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which quickened by every wind, and augmented by Meteor ology. b= 1.149338,
every increment of heat, is continually exerting some and
effort to return the humidity which the rain and the dew c=4.086$30.
may discharge on the Earth. If, moreover, w assume (x
(478.) The History of this department of Physics is Remarks -0.3815) N 2 = , we
Great names have been connected connected shall further have y'=s - it', and this being substi- replete with interest.
with it, and the subject has been a difficult one to purtuted in the equation for the hyperbola, will give
History. Men at one time thought that an essential differ(8 - x) = a + 2b r' + cx',
ence existed between the vapour produced above and the resolution of which quadratic gives
below the boiling point. In the lower temperatures, it (s + b)+((s + b) + ($2 – a) (c − 1)) was imagined that moisture was devoid of elasticity, and x =
hence arose the famous theory of Halley that water is - 1
dissolved by air. Deluc and Saussure achieved a great
step, therefore, when they proved that vapour of every
temperature had elasticity; and a step hardly less im
portant was accomplished by Saussure and Pictet, when The value of y' thus found, being substituted in the they found evaporation to be accelerated in vacuo. А equation
true and perfect theory of spontaneous evaporation was, y=1-x + y' ✓ 2,
however, reserved for DALTON, a name which seems to Formula will determine the function y in terms of c, and thus rise in importance the further we proceed.
(479.) It is a law of Nature, now absolutely demon- Water will for com- enable us to compute the hygrometric degrees in terms puting the of the observed tensions ; and Biot, at page 533 of the strated, that water has a tendency to assume the elastic assume thu hygrome. Ist volume of his Traité de Physique, has furnished us
form of vapour at all temperatures however low. Our form of
vapour at in terms of with a table of all the necessary results from zero to one
ordinary experience, indeed, must tend to convince us of
all tempera the ob- hundred degrees of Saussure's scale; and a simple inspec
its truth, since the product of every shower soon dis- atures. served ten- tion of which will at once afford the means of discover- appears, and ice and snow are wasted by its irresistible sions.
ing the relation between the degrees of the hygrometer, power. The sea too performs its part on a grand and
lutely necessary to diminish the enormous accessions of
evaporate spontaneously at all temperatures, is one of the Inquiry if (476.) Limited as these computations have been to
most interesting and important in the whole economy of the relation the temperature of ten centesimal degrees, it becomes Nature. The growth of plants and the existence of every of the ten- important to inquire whether the relations of the ten- living creature depends upon it. sions and
(480.) We have no materials for tracing experimen- No matehygrome
sions and the hygrometric degrees will remain the same tric degrees at any other temperature. We might, indeed, at first tally the phenomena of evaporation in different climates, rials for is general view, suppose, that when the index of the instrument though we know from its general relations to heat, that comparin for all tem• marked a hundred degrees, under circumstances of en.
it must exist in its greatest power in the Equatorial peratures. tire saturation at any given temperature, that at any will be found in a state of even comparative activity in climates.
gions, and, diminishing in some way with the latitude, different other degree of heat, and with as complete a saturation as that temperature will permit, the same indication of the icy regions of the Pole. Anomalies, however, and Anomalii the instrument would take place. There is reason,
some of a very remarkable kind, present themselves in however, for supposing that such a desirable condition these widely extended regions, and the same terrestrial will not hold good ;-that the affinity of the hair for parallel possesses them in different degrees. The Medimoistu is somewhat modified by temperature, and that terranean Sea, surrounded on all sides by land, is more thus the relation of the coordinates of the hyperbola must heated than the Ocean in the same degree of latitude, and change. Ingenious and interesting therefore as the method the winds which blow over it, being thus rendered drier, is by which these results for the observed temperature have promote a more copious evaporation than in the Atlantic
itself. been found, we cannot with perfect confidence extend
The annual evaporation at Whydah has been them to other temperatures; nor must we omit stating
estimated at 64 inches, but when the Harmattan blows explicitly, that the relative results which this and other in
that rate is augmented to 133 inches. With such anostruments of a like kind exhibit, however convenient they
malies therefore, and with so few facts to guide us, it is no may be for some Meteorological purposes where a rigid deavour to trace the laws, which, in spite of such great
wonder that difficulties arise on all sides when we enAbsolute
and Philosophical accuracy is not required, are by no results to be means to be compared with those absolute results, which
aberrations, undoubtedly govern evaporation in every preferred to an attention to the conditions of temperature and baro
climate. relative. metrical pressure afford. We must not, however, enter
(481.) Before we proceed to consider the experimental Remar's here on a discussion of the merits of Hygrometers, but determined, let us attend for a moment to the phenomena ratione
laws which the sagacity of the Philosopher has already hasten to the subject of evaporation.
disclosed by the evaporation of an atom of water when atom of Evaporation.
placed in the centre of a spherical manometer, sur- water
rounded on all sides with an infinity of concentric sphe- placed i Evapora- (477.) We come now to the consideration of that pro- rical beds of dry air. In such a condition of things, the of a 's! tion. cess of Nature, which by its incessant activity affords all process of evaporation must at once commence on all sides rical mi
the moisture with which the atmosphere is stored, and of the spherical drop, and the stratum of air in iinmediate meter.
Meteof contact with it, will be the first to become saturated with moisture just at the same rate, whether it be held ver- Meteor viens vapour. The vapour thus imparted to the first atmo- tically or horizontally, and whether it occupies the upper
ology spheric stratum, from its occupying a station in imme- or under side of the plate. The quantity evaporated diate contact with another bed of dry air, will at once from a wet ball will be the same as from an equal plane, exert its elastic power and expand according to the or by a well-known property of the sphere, as froin a mechanical nce which this second bed permits. In circle twice its diameter. From water also contained in Remarks this way, we may imagine moisture to be transmitted vessels of any magnitude and form, and of equal or concerning from one successive bed to another, but in continually unequal depths, the rates of evaporation, at least in tran- water evadiminishing quantities as the beds are further removed quil states of the atmosphere, and when the circum- porating from the centre; and a moment of time may be con- stances of temperature and situation are the same, will sels of difceived when the beds of the manometer most removed be proportional to the magnitude of the watery surface ferent mag. from the origin of the vapour, are just beginning to exposed to the action of the air. Muschenbroëk, in- nitudes and
forins. receive the first impressions of humidity, whilst the cen- deed, asserts, that the deepest vessel is always found, tral beds have already acquired all that the circum- after a certain interval of time, to have suffered the stances of their temperature will permit. As each par- greatest waste, and that the quantities evaporated are as ticle of vapour, however, tends continually to diffuse itself the cube roots of the heights, their orifices and other cirin the space on which it immediately borders, if the cumstances being the same. He remarks, however, quantity of vapour be less than the circumstances of that when the experiment was performed in a room, no temperature permit, the process of evaporation cannot sensible difference was perceptible.
Leslie also as- Different cease on the surface of the watery atom at the centre, nor serts, that the deepest vessel evaporates most copiously, opinions. can the humidity imparted from one successive stratum to and grounds his opinion on the Principle, that the shalthe other be checked, until the whole mass of air which lowest vessel receives more readily than the other, the occupies the manometer, shall have acquired all the chilling impressions which accompany evaporation; and moisture which the given temperature demands. that the larger mass being thus kept invariably warmer
(492.) In applying to the atmosphere the Principles than the other, must, as evaporation is always accelerated - 3 of the of this illustration, we shall be able to trace the general by heat, afford a more copious supply of inoisture bere Principles of evaporation. The air confined within the than the other. Lambert, however, like Leslie, a
manometer may be supposed to be enlarged into the refined experimenter as well as a great Mathematician,
(453.) To simplify the subject, we have supposed the À surface of water contained in a vessel of finite dimen-
(486.) The two great causes, however, which influ- Principal Herz (481.) It may be announced as a general propo- ence evaporation, and modify its results, are the tem- causes
sition, that the rate of evaporation is always propor- perature and movements of the atmosphere, into the which mo
tional to the area of the humid surface, and, in air effects of which we must more particularly inquire. results of sza of entirely quiescent, it appears, that position exerts but And first with respect to temperature. Every degree evaporation. and very little influence. A sheet of paper, observes Leslie, of heat seems to produce evaporation from water, nor Tempera
applied to a plate of glass in a close room, will lose its can the lowest temperature, in a proper condition of the ture.
Meteorair, entirely destroy this power. Evaporation, indeed, as (488.) In the next Table we have given Mr. Dalton's Meteor ology. Dalton remarks, continues to act below the point of con- results at the successive temperatures recorded in it, withology.
gelation as well as above it, and we owe to that pro- different velocities of the atmosphere, but in a perfectly. found inquirer, the best experiments we possess respecting dry state of the air. The three last columns of the the rate at which water evaporates under different de Table show the quantity of vapour in grains driven off grees of heat. In an atmosphere perfectly dry and calm in a minute from a circular vessel of water six inches in he obtained the following results.
diameter. TABLE CIF.
Rate of Evaporation
20° 21 22 23 24 25 26 27 28 29 30 31 32
Rate of eva Thus the rate of evaporation under the circumstances
the vapour produced. elasticity of
(487.) Concerning the changes which the rate of evapothe vapour ration undergoes in consequence of the air in contact with produced the evaporating water being either quiescent, or moving Least eva- with different velocities over it, it may in the first place be reporation marked, that air in a state of perfect repose and under conproduced
stant circumstances of temperature and pressure, produces when air is perfectly
the least possible degree of evaporation, and that, in such quiescent. a condition of things, the vapour, as it forms, accumulates
over the evaporating surface, and by successively check
ing the evaporating power, ultimately reduces it to A current of nothing. But a current of air with whatever velocity it air accele
may proceed, by bringing new portions of air less saturates it.
rated with humidity over the evaporating surface, must
again renew the evaporating power; and thus the more The more rapid the current, the more rapidly, cæteris paribus, rapid the
must the water disappear. As a proof of this we may current the adduce the important experiments of Mr. Dalton, who evaporation, found, that at the temperature of the boiling point, the other things least evaporation took place from water, when the evabeing the porating surface was placed in the middle of a room, same. with the doors and windows closed. Under these cirMr. Dalton's re
cumstances the rate of evaporation was 30 grains per sults.
minute. On placing the water in the chimney, with the
existing in the chimney increased the evaporating power Effects of
to 35 grains a minute. An increase in the fire, by augcurrents.
menting the current in the chimney, raised the evapora-
air moving with an infinite velocity, as Biot remarks,
at different heights is given by Mr. Howard. During
35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77
0.129 0.134 0.139 0.144 0.150 0.156 0.162 0.168 0.174 0.180 0.186 0.193 0.200 0.207 0.214 0.221 0.229 0.237 0.245 0.254 0.263 0.273 0.283 0.294 0.305 0.316 0.327 0.339 0.351 0.363 0.375 0.388 0.401 0.415 0.429 0.443 0.458 0.474 0.490 0.507 0.524 0.542 0.560 0.578 0.597 0.616 0.635 0.655 0.676 0.698 0.721 0.745 0.770 0.796 0.823 0.851 0.880 0.910 0.940 0.971 1.000 1.040 1.070 1.100 1.140 1.170
0.67 0.69 0.71 0.73 0.77 0.79 0.82 0.86 0.90 0.93 0.95 0.99 1.03 1.07 1.11 1.14 1.18 1.22 1.26 1.31 1.35 1.40 1.45 1.51 1.57 1.62 1.68 1.75 1.80 1.86 1.92 1.99 2.06 2.13 2.20 2.28 2.35 2.43 2.52 2.61 2.70 2.79 2.88 2.97 3.07 3.16 3.27 3.37 3.47 3.59 3.70 3.83 3.96 4.09 4.23 4.37 4,52 4.68 4.83 4.99 5.14 5.35 5.50 5.66 5.86 6.07
0.82 0.85 0.88 0.91 0.94 0.97 1.02 1.05 1.10 1.13 1.17 1,21 1.26 1.30 1.35 1.39 1.45 1.49 1.54 1.60 1.65 1.71 1.78 1.85 1.92 1.99 2.06 2.13 2.20 2.28 2.36 2.44 2.51 2.61 2.69 2.78 2.88 2.98 3.08 3.19 3.30 3.41 3.52 3.63 3.76 3.87 3.99 4.12 4.24 4.38 4.53 4.68 4.81 5.00 5.17 5.34 5.53 5.72 5.91 6.10 6.29 6.54 6.73 6.91 7.17 7.46
83 84 85
Meteor (489.) But this Table has been constructed on the indicates, its value must be 120; in a moderate breeze Meteorolezysupposition that no vapour previously exists in the air, 150, and in a high wind 180; so that denoting the
ology. a hypothesis which can never be verified in any con- results of evaporation under these different conditions
Formula Freerung dition of the atmosphere, since at every season, moisture by E, E', and E", we shall have
for estiLad a more or less abounds in it. In cases where the elastic
mating the spasition forces are considerable, the influences of this vapour are
rate of eva
30 insensible, but at lower temperatures its amount must
poration on 150
different by no means be neglected. If we suppose the water of evaporation to have a temperature of 50°, we shall find
velocities of 30
the air, the force of vapour at that temperature exactly th of
180 its force at 2120, and therefore, from what has been be
(f-f") = 6(f-) fore advanced, the rate of evaporation ought to be sloth
30 also. If, however, at the time of observation, an (492.) These formulæ will enable us to resolve some aqueous atmosphere already existed to that amount, or, interesting Meteorological problems. In the first place in other words, the air be completely saturated with mois- by knowing the functions f and f', that the maximum ture, no evaporation can possibly take place, and the tension of the aqueous vapour for the observed temperamount of vapour must remain unchanged. But if the ature, and the tension of the vapour existing in the force of the aqueous atmosphere should be less than atmospheric beds at the same time, and multiplying that which a complete saturation admits, there will their difference by the coefficient depending on the be room for the evaporating power to become active, state of the atmosphere as to wind, will at once deterand a quantity of vapour, dependent on the difference mine the rate of evaporation. If we take the case when of the temperature of the water and of the vapour a moderate breeze prevails, and suppose the temperaalready existing in the air, will be raised.
ture 70°, and the amount of the observed humidity 1. Delton (490.) To measure the effect of this humidity of the in the air to be 0.388 inches of the mercurial column,
atnosphere, Mr. Dalton endeavoured to discover the then the whole amount of the elastic force being 0.721 exact quantity of vapour existing at the moment of inches, we shall have observation. He took a tall cylindrical glass jar, dry on E' = 5(f-f') = 5 (0.721 – 0.388) = 1.665 grains, Example of the outside, and filled it with cold water fresh from a well. If dew were immediately formed on the outside, being the rate of evaporation per minute from a circular poration de he poured the water out, and allowed it to stand some surface of water six inches diameter and one inch deep, termined. time to augment its heat, at the same time carefully under the circumstances stated. drying the outside of the glass with a linen cloth. (493.) This may be regarded as an example of the This operation was continued till dew ceased to be formed, direct use of one of the formulæ, but they may be all when the temperature of the water and the force of applied to other useful purposes. If we wish to detervapour were determined. The experiment was performed mine the actual tension of the vapour really contained in either in the open air, or at a window, on account of the atmosphere at the time of observation, and we know the internal air being generally more humid than that by some previous experiment the rate of evaporation, which is without. Spring water being commonly, also, and supposing, moreover, a high wind to prevail
, we at about 50° of temperature, was made use of for the shall have three hottest months of the year, but, at other seasons, a
E" = 6 (f-f'), cold, artificial mixture was employed. To estimate the and from which may be deduced evaporating power, water was introduced into a tin
E' vessel of a given diameter suspended from the arm of a
6 balance, and the exact loss of weight from evaporation found. Then denoting the whole amount of the elastic and which will therefore give a measure for the actual Example of force of vapour at the term of saturation corresponding humidity of the air under the given circumstances of f the actual to the observed temperature by f, and the actual force of and E". If we suppose the first of these elements to be humidity of
the air deFapour already existing in the air by f', Mr. Dalton 0.524 inches, and the second 0.324 grains, the tempera- termined found under all conditions of the atmosphere, whether ture, as the value of findicates, being 60°, we shall by aid of
the rate of mortatica
perfectly quiescent or agitated by the wind, the rate of obtain from the particular formula we have selected evaporation to be constantly proportional to
0.312 f- f';
f' = 0.524
= 0.524 - 0.052 = 0.472 inches,
6 or, in other words, that if the whole tension of the and which may be regarded as an exact and simple vapour at the temperature of the boiling point be denoted means of determining the value of f'. by F, and that at this same temperature the weight of (494.) We might also discover the whole amount of
Whole water evaporated in a minute in dry air be denoted by humidity corresponding to the term of saturation for the tension of m for a given unit of surface, the quantity of vapour to observed temperature, by previously obtaining the values vapour if be obtained at any other temperature, but with the same of E and f'; but no real advantage would result from necessary. conditions of repose or agitation of the air, will be ex- it, as we are already in possession of a better method of pressed by the function
determining it, and even by a formula which will give m (f-f')
us its value at any temperature.
(495.) The rate of evaporation is also influenced by Rate of evaF
the density of the air, increasing as that density lessens, poration in(491.) The value of the coefficient m will of course and exhibiting its greatest degree of power in a va- fluenced by be different under different circumstances of the atmo
Mr. Daniell, in an ingenious course of experi
the air, sphere. In a calm and tranquil air, as our Table CIII. ments, has traced the relation between the increase of
Meteor. the evaporating power and the diminution of the atmo- free it from air, was weighed in a very sensible balance, Meteorology. spheric pressure. For this purpose he enclosed in a before it was exposed to the action of the sulphuric ology.
glass receiver, upon the plate of an air-pump, a vessel Acid under the receiver. Its temperature in this situaMr. Da
with sulphuric Acid, and another with water, and by tion was 45°, and the height of the barometer 30.4. In riments.
properly adjusting the surfaces of the two, was enabled half an hour it was again weighed, and the loss by eva-
Results, face; and the third to ascertain the Dew point, by means
39.40 three inches above that of the acid. A
delicate thermometer rested in the water upon the bottom of the glass, and another was suspended in the air. The
When the exhaustion was pushed to the utmost, the sides of the vessel were perpendicular to its bottom, gauge stood at 0.07, and the evaporation in the half which was perfectly flat. The height of the barometer
hour amounted to 87.22 grains. During this last was 29.6, and the temperature of the water 56°. In experiment, the water was frozen in about eight minutes, twenty minutes from the beginning of the experiment, while the thermometer under the ice denoted a temperwas
ature of 37o. moisture was found at 26°.
(501.) These results require, however, some correc- Correction Successive (497.) This being the greatest degree of cold which
tions for the variations of temperature which took place of these re experi- could be conveniently produced by the affusion of ether, during their progress. The rate ef evaporation having sults for men's.
Mr. Daniell repeated the experiment with a contrivance been proved proportional to the elasticity of vapour, we variations which admitted the application of a mixture of pounded must estimate the latter from the mean of the temper
that the method of estimating the temperature of the
temperature of 45°.
Evaporation in Grains. danger of error, that the sulphuric Acid, under these cir
1.24 cumstances, maintained the air in a state of alınost
2.97 complete dryness.
5.68 (499.) In a second experiment, the same trial was
9.12 made with atmospheres variously rarefied.. No deposi
15.92 tion of moisture was in any case perceived, with the
29.33 utmost depression of temperature it was possible to pro
50.74 duce; and the state of dryness was as great in the most
* It will be remarked by referring to our Table CIII. that Mr.
Dalton found the full evaporating force of water at the temperature (500.) In a third experiment, the water which had of 45°, to be 1.26 grains per minute, from a vessel of six inches been previously exposed to the vacuum of the pump to diameter.