581 



HYGROMETRY. 



Hygrorae- 



The co-efficients m and n being two constant quantities 

 to be determined by experiment, the equation will re- 

 sume the simplicity of its original form, by supposing 



~ = p ; we thus have, D = p (f t /). This equa- 



tion, however, will only express the value of D by a 

 near approximation. To render the expression con- 

 sistent with the properties of the curve, whose ordi- 

 nates represent the progressive reductions of tempera- 

 ture by evaporation from the moistened bulbs, it seems 

 necessary to give it the form, 



Investiga- 



Leslie'shy 

 grometer. 



58. The co-efficients p and r, particularly the former, 

 jj e ; n g important elements in any equation which should 

 express the quantity of moisture contained in the at- 

 mosphere, in terms of f t , f and D, we instituted a 

 ee ries of the most laborious experiments to determine 

 their values with accuracy. For this purpose we had 

 recourse to Mr Dalton's method of finding the point of 

 deposition, whose elastic force we formerly denoted by 

 <f> ; and, from the precautions we observed, we have 



reason to think, that our experiments were calculated 

 to determine that point within the fifth part of a degree 

 of Fahrenheit. We generally employed two jars of a 

 cylindrical shape, one of glass, and the other of tin- 

 plate. Each of them was about eight inches deep, and 

 six inches in diameter ; it being convenient to have them 

 of a considerable size, that their temperature, when 

 they are filled with cold water, may not be raised too 

 quickly by the superior temperature of the surrounding 

 bodies. We also used occasionally, for the same pur- 

 pose, a vessel of silver, which, on account of its resplen- 

 dent surface, is admirably fitted to shew the slightest 

 deposition of moisture on it. These vessels were filled 

 with water, cooled down several degrees below the 

 point of deposition, and placed near one another on a 

 table, at a sufficient distance from the thermometers 

 employed to indicate the temperature of the air t, and 

 the point of depression of the moistened bulb produced 

 by evaporation. The experiments were generally per- 

 formed in a large octagonal apartment, about 50 feet in 

 diameter, and 30 feet high; and in order that the 

 smallest deposition of moisture might be perceptible, 

 the jars were observed at the distance of 15 feet with a 

 powerful telescope. By this means, when no sensible 

 deposition appeared to the naked eye however closely 

 the jars were examined, the drops of moisture, which 

 formed on their surfaces, were so much magnified, as 

 to be seen increasing gradually in size. The jars were 

 wiped from time to time with a clean dry towel ; but 

 unless this operation was performed with the utmost 

 care, the telescope discovered large tracts of moisture, 

 which could not be discerned without its assistance. 

 The state of the thermometers was accurately observed 

 at the same time, and a rotatory motion frequently 

 given to the water. When the deposition of moisture 

 on the surface of the jars was no longer perceptible, 

 the temperature of the different thermometers was care- 

 fully noted. Sometimes the temperature of the water 

 in the jars was again reduced, by stirring bits of ice in 

 it till it was lowered a degree or two below the point of 

 deposition, to remove all chance of error, and ascertain 

 the limits within which it might range. The difference 

 in the result seldom exceeded the fourth part of a de- 



gree. It is proper also to remark, that the point of de- Hygrome. 

 position, as determined by the different jars, was always .^ ry '_ l _ l . 

 the same. "v^ 



59. The temperature of the air at the time of these 

 experiments was observed with two excellent mercurial 

 thermometers, one of which was made by Jones, and the 

 other by Adie ; and the temperature of the water in . 

 the jars was determined by a good thermometer of 

 Crichton. The reduction of temperature produced by 

 evaporation was ascertained by two mercurial thermo- 

 meters ; two thermometers filled with spirit of wine, 

 and covered with moistened tissue paper ; and two hy- 

 grometers of Mr Leslie. One of the mercurial thermo- 

 meters possessed the utmost delicacy, and had a range 

 of scale, which easily gave the 10th part of a degree. 

 The largest of the spirit of wine thermometers had a 

 bulb 2-f- inches in diameter ; and its tube, which was 

 two feet long, included a range of scale from 32 to 60. 

 This instrument, on account of its size, and the great 

 capacity of alcohol for caloric, was more slowly reduced 

 in its temperature than the other thermometers with 

 covered bulbs ; but, after a certain lapse of time, all of 

 them became stationary nearly at the same point, the 

 difference never exceeding a small fraction of a degree. 

 It is easy to perceive, indeed, that whether the thermo- 

 metrical ball be large or small, the inclosed fluid, what- 

 ever it may be, must ultimately reach the same tempe- 

 rature ; for since the quantity f t f, which regulates 

 the evaporation, is constant, the evaporation must al- 

 ways cool down the moistened surface to the same 

 pitch, and thus gradually abstract heat from the ther- 

 mometrical fluid, precisely in the same manner as if the 

 bulb consisted entirely of water, (due allowance being 

 made for the difference of capacity for caloric,) until the 

 reduction of temperature is counterbalanced by the in- 

 flux of heat from the air. If the specific heat of the 

 inclosed fluid be great, the process will be longer in 

 producing its maximum of effect; but the diminution 

 of temperature by evaporation, and the influx of heat by 

 communication, being in all cases to each other in the 

 same constant ratio, the reduction of temperature must 

 be always the same. Hence the diminution of tempe- 

 rature indicated by Leslie's hygrometer invariably cor- 

 responds to that of the most sluggish thermometer, with 

 a moistened bulb. 



60. To determine what influence the atmospherical i n fl llence 

 pressure might have on these results, Leslie's hygrorae- of atmo- 

 ter, having its bulb duly moistened, was placed with a spherical 

 quantity of sulphuric acid in a cup, under a large re- pressure on 

 ceiver, on the plate of an air-pump, and allowed to re- evapori 

 main till it became stationary. In a few minutes it 



sunk down to 26 ; the temperature at the time being 

 48^, and the atmospherical pressure 29.6. The air 

 was then exhausted till the gauge stood successively at 

 6, 12, 18, and 24> inches, when the following results 

 were obtained: 



From these experiments it may be safely inferred, 

 3 



