under " Excess " we
find 2-22, which added to the number ranging with
it, 544-4, will give 546-82, the weight of a cubic
foot of dry air ; but 2'22x'8 = l'77, the excess
of such weight above the weight of a cubic foot of
air of the existing temperature and humidity,
which will therefore be 546-82 1-77 = 545-05.
9Q
Finally, 545*05 x = 526'66 will be theweight of
oU
a cubic foot of air at a temperature of 50, of the
degree of humidity *8, and under a barometric
pressure of 29 inches.
These are all the deductions which are practised
at Greenwich ; the formulae and tables are for the
most part drawn from Glaisher's Hygrometrical
Tables and the Greenwich " Magnetical and Me-
teorological Observations/' which are published
yearly. The author, through the Council of the
Eoyal Astronomical Society, has obtained for
many years past a grant of these portly quarto
volumes, which have been most valuable as books
of reference, and he gratefully acknowledges his
obligation both to the Society and the Government
to the one for their recommendation, and to the
other for their liberality ; he trusts that his
HYGROMETRIC CONDITION OF THE AIR. 163
endeavour to explain and simplify the processes
employed will be found useful to a large class of
persons who may never have an opportunity of
consulting the records of the Greenwich meteoro-
logical observations.
It may not at first be supposed that it is pos-
sible for the air, when saturated with moisture, to
hold in solution a less amount of aqueous
vapour than when, the temperature being higher,
it is considerably removed from the point of satu-
ration; this will, however, be the fact whenever
the temperature of the dew-point in the latter
case is higher than in the former. Let us inves-
tigate the amount of moisture in a cubic foot
of air when the dry-bulb thermometer reads 63
and the wet 58 ; and also when the dry-bulb
reading is 40 and the wet-bulb 39.
Dry-bulb 63. Dry-bulb 40.
Wet-bulb 58. Wet-bulb 3Q.
Degree of humidity '72 -92
Dew-point 53.8 37*7
Weight of vapour in a cubic foot ... 4/6 2*6
The fact above stated will be realized when it
is considered that when the dew-point is 53 0< 8,
the air will hold in solution a greater amount of
aqueous vapour than when its temperature is 37' 7.
It would seem undisputed, however, that with
regard to the effect of the air viewed in relation
to health, the degree of humidity, rather than the
absolute amount of moisture in a given quantity
M 2
164 PRACTICAL METEOROLOGY.
of air, will be the matter most worthy of the
attention of the medical man. To assist in ar-
riving at a conclusion on this point without the
labour of reduction, Table VI. (Appendix) has
been inserted, which will be useful in a sanitary
point of view, and also for agricultural operations.
It gives by inspection the degree of humidity of
the air for the general range of temperature in
this country, the argument being the difference
between the readings of the dry- and wet-bulb
thermometers for every degree of temperature
from 32 to 79, the determinations corresponding
with those resulting from the dew-point as found
by Apjohn's formula.
110. Welsh's Sliding-mle. Mr. Welsh, late of
the Kew Observatory, invented a sliding-rule* for
facilitating hygrometrical calculations, which has
been described at full in the Report of the
British Association for 1851, p. 42. By means
of this instrument, the use of which may be learnt
in half an hour, we may obtain, with little trouble
and with sufficient accuracy, the dew-point, the
tension of aqueous vapour, the degree of humidity,
and the weight of vapour in a cubic foot of air,
the readings of the dry-bulb and wet-bulb ther-
mometers being known.
The author has tested this useful instrument at
* Sold by Adie, Fleet Street.
HYGROMETRIC CONDITION OF THE AIR. 165
various ranges of difference between the dry- and
wet-bulb, and has found it beautifully accurate,
the quantities agreeing with those deduced from
the Greenwich formulae to the third decimal. By
those who are not ready at calculation, it will be
found most useful, and those who are will obtain
a result in five minutes which in any of the usual
methods would occupy a much longer time.
111. Glaisher's Tables. Mr. Glaisher's "Hy-
grometrical Tables * " give all the results ex-
plained in this work by inspection, when the
difference between the dry- and wet-bulb thermo-
meters is void of fractions of degrees ; additional
labour is, however, requisite when decimals inter-
vene. The observations taken at upwards of fifty
stations in Great Britain, and forwarded monthly
to Mr. Glaisher as Secretary of the British
Meteorological Society, are all reduced by the
aid of these Tables, to bring them up to the same
standard, and to afford the means of direct com-
parison among themselves, and with the atmo-
spheric conditions registered at Greenwich, the
head-quarters of meteorological science.
112. Deductions worked out in full. The for-
mulae and tables explained in this part of the
work will be found sufficient to perform all the
* Published by Taylor and Francis, Red Lion Court ; price
2s. 6d.
166 PRACTICAL METEOROLOGY.
deductions which are ever required relative to the
hygrometric state of the air; they amount to six,
the first four being the most important. An
example will, at one point of view, illustrate the
manner by which all these important particulars
may be derived from the simple record of the
temperature of the air and that of evaporation.
Suppose the reading of the dry-bulb to be 56,
of the wet-bulb 50, the barometer reading
29*900 inches; required,
a,. The tension of aqueous vapour. Table II.
(Appendix), and Apjdhn's formula, page 146,
13. The dew-point corresponding to this tension
of aqueous vapour in Table II. (Appendix) is
44-5.
By Glaisher's factors, page 152, these two de-
ductions result thus :
56-(s6-5o) 1-9 = 56- 1 1-4=44-6,
the dew-point ; whence by Table II. the tension
of aqueous vapour = *294.
7. The degree of humidity,
_ tension at dew-point _ -294 __.-,
tension at 56 -449
&. The weight of vapour in a cubic foot of air;
HYGROMETRIC CONDITION OF THE AIR. 167
Table III. gives what it would be if the air were
saturated, viz. 3'3 grs., which, multiplied by the
factor opposite 11'5 (the difference between the
temperature of the air and the dew-point) in Table
IV,. will be 3-3 x -977 = 3-22 grains, or by the
second formula, page 160,
x -294=3-19.
448 + 56
e. The amount of vapour required to saturate
the air : the quantity of vapour held in solution
by air at 56, when saturated, = 5'; .'. 5' 3'22 =
1'78, the amount required.
. Table V. will give the weight of a cubic foot
of air in grains : thus (see page 161),
{537-5 + 2-95-(2-95 X '654)} ^2 = 5367.
113. Diurnal range of temperature of evapora-
tion and the dew-point. In the Philosophical
Transactions, Part 1. 1848, Mr. Glaisher has given
Tables of the diurnal range of the temperature
of evaporation as shown by the wet-bulb ther-
mometer, and also of that of the dew-point ; by
which, from one daily observation of either, the
monthly mean may be deduced, as in the case
of the monthly mean temperature explained in
48.
168
PRACTICAL METEOROLOGY,
Corrections to be applied to the monthly mean
readings of the wet-bulb thermometer placed
at the height of 4 feet above the soil, at any
hour, to deduce the true mean temperature of
evaporation for the month from the observa-
tions taken at that hour.
Local
mean time.
Jan.
Feb.
March.
April.
May.
June.
h
o
o
o
o
o
12 Midn.
+07
+ 1*2
+2*9
+ 3'8
4-3'S
I A.M.
+07
4*1*4
+ 2'0
+ 3'i
+4-2
2
+ 1*0
+ r6
+ 2*2
+ 3*4
+4*2
+4-8
3
4-i*i
+ 1-6
+2*4
+ 3'8
+4'4
4-5*3
4
+ **4
+ r8
+27
+4- 1
+4'4
+ 5'8
5
+1-6
-j-r8
+ 2*8
+4'3
+4-2
4-5'3
6
+ i'7
-j-i'9
+27
+ 3'9
+2*9
+ 3*5
7
4-17
+ 1*7
+ 2*6
+27
+ 1*2
+ I'2
8
+ 1*3
4- r 3
+ 2'0
+ ro
+ 0*1
-0'7
9
+o'8
4-0*7
+0*3
1*0
-17
2'I
10
O'O
-0*3
-!"3
2 '3
-3-0
3*1
1 1 A.M.
-1*4
-r6
-2-5
3*6
-3'8
-3'4
12 Noon.
2*1
2-5
-3*6
-4'3
-4-2
-4-0
I P.M.
2 '5
-3*0
-4*0
-4-8
4*4
-4-2
2
2'4
-2*9
-3'9
-4*8
-4'3
-4*4
3
-r8
-2-4
-3*6
-4'5
-4-0
-4-6
4
I'2
2'I
-2'8
-3*9
-4*4
-47
5
-0-6
-1-4
-1*9
_ 3 -o
-2*6
6
0'2
-07
1*0
-r8
-1-4
-27
7
O*2
O'O
-0*3
-0*4
-0*4
~~ r 5
8
O*I
+ 0'2
+0-6
+0-5
+0-8
O'O
9
+ 0'2
+0-5
+ 1*2
+ I'2
+ i'4
+07
10
-f'3
+0-8
+ 1*6
+ 1*8
II P.M.
+0-4
+ 1*0
+ r8
+2-4
+ 3'o
+ 2*6
HYGROMETRIC CONDITION OF THE AIR. 169
Table (continued).
Local
mean time.
July.
Aug.
Sept.
Oct.
Nov.
Dec.
h
o
o
o
o
o
12 Midn.
+ 3'i
+2-6
+ 2-2
+ i*9
+ i-3
4-0-5
I A.M.
+ 3-2
+2-9
+2-8
4-r8
+ i'4
4-0-7
2
-f3'5
+ 3'3
+37
4-2-1
+ r6
-1-0-8
3
+ 3-6
+ 3'4
+4'i
+ 2'3
+ r6
+0-9
4
+3-6
+ 3-5
+4*5
+2-4
4-r6
4-ro
5
+ 3'4
+ 37
+4'i
+2-3
+ i'5
4-0-9
6
+ 2'3
+ 3-o
+ 3-2
4-2-0
+ i'4
40-8
7
+ ro
+ 1-2
+ 2'I
+ i-5
4-ri
4-ro
S
-0-6
0-3
+0-9
+0-4
4-0-5
4-0-9
9
-r6
-1-7
-07
4-0-2
4-0-6
10
-2-6
-2-6
2 '5
2*0
-0-4
O'O
II A.M.
-3-2
-3-6
-3-6
-3'
-i'5
-0-8
12 Noon.
-3*5
-3*9
-4'3
-3*7
-2-3
-1-4
I P.M.
-3-6
-4*3
-4*4
-3'7
-'5
-1-6
2
-3'6
-4*5
-3'9
-3'
-2-6
-i'5
3
-3*4
-3'9
-3' 1
-1-9
-'3
-i'3
4
-3*3
-3-0
-2-6
I"2
-1-6
I'O
-2'6
-1-3
-2-4
-0-9
-0-9
-0-6
6
-r 9
-0-5
-1-9
-0-7
-0-3
-0-6
7
-0-8
+o-3
-0-9
O'l
O'l
-0-4
8
+ 0-2
+ ro
+0-1
+ 1-2
+0-3
O'Z
9
4-ri
+ i'4
+0-4
4-0-6
4-0-7
-l-o-i
10
+2-0
+ i'5
+ ro
+ ro
4-0-9
4-0-2
II P.M.
+**5
+ 2'I
+ i'5
+ i'3
4-ri
4-0-5
The next Table gives the corrections for the dew-
point for a few hours only out of the twenty-four,
those which are most generally chosen for hours
of observation.
170
PRACTICAL METEOROLOGY.
Corrections to be applied to the monthly mean
reading of the temperature of the dew-point,
4 feet above the soil, to deduce the true mean
temperature of the dew-point for the month
from observations taken at the hours 3 A.M.,
9 A.M., 3 P.M., and 9 P.M.
Jan.
Feb. March.
April.
May.
June.
3 A.M.
+ ri
+ 2-2
+ 1-2
+ 17
+2*9
+ 3'6
9 A.M.
+0-3
+ 0-2
+0-1
I'D
-r6
-i'5
3 P.M.
-0-8
-*?
-r8
-I- 9
-2-4
- 2 '5
9 P.M.
0*5 0*2
+ 0-2
+0-8
+ 0-9
+07
July.
August.
Sept.
Oct.
Nov.
Dec.
3 A.M.
+ r 9
+ r8
H-i
+ ro
+ 1-4
-f-o-8
9 A.M.
-0-6
1*4
-0-8
--0-3
+ 0'I
+ 0-2
3 P.M.
-r6
2'2
2*0
0'
1*2
-0-6
9 P.M.
H-o-6
-1-0-4
+0-1
+ 0'I
+0-5
+ 0'I
114. Observations in the higher regions of the
atmosphere. Observations taken at a much
greater height than 4 feet above the surface of
the earth would be most valuable for the promo-
tion of meteorological investigations, for we are
very little acquainted with the degree of humid-
ity of the higher strata of the air. From obser-
vation some few facts are rendered apparent ; it
is found, for instance, that in the hottest hours of
the day, when evaporation goes on most rapidly,
the air at a short distance from the surface of
HYGROMETRIC CONDITION OF THE AIR. 171
the earth is relatively drier than during the cooler
hours ; it follows that much of the moisture must
ascend into the air and be united with the upper
strata, seeing that it does not increase the degree
of humidity of the lower.
With the object of learning somewhat of the
conditions of the upper regions of the air, the
Kew Committee of the British Association for the
Advancement of Science projected four balloon
ascents in the year 1852, the results of which
were published in the 'Philosophical Transac-
tions ' for 1853. The observations, thermometric,
hygrometric, and barometric, were taken by John
Welsh, Esq., a gentleman of great experience,
accustomed to the practice of meteorology at the
Kew Observatory, and they form a valuable scien-
tific record.
Simultaneous observations during the time of
the ascent, as well as some hours before and after,
were undertaken in various parts of England,
Ireland, and France, and every care was taken to
determine the state of the air at the level of the
sea, by combining and reducing those near the
point of ascent, the Vauxhall Gardens, and below
the course of the balloon. Regnault's hygro-
meter was occasionally used in the ascent, but the
degree of humidity and the dew-point were in ge-
neral deduced from the readings of the dry- and
172
PRACTICAL METEOROLOGY.
wet-bulb thermometers, by Apjohn's 'formula, and
Dalton's Table of the tension of aqueous vapour.
The following register of meteorological obser-
vations is selected from the account of the last
ascent, Nov. 10, 1852, when the highest point
reached, 22,930 feet, far exceeded that of any of
the three previous ascents. The mean height of
the barometer at 120 feet above the level of the
sea during the time of the ascent was 29*978;
the temperature of the air was concluded to be,
at the same time and level, 49 '2.
Height
above the
sea in feet.
Baro-
meter.
bib".
Wet-
bulb.
Tension
of
aqueous
Dew-
point.
Relative
humid-
ity.
vapour.
5,880
24-17
347
33'4
197
317
9
7,070
23*12
347
31-2
167
2 7 -I
76
8,420
21-97
3*'5
3i'4
I8 7
30-2
92
10,630
20*17
26*9
24-2
132
20*6
79
13,860
1775
15-9
12-4
076
5-8
68
15,820
16-44
12-8
8-1
057
- r8
58
18,700
14*60
2'2
2'I
035
-14-0
"53
22,640
I2'4
-8-9
The following are Mr. Welsh's remarks on the
ascent of November 10th :
"The humidity, as in all the previous series,
increased from the earth to the first cloud, which
was at a low elevation and of but little density ;
upon leaving it, at about 1900 feet, a slight de-
HYGhOMETRIC CONDITION OF THE AIR. 173
pression took place. Immediately above this low
cloud a different current of air existed, shortly
after entering which the humidity again increased,
until, in the second cloud, it became nearly com-
plete; the decrease, after leaving the cloud at
5000 feet, becoming rapid, and attaining a mini-
mum at 6500 feet. A second well-defined maxi-
mum was reached at 8300 feet, followed at
10,000 feet by a secondary minimum. The humid-
ity diminished on the whole till about 15,800
feet, when a sudden increase commenced, which
continued from 16,500 to 17,600 feet, followed
by an equally sudden decrease at 18,000 feet, the
humidity subsequently increasing. The fluctua-
tions in this series were numerous, there having
been no fewer than four or perhaps five different
strata of vapour."
It is evident that we have not at present suf-
ficient data to draw safe conclusions as to the
hygrometric condition of the superior atmospheric
regions ; and we trust that there will be continued
series of observations undertaken, from time to
time, under similar circumstances, and thus aerial
voyages, hitherto barren of everything but risk,
will be turned to good scientific account.
115. General remarks. It may here be stated,
that doubts have lately been thrown upon the
accuracy of the dew-point temperature as deduced
174 PRACTICAL METEOROLOGY.
from the dry- and wet-bulb thermometer readings.
M. Regnault, a great authority in these matters,
considers that the wet- bulb temperature may not
indicate accurately that of the air immediately
surrounding it, and his condensing-hygrometer
was the result of an attempt to produce an instru-
ment which should give the dew-point directly,
and not be liable to the objections brought against
Daniell's, which we have already mentioned. It
would appear, that at high elevations in the balloon
ascent on Aug. 26th, whilst the indications of
Regnault' s hygrometer did not differ much from
those of the dry- and wet-bulb thermometers at
the height of 11,000 feet, the difference became
considerable at about 12,000 or 13,000 feet, thus
rendering it probable that at the latter heights the
relative humidity, as deduced from the dry- and
wet-bulb thermometers, was too great. The
general accordance, however, was restored at
15,000 feet.
Col. Sykes, in his "Discussion of Meteorological
Observations taken in India" (Phil. Trans. Part II.
1850), has entered upon the question at some
length. He found that, in India, a very high
degree of humidity resulted from observations
with the dry- and wet-bulb thermometers, such as
experience showed to be impossible ; " and I have
no hesitation," says he, " in expressing my belief
HYGROMETRIC CONDITION OF THE AIR. 175
that the results which I have obtained with the
labour of some months (by reducing numerous
observations with the dry- and wet -bulb thermo-
meters) do not represent the real fractions of
saturation of the air at the several places where
the wet-bulb was observed."
As, however, in this country, we have a com-
paratively small range of temperature, extremes
either of great heat or great cold being seldom
reached, and as all the stations in the United
Kingdom are at low elevations, we apprehend the
majority of observers will follow the plan of ob-
servation with the dry-bulb and wet-bulb ther-
mometers advantageously, both for the sake of
uniformity, and from the small amount of expense
and trouble which it involves. Nor are we without
patronage as regards its adoption, seeing that at
the late " Meteorological Congress " at Brussels,
which was attended by men of eminence from the
maritime nations of Europe and from the United
States, the dry- and wet- bulb thermometers were
recommended to be regularly observed ; the chair-
man of the Congress, M. Quetelet, had long be-
fore thus expressed his opinion on the point now
under discussion in his " Instructions pour Fob-
servation des phenomenes periodiques," issued by
the Academic Royale de Bruxelles :
" I/hygrometre donne des renseignemerits
176 *' PRACTICAL METEOROLOGY.
utiles; mais on le remplacera avantageusement
par le psychrometre, moins sujet & se deranger,
et dont les indications sont plus sures."
No doubt can exist in the mind of any one who
has examined and tested the Greenwich observa-
tions by Apjohn's formula?, and who has compared
them with the results arising from the use of
Glaisher's Tables, that the subject of the hygro-
metrical state of the air is one which still demands
the attention of physicists ; the discrepancies in
the results of various formula? show clearly that,
with regard to the deductions explained in this
work, the most we can at present expect are ap-
proximations, and those by no means of the closest
kind.
CLOUDS.
116. It may be fairly assumed that observa-
tions on the various kinds of clouds have not been
followed up by scientific men with that attention
which their evident connexion with atmospheric
changes demands. Indeed the philosopher must
acknowledge that the sailor and the fisherman on
the coast are far more weatherwise than himself,
and the clouds are that page of nature's book
which only they are competent to read ; dependent
as they are on observation of the face of the
heavens, they have attained by practice an apti-
tude in predicting atmospheric changes which
HYGROMETRIC CONDITION OF THE AIR. 177
puts science to the blush, and if we aim at ever
arriving at the power of prediction as regards
meteorological phenomena, we must mark and
record more extensively the changes in the cha-
racter of the clouds which float above us.
117. Their appearance. The clouds are spread
over the sky in forms of great variety, and, at
times, of surpassing grandeur. Their beauty of
colour, and the grand effects of contrast, defy imi-
tation by the painter's art ; they move above us
mysteriously floating without visible support, and
setting at naught the explanations which philo-
sophers have given. We are told by some that
they are vesicular vapour ; this is simply a hypo-
thesis; we can only affirm with certainty that
they consist of particles of aqueous vapour in a
peculiar state of aggregation, and that they float
in the lower regions of the atmosphere. That
electricity affects their state is pretty certain, but
facts are wanting on which to found a theory as
to its mode of operation.
118. Height. The clouds do not seem to as-
cend higher in the atmosphere than five miles.
Riccioli, who determined the height of clouds
trigonometrically, never found them to reach
higher than 8880 yards. Dalton found cirri, the
lightest form of cloud, from three to five miles
above the earth ; and all observations combine to
178 PRACTICAL METEOROLOGY.
fix the region of clouds in that stratum of the air
included between the sea-level and five miles
above it.
The existence of clouds does not seem to affect
the atmospheric pressure; the barometer may
remain stationary at times when heavy clouds are
rolling above.
119. Classification. In the year 1802, Mr. Luke
Howard published an elaborate Essay " On the
modifications of Clouds, and on the principles of
their production, suspension, and destruction "
The classification proposed by him has been gene-
rally adopted both in England and abroad ; the
following is a summary of his views, and an ex-
planation of his nomenclature, which has the
merit of being founded on the natural charac-
teristics exhibited by clouds in the forms they as-
sume, and on the causes from which they derive
their origin.
The simple modifications are thus named and
defined :
1. Cirrus. Parallel, flexuous, or diverging
streaks or fibres of cloud.
2. Cumulus. Convex or conical heaps, in-
creasing upward from a horizontal base.
3. Stratus. A widely extended, continuous,
horizontal sheet.
The intermediate modifications are :
HYGROMETRIC CONDITION OF THE AIR. 179
4. Cirro-cumulus. Small roundish masses of
fleecy cloud, in close horizontal arrangement or
contact.
5. Cirro-stratus. Horizontal or slightly in-
clined masses attenuated towards a part or the
whole of their circumference, or else a thin dif-
fused sheet.
The compound modifications are :
6. Cumulo-stratus. The cirro-stratus blended
with the cumulus, and either appearing inter-
mixed with heaps of the latter, or superadding a
wide-spread structure to it.
7. Cumulo- cirro-stratus or Nimbus the Rain-
cloud. A cloud, or system of clouds, from which
rain is falling; it is a horizontal sheet, above
which spreads the cirrus.
120. a. The Cirrus. This is the lightest of
all clouds, and generally occupies the highest
regions of that stratum of the air which alone is
frequented by cloud; accurate measurements
have proved this fact, which is confirmed by the
circumstance of cirri reflecting the sun's rays for
a long time after they have ceased to illumine
the clouds below. Cirri have been known to pre-
sent the same configuration for two successive
days, while a strong breeze has been agitating
the air beneath; hence it is probable that the
vapour of which they are composed aggregates in
N2
180 PRACTICAL METEOROLOGY.
a calm region, perhaps out of the reach of the
daily variations of temperature and evaporation
which disturb the lower strata of the atmosphere.
It may be that the electric states of dry air and of
moist are antagonistic ; the various fine ramifica-
tions may be the means of electric communication
between the moist air of the cirrus and the dry
air which surrounds it.
121. b. The Cumulus. We have already seen
that the sun's rays in traversing the atmosphere
communicate little, if any, of their heat; it is
by conduction from the earth's surface that the
lower stratum of the air is heated, and the heat
spreads by convection.
When the sun has risen, the surface of the
earth becoming gradually warmed communicates
heat to the stratum of air in contact with it,
which, consequently, is capable of holding in sus-
pension an increased amount of aqueous vapour ;
the current caused by the ascent of the heated air
urges upwards the vapour, which during the night
had remained in the air, till it arrives at a region
so cold that it becomes condensed in part, and
descends in fine particles, which are reabsorbed
before they reach the earth ; the meeting of the
descending globules of vapour and of the ascend-
ing current forms a cloud, which gradually in-
creases in size from attracting the vapour in its
HYGROMETRIC CONDITION OF THE AIR. 181
neighbourhood. Were the supply of vapour to
proceed from every quarter, the form of such cloud
would probably be spherical ; but as the portion
directed towards the earth is in contact with air
not saturated with moisture, or having none to
spare, we find the cumulus attaining only the
hemispherical form, which is the general charac-
teristic of this modification of cloud.
The Cumulus is formed only in the day-time,
as then only it is that the disturbance in the tem-
perature of the air above described can occur; it
vanishes towards the evening from the superior
strata of the air having increased in temperature,
while the temperature of the inferior is being
lowered, so that there will cease to be an upward
current.
122. c. The Stratus. This form is a horizontal
sheet of cloud which occurs mostly at night or in
the evening when the air is calm; it is at this
time that it is formed, by the layers of air next
the earth getting cooled from its contact, when
the earth has lost heat by radiation, to below the
Dew-point; it must therefore increase by addi-
tions to its upper surface, as higher portions of
the air begin to lose their heat in turn. The
Stratus comprehends mists which ascend from val-
leys, the surface of lakes and pieces of water, all
which are dispersed by the heat of the morning sun.
182 PRACTICAL METEOROLOGY.
123. Registration of amount of cloud. In a
register of meteorological phenomena, it is usual
to place upon record the kind of cloud, if any,
visible at the times of observation, and also the
amount of cloud which overspreads the sky. A
clear sky is registered 0, and a cloudy sky 10 ; and
the observer, in intermediate conditions, estimates
the amount of cloud and registers accordingly.
From the Greenwich observations it has been con-
cluded that a certain degree of regularity may be
traced in the cloudiness of the sky, and from them
has been formed a table of the " diurnal variation"
in the amount of cloud, similar to that of the
" diurnal range " of temperature. From this
table it would appear that the hours of night
are those in which the least amount of cloud
prevails, and that the greatest amount may be
expected at mid-day. The average amount of
sky covered by cloud in the several months of
the year is as follows :
February
March
April
/
7'S
6-4
C'Q
August
September ...
October
/
6-4
5'9
6-8
Mav . . ,
j y
6-c
November
7-2
June
TO
December .
7*4.
Hence it would appear that, from November to
HYGROMETRIC CONDITION OF THE AIR. 183
February, three-fourths of the entire sky are
covered by sun-repelling clouds.
The difference between the day and night, as
regards the clearness of the sky, is exceedingly
small during these months.
June is the clearest month in the year ; during
the night hours the least amount of cloud may be
expected in summer and autumn, especially in the
month of September, which is a month, as astro-
nomers can testify, exceedingly favourable for ob-
servation.
Observers usually trust to estimation in record-
ing the amount of cloud, which of course will only
give a rough approximation. The author has
been shown an ingenious contrivance by J. Camp-
bell, Esq., of the Board of Health, by which the
amount of sunshine has been made to register
itself during the hours of day-light, well-worthy
of mention in this place. A hollow globe of
glass, 3 inches in diameter, is filled with Canada
balsam (the aperture being covered with a piece
of bladder) and exposed all day in the open air*;
whatever may be the sun's declination, his rays
will be concentrated in the focus of this spherical
lens, which in this case will be distant from the
* The liquid need not be Canada balsam ; water, with a little
acid in it to keep it pure, will do.
184 PRACTICAL METEOROLOGY.
surface of the sphere 0*615 in.*. The globe is
fixed in a hollow hemisphere whose radius is
greater than that of the globe by this quantity ; a
piece of black ribbon is laid on this opposite to
the sun's diurnal arc for the day, and whenever
the sun shines, his rays will be brought to a focus
on it, and his progress will be marked by a line
burnt through the ribbon if his light has been
continuous, or by a broken line, or series of small
holes, if it has been intermittent ; a comparison
of the length of these with the length of ribbon
due to the number of hours the sun is above the
horizon for the day, will give the amount of sun-
shine ; and, as each strip of ribbon may be pre-
served, not only the number of hours of sunshine,
but the exact time of their occurrence may be
faithfully registered.
Mr. Campbell has given to the Royal Meteoro-
logical Society details of the construction and
mode of use of this instrument, in a paper which
is printed with the Report of the Council for 1857.
Foreign meteorologists register the " serenite
* To find the focus of a sphere for parallel rays in terms of
the radius, divide the index of refraction by twice its excess
above 1. Refractive index of Canada balsam = 1.549.
1 .p>4.o
. * * g x 1-5 = 2-116 inches, measured from the centre of the
' ' 1-098
sphere ;
2-116 1*5 =0-616 beyond the surface.
HYGROMETRIC CONDITION OF THE AIR. 185
du ciel " or ' c portion of sky clear," and reverse
the record used in England by letting stand for
a cloudy sky, and 10 for one of pure sunshine ; this
form was adopted by the Meteorological Congress
at Brussels, in the directions which they pub-
lished for the use of the naval officers of the
whole civilized world. (See Part III.)
RAIN. SNOW. HAIL.
124. One of the most important elements in
determining climate is a knowledge of the amount
of rain received in any district ; to the engineer
this is especially useful, as supplying data on which
he may safely construct his sewers, or calculate the
amount of water which will be afforded to a town
within a given area. Of all meteorological obser-
vations, the determination of the quantity of rain
which has fallen at any place within a given time
is the easiest ; of all instruments, the rain-gauge
is the least expensive and least liable to be out of
order.
125. Rain-gauge. The principle of the instru-
ment is the following : If we imagine the surface
of the ground, over which a shower of rain has
passed, to be perfectly level and impervious to
moisture, and that it is so surrounded with an
enclosure that the whole quantity of water shall
be retained, the rain would cover the surface to a
186 PRACTICAL METEOROLOGY.
certain depth, which, measured in inches, would
give the amount of rain that had fallen. In cal-
culating this depth by means of the rain-gauge,
we expose a small surface to the reception of the
rain, and measure the depth of what it receives,
proceeding on the supposition that the same
amount would have fallen into the gauge at any
portion of the rain-fall; this equable distribu-
tion of rain, however, seldom occurs, for a shower
may pass by the position chosen, and, although
much rain may fall at no great distance, not a
drop may reach the rain-gauge itself. Hence, to
obtain the exact amount of rain which falls in
any given district, several rain-gauges should be
dispersed in various parts, and the mean of the
whole amount received would be the true quantity
due to such an area.
Observers are generally satisfied with registering
the amount of rain received by their own gauges
at 9 A.M. every day. In engineering operations,
however, it is of the utmost importance to know
how much rain may be expected on any occasion
to fall per hour ; hence, in very violent falls, the
quantity should be ascertained immediately on
their cessation, and the time of duration noted in
the register.
Rain-gauges are of various constructions. In
some a glass tube, divided into inches and parts
HYGROMETRIC CONDITION OF THE AIR. 187
of an inch, proceeds externally from the bottom
of the vessel in which the rain is received, and is
read off; after registration, the water is discharged
by a stop-cock. The objection to this form of
construction is the exposure to breakage of the
glass tube on the occurrence of frost.
In others a float is elevated by the water, and
the scale which is attached to it shows the depth
of rain received.
Perhaps the most simple is the one which I have
adopted; it is guarded from evaporation, when made
of tin is exceedingly inexpensive, and is never liable
to be out of order (Plate I. fig.6.) . A circular copper
funnel, , 12 inches in diameter, is connected by a
pipe with a vessel, b, capable of holding a gallon
or more. To the bottom of this vessel is attached a
stop-cock, c,by means of which the rain is drawn off
and measured in a graduated glass cylindrical jar,
e d (fig. 7.). The divisions of the jar may be thus
obtained : if a represent the diameter of the re-
ceiving vessel, and b that of the jar, cthe depth of
rain in the vessel, considered cylindrical, and x
the required depth of the glass jar to measure
such amount, then, since area, multiplied by the
depth, gives the volume
7854a 2 c=*7854.5 2 # ; or, # 2 =Z> 2 #; or, oc=~c.
Now, suppose the diameter of the glass jar to
188 PRACTICAL METEOROLOGY.
be 2 inches, and it is required to find what depth
of the jar will measure J of an inch, we have
Nine inches of the jar, 2 inches in diameter, will
therefore measure one quarter of an inch of rain,
received by a surface 12 inches in diameter. One
twenty-fifth part of nine inches will consequently
measure one-hundredth of an inch ; and the thou-
sandths may be estimated.
Among the contrivances which have been
numerous for registering the amount of rain
received and the time during which it falls, the
most complete is " Osier's Pluviometer/' which
will be described in Part III., among the Instru-
ments in use at the Royal Observatory, Greenwich.
Crosley's, also in use at the Observatory, is a
self-registering rain-gauge. The collected water
falls into a vibrating bucket ; as soon as one side
is full, the bucket oversets and presents another
compartment, which, having received its portion,
discharges it in an opposite direction. The bucket
is thus, during the fall of rain, kept in a state of
vibration. An anchor with pallets is attached to
the axis on which it turns, which acts upon a
toothed wheel by a process exactly the reverse of
that of a clock-escapement. This wheel commu-
nicates motion to a train of wheels, each of which
HYGROMETRIC CONDITION OF THE AIR. 189
carries a hand upon a dial-plate, and thus inches,
tenths, and hundredths are registered.
126. Position of rain-gauge. The rain-gauge,
for general registration, should be only a few feet
from the ground, and in every case its height
should he stated, as it is invariably found that
more rain is received near the surface than at a
superior elevation. Indeed, it should be agreed
upon by observers that their gauges should all be
at the same height, and all equally free from the
interference of buildings or trees. Till some rule
of this kind is adopted, we are not in a position
to compare, so accurately as we might, the quantity
of rain which falls in different districts. At
Greenwich there are several rain-gauges at differ-
ent heights above the ground.
The following table will show the differences
between the quantities of rain received by them
during 1846 and 1847 :
Height above the
ground,
ft. in.
Inches of rain
received in
1846.
Inches of rain
received in
1847.
5
O
13^46
7'12
2 4
I
o
II
51
22-63
25-86
25-29
13-02
16-49
I7-6I
Some observers prefer a small receiver, and
consider that the amount of rain may be as readily
decided with it as with a larger one. The great
190 PRACTICAL METEOROLOGY.
point to be attended to in the construction of re-
ceivers is that the area of the aperture for the
reception of the rain be very carefully ascertained ;
the circular form is the best, as it admits of the
rim being carefully turned in a lathe to the re-
quired dimensions ; the rim should project over
the body of the funnel to retain the water which
splashes up, especially when the shower falls
obliquely, and the divisions of the measuring -jar
should be accurately tested by weighing the water
received ; indeed some observers always estimate
the amount by weight, and the following con-
siderations will show that this may be done with
great accuracy and facility.
127. Weight of rain-water. A cubic inch of
distilled water at a temperature of 62 Fahr., ac-
cording to the enactment of the British legisla-
ture, is a standard of weight ; this quantity has
been determined to weigh 252*458 grains, of
which 437^ make one ounce avoirdupois ; hence
the volume of an ounce of water will be repre-
437*5
sented by inches. Let w = the number
252*458
of ounces in a quantity of water which we wish
to measure ; then, t w, will equal its
252*458
volume.
Let a represent the area of a cylindrical vessel,
HYGROMETRIC CONDITION OF THE AIR. 191
placed horizontally, and d the depth of water in
it, then the volume of this quantity = da ; con-
sequently,
da= 437 ' 5 x w
252*458
is true for any quantity whatever. Putting r for
the radius of the receiving vessel, supposed cir-
cular, and TT for 3*1416, we have
d= 437-5 x g = 437-5 x _g_.
252*458 a 252^458 r TT
The only variable quantity in this expression
is w } which may be obtained by weighing the
amount of rain received ; by combining the
others, substituting for r the radius of any vessel
we are making use of, we obtain a constant
factor, which multiplied into w will give d.
128. Example. At the Ordnance Map Office,
Southampton, the rain is estimated by weight ;
the radius of the receiver is 2'75 inches, which,
substituted for r in the formula, gives the factor
07294. On the 5th of February, 1854, 2'63
ounces by weight fell on the receiver: then
07294 x 2-63 = 0-192 inch, depth of rain fallen.
For the same shower the rain-gauge at my
observatory * gave as the amount, 0*195.
* For this comparison I am indebted to Sergeant-Major
Steel, of the Royal Sappers and Miners.
192 PRACTICAL METEOEOLOGY.
As rain-gauges are usually of 6, 8, or 12 inches
diameter, the following factors have been com-
puted for such dimensions.
129. Rule. Weigh the rain received, in an
accurate balance ; multiply the weight in ounces
avoirdupois by the factor determined for the
diameter of the receiver, and the result will be
the number of inches received.
Diameter 6 inches; factor ;o6 129.
Diameter 8 inches ; factor "03448.
6" Diameter 12 inches; factor '01532.
130. Relative amount of rain. In the Reports
of the British Association for 1851 and 1854 will
be found contributions by the author towards the
comparison of the climates of different places in
England ; those selected being Southampton, in
the centre of the southern line of coast; Fal-
mouth, on the extreme south-west ; Stone, be-
tween Oxford and Aylesbury, a central situation ;
and York, a northern position, also inland. The
fall of rain for five years is here given from those
papers, together with the corresponding quantities
at Greenwich ; the columns headed " Days " de-
note the number of days in the year on which
rain fell ; those headed " Amount " the quantity
of rain in inches.
HYGROMETR1C CONDITION OF THE AIR. 193
i
849.
i
850.
851.
Days.
Amount.
Days.
Amount.
Days.
Amount.
Greenwich . .
Southampton
Falraouth ....
Stone
CO ON "H C
U-> CO ON T
00 SO f
CO COOO C
H CO CO C
141
128
163
171
197
387
10*2
I 4 6
140
184
176
24'5
37*2
22*6
York
160
I CO
3 5
*jy
1 / y
*3
A<J ^
1
852.
]
853.
1
'otal.
Days.
Amount.
Days.
Amount.
Days.
Amount.
Greenwich . .
Southampton
Falmouth ....
Stone
153
166
I 9 I
180
34'4
497
184
146
203
216
29*0
29-2
39' 1
27'8
777
719
932
883
127-4
168-7
203-6
York
3
15
12 3
7 5
in 4
The number of days on which 1 inch or more,
and on which O5 inch or more, of rain fell within
twenty-four hours, during these years, were at
1849-
1850.
1851.
1 in.
0-5 in.
lin.
0-5 in.
1 in.
0-5 in.
Greenwich . .
Southampton
Falmouth ....
Stone . . .
I
2
2
12
21
23
4
5
4
5
7
20
18
5
3
3
2
6
9
14
7
7
York
1852.
1853.
Total.
1 in.
0'5 in.
1 in.
0-5 in.
lin.
0'5 in.
Greenwich . .
Southampton
Falmouth ....
Stone
4
8
10
16
39
32
J 9
9
4
2
3
II
21
!9
13
5
12
19
22
I
2
52
JIO
106
4f
26
York
2
194 PRACTICAL METEOROLOGY.
The greatest amounts of rain known to have
fallen in twenty -four hours during the five years
were : at Greenwich, 2'63 inches ; Southampton,
2-1 in.; Falmouth, 1'96 in.; Stone, 1-3 in.;
York, 1-99 in.
131. Deductions. From the above Tables we
may deduce a few valuable results.
. With regard to the number of falls of rain
beyond half an inch in twenty-four hours, South-
ampton and Falmouth are about equal; the
number of such days at Greenwich and Stone is
about half of those at the former places ; at York,
one quarter.
/3. The entire quantity of rain, during the five
years, is greatest at Falmouth, 203'6 inches, which
is 25 per cent, more than at Southampton, 66
per cent, above Greenwich and Stone, and nearly
double the amount at York.
7. The least number of rainy days occurs at
Southampton ; both at that place and Falmouth
the rain falls in much larger quantities than at
either of the others, which is explained by their
proximity to the sea, and confirmed by the Table
showing the number of days on which quantities
exceeding one inch, and half an inch, were
received.
On an average of thirty years it has been found
that the mean annual fall of rain at Paris is 26'6
HYGROMETRIC CONDITION OF THE AIR. 195
inches the yearly amounts varying from 16*9 to
27-9 inches.
In the torrid zone rain falls in the greatest
abundance ; in parts of India 120 inches is an
average yearly fall ; in the year 1852, at Maha-
baleshwar, 290 inches were registered, and it is
said that on the hills north of Calcutta the enor-
mous quantity of 600 inches had been reached in
one year, 25 inches having fallen in a day.
132. Snow. We are not at present able to say
whether the flakes of snow are formed at once
from the congelation of vapour in the cloud whence
they fall, or whether the particles of vapour, at
first minute, unite with other frozen particles as
they pass through the successive strata of air, and
thus gradually increase in magnitude. In the
winter of 1853-4, some observations were made
on the form of the snow-flakes, which were found
more perfect at that season than ever before
known; some photographic drawings of these
flakes were distributed among the Members of
the British Meteorological Society, for the pur-
pose of directing their attention to the subject.
On the morning of January 21, 1855, the thermo-
meter being at 19, a slight fall of snow occurred
in a perfect calm ; the flakes on this occasion were
examined by the author with a microscope, and
they presented most of those beautifully regular
o 2
196 PRACTICAL METEOROLOGY.
hexagonal forms of crystallization, which have been
hitherto remarked only in the Arctic regions.
Snow is considered to yield y^th of an inch of
water for every inch in depth : thus, if the snow
when melted and measured yields 1 inch of water,
it is concluded that the fall was 10 inches in
depth.
133. Hail. The explanations which have been
given of the phenomenon of hail are very unsatis-
factory ; that electricity is concerned in its forma-
tion is beyond a doubt. The melted hail is esti-
mated in the rain-gauge as so much rain. Ob-
servers should be careful to record the phenomena
attendant on hail-storms, such as the size and
weight of the hail-stones, &c. ; these at times have
been known to weigh more than half a pound.
Dr. Noggerath, at Bonn, found the weight of
hail-stones which fell on the 22nd of May, 1822,
to be no less than 12 and 13 ounces.
3. The Barometric Condition of the Air.
134. Fluctuations of the barometer. To the in-
dications of the barometer alone are we indebted
for any knowledge of what is taking place in the
regions of the atmosphere far above us ; the ther-
mometer and hygrometer give us local determi-
nations of the heat and humidity of that stratum
of air which is in proximity to the ground, but,
BAROMETRIC CONDITION OF THE AIR. 197
as Humboldt observes, u Important changes of.
weather do not usually arise from a local cause
situated at the place of observation itself; their
origin is to be looked for in a disturbance of the
equilibrium of the currents of the atmosphere,
which has begun afar off, and generally not at the
surface of the earth, but in the higher regions*."
The barometer marks the heavings and pulsations
of the atmosphere; the preponderance or defi-
ciency of air vertical to the place of observation ;
the changes from a less pressure to a greater,
and the reverse. We may regard the object of
deductions from barometric observations to be
twofold; the determination of the transmission
of extensive atmospheric waves, and the registra-
tion of local variations in the atmospheric press-
ure, which occur regularly in successive periods
of no great length.
135. Atmospheric waves. The former branch
of the subject has engaged the attention of Mr.
Birt, who has published the result of his investi-
gations from time to time in the " Reports " of
the British Association. The method of tracing an
atmospheric wave is by comparing the barometric
registers kept at different points on the earth's
surface, and observing the time when some re-
markable rise and depression may have been re-
* ' Cosmos,' Sabine's translation.
198 PRACTICAL METEOROLOGY.
, corded in immediate succession at several places ;
there will be frequently found, when the diurnal
and local interferences are eliminated, a wonder-
ful agreement in the amount of such variation in
pressure in situations far removed from each other ;
the depressions, or " troughs " of the wave, will
be separated from the greatest pressure, or "crest"
of the wave, by the same intervals of time, and
the same, or nearly the same, amount of baro-
metric pressure, at stations separated from each
other by distances more or less considerable ;
such waves have been traced from the western
shore of the Atlantic to the eastern, and their
identity proved.
136. Diurnal variation of the barometer. At-
mospheric waves of smaller size, and following a
law dependent upon the sun's hour-angle, have
been proved, by well-sustained observations, to be
of a definite and regular character.
That a tidal wave of air, as well as water, should
follow the moon in her daily course, was long sus-
pected; it appears to have been proved by the
observations at St. Helena since 1842, that the
attraction of the moon causes the mercury in the
barometer to stand, on the average, -004 inch
higher when the moon is on the meridian above or
below the pole, than when she is six hours distant
from the meridian. The small portion of this
BAROMETRIC CONDITION OF THE AIR. 199
effect which would reach our latitude, would seem
to be masked, at Greenwich, by the hourly fluctua-
tions of the barometer, which are far less steady
than within the tropics. The recorded diurnal
oscillations of the barometer at Greenwich have
been the foundation of a table of corrections to be
applied to observations taken at any hour of local
time, to obtain the mean height of the barometer
for the month; in the same manner as the cor-
rections for temperature explained in 47.
The corrections for the hours of 3 A.M., 9 A.M.,
3 P.M., and 9 P.M., are here given.
Local
mean time.
Jan.
Feb.
March.
April.
May.
June.
h
in.
in.
in.
in.
in.
in.
3 A.M.
+'005
-j-'OI2
+ 023
+ *OIO
+005
+ 004
9
008
008
oio
'Oil
-007
*OI2
3 P.M.
+'004
+006
+003
+009
+ 006
+ 007
9
'007
008
-015
009
-006
+ '003
Local
mean time.
July.
August.
Sept.
October.
Nov.
Dec.
h
in.
in.
in.
in.
in.
in.
3 A.M.
+ 005
+ 011
-|-'OIO
+ 'OI5
+ 008
+ '010
9
'010
008
on
'009
'005
*OIO
3 P.M.
+ 005
+ 005
+008
+005
+ '010
+ 010
9
*00 1
'010
-009
-014
-017
009
On frequent occasions I have applied these
corrections separately to the monthly means of
my 9 A.M., 3 P.M., and 9 P.M. observations, taken
at Southampton, and the results were in no case
200 PRACTICAL METEOROLOGY.
consistent ; nor is this surprising, when we regard
the situation of Southampton at the head of an
estuary which is divided into two arms by the
Isle of Wight. Most probably the local variations
of atmospheric pressure are peculiar, but what
these may be can only be determined by a far
more extensive series of observations than I have
had the leisure to undertake.
137. Maxima and Minima. At Greenwich, the
barometer readings show a double maximum and
minimum in the twenty-four hours ; unlike the
curve of mean temperature, projected in Plate IV.
fig. 2, the daily average readings of the barometer
projected for any month, would form a curve with
two ascending branches and two descending
with two apices, and two corresponding depres-
sions, while four times a day the reading of the
barometer would be at its mean value; one of
these mean readings will occur with the greatest
steadiness some time between mid-day and 2 P.M.,
according to the season.
The horary variations of the barometer within
the tropics are exceedingly regular ; they present
two maxima, at 9 A.M. and 10| P.M. ; and two
minima, at 4 A.M. and 4 P.M., which latter are
nearly the hottest and coldest hours of the day.
138. Explanation. At stations in the interior
of great continents far removed from the sea, the
BAROMETRIC CONDITION OF THE AIR. 201
double maximum and minimum disappears, and
the diurnal variation exhibits a single maximum
and minimum, similar to that of the diurnal curve
of mean temperature ; the turning-points of the
barometric curve seem nearly to correspond with
the times of greatest and least heat. The expla-
nation of this phenomenon would seem to be, that
the air superincumbent on that portion of the
earth which is heated by the sun, rises, and ex-
tending in height overflows laterally, causing
additional pressure all around, but a diminished
pressure at the base of the column, which will
reach its minimum at a period of the day not far
removed from that of the maximum of heat. As
the surface cools in the latter part of the day, the
air above it cools also and descends, receiving the
overflow from the adjacent air, which has become
heated in its turn on the rotation of the earth ;
and thus the pressure will gradually increase, and
arrive at its maximum at a period of the twenty-
four hours not far removed from the minimum of
heat.
It is presumed that the double barometric
maxima and minima, observed at stations in the
neighbourhood of an abundant supply of water for
evaporation, may be explained by separating the
pressure of air from that of water, when it would
be found that one of the maximum and one of the
202 . PRACTICAL METEOROLOGY.
minimum points would be due to the rise and
fall of the gaseous atmosphere, and the others to
the variations in the amount of aqueous vapour
mixed with it. Comparisons have been instituted
between the amount of gaseous and vapour press-
ure, and the projections of the curve of these
pressures separately, combined with the direction
of the wind, have tended to give a confidence in
this explanation, which will be increased by in-
specting the curves projected by Col. Sabine, in
the British Association's Report for 1845, from
the data supplied by the meteorological observa-
tions at Bombay.
The same principles would appear to account
for the annual variations in the barometer, the
monthly means being dependent on the amount
of aqueous vapour, combined with the gaseous
atmosphere.
139. Character of instruments. As the object
of this work, however, is rather to show how to
observe phenomena, than to enter at large upon
their explanation, we must proceed to explain the
instruments of observation ; the very subjects we
have been discussing, viz. the turning-points of
the barometric curve, will show how necessary it
is that such instruments should be capable of very
nice indications, for at Greenwich the difference
between the highest and lowest reading of the
BAROMETRIC CONDITION OF THE AIR. 203
barometer will, in some months, not amount to
O02 inch in the twenty-four hours, as far as
diurnal variation is concerned. With regard to the
times of maximum and minimum pressure, much
would seem to depend on the sensitiveness of the
instrument. The water-barometer, constructed by
the late Professor Daniell, arrived at its maximum
an hour before the standard of the Royal Society,
while this preceded a mountain barometer by the
same interval : had a marine barometer been re-
marked at the same time, it would probably have
been still further behind, as it is, from its con-
struction, the least sensitive of any. We see,
then, the importance, in all barometric registers,
of inserting a full and complete description of the
barometer employed, the size of its tube, the mode
of measurement by the scale, the maker's name,
and the fact whether it has or not been compared
with some well-known and acknowledged stand-
ard, such as the flint-glass barometer of the Royal
Society : we then have the means of judging of
the worth of a series of observations, which no
labour can render valuable if the instrument be
not one whose character will stand a severe test.
140. Corrections of the barometric reading. In
fig. 1, Plate I. we have a representation of the
barometer in its simplest form ; after the lapse of
200 years, the standard barometer, the tube of
204 PRACTICAL METEOROLOGY.
which is 1 inch in diameter, just erected at Kew
under the sanction of the most eminent physicists,
is an exact reproduction of the original experiment
by which Torricelli convinced himself of the weight
of the air. All the varieties of construction have
aimed only at ensuring, in the most accurate
way, a correct measurement of the height of the
column c d\ in arriving at this determination
with one reading of any scale, we encounter diffi-
culties and incur errors, the nature of which must
be detected and their value ascertained.
The vacuum at the upper part of the tube, if
the instrument is well constructed, though the
most perfect that can be produced, is not com-
plete, for the space above the column is filled with
the vapour of mercury, though of a very low ten-
sion dependent upon the temperature ; to prevent
the rise of particles of air which may be diffused
throughout the mercury, or may have been at-
tached to the sides of the tube, the mercury should
be boiled in the tube, and the perfection of the
vacuum may be tested by inclining the tube and
driving the mercury to the closed end, on striking
which it will give a sharp and sudden tap if no
air or moisture exist above the mercurial column.
141. Capillarity. If a piece of glass tube, not
more, we will suppose, than *4 inch in diameter,
be inserted in water, the water will rise within it
BAROMETRIC CONDITION OF THE AIR. 205
by capillary attraction to a height greater or less
according to the size of the tube, the surface of
the water within being concave ; on the contrary,
if the same tube be plunged into mercury, it will
repel the metal all around, and the surface of the
mercury within the tube will be convex, the top
of the curve being depressed below the level of
the liquid in the vessel. Now, unless the tube
of the barometer is so large * that the capillary
action may be disregarded, it is evident that a
correction must be applied to the observed height
of the mercury in the barometer to reduce it to
the true. This correction is always +, and is
usually determined by the maker ; if it be not, it
may readily be obtained from tables when the
diameter of the tube is known.
142. Temperature. The scales of barometers
adapted to scientific use are of brass throughout,
extending from the cistern to the top of the tube ;
an increase of heat will be followed by an expan-
sion both of the mercury and the scale. If the
two metals expanded equally for equal increments
of heat, no error would arise ; but mercury ex-
pands more than any other metal known. Now
supposing the atmospheric pressure to remain the
same, but that the temperature has risen within
* The diameter of the tube of the Greenwich standard is
0-565 inch ; the correction for capillarity is 0'002 inch.
206 PRACTICAL METEOROLOGY.
a given period from 40 to 60, the index would
show (at a height of about 30 inches) a rise of
0'054 inch, which would be due, not to increased
pressure, but to the excess of the expansion of the
mercury over that of the brass scale. It has been
agreed to reduce all observations to a standard
temperature, viz. 32 Fahrenheit the freezing-
point of water and for this purpose corrections
are given, as in Table VII. (Appendix), which
may be obtained by inspection.
In most barometers a thermometer with its bulb
in the cistern shows the temperature of the mer-
cury, and it is presumed that this is the same
throughout the column. Sir John Herschel* ob-
jects to this arrangement, on the ground that the
thermometer does not give the temperature of the
whole mass including the column. In barometers
lately constructed by Negretti and Zambra, the
thermometer bulb is of the same diameter as the
barometer tube, and cased in brass, so as to be, as
much as possible, similarly circumstanced; and
it is presumed that, unless in cases of sudden
changes, the temperature of the two portions of
mercury will be the same. It is advisable that
the barometer be suspended in a room whose
temperature is not liable to sudden variation, that
error may not arise from this source.
* Admiralty Manual of Scientific Inquiry.
BAROMETRIC CONDITION OF THE AIR. 207
143. Capacity. When the atmospheric press-
ure diminishes, the mercury sinks in the tube and
rises in the cistern. The height measured by the
scale, supposing it to be fixed, will not then be
the true, as its divisions presume the level of the
surface to be constant and not fluctuating in fact,
there will only be one point at which the measured
distance will exactly agree with the real distance
of the top of the column from the surface of the
mercury in the cistern. This is termed the neutral
point t and is ascertained experimentally by the
maker during the progress of construction and
engraved on the scale, together with the propor-
tion between the area of a section of the tube and
a section of the cistern. It is evident that the
surface of the mercury in the cistern will be lower
than the zero-point of the scale when the reading
is above the neutral point, from the abstraction of
a portion of its contents to supply the rise in the
tube ; and that it will be higher when the reading
is below the neutral point. If the capacities be
as one to forty-two, one forty-second part of the
difference between the neutral point and any
particular reading must be added in the former
case, and subtracted in the latter, to obtain a
corrected height.
To the barometer with which my observa-
tions at Southampton were taken, which was
208 PRACTICAL METEOROLOGY.
a very excellent one made specially for me by
Newman, it was necessary to apply the capacity
correction. I have tabulated the capacity and
capillarity corrections and index error, so that
the algebraic sum of a constant quantity and the
temperature correction, applied to the reading of
the vernier, will always give me the true height.
In lieu of the glass cistern and leathern bag
with which many barometers are supplied, a
double iron cistern with a solid bottom is intro-
duced, and, with great simplicity, the mercury is
secured for travelling by stopping off the greater
portion after the instrument is inverted. It has
accompanied me several hundred miles, and, with-
out requiring special care, has returned uninjured.
I have applied it to the purpose of measuring
heights with great success, and have the utmost
confidence in its indications. During the months
of January, February, and March, 1854, simulta-
neous readings were taken with it at my observa-
tory at 9 A.M., and with one of Barrow's (described
hereafter) at the Ordnance Map Office, Southamp-
ton : the differences between the mean readings
for these months were (mine being in excess)
024, -030, '025 ; the mean of these is -026,
exactly the difference which, by calculation, is due
to the difference of level between the two posi-
tions.
BAROMETRIC CONDITION OF THE AIR. 209
The marine barometers supplied to Government
by Mr. Adie, are so divided that the inches are
not really inches, but their representatives, each
one being an inch minus the capacity correction.
The barometer has a portion of its tube con-
tracted to a very fine bore, to prevent the oscil-
lation, or "pumping," of the mercury from the
motion of the vessel. In the barometer under
consideration, which the Governments of England
and the United States have adopted at the re-
commendation of the Kew Committee of the
British Association, a pipette, or Gay-Lussac's
air-trap, is inserted a little below the contraction,
which prevents the entrance of air into the upper
part of the tube ; the tube is fitted into the cast-
iron cylinder with cement, a portion of the upper
part of the cistern being covered with strong sheep-
skin leather, which will admit the air but not
allow the mercury to pass, so that the instrument
will sustain no injury by being laid horizontally;
the attached thermometer bulb is included within
the brass case, but does not dip into the cistern
itself. All these as they are sent out will have
been verified by comparison with the standard
barometer at Kew. The appearance of the in-
strument, whose scale is of brass, is not unlike
the usual mountain barometer.
The British Meteorological Society recommend
210 PRACTICAL METEOROLOGY.
a barometer by Barrow, of Oxenden street all
those in use among the members having been
compared either directly or intermediately with
the Royal Society's Standard, and their index
errors determined. The tube is enclosed in a hol-
low brass cylindrical case, on which is engraved
the scale, which, as usual, reads to -002 inch ; the
cistern consists of a hollow cylinder of glass closed
by a leathern bottom. A small ivory index points
downward towards the surface of the mercury, and
the first step in taking an observation is, by means
of a screw which acts on the leathern extremity of
the cistern, to adjust the level of the mercury
until it exactly touches the ivory point ; the same
action either raises or depresses the column of
mercury, and, as the extremity of the ivory point
is the zero of the scale, the reading will show the
real height of the mercurial column above the
surface of the liquid metal in the cistern, subject
to only two corrections, viz. for temperature and
capillary action. The barometer is attached to
a mahogany slab, projecting forwards about 2
inches, and is free to turn on its axis in any
direction. In reading off" the scale a moveable
ring is made to form a tangent to the curved sur-
face of the mercury in the tube, a piece of white
paper to reflect the light being placed behind
it; with very little trouble the temperature and
BAROMETRIC CONDITION OF THE AIR. 211
capillarity correction may be combined in one,
together with a small zero correction to reduce
the reading to the R. S. standard. When a table
is thus formed, the absolute height of the baro-
metric column may be ascertained from the reading
by the application of one correction only, and
that, it is presumed, with the accuracy which has
hitherto been attained by standard barometers at
three times the price.
144. Mountain Barometer. The beautiful
mountain barometers of Troughton and Sims are
adjusted by means of a screw which urges the
mercury upwards, so that it may fill the whole
tube and render the instrument portable, and it
also serves, at every observation, to adjust the
surface of the mercury in the cistern to the zero
of the scale; which, however, is not an ivory point.
In the brass box which covers the glass cistern of
mercury, near the bottom of the tube, are two slits
made horizontally, precisely similar and opposite
to each other, the plane of the upper edges of
which represents the beginning of the scale of
inches, or the zero of the barometer ; before read-
ing off the height of the column, the mercury is
brought by the screw in such a position as exactly
to shut out the light between its surface and the
edges of the slits ; the column then read off will
give the true height of the mercury. The scale of
.212 PRACTICAL METEOROLOGY.
the mountain barometer employed in determining
heights, must have a greater range of inches en-
graved on it than is necessary for one used only
for general purposes ; an extent from 28 to 31
inches will embrace all the fluctuations to which
the stratum of atmosphere, not far removed from
the sea-level, will be subject.
145. Measurement of heights. The measure-
ment of heights by the barometer is a most use-
ful application of the instrument, and various have
been the formulae proposed for the solution of the
problem ; they have been partly empirical, and
partly dependent on the principle that, inasmuch as
the heights increase in arithmetical as the densities
diminish in geometrical proportion, if we suppose
the densities to be a series of natural numbers, the
heights corresponding to the densities will be the
logarithms of those numbers. The mathematical
reasoning on the subject must be omitted in this
place* ; and we shall give one formula only, select-
ing that of La Place, which was used by Mr. Welsh
in calculating the heights reached in the balloon
ascents of 1852. It has the advantage of being
solvable by a table of common logarithms, and it
takes in every minute consideration in the correc-
tions which it demands.
* Narrien's ' Practical Astronomy and Geodesy' may be con-
sulted on the subject.
BAROMETRIC CONDITION OF THE AIR. 213
Let 2= the height required; h and h' the readings
of the barometer corrected for temperature; t and t'
the temperature of the air, at the lower and upper
stations respectively ; L the latitude ; then
2=s 20939151^ .
20886900 n
where the value of n is derived from the formula
An example worked out in full will show the ap-
plication of the formula.
November 10th, 1852, on the earth's surface,
120 feet above the sea-level, at 2 hours 30 minutes
35 seconds P.M., the barometer reading was
29-978 inches =h ; the air-temperature 49*7=/ ;
at the elevation the balloon had reached at that
instant the barometer stood at 23-45 = A', the
temperature being 35'6=' ; required the height
of the balloon, in feet, above the station at which
the corresponding observations were made :
h log 1-4768027
Ji ar. co. 8-6298572
0*1066599 1S 9-0280016
60159 4-7793006
1-0237 0-0101727
1*000638 0^0002769
^=6573 3'8775 l8
20939I5 1 ., 7-3209559
20880327 ar. co. 2-6802630
3=6591 3-8189707
214 PRACTICAL METEOROLOGY.
As it may frequently occur that an approxima-
tion to the true altitude of any position above
another may be desirable without the trouble of a
long computation, the author, after having given
some considerable attention to the subject, has
calculated the following Table, which he believes
will be found correct for heights not exceeding
5000 feet, beyond which elevations in this country
do not rise. The theorem accompanying it is
from Sir George Shuckburgh, as is also the height
of a column of air which shall cause a rise of 1 inch
of mercury at a temperature of 32, viz. 868 '5 feet ;
the other quantities are tabulated on the assump-
tion, founded on Regnault's determination, that
air expands ^ of its bulk for every increase of
1 of temperature.
Let Zj h, h'j t and t' be as before, and T the
tabular quantity opposite -it , or the mean of the
<w
temperatures of the air at the upper and lower
stations; then
_ 3 o(*-iOT
Table showing the height, in feet, of a column of
air equivalent in weight to a column of mercury
1 inch in height, at every degree of tempera-
BAROMETRIC CONDITION OF THE AIR.
215
ture from 30 to 81, the barometric pressure
being 30 inches.
Temp.
Feet.
Temp.
Feet.
Temp.
Feet.
Temp. 1 Feet.
30
865-1
43
888-0
56
911*1
69
9 34' i
3 1
866-8
44
889-8
57
912*9
70
935-8
32
868-5
45
891-6
58
9H7
71
937-5
33
870-3
46
893-4
59
916-5
72
939'3
34
872-1
47
895-2
60
918-2
73
941*1
35
873-9
48
897-0
61
919-9
74
942-9
36
875-7
49
898-8
62
921-6
9447
37
877*5
900-5
63
9 2 3'4
76
946-5
38
879-3
51
902-2
64
925-2
77
39
88n
52
93'9
65
927*0
78
950-1
40
882-8
53
957
66
928-8
79
951-8
4 1
884-5
54
907-5
67
930-6
80
953*5
42
886-2
55
99'3
68
932-4
81
To test this method I will select an example,
which may be found worked out in Mr. Sims's
treatise on mathematical instruments by Baily's
formula.
The following observations were made in the
Transit Room of the Royal Observatory, and at
the base of the statue of George II., in Greenwich
Hospital, to determine the difference of altitude.
Upper Station. Lower Station.
Detached therm. ... ji'$ = ' '
Attached ditto ...... 70 70
Barometer ............ 29-870 in. =k'
here ,= 30x^44x038
29-942
The difference of altitude as obtained by levelling
with the spirit-level (Phil. Trans. 1831, Part I.)
= 135-57 feet.
7i'5 =
216 PRACTICAL METEOROLOGY.
The observations for determining heights with
the barometer should in strictness be taken at
the upper and lower stations simultaneously ; for
which purpose two observers and two instruments
are necessary. When one person takes the ob-
servations, the barometer reading should be first
recorded at the lower station, then at the higher,
then without loss of time the lower reading should
be noted on descending; should the barometer
be steady, the mean of the two readings at the
lower position will come very near the truth.
146. Reduction to the sea-level. The Table,
page 215, will serve to reduce barometer readings
taken at any known elevation above the sea, to
what they would have been if taken at the sea-
level. Let T be the tabular number opposite the
temperature of the air ; h the reading of the baro-
meter at / feet above the sea-level, and x the cor-
rection required ;
then x =x.
T 30
Suppose the barometer to read 29-500 at 60 feet
above the sea, the air-temperature being 50 ;
what would be the reading at the sea-level ?
x 5_x^-^='o65 correction required :
900-5 30
therefore the reading at the sea-level will be
29-500+ -065 =29-565.
BAROMETRIC CONDITION OF THE AIR. 217
147. Adie's Sympiesometer. As the transport
of the barometer from place to place is attended
with considerable trouble and demands great care,
other instruments less liable to derangement have
been contrived to express the pressure of the air.
The sympiesometer of Mr. Adie of Edinburgh
accomplishes this by allowing a portion of air or
hydrogen gas to be compressed into a small cham-
ber by the pressure of the atmosphere on the
surface of a fluid, as shown in Plate II. fig. 4.
ABDC is a glass tube from 9 to 18 inches in
length, terminating upward in a bulb C, and, at
the lower end, in the cistern AB j the tube is
partly filled with oil, or coloured sulphuric acid,
which, on being urged upward by the atmospheric
pressure, compresses the air in the upper part of
the tube into a space small in proportion as the
pressure is great. To correct the error which
would arise from the change of volume in the en-
closed air produced by change of temperature, the
scale EF, which carries an index, ,is moveable, and
before reading off this index must be brought, on
the scale GH, to correspond with the air-tempera-
ture, as shown by the thermometer IK. The index
c, which is moveable on the tube, being then
brought to the upper surface of the liquid column,
will cut the scale EF at the reading due to the
atmospheric pressure at the time of observation.
218 PRACTICAL METEOROLOGY.
This instrument is much used at sea ; not that
it will compare in correctness with a barometer,
but being easily affected by atmospheric changes
it gives an early notice, and thereby directs atten-
tion to the marine barometer itself, the indications
of which are sluggish, from its contracted bore.
148. The Aneroid Barometer. This elegant in-
strument was invented a few years since by M.
Vidi of Paris. In its latest form it consists of a
cylindrical case about 4 inches in diameter and
1^ deep, in which lies a thin metal box, near to
and parallel with the curved boundary of the case,
its two ends being distant about half an inch
from each other. From this box the air has been
exhausted, and the pressure of the external atmo-
sphere on it causes it to alter its form, and to in-
crease in length to a small extent, as the pressure
becomes greater ; by the intervention of a system
of levers the small expansion is rendered very
evident ; the last of these is curved, toothed, and
fits into a pinion concentric with the index, which
turns with it and traverses a dial. The index
must first be set to the correct reading by a
standard barometer, for which provision is made
in the construction; after this the instrument
will give a very near approximation to the reading
of a good barometer, nor will this reading be
very much affected by the usual range of tempe-
BAROMETRIC CONDITION OF THE AIR. 219
rature. If the hand gets shifted by a shake, or
in any other way, it can be re-set by a turn of
the screw at the back of the instrument. Its
portability renders the aneroid an agreeable com-
panion for the tourist ; its indications will show a
rise of even a few feet, and hence, for general
purposes, its utility in determining heights, when
extreme accuracy is not required and the range is
inconsiderable, is very great.
As no correction need be applied for tempera-
ture as regards instrumental variation, a simpler
formula may be adopted than for a mercurial ba-
rometer; perhaps the best is one by Poisson,
from which Prof. Patton (in the Journ. of the
Geogr. Soc. vol. xxi.) has derived the following
rule.
Multiply the number in the following Table
opposite to the mean of the temperatures of the
air at the two places (in degrees of Fahrenheit)
by the difference of the barometric heights, and
divide by their sum. The quotient will be the
height in feet.
220 PRACTICAL METEOROLOGY.
TABLE.
o
32
52416
52
54745
o
72
57055
33
5 2 532
53
54862
73
57192
34
52649
54
54979
74
57308
35
5 2 765
55
55095
75
57424
36
52882
56
552H
76
5754 1
37
52993
57
55328
77
57658
38
53"5
58
55444
78
57774
39
53231
59
5556i
79
57890
40
53348
60
55677
80
58007
4i
534 6 4
61
55794
81
58124
42
5358i
62
55901
82
58240
43
53697
63
56027
83
58356
44
538H
64
56i43
84
58472
45
53930
65
56260
85
58539
46
54046
66
56376
86
58706
47
54163
67
56493
87
58823
48
54280
68
56609
QO
55
58939
49
54396
69
56720
89
5955
5o
54512
70
56842
9
59 r 72
5i
54629
7i
56959
9 1
59288
This Table is applicable to observations with a
mercurial barometer if they be first reduced to
a standard temperature, which may be done by
means of Table VII. (Appendix), as explained in
p. 206.
149. Heights determined by the boiling-point
of water. When the tension of vapour of water
is equal to the atmospheric pressure, ebullition
takes place; it follows that if the pressure be
reduced, water will boil at a lower temperature
than that necessary to produce ebullition under
the higher pressure. On this ground has been
BAROMETRIC CONDITION OF THE AIR. 221
founded a method of determining heights by
noting the temperature of the boiling-point at
different elevations above the earth's surface.
The formula of De Luc, reduced to English
measures, is the one generally adopted. It pro-
ceeds on the assumption that the boiling-point
will be reduced in temperature one degree for
548 feet of additional elevation; then letting H
stand for the number of English feet in vertical
height between two stations; b and b 1 the boiling-
points at the lower and upper stations respectively;
t the mean temperature of the air,
150. Example. In the Phil. Trans. Part II.
1846, will be found a paper on the subject by
Professor Christie, of the Royal Military Academy,
from which is extracted the following example:
in that paper the apparatus used is described at
full, and the results of various measurements
on the Swiss Alps recorded.
At Geneva the observed boiling-point of water
was 209-335; on the Great St. Bernard,
197" 64 ; the mean temperature of the air being
63'5 : required the height of the Great St. Ber-
nard above Geneva. By substitution, we get
H=548x 1 1-695 x 1*07=6857-5 feet.
Prof. J. D. Forbes' s observations on the Alps
222 PRACTICAL METEOROLOGY.
have convinced him that, for heights not above
12,000 ft., accuracy enough can be got by simply
multiplying 543'2 ft. by the number of degrees
that the boiling-point is below 212.
151. Barometric indications. That change of
weather is indicated by the barometer is un-
doubted ; with our present knowledge of the laws
of the atmosphere we are far from being able to
predict with certainty from its indications. A
few principles may be received as of general,
though by no means universal, application ; and
they are here given as the result of the experience
and observation of former ages; not that too
much reliance ought to be placed on them, as no
rule will hold good in every instance.
a. Changes of weather are indicated by changes
in the height of the column, and not by its abso-
lute height. However, when the mercury is low,
wind and perhaps storms may be anticipated.
/3. Generally the rising of the mercury indi-
cates the approach of fair weather; the falling of
it shows the approach of foul weather.
y. In sultry weather the fall of the mercury
indicates coming thunder. In winter the rise of
the mercury indicates frost. In frost its fall in-
dicates thaw, and its rise indicates snow.
S. Whatever change of weather suddenly follows
a change in the barometer, it may be expected
BAROMETRIC CONDITION OF THE AIR. 223
to last but a short time. Thus, if fair weather
follow immediately the rise of the mercury, there
will be very little of it ; and in the same way, if
foul weather follow the fall of the mercury, it
will last but a short time.
e. If fair weather continue for several days
during which the mercury continually falls, a long
succession of foul weather will probably ensue;
and again, if foul weather continue for several
days, while the mercury continually rises, a long
succession of fair weather will probably succeed.
. A fluctuating and unsettled state in the
mercurial column indicates changeable weather.
The following 'Card to accompany Weather-
Glasses^ is taken from the Board of Trade
Meteorological Papers, in the use of which it
must be remembered that the Barometer fore-
tells weather rather than indicates its present
state.
The Barometer rises for
North-easterly wind (including
from N.W., by the N. to the
Eastward), for dry or less
wet weather, for less wind,
or for more than one of
these changes.
Except on a few occasions,
when rain (or snow) comes
from the North- eastward with
strong wind.
The Barometer falls for
South- westerly wind(including
from S.E., by the S. to the
Westward), for wet wea-
ther, for stronger wind,
or for more than one of these
changes.
Except on a few occasions,
when moderate wind with
rain (or snow) comes from
the North-eastward.
224 PRACTICAL METEOROLOGY.
4. Electric Condition of the Air.
152. Nature of atmospheric electricity. Elec-
tricity pervades the atmosphere at all times, and
its presence may be detected with facility; its
more stupendous phenomena, as exhibited in the.
thunder-storm, are not of frequent occurrence,
but observation has shown that a certain electric
action is constantly in progress, even when the
sky is serene, though the source and the mode
of operation of atmospheric electricity are most
mysterious.
In experiments on atmospheric electricity, as
usually conducted, a pointed metallic wire is ele-
vated to some considerable height above the
earth's surface; this wire serves to conduct the
electricity within the reach of the observer, who
may then apply his tests and his measurements
for determining its nature and intensity. The
phenomena developed by electricity thus collected
from the air are precisely those of statical or
frictional electricity, and the various experiments
of the lecture-table may be reproduced by that
collected from the air.
Experiments would seem to demonstrate that
the electricity developed on the surface of the
earth is negative, while the atmosphere is in
general positively charged; the disturbance of
the equilibrium between these two opposite states,
ELECTRIC CONDITION OF THE AIR. 225
gives rise to all the phenomena of atmospheric
electricity. Sometimes, as in foggy weather, the
nature of the electric charges will be changed,
the earth becoming positive and the air negative.
During a storm of rain, though the air may
exhibit symptoms of being positively charged, the
drops of rain, on examination, will be frequently
found to be electrified negatively.
The clouds which float far above the earth's
surface may be, and indeed often are, in different
electrical conditions. Those similarly electrified
will repel, those in opposite states will attract
each other. If a stratum of dry air intervene
between two clouds in opposite electric conditions,
the equilibrium will be restored, when they are
so near each other that the force, or intensity, of
the electricity overcomes the resistance of the in-
tervening air, by a violent discharge, producing
the phenomena of thunder and lightning; as
clouds, however, are very imperfect conductors of
electricity, the equilibrium will only be established
between certain portions of the opposing clouds,
and can only be completed by successive dis-
charges ; and these are indicated by the repeated
flashes and reports which constitute a thunder-
storm.
If between two clouds in opposite electric
states a stratum of moist air intervene, their con-
Q
226 PRACTICAL METEOEOLOGY.
ditions will be equalized quietly and insensibly by
conduction ; this would appear to be the method
by which the change of condition in the lower
strata of air, which is all we are at present pre-
pared to record, is brought about, the tendency
being to equalize the positive and negative elec-
tricity that prevails, the one in the air, the other
on the surface of the earth.
Observers of the phenomena of atmospheric
electricity have endeavoured to ascertain the
nature and amount exhibited, the periods of the
day and year when the intensity is greatest and
least, and to trace, if possible, a connexion be-
tween the development of electricity and other
atmospheric phenomena ; especial care being
taken to record changes in character and intensity
during storms of rain, hail, thunder, and violent
gales of wind.
153. Quetelet's deductions. M. Quetelet has
carried on a series of daily observations on atmo-
spheric electricity at Brussels for ten years, and
some of his conclusions confirm those to which
Saussure arrived at an early period.
a. He concludes when the air is calm and clear,
and free from the interference of high buildings,
elevations, or trees, which might serve as con-
ductors, it is always charged with electricity, which
is for the most part positive.
ELECTRIC CONDITION OF THE AIR. 227
/S. That the electricity of the air attains two
maxima of intensity at a short period after sun-
rise and sunset and two minima; the first
occurring at about 2 or 3 P.M., arid the second
during the night. The times of maximum inten-
sity observed at Kew are, in the summer, 10 A.M.
and 10 P.M. ; in the winter, 10 A.M. and 8 P.M. :
the times of the minima are, in the summer, 2 A.M.
and 10 P.M. ; in the winter, 4 A.M. and 8 P.M.
7. In the winter the atmospheric electricity
exhibits greater intensity than in the summer;
the intensity in January being to that of July in
the proportion of 13 to 1.
Electrical Apparatus at the Kew Observatory.
154. The Observatory at Kew is a building the
property of the Government, which has been lent
to the British Association for the Advancement of
Science for some years, for the purpose not so
much of carrying on an uninterrupted series of
observations on atmospheric phenomena, as for
enabling the members to form a sound judgment
on various instruments submitted to trial and
comparison in that place. It was for many years
under the superintendence of Mr. Ronalds (as-
sisted by Mr. Birt), whose skill and ingenuity
have displayed themselves in various contrivances
connected with observation. It is not consistent
Q2
228 PRACTICAL METEOROLOGY.
with my design to notice all that this gentleman
has done for science, but I cannot forbear alluding
to a very neat contrivance by which the variations
of the magnet are daguerreotyped on a plate of
prepared metal, which is moved by clock-work
and thus forms an accurate register of its oscilla-
tions. The whole apparatus is very compact, and
the Observatory at Toronto has lately been supplied
with one by the British Government. The account
may be found in full in the Philosophical Trans-
actions, Part I. 1847. .
Mr. Ronalds has invented a self-adjusting baro-
meter, in which the expansion of the mercury by
heat is counteracted by bars of zinc, a metal which
expands about one-sixth part of the expansion of
mercury at the same temperature j as these dilate,
they put in motion a system of compound levers,
to which the barometer-tube is attached, and thus
the surface of the mercury is always kept at the
same distance from the zero of the scale as it
would be were it not to be subject to expansion
with an increase of temperature.
The branch of atmospheric investigation in
which the merits of Mr. Ronalds are most ex-
tensively recognized, is electricity. In the year
1843, the Observatory was completely fitted with
every requisite for judging of the electric state of
the atmosphere, and the contrivances were found
EfECTRIC CONDITION OF THE AIR. 229
to answer their purpose so well, that those adopted
at Greenwich, Toronto, and elsewhere, were copied
from them, and constructed under Mr. Ronalds's
superintendence. A description, therefore, of those
in use at Kew will be all that can be required to
give a general view of what is doing in the best
observatories throughout the British Empire, in
the registry of the electric state of the atmosphere.
Mr. Ronalds some years since completed a
contrivance by which the state of the electrometer
is registered by the daguerreotype. The pre-
pared plate of metal is suspended vertically, and is
drawn up by clock-work, and the gold-leaf electro-
meter is interposed between it and the light. On
the opening of the leaves a mark is left on the sur-
face of the metal plate, exactly at that spot which
corresponds to the time when the occurrence
took place and the duration of the electric action.
This instrument is described in the paper just
referred to. The electric apparatus now about to
be described is marked by complete efficiency, as
well as by the compactness and simplicity of all
the arrangements. The dome in which it is
located rises high above the rest of the observa-
tory, and there are no buildings or trees to inter-
fere with the full development of the electricity
existing in the air. We may therefore conclude
that the records deserve our full credence, both
230 . PRACTICAL METEOROLOGY. *
from the nature of the instruments and the cha-
racter of the observers, and from the favourable
situation in which the observations have been
taken.
Mr. Ronalds's attention was for some time
directed to experiments on " frequency " of at-
mospheric electricity, that is, the rate at which
a new charge rises to its maximum after the
former charge of an atmospheric insulated con-
ductor has been destroyed. The observations were
taken at such periods of the day as sunrise, noon,
and sunset.
For the record of the rapidly succeeding and
varied electric phenomena during the passage of a
storm, he introduced what he terms a " storm-
clock," without which it would be impossible for
one observer to register the observations. It con-
sists of a time-piece, which carries an index down
a long sheet of paper laid on a desk; this it
accomplishes in half an hour, and the observer
has simply to record the events as rapidly as they
occur opposite to the point of the index, which
can evidently be done much more readily than by
reading the chronometer and setting down the
time at successive instants. In the hurry of the
moment mistakes are often made, and several
phenomena are entirely lost ; whereas one ob-
server, by means of this contrivance, accomplishes
ELECTRIC .CONDITION OF THE AIR. 231
as much work as two could effect in the usual
method.
Plate VII. represents the dome of the Observa-
tory at Kew, with the electrical apparatus in situ ;
through the centre of the dome a circular aper-
ture has been cut, in which is fitted a mahogany
varnished cylinder, a, a. G, G is a strong
cylindrical pedestal, which serves as a closet for
articles connected with the observations. It is
surrounded by a stage, which, as well as the steps
by which the observer ascends, is detached. C, C
is the safety-conductor, for conveying the electri-
city away from the building. The principal con-
ductor, D, D, is a conical tube of thin copper 16
feet high; E is a brass tube into which it is
firmly secured; F is a hollow glass pillar, the
lower end of which is trumpet-shaped and ground
flat. A collar of thick leather is interposed be-
tween F and the table, and such is the firmness
of the whole that the conductor has resisted gales
which have uprooted trees in the neighbourhood.
H is a spherical ring carrying four arms at right
angles to each other, three of which are shown in
the engraving; I, I are two of these, k is a
lamp for warming the glass tube F, in order to
produce perfect insulation; K is a chimney of
copper, closed above, passing through the table
and entering but not touching F. By this ar-
232 PRACTICAL METEOROLOGY.
rangement the lower part of F is generally
warmed too much and the upper too little ; but
the pillar F, being conical, some zone always
exists between the two ends, which is in the best
possible state for electrical insulation. L is a
set of finely-pointed platinum wires soldered to
D. M is a Volta's small lantern. N is an in-
verted copper dish or parapluie, fitted by a collar
and stays on E, and of course insulated by F ; its
least distance from the mahogany cylinder is
3 inches. It will be seen that, by this arrange-
ment, the active parts of all the electrometers
and the conductor itself are insulated by the glass
pillar. is a Volta's electrometer, No. 1; P, Volta's
electrometer, No. 2 ; Q, Henley's electrometer ;
S, a galvanometer by M. Gougon. No. 1 is the
most sensitive, and comes into action first ; No. 2
then exhibits symptoms of electric action ; when
this has arrived at the maximum of its scale,
Henley's is found to be affected, and the record
of these three will give the force of electricity of
the air under all circumstances. R is a dis-
charger; or, as it is termed in the Greenwich
observations, a Ronalds's spark-measurer. The
length of the spark is measured by means of a
long index, which exhibits the distance of the
two balls x and y from each other on a multi-
plying scale, y being connected with a rod which
ELECTEIC CONDITION OF THE AIR. 233
is raised and lowered by means of the glass lever,
z. Each division of the scale represents one
twentieth of an inch in the length of a spark;
the divisions, of course, are not equal, and they
serve to estimate fortieths of an inch, or even
less.
The results of Mr. Ronalds' s observations have
been, from time to time, published in the Reports
of the British Association for the Advancement of
Science. In that for 1849 will be found an ela-
borate discussion by Mr. Birt of all the observa-
tions taken at Kew through a period of several
years ; to abridge this paper with advantage would
be inconsistent with the plan of this work, but refer-
ence may be easily made to it by those who wish
further information on this very important subject.
A very efficient apparatus, on the model of those
instruments in the Kew Observatory, has been
constructed at a comparatively small expense ;
the whole, mounted on a tripod stand, very light
and well adapted for transport, may be protected
by a covering of wood of sufficient height to allow
the observer to stand upright, and of such a breadth
as to allow him to walk round the apparatus*.
155. Peltier's Electrometer. The instrument
with which M. Quetelet's observations were made,
is M. Peltier's induction electrometer ; it is por-
* See Brit. Assoc. Report, 1851.
234 PRACTICAL METEOROLOGY.
table, being of small size, simple in its construction,
certain in its results, and any number may be
made perfectly comparable with each other. The
electricity obtained from the air is made to de-
flect a magnetic needle, and the arc of deflection
shows the intensity, the value of each degree having
been previously ascertained. Though thig instru-
ment was exhibited by Prof. Wheatstone at the
meeting of the British Association in 1849, and
described by him in the ' Report/ I am not aware
that it has been used in England ; yet from the
report of its performance, it would seem well to
deserve the attention of meteorologists.
OZONE.
156. Nature of Ozone. In the year 1848, Dr.
Schonbein, of the University of Bale, discovered
a new principle to which he gave the name of
ozone ; and as observations, with reference to the
amount which may be presumed, on the applica-
tion of the proper test, to exist in the atmosphere,
are now taken extensively in England, and still
more so in Germany, it demands explanation in
this place. Its name is derived from the peculiar
smell which distinguishes it, when produced arti-
ficially by the electrifying machine.
Of the nature of ozone, as it exists in the atmo-
ELECTRIC CONDITION OF THE AIR. 235
sphere or as it may be produced by electricity or
chemical combinations, various opinions are held.
Dr. Faraday considers it to be oxygen in an allo-
tropic state, that is, with a capability of imme-
diate and ready action impressed upon it ; its
discoverer is disposed to view it as a bin-oxide
of hydrogen ; as yet, the mode by which oxygen
passes into ozone is inexplicable.
157. To procure Ozone. To procure ozone, let a
piece of newly-scraped phosphorus, half an inch
long, be put into a two-quart bottle containing just
water enough to cover it to the height of a quarter of
an inch ; raise the temperature of the water to 63,
and ozone will be produced in considerable quantity
in the course of five or six hours ; the bottle mean-
while being kept closed, but not very tightly, lest
the phosphorus should inflame and burst it. The
bottle must then be rinsed with water to dissolve
the phosphorous acid, and the air that is left will
be stongly impregnated with ozone, which may
be recognized by a peculiar smell similar to, but
more powerful than, the electric odour. Ozone is
produced also when electricity is discharged into
the air from a powerful machine by a moist
wooden point; there will be a feeling as of a
current of vapour escaping, and the test, to be
presently described, will show the presence of
ozone, and at the same time the peculiar odour is
236 PRACTICAL METEOROLOGY.
recognized. Now, as electricity may be shown to
be generally more or less present and active in
the air, it is presumed to be the agent by which
oxygen is converted into ozone, or, to seize Dr.
Faraday's idea, by which oxygen is rendered more
energetic in its action. This, we shall see, has a
most beneficial effect in purifying the atmosphere ;
for ozone unites most readily with fcetid gases and
miasmata, which are thereby deprived of their
deleterious qualities. Hence the importance to
the meteorologist and the physician of studying
its properties ; it may be that we may light upon
some combinations which may render us com-
petent to reduce at will to innocent compounds
the most deadly effluvia.
158. Test for Ozone. The test for ozone may
be thus prepared : 200 parts of water, 10 of starch,
and 1 of iodide of potassium, are to be boiled
together for a few seconds; bibulous paper is
dipped into the solution, and then dried. If a
strip of this paper is exposed to an atmosphere
suspected to contain ozone, it will, should ozone
be present, assume a brown tint, which will be
turned to blue when the paper is dipped in water ;
the amount may be judged of by the time it is
exposed compared with the depth of the tint. The
ozone seizes on the potassium, and leaves iodide
of farina, indicated by the blue colour.
ELECTRIC CONDITION OF THE AIR. 237
159. Schonbein's Ozonometer. Dr. Schonbeiu's
ozonometer* consists of twelve bundles of paper,
prepared with iodide of potassium and starch ;
each bundle contains sixty strips, and serves for
one month's observations; a spare set is added
for additional observations during thunder-storms,
or whenever the air may appear to be overcharged
with electricity. At nine o'clock every morning,
a strip of the prepared paper is to be suspended
in a spot to which the air has free access, but not
the sun. It must be removed from dung-heaps,
stables, &c*, where gases are developed, which
would vitiate the observation. At nine o'clock in
the evening the exposed strip is dipped in water ;
it will be found to assume a purple tint. The
depth of this tint is compared with the correspond-
ing colour on a scale, on which there are ten
gradations, and the number is to be inserted in
the register with which it agrees in depth. Another
slip of paper must be exposed at 9 P.M., and
examined and registered in like manner at 9 A.M.
on the following morning. At the close of each
month, the mean is to be deduced, by dividing the
sum of the numbers registered, by the number of
observations.
160. Properties. The properties of ozone are
* To be obtained of the agents, Casella and Co., Meteorolo-
gical Instrument Makers, 23 Hatton Garden.
238 PRACTICAL METEOROLOGY.
the peculiar odour, resembling, when the ozone is
diluted, the electric smell when concentrated,
that of chlorine ; animals expire when placed in
it ; and it has the effect of rendering respiration
difficult and producing catarrhal effects on the
human subject. It is insoluble in water ; it dis-
charges vegetable colours like chlorine ; acts most
powerfully on metallic bodies, producing a very
high degree of oxidation. It decomposes rapidly
phosphuretted and sulphuretted hydrogen gas ;
this property is the one which combined observa-
tions will, it is hoped, tend to develope. We know
that immense amounts of deleterious gases arise
from the decomposition of animal and vegetable
matter. Ozone, even at a low temperature, com-
bines energetically with these, and neutralizes
their effects. Schonbein has proved by experi-
ments, that air containing -^-^ of ozone can dis-
infect 540 times its volume of air produced from
highly putrid meat -, that is to say, such a foetid
atmosphere may be completely purified by a
quantity of ozone equal to 3 5 4 i u d o 1 f its, volume.
Now, in bad localities, it is evident that we may
expect the test to show little or no ozone, while,
as Faraday found at Brighton, the pure air from
the ocean abounds with it. Schonbein met with
it in abundance during a storm on the Jura, and
could even recognize its smell ; so that the puri-
ELECTRIC CONDITION OF THE AIR. 239
fication of the air by storms would seem now to
be philosophically proved. The electric discharge,
of which thunder and lightning are the sensible
indications, produces in large quantities this valu-
able disinfecting agent.
161. Ozone Observations. At an interview with
Dr. Schonbein, at Bale, in 1853, I engaged to
bring the subject of ozone observations before the
meteorologists in this country, for which pur-
pose I addressed a circular to several meteoro-
logists ; and the readiness with which the matter
was taken up, especially by medical men, showed
a deep interest in it. Many observers under-
took, for twelve months, simultaneous observations
daily, at times which coincided with those taken
throughout Germany ; the whole were forwarded
to me for transmission to Dr. Schonbein.
I cannot say that the examination of these re-
ports forwarded to me, have led me to deductions
of any value to science ; my own experience, con-
firmed by that of other observers, shows a want of
accordance between observations taken at places
even at a few hundred yards from each other. In
some large towns, as London and Manchester, no
ozone is developed; and it has sometimes hap-
pened that the maximum of discoloration of
Schonbein' s test paper takes place some time before
that fixed for registering, and the colour has par-
240 PRACTICAL METEOROLOGY.
tially faded before the amount has been re-
corded.
162. Moffatt's Ozonometer. Dr. Moffatt, of Ha-
warden, Cheshire, has paid great attention to the
subject for many years; his test paper is suspended
within a large box from which light is excluded,
but which is perforated at the bottom for the
admission of air ; any arrangement, however, will
do that shelters the paper from wet and from the
sun's rays; a convenient one is Sir J. Clarke's
' Ozone cage/ made of wire-gauze ; the amount of
ozone is determined by the strength of discolora-
tion in a given time without immersion in water.
Schonbein's tests after immersion in water are
valueless ; Moffatt's papers, not being moistened,
if kept in the dark or between the leaves of a book,
may be retained for years, and it has lately been
shown that they are more sensitive than Schon-
bein's; they are consequently recommended by the
Council of the British Meteorological Society.
In the Report of the Council of the British Me-
teorological Society for 1854, Dr. Moffatt gave
various deductions on the subject of 'ozone from
his own observations, which are deserving of con-
sideration ; indeed the whole investigation is
worthy of successive observations and experiments,
especially in positions far removed from the gases
which infect the stratum of air near the ground.
ELECTRIC CONDITION OF THE AIR. 241
It is contemplated to erect, near the Kew Obser-
vatory, a lofty mast, on the summit of which the
effect of the air on the ozone test paper may be
supposed to be beyond the reach of local influ-
ences.
242
PART III.
PRESENT STATE OF METEOROLOGICAL
SCIENCE IN ENGLAND.
SEVERAL causes have of late years combined to
direct attention, in this country, to the difference
of climate, the amount of- rain, and other atmo-
spheric phenomena, on which information can be
supplied only by the unassuming labours of prac-
tical meteorologists. The presence of cholera in
England during the year 1849 led many scientific
men, especially the members of the medical pro-
fession, to inquire whether, during that unhealthy
season, any deleterious changes were traceable in
the conditions of the atmosphere. The removal
of protective laws from the produce of native
agriculture has compelled the farmer to call in
the aid of science to increase the quantity and
improve the quality of his crops, the confined
limits of our small island opposing his extending
his operations; hence he has anxiously sought
information as to the mean temperature of his
locality, in order that he might commit to the
soil those productions only to which the climate
may be considered favourable. It has followed
PRESENT STATE OF THE SCIENCE. 243
that the science of meteorology, before confined
to students of natural philosophy, has been
favourably viewed by the agricultural class of the
community, and the pages of their journals are
open to communications on that subject.
163. British Meteorological Society. At the
commencement of the year 1850, a small number
of practical meteorologists, whose attention to the
science was not of sudden growth, were induced
to take into consideration the possibility of en-
larging their sphere of operation of collecting
facts and observations in such number as to form
the groundwork for generalization.
The result of their deliberations was the forma-
tion of the British Meteorological Society, which
now numbers some hundred members, a large
portion of whom are practical meteorologists.
They have sent out some valuable reports, and
have decided upon a form of registration which
includes columns for all phenomena likely to
come under observation. Those who wish to
unite with the established corps of observers
would do well to communicate with the Secretary,
James Glaisher, Esq.*, whose services in the
cause of meteorological science are well known.
One of the most important steps taken by the
Society has been the verification of instruments
* 13, Dartmouth Terrace, Lewisham, Kent.
244 PRACTICAL METEOROLOGY.
and the comparison of their readings under simi-
lar circumstances ; the object being to remove all
sources of error arising from imperfection of
barometers and thermometers in use among the
members.
164. Registrar-General's Eeports. For some
years past the Registrar-General has subjoined to
the published weekly returns of births and deaths
in London, the results, for the week, of meteoro-
logical observations taken at Greenwich. These
include the mean daily readings of the barometer,
thermometer, and hygrometer; the difference in
temperature of each day from the mean of the
preceding forty-three years ; the force and direc-
tion of the wind; the amount of rain ; notices of
the electric state of the atmosphere ; remarks on
the amount of cloud, on sudden changes in tem-
perature, on the strength of the wind, or any
other phenomena deserving of notice ; the indica-
tions of a thermometer exposed to the full rays
of the sun ; the reading of another sunk 2 feet
below the surface of the river Thames. As these
papers have an immediate and extensive circula-
tion, and as they classify the causes of death by
referring each to its peculiar class of disease,
they supply the means of comparing the prevalence
of any one in particular with any peculiarity in the
state of the air. At the end of the year a digest
PRESENT STATE OF THE SCIENCE. 245
of the whole is regularly published on a single
sheet, for facility of reference.
The weekly returns from the Observatory at
Greenwich are incorporated, every three months,
with a digest of all the returns of births, marriages,
and deaths which have been received during that
time. The meteorological portion of this Quar-
terly Report embraces, beside, returns from about
fifty places scattered all over the country; and
such has been Mr. Glaisher's care in the com-
parison of instruments and systematic training of
all the observers, that considerable confidence may
be placed in the results. As this is the most
valuable combination of observers in the science
of which this country can boast, it may be pro-
per to explain the grounds of that confidence
which they may fairly claim.
The individuals who have undertaken the ob-
servations are, with few exceptions, either gra-
duates of one of the Universities, or fellows of
some learned Society ; it may therefore be pre-
sumed not only that they are competent to record
phenomena, but that their character would im-
press the returns with a stamp of trustworthiness
and authority. Their barometers have been con-
structed by the best makers, and have been com-
pared with the Royal Society's flint-glass standard,
either directly or intermediately. The scales are
246 PRACTICAL METEOROLOGY.
of brass, leading from the cistern throughout the
whole length of the tube. The indices read to
002 of an inch by means of the vernier, and by
estimation to -001. Before the results are for-
warded to Mr. Glaisher, the barometric readings
are all reduced to one standard temperature, viz.
32 of Fahrenheit, the freezing-point of water.
The wet-bulb and dry-bulb thermometers have
also been compared with standards, and from
simultaneous 'observations with them, are deduced
the dew-point, the tension of aqueous vapour, the
degree of humidity, the weight of vapour in a
cubic foot of air, the weight of vapour requisite
to complete the saturation of a cubic foot of air,
and the weight of a cubic foot of air of the mean
temperature and density of the month.
The thermometers indicating the greatest and
least temperature occurring in the preceding
twenty-four hours, are registered at 9 A.M. They
are for the most part of Rutherford's con-
struction; but for the maximum reading, Ne-
gretti and Zambra's maximum thermometer is
now preferred.
The rain is measured at 9 A.M., and the quantity
recorded is the produce of the preceding twenty-
four hours. The force of the wind is estimated
by some observers from the indications of Dr.
Lind's anemometer. All concur in recording, as
PRESENT STATE OF THE SCIENCE. 247
nearly as possible, the approximate value to be
given to the amount, by reckoning a gale as 6, a
calm as 0. The nomenclature of the clouds is
that first proposed by Luke Howard, Esq. A
clouded sky is represented by 10, and a clear sky
by ; the interpolations are arrived at by esti-
mation. At most of the places, too, a register is
kept of the amount of ozone.
Though some parts of England are still without
a representative, the positions of observers are
tolerably well distributed. Thus then we may
conclude, I apprehend, fairly, that well-founded
reliance may be placed on the monthly reports of
those gentlemen whose names are appended to
them, on every ground, whether we regard their
position in society, the valuable instruments in
which they have invested no small outlay, or the
unison of action which characterizes their pro-
ceedings.
The three months' observations are forwarded
to Mr. Glaisher, with every particular requisite to
reduce them to what they might be supposed to
have been had they been taken at the level of the
sea. They are then arranged in groups by him,
according to the latitude, and from them are de-
duced certain results regarding the climate of the
various parts of England, coincidences or irregu-
larities of atmospheric phenomena, and of natural
248 PRACTICAL METEOROLOGY.
occurrences, such as the arrival and departure of
migratory birds, the time of flowering of plants,,
the progress and prospects of agriculture, falls of
snow, thunder-storms, appearance of meteors and
aurorse, on all which subjects the observers are
expected to report for their own localities.
Mr. Glaisher's Quarterly Reports, diffused
throughout the community by means of the news-
papers and the scientific periodicals, have been
the means not only of spreading information
valuable to the man of science, the physician, the
agriculturist, and the engineer, but of exciting
and keeping alive an interest in the study of
atmospheric phenomena. The labour of compa-
rison and reduction is very great, and that
gentleman has received, as he has well deserved,
the thanks of all those who desire the advance
of science, for his indefatigable labours in the
cause.
165. Conference at Brussels. In consequence
of certain representations made to the British
Government by that of the United States, a con-
ference was held at Brussels in August 1853, to
bring before representatives from the maritime
states a plan which had been submitted by Lieut.
Maury, of the United States Navy, for extending
the field of research into the laws which govern
the circulation of the atmosphere and control the
PRESENT STATE OF THE SCIENCE. 249
currents of the ocean, by combining the marine
of all nations in one uniform system of observa-
tion. The methods of observation, the character
of the instruments, the scales to be adopted, all
underwent ample discussion ; the points recom-
mended by the Conference for the adoption of
their respective governments were :
a. An improvement in the construction of in-
struments, especially the Barometer and Thermo-
meter, over those inferior instruments hitherto
found on ship-board.
/3. A comparison, inter se, between the instru-
ments used by all the countries whose repre-
sentatives were present at the Conference.
7. An ' abstract log' was agreed upon, to be
filled up systematically by all the captains in the
navy of each nation, and as many captains in the
merchant service as should volunteer their co-
operation.
S. A depot was recommended to be established
in each country for the joint contributions of
every ship furnished with the suitable instruments,
so that the whole set of observations may, from
time to time, be discussed, and the results rendered
available for future navigators*.
The ' abstract log ' requires observations at least
* Report presented to Parliament and ordered to be printed,
8th of February, 1854.
250 PRACTICAL METEOROLOGY.
five times daily but recommends fourteen of the
ship's place ; direction and rate of currents ; ob-
served magnetic variation ; the direction and force
of the wind, on Admiral Beaufort's system of
registration, which is best adapted for nautical
persons (though in the papers published by the
Board of Trade the Greenwich notation, 06,
will henceforward be used) ; the height of the
barometer ; the dry- and wet-bulb thermometers ;
the forms and direction of clouds ; the proportion
of sky clear ; the hours of fog, rain, snow or hail ;
the state of the sea ; the temperature at the sur-
face ; the specific gravity and temperature at any
depth of the water of the ocean ; the state of the
weather; and an ample column is left for re-
marks, the subjects suggested for which are
tempests, tornadoes, whirlwinds, typhoons or
hurricanes, waterspouts, temperature of rain,
shooting stars, aurorse, halos, rainbows, meteors,
appearance of birds, insects, fish, sea-weed, or
drift-wood out at sea, and tidal observations;
thunder, lightning, and electrical phenomena.
The high scientific standing of those who as-
sisted at the Conference, is a sufficient guarantee
for the importance to science of these objects of
research, which have been thus enumerated in
full, as suggestive to observers in general of the
kind of phenomena worthy of registration.
PRESENT STATE OF THE SCIENCE. 251
The first movement of this truly noble assembly
was to elect M. Quetelet of Brussels, the repre-
sentative of Belgium, their President ; and at
thirteen consecutive meetings, at which from the
report it is evident that much important business
was got through, the entire subject was discussed
in detail, and decisions were arrived at only after
the most mature consideration.
166. Recommendation adopted by Government.
The British Government, following up the re-
commendation of this Conference, has created a
special department of the Board of Trade to carry
out the objects proposed, under the superin-
tendence of Rear- Admiral FitzRoy, who thus
explains the object in view : " All the valuable
meteorological data which have been collected at
the Admiralty, and all that can be obtained
elsewhere, will be tabulated and discussed in this
new department of the Board of Trade, in ad-
dition to the continually accruing and more exact
data to be furnished in future. A very large
number of ships, chiefly American, are now en-
gaged in observations, stimulated by the advice,
and aided by the documents so liberally furnished
by the United States Government, at the instance
of Lieut. Maury, whose labours have been in-
cessant. Not only does that Government offer
directions and charts gratis to American ships,
252 PRACTICAL METEOROLOGY.
but also to those of our nation, in accordance
with certain easy and just conditions. In this
country, the Government, through the Board of
Trade, will supply a certain number of ships
which are going on distant voyages with ' abstract
logs' (or meteorological registers) and instruments
gratis, in order to assist effectively in carrying
out this important national undertaking. In
the preface to a late edition of Johnston's ' Wind
and Current Charts/ published last June at
Edinburgh, Dr. Buist says, ( It has been
shown that Lieut. Maury's charts and sailing
directions have shortened the voyages of Ame-
rican ships by about a third. If the voyages of
those to and from India were shortened no more
than a tenth, it would secure a saving, in freight-
age alone, of 250,000 annually. Estimating
the freights of vessels trading from Europe with
distant ports at 20,000,000 a year, a saving
of a tenth would be about 2,000,000; and
every day that is lost in bringing the arrange-
ments for the accomplishment of this into opera-
tion, occasions a sacrifice to the shipping interest
of about 6000, without taking any account of
the war navies of the world/ It is obvious that,
by making a passage in less time, there is not only
a saving of expense to the merchant, the shipowner,
and the insurer, but a great diminution of the risk
PRESENT STATE OF THE SCIENCE. 253
from fatal maladies, as instead of losing time, if
not lives, in unhealthy localities, heavy rains, or
calms with oppressive heat, a ship properly na-
vigated may be speeding on her way under
favourable circumstances. There is no reason
of any insuperable nature why every part of the
sea should not be known as well as the laud, if
not indeed better than the land, generally speak-
ing, because more accessible and less varied in
character."
167. Instruments supplied. One of the first
steps taken by the Board of Trade in furtherance
of the object recommended by the Brussels Con-
ference, was to apply for advice and assistance to
the Kew Committee of the British Association for
the Promotion of Science. Under their auspices
a cheap thermometer and barometer have been
constructed, which not only our Government, but
that of the United States, have adopted.
Messrs. Negretti and Zambra, and Messrs.
Casella and Co., have at the low price (wholesale)
of 5s. 6d., engaged to supply any number of ther-
mometers on the model proposed by the Kew
Committee ; they are 10J inches in length, with a
range of graduation from 10 to 130 Fahr. ; the
tube is enamelled, and the divisions etched on it
with fluoric acid; the figures are stamped on a
brass scale, or else marked on a porcelain scale by
254 PRACTICAL METEOROLOGY.
Messrs. Negretti and Zambra's new process, and
the whole is enclosed in a copper case.
One of the conditions of supplying the Govern-
ment demand was that each instrument should be
examined and tested at the Kew Observatory ; the
numbers in the course of verification there at the
latter part of 1854, were, for the United States'
Navy 1000 thermometers and 50 barometers,
for the Board of Trade 500 thermometers and
60 barometers. I have in my possession one of
these thermometers by Casella, numbered 1020
and marked K. 0., to indicate its having passed
the ordeal, and the very small corrections which it
requires(viz. at 32, -0-3; at 42, -0'l; at 52,
+0-1; at 72, 0; at 92, -0'l) will clearly
indicate the important advance toward correct
instrumental appliances to which this movement
has given rise.
The barometer selected has already been de-
scribed, p. 209 ; its price, including the cost of
packing case, the brass arm for suspension, and
10s. for verification at the Observatory, will be to
Government when supplied wholesale, 3 15s. 6d.;
this instrument has also to undergo severe scru-
tiny, and to be compared with a new standard
which has lately been erected in the Kew Obser-
vatory. The diameter of the tube of this standard
is one inch ; the zero-point is brought down to
PRESENT STATE OF THE SCIENCE. 255
touch the surface of the mercury in the cistern,
and the height of the column is determined by two
telescopes moving on a vertical brass scale at the
distance of some feet from the tube containing
the mercury. The point which is brought to the
surface of the metal in the cistern is at a known
and well- determined distance from a cross above
it ; in the lower telescope a horizontal wire is
made to bisect this cross, and in the upper another
wire measures the distance from the cross to the
top of the column.
A large receiver, in which the atmospheric
pressure is varied at pleasure, is used for com-
paring the various barometers, the whole being
under the superintendence of Mr. Stewart; through
the Kew Observatory facilities are granted to
private observers, as well as public institutions, for
having the errors of their instruments determined,
before entering upon a course of registration of
atmospheric phenomena. Here also may be ob-
tained, at a moderate cost, standard thermometers
which have been divided on the stem with great
accuracy, and with the minutest regard to every
consideration which may tend to bring the instru-
ment as near as possible to perfection.
The Royal Observatory , Greenwich.
168. For a period of not less than 160 years,
256 PRACTICAL METEOROLOGY.
systematic observations of the places of the sun,
moon, planets, and fixed stars, have been recorded
at the Royal Observatory with an accuracy not
surpassed, if indeed equalled, elsewhere. The
Government of this country have evinced a sound
discretion in the appointment of the most eminent
mathematicians to the office of "Astronomer
Royal," an office now held by G. B. Airy, Esq.,
whose mind is ever actively engaged, not only in
sustaining the character of the Observatory for
accuracy of observation by the introduction of
instruments superior to any hitherto employed
(witness the new 14-feet transit-circle, and the
alt-azimuth circle), but in adding to its efficiency
in every collateral branch of science. Under his
auspices the magnetical and meteorological ob-
servatory was originally established ; and of late,
the new system of automatic registration of the
magnetical and meteorological instruments by
means of photography (invented and brought to
perfection by Charles Brooke, Esq., M.B., F.R.S.),
has been introduced, and now forms a most
striking feature in that department of observation.
Successive Governments have shown a liberality
in promoting the objects for which the Observa-
tory was originally founded, namely to assist the
navigator in traversing the pathless ocean.
In the year 1837 it was determined to erect a
PRESENT STATE OF THE SCIENCE. 257
magnetic observatory, for the purpose of investi-
gating the laws of magnetism, on the full under-
standing of which the mariner's compass depends
for improvement, and the chart by which the
navigator is guided for its accuracy ; conjointly
with these investigations an elaborate system of
meteorological observation was commenced, in
the expectation of discovering some of those
causes which produce the variations in the condi-
tions of the atmosphere, a kind of knowledge
auxiliary to navigation, in which so much depends
upon that variable element, the wind. Green-
wich, moreover, was understood to be well ap-
pointed in a trained corps of observers, renowned
for the accuracy and care which they had em-
ployed in the most exact science ; and the pub-
lished Reports, which originated in 1840 and
have extended to 1851, have proved the wisdom
of the choice of that locality for magnetical and
meteorological instruments.
The magnetic observatory is a small detached
building, its nearest angle being' 230 feet from
the nearest part of the astronomical observatory,
and 1 70 feet from the nearest outbuilding ; the
material is wood, and iron has been carefully ex-
cluded from its construction ; the form is that of
a cross, with four equal arms nearly in the direc-
tion of the cardinal magnetic points ; its extreme
s
258 PRACTICAL METEOROLOGY.
length and breadth are each 40 feet, and the
breadth of each arm, which is 10 feet high, is 12
feet. The only iron to be found throughout the
whole building is in the fire-grate in the ante-
room, the mean-time clock, the sidereal-clock, and
the check-clock.
Though the magnetical instruments do not
form a part of the meteorological establishment of
the Observatory, strictly speaking, yet, as their
movements are registered photographically, and
in combination with those devoted to atmospheric
changes, I consider it necessary to say somewhat
respecting them, in connexion with those which
record more particularly atmospheric variations.
On Photographic Registration of Meteorological
Phenomena.
169. On this subject I shall endeavour to be
sufficiently explicit to convey a clear notion of the
ingenious contrivances by which automatic regis-
tration is attained ; but it is not my intention to
describe them so minutely as would be requisite
were 1 about to give directions for their construc-
tion. Those who may wish to adopt a similar
apparatus, may consult advantageously the Philo-
sophical Transactions, Part I., 1847, and Part I.,
1850; and the Greenwich Magnetical and Meteoro-
logical Observations, 1847.
PRESENT STATE OF THE SCIENCE. 259
On the first introduction of photographic re-
gistration by Mr. Brooke, that gentleman adopted
the light of a camphine lamp, as producing the
most powerful photographic effect. For this has
now been substituted a mixture of common coal-
gas and naphtha, which is found to be quite equal
in brilliancy, and far more manageable. The gas,
on admission into the magnetic observatory, is
received into a tin box divided horizontally into
two compartments, the lower of which contains
water, and half of the other is filled with naphtha*
This upper half is partitioned off into eighteen
cells by vertical divisions, each attached alter-
nately to different sides of the box.
Fig. 3, Plate VIII. is a section, and fig. 4 a plan
of this box or receiver ; c is the portion partially
filled with water, which is heated by the jet of
gas /; d is the naphtha compartment, half-filled
with 'that substance; as the water heats the
naphtha, the upper part of the compartment at e
becomes filled with vapour, and the gas entering
at a traverses the compartment in the direction
of the arrows, as shown in fig. 4, unites with this
vapour, and the two gases mutually diffused issue
at b, and thus combined are distributed through-
out the building.
The paper on which the photographic trace is
received is a strong woven paper, of equal texture
260 PRACTICAL METEOROLOGY.
throughout ; in manufacturing it, all foreign sub-
stances which might combine injuriously with the
chemical substances used in its future prepara-
tions have been carefully excluded.
A sufficient quantity of paper for the consump-
tion of three or four weeks is treated in the fol-
lowing manner : " To a filtered solution of 4
grains of isinglass in 1 fluid ounce of boiling-
distilled water are added 12 grains of bromide of
potassium and 8 grains of iodide of potassium.
The solution, either when hot or cold, is evenly
laid upon the paper with a camel's-hair brush, in
such quantity as thoroughly to wet its surface,
but not to run off; the paper is then dried
quickly before the fire. The paper thus treated
is preserved by keeping it in a dry place and in a
drawer.
" When a cylinder is to be charged with photo-
graphic paper, the room is darkened and illumi-
nated only by a candle, whose flame is surrounded
by a cylinder of yellow glass. The paper is laid
flat in an earthenware dish, and is washed with an
aqueous solution of nitrate of silver, made by dis-
solving 50 grains of crystallized nitrate of silver
in 1 fluid ounce of distilled water, which is laid
on in quantity not sufficient to run. The paper
is then in a state fit to be placed upon the
cylinder.
PRESENT STATE OF THE SCIENCE. 261
" When the paper is to be taken off the cylinder,
the room is illuminated in the same way, the
cylinder is detached from its mounting, the ex-
ternal cylinder is drawn off, and the paper is un-
folded and laid flat in a dish. In this state it
exhibits no trace of the action of the light. It is
then washed with a solution of gallic acid, to
which a few drops of acetic acid are added, till it
is moderately wet all over; the impression begins
soon to appear, and in a few minutes acquires its
full strength. The paper is then repeatedly
washed with water till the water runs off quite
clear. Solution of hyposulphite of soda (formed
by dissolving 1 drachm of the hyposulphite in
5 ounces of distilled water) is then poured upon
it, and water is added in considerable quantity ;
after this has remained about five minutes, the
paper is washed repeatedly with water. The
trace is then securely fixed, and light may be
admitted into the room. The sheets are then
usually preserved for gradual drying within the
folds of linen towels."
The cylinders alluded to in the above extract
are those around which the paper is wrapped to
receive the photographic trace. They are, in fact,
French glass-shades (such as are used to protect
works of art), 11 inches in length, and 14J
inches in circumference. The shade, after having
262 PRACTICAL METEOROLOGY.
been blackened in the inside, is cemented into a
cap 1 inch deep, having a brass pin projecting
from the centre. A second shade, a little larger
than the former, is placed over the paper when it
has been attached in a moist state to the first ;
this latter cylinder is kept in its place by a few
turns of tape round the collar part, which is
moistened with water; damp list is also placed
between the hemispherical parts of the shades.
This provision is necessary to prevent the paper
from becoming dry during the time it is subject
to the photogenic action, for dryness would very
materially its sensibility. When the axis of
the cylinder is required to be horizontal, as in
the registration of horizontal movements, the pin
which is in the line of the axis, and the cylinders
themselves, rest on friction-rollers ; a bent wire on
the axis is caught by a fork attached to the hour-
hand of a time-piece, which is about the size of a
ship's chronometer, and thus the cylinder is
carried round once in twelve hours, or any other
period which may be determined, with such
smoothness and ease as not to alter the rate of
the time-piece in the slightest degree. One-
twelfth of the circumference of the cylinder will
evidently measure one hour, and about j^th of an
inch will be the measure of five minutes of time.
Fig. 1, Plate VIII. a, vertical cylinder charged
PRESENT STATE OF THE SCIENCE. 263
with photographic paper; b, wooden cap; c t
central pin.
Fig. 2, d, the paper unwound and divided into
twelve parts, marking the hours ; e, f, the trace
of the movement of the mercury in the barometer
during that time ; g g, (g' g 1 in fig. 1), photogra-
phic base-line.
The time-piece, in moving the vertical cylinder,
lies flat underneath it. In the case of a hori-
zontal cylinder it is placed with its face vertical,
and facing the cap (see fig. 5, B).
For the sake of convenience, each cylinder is
made to perform double duty. The barometer
and the vertical-force magnetometer are registered
on the same cylinder, and their traces are allowed
to cross each other in opposite directions, which,
with a careful adjustment, can easily be effected
without interference. The declination and hori-
zontal-force magnetometers are registered in the
same way, on a cylinder whose axis is horizontal ;
and the dry- and wet-bulb thermometers share a
cylinder between them.
To describe now the manner in which the pho-
tographic trace is left, commencing with the de-
clination magnet. The light by which the trace
is made, is placed slightly out of the direction of
a straight line joining the suspension-skein of
the magnet and the centre of the photographic
264 PRACTICAL METEOROLOGY.
sheet. The chimney which covers the light (a jet of
gas united with the vapour of naphtha) is perforated
by a slit j 3 ^ths of an inch long and yj^th of an inch
broad (see fig. 5, d) ; the light from this slit falls
on a metallic concave mirror (e) which is carried by
the suspension apparatus of the magnet, and moves
with it ; by it the light is made to converge about
the centre of the cylinder of photographic paper,
at a distance of nearly 12 feet. To reduce the
image of the slit to a neat spot of light, a cylin-
drical lens of glass, b, is interposed. Now, as the
magnet, and with it the mirror, turns in azimuth,
the image of the slit runs along this lens ; and, at
whatever part it falls, it is concentrated into a
definite and brilliant spot of white light, which
leaves a photographic trace on the prepared paper ;
and as this is constantly carried round by the
time-piece, the effect produced will be a continu-
ous Hoe around the cylinder, c, with deviations to
the right or left indicating the horizontal move-
ment of the magnet.
As in practice it is found that the length of the
paper is not always the same, it is therefore
necessary to have a time-scale for each portion
after it has been detached from the cylinder ; this
is effected by shutting-off the light for an instant,
which causes a break or light space in the photo-
graphic trace. The time is noted accurately, and
PRESENT STATE OF THE SCIENCE. 265
the same thing is repeated, we will suppose, one
or two hours afterwards; the distance between
these breaks supplies data for a time-scale for
that special register.
To measure the ordinates from this time-scale,
which may be considered as a line of abscissae,
the actual deviation of the magnet at particular
instants, four times daily, is read off by a theodolite
(carefully adjusted for the purpose) in degrees,
minutes, and seconds of arc ; these readings, com-
pared with the length of the ordinates at those
times, supply the means of reducing all the others
to the same standard.
As it can never be expected to obtain glass
cylinders with perfectly cylindrical surfaces, or
perfect surfaces of revolution, there is a probabi-
lity that the line of intersection of a plane per-
pendicular to the axis of the cylinder with the
paper on the surface, will not be a perfectly
straight line when the paper is opened out. To
obtain a base-line on each sheet the following
plan is adopted : An independent ray of light,
impinging perpendicularly to the axis of the
cylinder from a light 6 inches distant, is re-
ceived by the cylindrical lens, and marks a strong
line all round the cylinder, which, when the paper
is unrolled, becomes the line of abscissae on which
the times are set off; while perpendicular ordi-
266 PRACTICAL METEOROLOGY.
nates from it will be proportional to the move-
ment which is the subject of measure (see g g,
in fig. 2).
The arrangements for the horizontal-force mag-
net are precisely the same as those described for
the declination magnet. Every part of the cylin-
der apparatus, except that on which the light falls,
is covered with a double case of blackened zinc,
having a slit on each side on the same horizontal
plane as the axis of the cylinder ; and every part
of the path of the photographic light is protected
by blackened zinc tubes from the admixture of
extraneous light.
The vertical-force magnet traces its line of
movement by reflected light on a cylinder charged
with paper, whose axis is vertical ; the other por-
tions of the apparatus resemble so nearly that
already described that a further account is unne-
cessary. On the east side, the same cylinder re-
ceives the trace of the barometer. At the di-
stance of 30 inches is a large siphon barometer,
the bore of the upper and lower extremities of its
arms being about 1^ inch ; a glass float in the
quicksilver of the lower extremity is partially sup-
ported by a counterpoise acting on a light lever
(which turns on delicate pivots), so that the
quicksilver constantly supports a definite part of
the weight of the lever. This lever is lengthened
PRESENT STATE OF THE SCIENCE. 267
(see fig. 6), to carry a vertical plate of opaque
mica with a small aperture, whose distance from
the fulcrum is so regulated with regard to the
distance of the point of action of the float-wire,
that its movement is four times the movement of
the column of the cistern -barometer. Through
this hole the light of a gas-jet, collected by a
cylindrical lens, shines upon the photographic
paper. Another pencil of light from the same
jet shines through a fixed aperture, with a small
cylindrical lens, for tracing a photographic base-
line upon the cylinder of paper, similar to that for
the cylinder of the declination magnet.
Such parts of the apparatus adapted to pho-
tographic registration of the declination mag-
net and barometer only are shown in the engra-
ving, as are requisite to explain the mode of its
action.
In fig. 5, A is the cylinder covered with pho-
tographic paper, the axis of which is horizontal ;
B is the time- piece which gives it a rotatory motion;
b is a cylindrical lens, bringing to a point the
light from the jet d, which has been reflected by
the mirror e ; / is the magnet suspended by the
silk thread g ; as this turns in azimuth, the mir-
ror e turns with it, and the reflected image of the
slit in the chimney covering the jet of light runs
along the cylindrical lens, by which it is brought
268 PRACTICAL METEOROLOGY.
to a point on the paper, on which it leaves a trace,
as shown at c. In the course of one rotation of
the cylinder this trace will have gone all round
it, with deviations to the right or left indicating
the movement of the magnet in azimuth during
the time occupied by the rotation.
Fig. 6 shows the arrangement of the barometric
apparatus. Q e is a lever whose fulcrum is e, the
counterpoise /nearly supporting it; s is an opaque
plate of mica, with a small aperture at p, through
which the light passes, having before been refracted
by a cylindrical lens into a long ray, the portion
only of which opposite the aperture p, impinges
on the paper ; d is a wire supported by a float on
the surface of the mercury; G, H is the baro-
meter; P, the vertical cylinder charged with
photographic paper ; r, the photographic trace ; I,
the time-piece, carrying round the cylinder by the
projecting arm t.
It is evident that the respective distances of the
float and the aperture p from the fulcrum may be
regulated so that the rise and fall of the float may
be multiplied to any extent required. At Green-
wich, the extent of the photographic record is
four times the actual rise and fall of the mercury
in the cistern. These contrivances were shown
in the Great Exhibition of 1851, Mr. Brooke
having supplied his apparatus.
PRESENT STATE OF THE SCIENCE. 269
The dry- and wet-bulb thermometers are re-
gistered by the same means as the instruments
* already described. They are very large, for ther-
mometers of the usual size would not sufficiently
shut off the light. The fluid employed is quick-
silver, and the bore of the tube is ^ths of an inch ;
the tube is cylindrical, and 8 inches long ; the bulb
of the wet-bulb thermometer is covered, in the
usual way, with muslin, to which moisture is
communicated by the capillary passage of water
through lampwicks. They are capable of elevation
by means of a coarse screw, so that the mean tem-
perature for the time of observation may be
brought near the centre of the cylinder ; but the
bulbs are so adjusted as to stand about 4 feet from
the ground, the small variation in height being
simply for the purpose of having the trace recorded
upon a convenient part of the paper. Plates cover
the thermometer-frames, with apertures so narrow
that the column of mercury shuts out the light.
Across these plates a fine wire is placed at every
degree, and a coarser wire at every 10, and also
at 32, 52, and 72, so that there may be no
chance of mistaking the reading of the degrees of
temperature. The light of a jet of gas is condensed
by a cylindrical lens whose axis is vertical, into a
well-defined line of light, which shines through
the thermometer-stalk upon the cylinder of paper,
270 PRACTICAL METEOROLOGY.
which is close to it. As the cylinder of paper
revolves under this light, it leaves a broad sheet
of photographic trace, the breadth of which varies
with the varying height of the quicksilver in the
thermometer-tube ; but, inasmuch as the light is
intercepted by the wires placed across the tube at
every degree, there are spaces traced by the wires
in which there is no photographic action. These
appear on the paper in the form of light lines on
a dark ground, and serve the purpose of reading
off the thermometers, which is facilitated by the
broader lines marking the decades of degrees ; nor
is any photographic base-line needed, for the wires
form the only register required. The cylinder
receives the trace of the wet-bulb on one side, and
of the dry on the other. Its axis is of course
vertical, and it is made to revolve once in forty-
eight hours ; the paper, when removed, will there-
fore show the variation of both thermometers
during the last twenty-four hours, one-half of the
photographic trace being due to the dry-bulb, the
other to the wet-bulb thermometer. The circum-
ference of this cylinder is 19 inches.
Such, then, are the arrangements for the auto-
matic registration of meteorological and magnetical
instruments now introduced into the Greenwich
Observatory, and their value, as indicating the
minutest movements, is very great; while the
PRESENT STATE OF THE SCIENCE. 271
labour of watching each instrument and recording
its variations every two hours is entirely dispensed
with. The consequence of the introduction of self-
registration has been that two observers are more
efficient than four under the old system. "We are
not aware, as yet, of the effect of time on the pho-
tographic trace ; to ensure permanency, therefore,
the variations of the instruments are inked in by
a definite line along the edge. The papers are
kept carefully arranged in the daily order, and
ready for immediate reference, with the other
records of the establishment.
Of the radiation thermometers, which measure
the amount of heat radiated from the earth's sur-
face, of those sunk beneath in the soil, of the ther-
mometer 2 feet below the surface of the river
Thames, and of the actinometer, which measures
the direct heat of the solar rays, little need be
said ; I therefore pass on to the
Anemometers.
170. To have the means of registering the
amount and direction of the wind for every hour
of the day had long been a desideratum with
scientific men, and much ingenuity has been
shown in the mechanical contrivances which have
been entered upon for that purpose. The instru-
ment which has met with the gredtest approba-
272 ^ PRACTICAL METEOROLOGY.
tion in England is Osier's anemometer, one of
which has been erected lately at the new Royal
Exchange, and another has been in use at Green-
wich for many years ; its indications are constantly
recorded and are considered by competent judges
to be very trustworthy, the instrument having
undergone various changes and improvements
since its first erection. The instrument traces on
a sheet of paper the direction and pressure of the
windj and the amount of rain which may have
fallen in twenty-four hours. A copy of this
register is shown in Plate IX. The anemometer
itself consists of a vane V, turned by the wind,
attached to a hollow vertical spindle WX : the
paper is divided longitudinally by lines, the central
showing the direction of the wind marked S.,
W., N., E., S.; the upper part receives the trace
which indicates the amount of rain; the lower
part shows the amount of pressure of the wind on
a square foot of surface exposed to its full force.
The register paper is placed on a board M, and
accurately fixed every day at 10 A.M. This board
is carried along by the clock shown at C, at the
rate of about an inch per hour. The engraving
shows the original contrivance to effect this object,
but in consequence of the continual failure of this
chain-apparatus, another construction has been
adopted ; the movement of the board has now been
PRESENT STATE OF THE SCIENCE. 273
effected at Greenwich by rack- work connected with
the pinion of a clock.
The pencil 1 is the index of direction; this
pencil is operated upon by the vane V, turning the
hollow spindle ; there is a pinion at r, which, as
the vane turns in the direction of the wind, acts
on the rack -work of a transverse bar, e /, and so
causes it to move on the one side or the other.
The centre of the board lies due north and south;
if, therefore, the wind blows from the north for
twenty-four hours, it is evident that the trace
will be along the centre of the board throughout
its whole length ; if the wind at a certain time
veers to the east, the transverse board, and with
it the tracing-pencil 1, will be turned aside by the
action of the pinion in the cogs, and the line now
described will be parallel to the direction of the
other, at a distance from it equal to one-fourth of
the number of cogs which would come into action
at an entire revolution of the spindle ; the trace
in this direction will continue till the wind again
shifts, and the number of horary divisions through
which it extends will show the time during which
the wind was blowing from that quarter.
The first adjustment for azimuth was obtained
by observing, from a certain point, the passage of
a star behind the vane-shaft, and from that ob-
servation computing the azimuth ; then, on a calm
274 PRACTICAL METEOROLOGY.
day, the vane was drawn by a cord to that position,
and the rack was so adjusted that the pencil's
position on the sheet corresponded to that azimuth.
For the pressure of the wind the shaft of the
vane carries a plate 1 foot square (T, in Plate IX.),
which is supported by horizontal rods n, m, sliding
in grooves ; this plate is urged in opposition to
the wind by three springs enclosed in the box t,
so arranged that only one comes into play when
the wind is light ; and the others necessarily act
in conjunction with the first as the plate is urged
more and more forcibly by the wind. A cord from
this plate passes over a pulley and communicates
with a copper wire running down the centre of
the spindle, which is finally brought to pull upon
the spring-lever v, and thus the pencil 2, which is
attached to it, is drawn in a direction transverse
to the motion of the board, the further from its
zero line in proportion to the force with which the
plate is driven back by the wind. A series of lines
numbered 2, 4, 6, 8, &c., shows the amount of
pressure on the square foot ; the intervals of these
lines are adjusted by applying weights of 2 Ibs.,
4 Ibs., 6 Ibs., &c., to move the pressure-plate in the
same manner as if the wind pressed it. The pin 3
registers the amount of rain, which is thus re-
corded. The water which has been collected by the
gauge passes into the vessel , which is supported
PRESENT STATE OF THE SCIENCE. 275
by spiral springs b, b, which shorten as the
quantity increases ; into the bottom of this vessel
is fixed a tube c, open at both ends, in a vertical
position, over the top of which is loosely placed a
larger tube e, closed at the top ; when the water
has risen to the level of the inner tube it begins
to discharge itself gradually into a tumbling
bucket d, which is enclosed in a globe under the
receiver ; when full, the bucket falls over and dis-
charges its contents, which run through the waste-
pipe /, and cause an imperfect vacuum in the globe,
sufficient to produce a draught through the pipe c,
which thus acts as the longer leg of a siphon, and
the water continues to flow from the receiver
through the interval of the two pipes c and e till
the whole is drawn off, when the spiral springs
b, b immediately elevate the receiver to its ori-
ginal position.
Now, if we suppose the quantity of water
necessary to produce the action thus described to
be equivalent to one-fourth of an inch of rain, the
mode of registration will be easily understood.
The pin 3 is connected by means of the cord y, g,
with the receiver, which cord is kept tightened by
the spring h ; as the apparatus descends from the
weight of water during the fall of rain, this pin
advances further and further from the zero of the
scale which is shown upon the registering paper,
276 PRACTICAL METEOROLOGY.
until a quarter of an inch has fallen, when, as this
is drawn out and the receiver ascends, the pin is
drawn back to its original position, and the same
process is repeated.
The register represented in Plate IX. is sup-
posed to record the phenomena of twenty-four
hours. It will be seen that rain continued to fall
for nearly four hours, when, a quarter of an inch
having been received, the trace was brought back to
the zero line ; five hours afterwards another quar-
ter of an inch had been collected ; in two hours
more about two-tenths of an inch, when the rain
ceased, and none fell for four hours, as is denoted
by the line traced parallel to the zero line ; rain
then fell for an hour; a cessation of three hours
followed; two hours after another quarter of an inch
was collected ; and, for the remainder of the time,
a gentle fall is indicated by the gradual departure
of the trace from the zero line ; the amount of rain
collected in the twenty-four hours will therefore
in this case be '25 + '25 + '25 + -06 = -81 inch.
On the same paper the traces of the force and
direction of the wind may be seen and readily
understood. The point of the compass from which
the wind blew at any hour is registered along or
near the centre of the paper, and the force at the
lower part ; the zero being the bottom line, and
the increase of force being indicated by the de-
PRESENT STATE OF THE SCIENCE. 277
parture of the trace from this line towards the
inner portion of the paper.
These explanations serve to exhibit the general
principles on which this beautiful apparatus is
constructed, though the details may occasionally
differ. The anemometer and pluviometer have
been many years in use at Greenwich, and their
registrations are considered very satisfactory. The
noble building of the Royal Exchange has been
supplied with an anemometer on the same con-
struction, except that the register is vertical ; and
the anxious merchant, by inspecting the register,
can easily satisfy himself whether the wind of the
preceding night or day has been favourable to the
arrival of some richly-laden vessel, of which he
may be in daily expectation.
171. WhewelTs Anemometer. Another anemo-
meter, invented by the Rev. Dr. Whewell, Master
of Trinity College, Cambridge, is likewise in con-
stant action at Greenwich ; it is also self-register-
ing, and indicates the rate of movement of the air
and the directions in which that movement takes
place. A horizontal brass plate (Plate X.) is con-
nected with a vertical spindle, which passes through
the axis of a fixed cylinder, having a vertical bear-
ing upon a plate at the bottom of it, and a collar
bearing in a horizontal plate at the top of the
cylinder. The vane, V, turns the whole of the
278 PRACTICAL METEOROLOGY.
apparatus above the cylinder, which consists of a
fly, F, and a system of wheels working into each
other ; as the fly turns round, with greater or less
rapidity according to the motion of the air, these
wheels are set in action, and communicate motion
to a vertical screw 15 inches in length ; the revolu-
tion of this screw causes a pencil, P, which is con-
nected with a nut, to descend. The cylinder, C,
is covered with paper, on which are marked the
points of the compass, and on this the pencil
leaves a trace, the length of which is proportioned
to the force of the wind. The fly has eight sails,
like those of a windmill, inclined at an angle of
45 to the direction of the wind ; upon the axis is
an endless screw, which works a vertical wheel, of
100 teeth ; another endless screw on its axis works
a horizontal wheel, of 100 teeth, which is attached
to the great vertical screw, S, by which motion is
given to the pencil ; the descent of this pencil is
measured by a vertical scale, and a calculation is
made, from accurate measurements of the different
parts of the apparatus, of the amount of horizontal
movement of the air which is due to an inch of
the screw's downward movement. The following
are the measures of the principal parts of this
anemometer :
Length of each sail from axis to end 2^30 in.
Length of the flat part of each sail i -92 in.
PRESENT STATE OF THE SCIENCE. 279
Inclination of each sail to the wind. . , 45
Forty-five revolutions of the vertical screw
correspond to 2 in.
Number of teeth in the vertical wheel ... 100
Number of teeth in the horizontal wheel ... i oo
Therefore, 10,000 revolutions of the fly cause the
pencil to descend through the distance of one
thread of the vertical screw, or through a space
equal to ^jths of an inch = O044 in.
Assuming that the effective radius of the sail
is 1'7 in.,
in.
Tie circumference described is 1 7 in. x 2?r= 10-68
Therefore the motion of the wind in one
revolution is 1 0*68
In 10,000 revolutions 106,800
corresponding to '044 in. of the vertical screw, or
to one revolution of the screw. From this it
follows, that the motion of the wind corresponding
to the descent of the pencil through 1 inch is
200,250 feet, or 37'9 miles.
The results of Osier's anemometer give the
force and direction of the wind, and those of
Whewell's give the amount of horizontal move-
ment in the air, for twenty-four hours ; these are
amongst the weekly published results of the
Greenwich observations. There is also Robin-
son's anemometer at work at Greenwich.
280 PRACTICAL METEOROLOGY.
We have thus then taken a view of the instru-
mental means and the organization with which, in
England, we are provided for the purpose of re-
cording atmospheric phenomena ; nor are these all,
for at Oxford, under the late Manuel J. Johnson,
Esq., of the Radcliffe Observatory, an efficient
system of photographic registration was intro-
duced, the arrangements for which differ in several
respects from those adopted at Greenwich. The
bulbs of the dry and wet thermometers are in the
open air, but the tubes are led horizontally through
the wall of the building, within which they are
bent suddenly in a vertical direction ; the mercury,
as it rises or falls, cuts off the light which shines
through a slit, and leaves a negative trace of tie
variation in temperature on a prepared sheet rf
photographic paper, which is carried on hori-
zontally behind the instrument by means of clock-
work. In the same manner the barometric indi-
cations are traced ; and an entirely new feature is
introduced by the photographic registration of the
amount of rain and the time of fall.
The rain from the funnel is received in one leg
of a siphon tube, and the weight of the water
causes a column of mercury to ascend in the other ;
this column shuts out the light and records the
amount received, in the same way as the mercury
acts in the barometer and thermometer.
PRESENT STATE OF THE SCIENCE. 28]
As the photographic registers are all reduced
to the same size by the intervention of lenses,
positive copies of any or all of the negative ori-
ginals may be readily taken on one sheet of paper,
and multiplied to any extent; we are thus en-
abled to see at once the connexion between the
thermometric, barometric, and hygrometric curves,
impressed by nature without the trouble of geo-
metrical construction. The manner in which these
combined curves are presented to the eye at a
glance is most striking, and they indicate most
important relations between various meteoro-
logical phenomena which have yet to be fully
developed.
At Cambridge, Liverpool, and some other places
few in number, very valuable and efficient ob-
servers have recorded and published registrations
more extensive than can be expected from private
observers, but far inferior to the elaborate system
pursued at Greenwich. Private individuals in
various localities record phenomena without pub-
lishing their results, or joining any Society which
has the cultivation of meteorological science in
view. I apprehend their number is not great, or
they would be more generally known.
From the view of what is accomplished by
extra-observatorial efforts, we are compelled to
arrive at the conclusion that much remains to be
282 PRACTICAL METEOROLOGY.
done before we shall become acquainted with
simultaneous movements in the air, or variations
in its thermo-hygrometric state, even within so
narrow a district as our own country. We shall
now, as a consequence of the late movement,
enlist on our side the officers of the Royal Navy
and of the Mercantile Marine. The ships of
Great Britain traverse the ocean in every direction,
and at all periods of the year; they are com-
manded by men accustomed to watch natural
phenomena, and the regularity of life at sea is
favourable to the systematic registration of the
barometer and thermometer : to render the re-
cords valuable, the instruments will henceforth be
of a superior character to those usually found on
ship-board, calculated not only to show differential
but absolute values.
Those observers whose reports are published
every three months at present, from want more of
time than inclination, confine their observations
within too narrow a range. In addition to the
pressure, temperature, and hygrometric state of
the air, it would be highly advantageous could we,
for all localities, ascertain in addition the rapidity
of evaporation, the range and intensity of solar
radiation, and the state of electric tension ; all
which, in their varied combinations, go to make
up that general result which we call climate, and
PRESENT STATE OF THE SCIENCE. 283
which, unitedly, produce effects upon the natural
world and the human frame, varying according to
the preponderance of one or the other element.
A knowledge of all these would lead us, most pro-
bably, to conclusions approaching the truth as to
the adaptation of one particular series of crops
to certain parts of the kingdom, and of the fitness
of certain places for those who are suffering from
peculiar diseases. We do not, moreover, at pre-
sent distinguish the rainy hours in a day, but
simply record the daily fall ; and this leaves* us
deficient in one important element. Upon the
whole, we may conclude that meteorological science
is in a state of infancy ; that it is,- and must long
continue to be, only a science of observation j that
recorded phenomena are at present too few, and
those taken over only a small portion of the
earth's surface ; nay, the two-thirds of that sur-
face occupied by the ocean, though exercising a
most important influence on atmospheric changes,
may, as regards correct observation, be considered
as till lately a blank too few are they and insig"
nificant to enable us to draw conclusions or deduc-
tions which shall hold good over a large extent.
Meteorology is precisely in that position in
which geology was found eighty years ago, or
microscopic science at a still later period; and
yet, since that time, how many facts then obscure
284 PRACTICAL METEOROLOGY.
have been elucidated in the structure of the earth !
for how many sound principles has geology gained
universal reception ! How many secrets of nature
has the microscope disclosed ! how many wonder-
ful processes of nature has it unveiled !
APPENDIX.
TABLE I. Correspondence of the different Thermome-
trical scales.
TABLE FOR THE CENTIGRADE THERMOMETER.
i
Centigrade.
Reaumur's.
Fahren.
belt's.
Centigrade.
Reaumur's.
Fahren.
belt's.
100
80-
212'
73
58-4
163-4
99
79-2
210-2
72
57-6
161-6
98
78-4
208*4
7i
56-8
159*8
97
77'6
206-6
70
56-
158-
96
76-8
204-8
69 .
55'*
156-2
95
76-
203-
68
54'4
154*4
94
75-2
201'2
67
53-6
152-6
93
74'4
199-4
66
52-8
150-8
92
73-6
I 97 -6
65
52-
149-
9i
72-8
I 9 5-8
64
51-2
147-2
90
72-
194-
63
50-4
145*4
89
71-2
I92-2
62
49-6
143-6
88
70-4
190-4
61
48-8
141-8
87
69-6
l88'6
60
48-
140-
86
68'8
I86'8
59
47-2
138-2
85
68-
I8 5 -
58
46-4
136-4
84
67-2
183-2
57
45'6
J 34' 6
83
66-4
181-4
56
44-8
132-8
82
65-6
179-6
55
44*
131-
81
64-8
177-8
54
43-2
129-2
80
64-
I 7 6-
53
42-4
127-4
79
63-2
I74-2
5 2
41-6
125-6
78
62-4
172-4
5 1
40-8
123-8
77
61-6
I70-6
5
*
122*
76
60-8
168-8
49
39' 2
120*2
75
60-
167-
48
38-4
118-4
74
59'^
165-2
47
37-6
Il6'6
286
PRACTICAL METEOROLOGY.
TABLE for the Centigrade Thermometer (continued).
Centigrade.'
Reaumur's.
Fahren. '
belt's.
Centigrade.
Reaumur's.
Fahren.
heit's.
46
36-8
114-8
7
5'6
44-6
45
36'
II 3 -
6
4-8
42*8
44
35*a
111*2
5
4'
41'
43
34'4
109-4
4
3'*
39' 2
42
33'6
107-6
3
2-4
37'4
4i
32-8
I05-8
2
r6
35-6
40
32*
I0 4 *
I
0-8
33*8
39
31-2
I02'2
0*
3 2 '
38
3o*4
IOO-4
I
- 0*8
30-2
37
2Q-6
98-6
2
- r6
28-4
36
28-8
96-8
- 3
- 2*4
26-6
35
28*
95*
- 4
- 3' 2
24-8
34
27-2
93-2
- 5
4'
23-
33
26*4
91-4
- 6
- 4-8
21*2
3 2
25-6
89-6
- 7
- 5'6
19-4
3 1
24-8
87-8
- 8
- 6-4
I 7 -6
3
24-
86*
- 9
- 7^
I 5 -8
29
23-2
84*2
10
- 8-
H*
28
22-4
82-4
ii
- 8-8
I2'2
27
21-6
80-6
12
_ 9-6
I0'4
26
20-8
78-8
-1
-10-4
8-6
25
20*
77'
-I 4
I I'2
6-8
24
I 9 -2
75-2
15 , 12*
5'
23
l8'4
73'4
16
-12-8
3' 2 .
22
I7'6
71-6
-17
-13*6
i '4
21
16-8
69-8
-18
-14-4
- 0-4
20
16-
68*
19
15-2
2'2
19
15-2
66-2
20
16-
- 4-
18
14-4
64-4
21
-16-8
- 5-8
J 7
13-6
62-6
22
-17-6
- 7'6
16
I2'8
60-8
-23
-18-4
- 9'4
15
12*
59'
-24
-19-2
II'2
*4
II"2
57*2
- 2 5
20*
-13'
*3
10-4
55'4
-26
20-8
-14-8
12
96
53-6
-27
-21*6
-16-6
II
8'8
51*8
-28
22-4
-18-4
10
8-
50-
-29
-23-2
2O"2
9
7-2
48-2
-30
-24-
22-
8
6-4
46-4
-3i
-24-8
-2 3 -8
APPENDIX.
287
TABLE for the Centigrade Thermometer (continued}.
Centigrade.
Reaumur's.
Fahren-
heit's.
Centigrade.
Reaumur's.
Fahren-
heit's.
-32
-25-6
-2 5 -6
-37
29*6
-34'6
-33
26-4
-27-4
-38
-30-4
-3 6 "4
-34
-27-2
-29-2
-39
-31-2
-38-2
-35
-28'
-3 1 '
-40
-32-
-40-
-36
-28-8
-32-8
TABLE FOR REAUMUR'S THERMOMETER.
Reaumur's.
Centigrade.
Fahren-
heit's.
leaumur's.
Centigrade.
Fahren-
heit's.
80
lOff
212-
54
67-5
'53'S
79
9875
209-75
53
66-25
151-25
78
97*5
207-5
52
65'
149-
77
96-25
205-25
51
63'75
76
95"
203-
5
62-5
I44-5
75
93"75
200-75
49
61-25
142-25
74
9 2 "5
198-5
48
60-
140-
73
9 I<2 5
196-25
47
58"75
I37-75
72
90-
194-
46
57"5
JJS'S
71
88-75
101-75
45
56*25
133-25
70
87-5
189-5
44
55"
69
86-25
187-25
43
53*75
128-75
68
85-
I8 5 -
42
5 2 '5
126-5
67
8375
182-75
124-25
66
82-5
180-5
40
5"
122"
65
81-25
178-25
39
119*75
64
80-
176-
38
47*5
JI 7'5
63
78-75
173-75
37
46-25
115-25
62
77'5
171-5
36
45"
113-
61
76-25
169-25
35
4375
110-75
60
75"
167-
34
42-5
108-5
59
7375
164-75
33
41-25
106-25
58
72-5
162-5
3 2
40-
104-
57
71-25
160-25
3 1
3875
101-75
56
70-
158-
30
37-5
99'5
55
68-75
15575
29
36-25
97-25
288
PRACTICAL METEOROLOGY.
TABLE for Reaumur's Thermometer (continued).
Reaumur's.
Centigrade
Fahren-
heit's.
Reaumur's
Centigrade
Fahren.
heit's.
28
35"
95'
- 3
~ 375
25-25
27
3375
92-75
- 4
- 5'
23*
26
32-5
90-5
- 5
- 6-25
20-75
25
3 r2 5
88-25
6
- 7'5
18-5
24
3'
86-
- 7
- 875
16-25
23
2875
8375
- 8
10'
14'
22
27-5
81-5
- 9
-11-25
1 175
21
26*25
79-25
10
-12-5
9'5
20
25*
77'
ii
-1375
7-25
'9
2375
7475
12
-15*
5*
18
22-5
72-5
-13
16-25
275
17
21-25
70-25
-14
-i7'5
0-5
16
20*
68-
-*5
-18-75
- 1-75
15
1875
6 575
-16
20*
- 4'
H
17*5
63-5
-17
-21-25
- 6-25
i3
i6'25
61*25
-18
-22-5
- 8-5
12
15*
59*
-19
- 2 375
10-75
II
1375
5675
20
-25'
-13-
10
12-5
54' 5
21
26-25
-i5'*5
9
11-25
52-25
22
-27-5
-7*5
8
10'
5'
-2 3
-28-75
- J 975
7
8-75
4775
-24
-3'
22*
6
7*5
45'5
- 2 5
-3I'25
-24-25
5
6-25
43'25
-26
-32-5
-26-5
4
5'
4i*
-27
-3375
-28-75
3
375
3875
-28
-35'
-31'
2
2'5
36-5
-2 9
-36-25
-33' 2 5
I
1-25
34' 2 5
- 3
-37'5
-35'5
0'
32'
-31
-3875
-3775
I
-1-25
29-75
- 3 2
-40-
-40-
2
-2-5
27-5
-33
-41-25
-42-25
APPENDIX.
289
TABLE FOR FAHRENHEIT'S THERMOMETER*.
Fahren-
heit's.
Reaumur's.
Centigrade.
Fahren-
heit's.
Reaumur's.
Centigrade.
212
8o - oo
lOO'OO
176
64*00
8o'oo
211
79*55
99*44
'75
63*55
79'44
210
79-11
98-88
'74
63-11
78-88
20 9
78-66
9^33
173
62-66
78-33
208
78-22
9777
172
62*22
7777
207
7777
97-22
171
61-77
77-22
206
77*33
96-66
170
61-33
76-66
205
76-88
96-11
169
60-88
76*11
204
76-44
95*55
168
60-44
75*55
203
76-00
95-00
167
6o'oo
75-00
202
75*55
94*44
166
59*55
74*44
201
75-11
93-88
165
59* 11
73*88
200
74-66
93*33
164
58-66
73*33
199
74-22
92-77
163
58-22
7277
198
7377
92-22
162
57*77
72-22
197
73*33
91-66
161
57*33
71-66
196
72-88
91-11
160
56-88
71*11
195
72-44
90-55
'59
56-44
70*55
194
72*00
90*00
158
56-00
70-00
193
7i'55
89-44
157
55*55
69-44
192
71-11
88-88
156
55-11
68-88
I 9 I
70-66
88-33
'55
5466
68-33
190
70-22
8777
J 54
54-22
67-77
I8 9
69-77
87*22
'53
53*77
67-22
188
69*33
86-66
152
53*33
66-66
187
68-88
86-u
151
52-88
66-u
186
68-44
85*55
150
5 2 '44
65*55
185
68-00
85-00
149
52-00
65-00
184
6 7*55
84-44
148
5i*55
64-44
183
67-11
83-88
147
51-11
63-88
182
66-66
83*33
146
50-66
63*33
181
66-22
82-77
"45
50-22
62-77
180
65*77
82-22
144
49*77
62-22
179
65'33
8r66
*43
49*33
61-66
178
64-88
81-11
142
48-88
6rn
177
64'44
80-55
141
48*44
60-55
* All the decimals in this Table are circulating decimals.
U
290
PRACTICAL METEOROLOGY.
TABLE for Fahrenheit's Thermometer (continued}.
Fahren.
belt's.
leaumur's.
Centigrade.
Fahren-
heit's.
Reaumur's.
Centigrade.
140
48-00
60*00
JOI
30*66
38*33
I 39
47*55
59*44
100
30*22
37*77
I 3 8
47-11
58-88
99
29*77
37*22
137
46-66
5^33
98
29*33
36-66
I 3 6
46-22
57*77
97
28-88
36*11
135
4577
57-22
96
28*44
35*55
134
45*33
56-66
95
28*00
35-00
133
44-88
56-11
94
27*55
34*44
I 3 2
44-44
55'55
93
27-11
33-88
131
44-00
55-00
92
26-66
33*33
130
43*55
54*44
9i
26-22
3277
I2 9
43* 11
53-88
90
25-77
32*22
128
42-66
53*33
89
25*33
31*66
127
42*22
5^*77
88
24-88
31*11
126
41-77
52-22
87
24-44
3*55
125
4i'33
51*66
86
24*00
30*00
124
40-88
51-11
85
23*55
29*44
123
40-44
5*55
84
23-11
28-88
122
40*00
50*00
83
22*66
28-33
121
39*55
49*44
82
22'22
27-77
120
39' 11
48-88
Si
21-77
27-22
119
38-66
48*33
80
21*33
26-66
118
38-22
47*77
79
20-88
26-11
117
37*77
47-22
78
20-44
25'55
116
37*33
46-66
77
20*00
25-00
"5
36*88
46*11
76
19*55
24-44
114
3 6 '44
45*55
75
I9*II
23-88
113
36*00
45-00
74
I8'66
23*33
112
35^55
44-44
73
18*22
2277
III
35* 11
43-88
72
17*77
22-22
1 10
34-66
43*33
7i
!7'33
21-66
109
34-22
42*77
70
16-88
21-11
108
33*77
42*22
69
16-44
20*55
107
33*33
41-66
68
16*00
20*00
106
32-88
41-11
67
15*55
19-44
105
32*44
4'55
66
15-11
18-88
104
32*00
40*00
65
14*66
18-33
103
3i'55
39*44
64
14-22
17*77
102
31-11
38-88
63
13*77
17*22
APPENDIX.
291
TABLE for Fahrenheit's Thermometer {continued}.
Fahren.
belt's.
Reaumur's.
Centigrade.
Fahren.
belt's.
Reaumur's
Centigrade.
62
i3'33
16-66
23
-4-00
-5-00
6l
12-88
16*11
22
-4'44
-5'55
60
12-44
i5'55
21
-4-88
-6-II
59
I2'00
15-00
20
-5'33
-6*66
58
"'55
14-44
19
-5*77
-7*22
57
irn
13-88
18
-6-22
-777
56
10*66
i3'33
17
-6-66
-8-33
55
IO'22
12-77
16
-7-11
-8-88
54
977
12*22
15
-7'55
-9-44
53
11-66
H
-8-00
lO'OO
5 2
8*88
II'II
13
-8-44
-''55
5 1
8-44
10-55
12
-8-88
ii'ii
5
8'oo
lO'OO
II
~9'33
-ir66
49
7-55
9'44
10
-977
12-22
48
7-11
8-88
9
I0'22
-12-77
47
6-66
8-33
8
10-66
-13*33
46
6-22
7*77
7
ii'n
-13-88
45
577
7-22
6
-"'55
- H'44
44
5'33
6-66
5
I2'OO
15-00
43
4-88
6-u
4
- I2'44
-i5'55
42
4*44
5"55
3
-12-88
-16-11
4i
4*00
5-00
2
-I3-33
-16-66
40
3-55
4"44
I
-1377
-17-22
39
3-11
3'88
14*22
-17-77
38
2-66
3"33
I
- 14-66
-18-33
37
2*22
2-77
2
-15-11
-18-88
36
I- 77
2'22
3
-*5'S$
-19-44
35
I'33
1-66
-4
i6'oo
20*00
34
0-88
I'll
-5
-16-44
-20-55
33
0-44
'55
-6
-16-88
21*11
32
O*
o*
-7
-i7'33
-21-66
3 1
-0-44
-0-55
-8
-17-77
22-22
3
-0-88
i-ii
-9
l8'22
-22-77
29
-i*33
-1-66
10
-18-66
~ 2 3'33
28
-1-77
2'22
ii
Ip-II
-23-88
27
2*22
-277
12
-I9'55
-24-44
26
-2-66
~3'33
- J 3
20*00
25-00
25 -3-11
-3-88
-14
-20-44
~ 2 5'55
2 4 1 -3*55
-4-44
- J 5
-20-88
26-11
u
292
PRACTICAL METEOROLOGY.
TABLE for Fahrenheit's Thermometer (continued").
Fahren-
heit's.
Reaumur's.
Centigrade.
Fahren.
heit's.
Reaumur's.
Centigrade.
-16
-21-33
-26-66
-29
-27-11
-33*88
-17
2177
-27-22
-3
-27*55
-34*44
-18
22'22
27-77
-31
-28-00
-35-00
-19
-22-66
-28-33
-32
-28-44
-35*55
20
-23-II
28-88
-33
-28-88
36-11
21
~ 2 3*55
-29-44
-34
-29*33
-36-66
22
24-00
30*00
-35
-2977
-37-22
-23
-24-44
-30*55
-36
30*22
-37*77
-24
-24-88
-31-11
-37
30*66
-38-33
-2 5
-25*33
-31-66
-38
-3I-JI
-38-88
-26
-25*77
-32-22
-39
-31*55
-39*44
27
26'22
-3277
40
32-00
40*00
-28
-26-66
-33*33
TABLE II. Tension, or Elastic Force, of Aqueous Vapour
in inches of mercury, for every degree of temperature
from to 95.
Temp.
Tension.
Temp.
Tension.
Temp.
Tension.
Temp.
Tension.
o
o
o
044
12
074
24
129
36
212
I
046
*3
078
25
135
37
220
2
048
14
082
26
141
38
229
3
050
15
086
27
147
39
238
4
052
16
090
28
''53
40
247
054
17
094
29
160
4*
257
6
057
18
098
30
167
42
267
7
060
'9
103
31
174
43
277
8
"062
20
108
32
181
44
288
9
065
21
*"3
33
188
45
299
10
068
22
118
34
196
46
311
ii
071
23
123
35
-204
47
323
APPENDIX.
293
TABLE II. (continued).
Temp.
Tension.
Temp.
Tension.
Temp.
Tension.
Temp.
Tension.
o
o
48
'335
60
518
72
785
84
1-165
49
348
61
'537
73
812
85
I'203
5
361
62
556
74
840
86
1-242
51
"374
63
576
75
868
87
1-282
S 2
388
64
596
76
897
88
1-323
53
'43
65
617
77
927
89
1*366
54
418
66
639
78
958
90
1-410
55
"433
67
661
79
99 o
9 1
i'455
56
"449
68
684
80
1-023
92
1-501
57
465
69
708
81
1-057
93
1-548
58
59
482
500
70
7i
'733
"759
82
83
1-092
1-128
94
95
1-596
1-646
TABLE III. The weight (in grains Troy) of vapour in a
cubic foot of saturated air ; at all temperatures between
10 and 89.
Temp.
Weight.
Temp.
Weight.
Temp.
Weight.
Temp.
Weight.
o
o
10
8
25
1-6
4
2-9
55
4*9
ii
"9
26
r 7
4i
3-0
56
5-0
12
'9
27
1*7
42
3'i
57
5'2
13
*o
28
r8
43
3'2
58
5 '4
H
o
29
1-9
44
3'3
59
5'6
15
I
3
2'0
45
3 "4
60
5-8
16
I
3 1
2'I
46
3-6
61
6-0
17
I
32
2*1
47
3'7
62
6-2
18
"2
33
2'2
48
3'8
63
6-4
'9
*3
34
2*3
49
4*o
64
6-6
20
i*3
35
2'4
5
4*i
65
6-8
21
*4
36
2'5
5 1
4-2
66
7-0
22
i '4
37
2-6
52
4'4
67
7*3
23
r 5
38
2'7
53
4' 5
68
7*5
24
r .S
39
2-8
54
4*7
69
7-8
294
PRACTICAL METEOROLOGY.
TABLE III. (continued).
Temp.
Weight.
Temp.
Weight.
Temp.
Weight.
Temp.
Weight.
70
8-0
75
9'4
80
11*0
8S
I2'8
71
8-3
76
97
81
irj
86
13-2
72
8-s
77
IO'O
82
117
87
I3'6
73
8'8
78
10-3
83
I2'O
88
14-0
74
9-1
79
10*6
84
I2'4
89
14-4
. Factors to be multiplied into the quantities in
TABLE III., when the air-temperature and dew-point
temperature differ by the number of degrees in the first
column.
Diff.
Factor.
Diff.
Factor.
Diff.
Factor.
Diff.
Factor.
o
o
o
I
'999
9
982
17
966
25
951
2
996
10
980
18
964
26
*949
3
'994
ii
978
J 9
962
27
"947
4
992
12
976
20
960
28
'945
5
990
13
'974
21
958
29
'943
6
988
J 4
972
22
956
30
942
7
986
15
970
23
'954
3 1
'939
8
984
16
968
24
952
32
937
APPENDIX.
295
TABLE V. Showing the weight in grains Troy of a cubic
foot, of air saturated with moisture, under the pressure
of 30 inches of mercury, at any temperature between
31 and 90; and the excess of the weight of a cubic
foot of dry air, under the same pressure, over that of a
cubic foot of saturated air, also in grains, throughout
the same range of temperature.
Temp.
Wt. of a cub.
ft. of sat. air.
Excess.
Temp.
Wt. of a cub.
ft. of sat. air.
Excess.
o
o
3 1
566-8
"2
61
53'*7
3'5
32
565-6
2
62
530-6
3-6
33
564-4
'3
63
529-4
3-8
34
563-2
'3
64
528-3
3-8
35
562-0
'4
65
527-1
40
36
560-8
'4
66
526-0
4'i
37
559-6
67
524-9
4-2
38
55^4
6
68
523'7
4'4
39
557'2
'7
69
522-6
4'5
40
556-0
'7
70
521-4
4'7
4 1
554'9
"7
7i
520-3
4-8
42
553-7
8
72
5*9'*
5'o
43
552-5
'9
73
518-0
5-1
44
55 J "4
'9
74
516-8
5'4
45
55"2
2-0
75
5*57
5'5
4 6
549 "
2' I
76
5H" 6
5'6
47
547-9
2' I
77
5 J 3"4
5-8
48
54 6 '7
2'2
78
5 I2< 3
6-0
49
545-5
2'3
79
511-1
6-2
5
5 1
544'4
543'2
2'4
2'5
80
81
5 IO '
508-8
6-3
66
S 2
542-i
2'5
82
5077
6-7
53
540-9
2'7
83
506-5
7-0
54
539-8
2'7
84
55'3
7-3
55
538-6
2-8
85
504-2
7'4
56
537'5
2-9
86
503-0
7*7
57
536-3
3-1
87
501-9
7-8
58
535'i
3"i
88
500-7
8-1
59
534'Q
3'3
89
499-6
8-3
60
532-8
3'4
9
498-4
8-6
296
PRACTICAL METEOROLOGY.
TABLE VI. Showing the degree of humidity of the
atmosphere, deduced from the readings of the dry-bulb
and wet-bulb thermometers, for the usual range occur-
ring in England ; complete saturation being 1 .
D W.
3 2
33
34
35
36
37
38
39
40
4i
1
2
3
4
8 7
'75
6 5
'57
89
78
69
61
89
79
71
63
90
80
72
65
91
82
'74
66
91
8 3
'75
68
91
83
'11
92
84
"77
70
92
84
76
69
92
84
"77
70
42
43
44
45
46
47
48
49
50
5i
i
2
3
6
8
92
85
78
72
60
'49
92
84
78
71
"59
'49
92
84
77
71
59
"49
92
85
78
72
60
'5
'93
86
'79
1\
'5 1
'93
86
'79
11
'5 1
'93
86
"79
'73
62
'5*
*93
86
'79
*73
62
'53
'93
86
80
'74
63
'53
'93
86
80
74
63
"54
5*
53
54
55
56'
57
58
59
60
61
i
2
3
4
6
8
10
*93
86
80
'74
64
'54
46
'93
86
80
'74
64
'55
"47
"93
86
80
74
64
'55
"47
'93
87
8 1
'H
56
48
'93
87
81
3
56
4 8
'93
87
81
'75
65
'57
'49
'93
87
8 1
76
66
'57
'49
'94
88
82
76
66
'57
'49
'94
88
82
76
66
58
*5
'94
88
82
77
67
58
"5
62
63
64
65
66
67
68
69
70
71
i
'94
:tj
'94
*94
'94
*94
'94
'94
'94
'94
2
3
t
8
10
12
88
82
77
67
58
50
'44
so
82
'77
67
*59
"Si
'44
00
82
77
67
59
'5 1
'45
'50
83
78
68
'59
'Si
'45
00
83
78
68
60
"5*
'45
00
83
78
68
60
3
*88
'83
78
68
60
"5*
46
88
83
78
68
60
'S3
'47
88
83
78
69
61
'S3
"47
88
83
78
6 9
61
"53
'47
APPENDIX.
297
TABLE VI. (continued}.
D-W.
7 2
73
74
75
76
77
78
79
80
81
o
I
94
'94
*94
'94
'94
'94
'94
'95
"95
'95
2
"89
89
89
89
89
89
89
90
90
90
3
'84
84
84
84
84
84
84
8 S
8s
4
'79
*79
'79
79
79
'79
'79
80
80
80
6
6 9
70
70
70
7i
71
71
71
71
72
8
61
62
62
62
63
63
63
64
10
'54
"54
'55
'55
'55
56
56
56
s
12
48
48
48
'49
'49
50
50
50
50
50
Explanation. Enter the column headed D W. with the
difference between the readings of the dry-bulb and wet-bulb
thermometers ; ranging with it under the reading of the dry-
bulb thermometer, found in one of the horizontal columns,
will be the degree of humidity required.
TABLE VII. Corrections to be subtracted from the read-
ings of Barometers, with brass scales extending from
the cistern to the top of the mercurial column, to re-
duce the observations to 32 Fahrenheit.
Inches.
Temp.
28
28-5
2 9
2 9'5
30
30-5
31
o
29
ooi
ooi
*OOI
ooi
ooi
ooi
ooi
3
3i
004
006
004
006
004
007
004
007
004
007
004
007
'004
007
3*
009
009
009
009
009
QIC
oio
33
*OJI
012
*OI2
*OI2
012
012
012
298
PRACTICAL METEOROLOGY.
TABLE VII. (continued}.
Temp.
Inches.
28
28-5
29
29-5
3
3'5
3i
34
014
014
014
015
015
015
015
35
016
017
017
017
018
018
018
36
019
019
'020
020
020
021
021
37
02 1
'022
*O22
*022
023
023
'024
38
024
024
025
025
026
O26
'026
39
026
027
027
028
028
029
029
40
029
'029
'030
'030
031
031
3 2
4 1
031
032
033
033
034
034
*35
42
034
034
*35
036
036
37
037
43
036
037
'038
3 8
039
'040
'040
44
039
'040
'040
'041
042
042
43
45
041
'042
043
044
044
045
046
46
044
045
045
046
47
4 8
049
47
046
047
048
049
050
5 I
'Q5 1
48
049
'050
5 I
052
5 2
053
054
49
051
5 2
053
054
55
'056
057
5
054
055
'056
057
058
059
060
5 1
o<;6
057
'058
059
060
06 1
062
5 2
059
060
06 1
062
063
'064
065
53
'061
063
'064
06 5
066
06 7
068
54
064
065
066
06 7
068
'070
071
55
066
068
'069
070
071
072
073
56
069
'070
071
073
074
075
076
57
071
073
'074
*75
076
078
079
58
074
075
'077
078
079
08 I
082
59
076
078
079
080
082
083
085
60
079
080
082
083
085
086
087
61
08 1
083
08 4
086
087
089
090
62
084
085
087
088
090
'091
093
63
086
088
'089
091
093
094
'096
64
'089
090
9 2
094
*95
097
098
i 65
'091
093
095
096
098
*IOO
*IOI
66
094
096
97
099
*IOI
'102
104
67
'096
098
'100
'102
103
105
107
68
099
101
'102
I0 4
106
108
'109
69
101
103
105
107
109
no
112
70
104
106
108
109
in
"3
"5
APPENDIX.
299
TABLE VII. (continued).
Temp.
Inches.
28
28-5
2 9
29-5
3
3'5
3i
71
106
108
no
'112
114
116
118
72
109
in
'"3
115
117
119
'120
73
in
113
'"5
117
119
121
123
74
114
116
118
'120
122
I2 4
126
75
116
118
120
'122
125
127
129
76
119
121
123
125
127
I2 9
I 3 I
77
121
123
126
128
I 3
I 3 2
'34
78
124
126
128
I 3
133
135
137
79
126
128
I 3 I
133
135
137
140
80
129
131
J 33
I 3 6
138
140
*H3
81
131
134
136
I 3 8
I 4 I
*43
145
82
T34
I 3 6
138
141
''43
'146
148
83
I 3 6
139
141
143
146
148
''5 1
84
J 39
141
144
'146
149
151
'54
85
141
144
146
149
151
J 54
156
86
I 44
I 4 6
149
'IS 1
!54
i 56
159
87
I 4 6
149
151
154
''57
!62
88
149
151
154
*'57
''59
162
165
89
ISI
'154
156
159
162
165
167
90
153
I 5 6
159
162
!6 4
167
170
PLATE I.
Fig. 1. The simplest form of a Barometer, p. 11
and p. 203.
Fig. 2. Siphon tube for experimental proof of Ma-
riotte's law, p. 13.
Fig. 3. Diagram for proof of the law of density,
p. 16.
Fig. 4. Constructing a Thermometer, p. 40.
Fig. 5. Determining the Boiling-point of a Ther-
mometer, p. 46.
Fig. 6. Rain-gauge, p. 187.
Fig. 7. Graduated jar for the Rain-gauge, p. 187.
PLATE II.
Fig. 1. Sixe's Register Thermometer, p. 51.
Fig. 2. Butherford's Register Thermometer, p. 54.
Fig. 3. Negretti and Zambra's Maximum Thermo-
meter, p. 55.
Fig. 4. Sympiesometer, p. 217.
Fig. 5. Lind's Wind-gauge, p. 118.
Fig. 6. Wind-star, p. 120.
. ir.
f
5.
. 3.
I
t
J
E
G
0-
10-
-Si
20.
-
40-
50-
*
'-30
60-
!
-
BO-
90-
RI
g
200-
110-
J
i
I"
tr
J
D
P
-27
j]
-2fi
B^
rj
I'}//. 2. Sijcek Jteffi'-tter Therm wnpf-er. _ ^". Jlittfierfords. _
4. ^ddie's t $ymf)iesometer . d.Xind's Wirid -Cfiuige .
J.Basire, sc.
lcm.drm: JoTin "VaT3-"Vbarst,l855 .
PLATE III.
Curves of the monthly mean temperature at Green-
wich and Southampton for the years 1848-
1853, p. 78.
x :
>
<
^
s
5;
S
I
s
\
%
t
::
s
_
1
%
$
/;/-
:
._
'-.
War
\
v \
I
Ap
\
^
s
W/r
x
\
-
^
^*
SB
*
./,//
^
s
?s
\
J
Tulv
x
X
s
'x
;
tS:
'
i
I
s, r .
'
'
/
51
,-l
,-
x
x'
^
y
<
.I'.'l-
--
^*
r
^v
^
r
'
&
AV
'
s
>
-
\
I
y
I
-
i
/
N,
i
(:/,
x -
^
Hu-
"
~
\
s
ll'
\
\
N
\
t-
*
s
M,,v
^
^^
^r
~-
^,
-
1
\
~\
s
\
K
C
s|
July
-^
..^
^
Ol
to
-
1
;
s
djlf]
/
/
'
^
Sep
/
x
/
>
^
Out
,--
---
--"
s
x
^
^
X
^
/),;
..
/
;
/
/
s|
Jan*
'
?
1
\
s-
N
1*
Feb.
^
.-
*
r
?s-
^~-
->
<i
Mar.
\
>
/'
^
<
.!/>.
s.
!Q
N
*>
$:
-^
>i
May
x.
x^
\
s
<i
N
^ N
..
te
^
**v
^
|s
s|
July
\
s
\
^
.In,/
^/i
1
Sep.
/"
.
/
','
^
Oat
^
_^-
^
^
I
Jffav.
..-.
^
^
\
I),-,-.
2
*'
/
;
g
-
fe
\
j
fi
\
\
s
fe
^
Q
8
5
\
PLATE IY.
Fig. 1. Greenwich Thermometer-stand, p. 82.
Fig. 2. Curves of the mean daily temperature at
Greenwich for January and July, p. 62.
Fig. 3. Dalton's apparatus for measuring the elastic
force of vapour, p. 128.
Fig. 4. lire's apparatus for measuring the elastic-
force of vapour, p. 130.
^-~
\,
/
^
/
\
'
1
\
1
\
M,
,m
It,,,
/"'
ihn
f t,
/ ./
,//v\
,
\
I
\
''
\
'
N
\
:
.
\
/^
\
/
\
;/ rt
//
,-tHfi
./,,;,
turt
A
r>
K
/
.
* . .
'-^
^
J.-Basire, fc.
london. JolnaYan."\ 7 oorst,l85o .
PLATE V.
Curve of the mean annual temperature at Green-
wich for the years 1771-1853, p. 84.
1813
14
IS
16
17
u
I'l
1820
2L
22
23
24
25
28
28
29
1830
31
32
33
34
35
36
37
38
30
1840
tl
42
43
44
In
46
47
48
49
IH:,O
1853
47-2
^.4
47-1
46-6
47-9
50
48-3
48-2
49-2
51-2
48-9
49-8
45-5
48
45
46-4
48-L
47-8
46-7
48
48
48
48
48-9
47-2
47-8
47-2
48-6
45-7
48-3
49
41-5
48-2
49-4
47-6
50-5
48-4
481
48
48-6
49-1
464
\* * * *S
177L
i i
74 c^
r; $
-! ^
^ ?%
/;.-; ^
,vv ?.$
* : 1
/,/; T- \
3 bv.
9L ^ g>
ffP ^
1800 f %
03 ^
O4 ^
OS ^
(6- ^
^
W N
Off ^
181O
^
45-8
48-9
46-4
ff-7
fff-3
DM
a
483
49-6
50-1
m
478
491
49
49-,
48
47-3
464
177
47-8
i:n;
/.'/
47-6
1-3
/!>>
SO&
I."-:
\50-6
17-
'
-,
v
;
^
_
X
-'
.
"
'-
^
^
\
--
'
'
/
-
/
-
L
-
-
p
\
'*-
r \
,
^-1
s
Jr
\/2
7
j^
2
\\
,
r
\
.
,
I
y
S
'"
/
3
i
:
.
'
,.,
i
'
I
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'
E
, -
\
,
fcj^^viQj'is'iK
/
JL
'- '
M2
PLATE VI.
Fig. 1. Daniell's Hygrometer, p. 137.
Fig. 2. Wet- and Dry-bulb Thermometers, or Ma-
son's Hygrometer, p. 144.
Fig. 3. Saussure's Hygrometer, p. 127.
Fig. 4. Regnault's Hygrometer, p. 139.
Fig. 5. ConnelTs Hygrometer, p. 143.
ft. 71.
^YGROME TERS.
J.Basire.sc.
: John. "Van. Voorst, 1865.
PLATE VII.
Sectional view of the Dome of the Kew Observatory,
with the electrical instruments, p. 231.
c
i : }
v_
C
V
G
K G
J
L
C
LoncLon.: JoTm \ r a a. Voorst. l855 .
PLATE VIII.
Fig. 1. Vertical cylinder with' the photographic
paper, p. 262.
Fig. 2. The paper unrolled, with the trace upon it
for 12 hours, p. 263.
Figs. 3 and 4. Section and Plan of the Naphtha and
Steam Box, p. 259.
Fig. 5. Eegistration of the movements of the Decli-
nation Magnet, pp. 264 and 267.
Fig. 6. Registration of the Barometer, pp. 266 and
268.
Larulou: John. Van. Voorst, 1.855 .
PLATE IX.
Osier's Wind-gauge and Rain-gauge, with a speci-
men of its register, p. 272.
SBL
IX
xii
'-"
;// ,11
II,.-
IV.'i
Register .
Lcuidou : John Van Voorst , l855 .
PLATE X.
Whewell's Wind-gauge, p. 277.
II 7/ri, >//.-
Landau: JoliTL Van. Voorst, 1800
PLATE XI.
Fig. 1. Negretti and Zambia's Minimum Thermo-
meter, p. 56.
Fig. 2. Robinson's Anemometer, p. 116.
Fig. 3. Hicks's Register Thermometer, p. 58.
Pl.JL.
Fig.2.
/yV/./. .Veqretti & Zamivas Jfinuruun. T/icrni^ineter^Pig.2. RoblnsorCs
WintLffUa0eFig. &. Flic fat's Mthci/snirn A- Mi'ninium,
London: John Van \E>cjrst,lBeO.