H Y G R O M E T R Y. 



heit. The same air would consequently, at the depres- 

 sed temperature of 59, shrink into a state of absolute 

 saturation ; and if cooled lower, it would even deposit 

 a portion of its combined moisture, losing the eightieth 

 part of its weight at the verge of freezing " Such is 

 the result deduced by Mr Leslie. We shall now com- 

 pare it with the weight of the moisture in a cubic inch 

 of air, aa determined by our formula. The moistened 

 ball having a temperature of 68", while it indicates 52, 



the temperature of the air / is 68+ 9 *f 2 , or 77.2 ; 



5O 



the height of the barometer is not taken into account, 

 but we shall suppose it 30. We thus have, 





Th point 

 ttf 



tr-. ' I :., 



O-SSJtityrf 



nearly UK 

 -ne.ky 

 the two mc- 



447.4 + 77.2 



.10953 X .91631 .26694 X SO _ 

 = " 52+.rj 



This result would correspond by the Table to 66' 

 nearly, being 7 higher than the point of deposition 

 found by Mr Leslie. The absolute quantity of mois- 

 ture, however, in a cubic inch, as deduced by his me- 

 thod, agree* very nearly with the result given by 

 oar formula. According to his view, air absolutely 

 humid holds at the temperature of 59, the eightieth 

 put of hs weight of moisture ; now a cubic inch of at- 

 mospheric air, at the same temperature, weighs .3121 

 grains, and the eightieth part of that quantity is 

 .003901 2J gnins, which being corrected for the actual 

 temperature of the air, 77*.f , became* .003760? grains, 

 dMtfing very little from the quantity given by the for- 

 mula. In general, the coincidence of result is still more 

 striking. 



68. The method which Mr Leslie employed to de- 

 termine the relation between the degree* denoted by 

 hi* instrument, and the absolute quantity of moisture 

 in the atmosphere, is very ingenious, though on ac- 

 count of the necessity of employing logarithms, for the 

 interpolation of the term* of the series in which that 

 relation is expressed, it must often be attended with a 

 rood deal of trouble and inconveniency. By comparing 

 toe relative capacity of air for caloric, with the measure 

 of beat requisite to convert a given portion of water 

 into vapour, he concluded that atmospheric air, at the 

 temperature of the moistened ball, would take up the 

 1 6,000th part of it* weight for each degree marked by 

 the hygrometer. He was led to this conclusion, by as- 

 suming, from several concurring observations, that the 

 capacity of air for caloric was J of that of water, being 

 only a fifth part of what it is usually reckoned ; and as 

 the quantity of caloric necessary to convert water into 

 team is equal to 6000 degrees of hi* instrument, that 

 measure of beat would suffice to raise an equal mass of 

 air, 16000 millesimal degree*, or those 6000 degree* 

 increased in the proportion of 8 to 3. But, according 

 to the view taken by Mr Leslie at the state of equi- 

 poise, the portion of beat deposited by air in touching 

 the moistened bulb, or the depression of temperature 

 which it suffer* by this contact, is equal to the oppo- 

 site measure of heat abstracted by it, on dissolving it* 



587 



corresponding share of moisture. Wherefore, he con- 

 cludes, at the temperature of the wet ball, atmospheric 

 air would take up moisture amounting to the 16,000th 

 part of its weight, for each degree marked by the hy- 

 grometer. 



69. This view of the relation subsisting between the Objections 

 evaporating process, and the reduction of' temperature to^Mr Les- 

 which it induces, is somewhat different from the one 



we have taken in 5. According to the explana- [hTnsture 

 tion we have there given of that relation, the heat ne- O f the eva- 

 ceasary for converting the moisture applied to the bulb, pointing 

 into the state of vapour, is derived immediately from process, 

 the water itself, at the moment the vapour is disenga- 

 ged from its surface. The water having thus lost a 

 portion of its heat, instantly abstracts caloric from the 

 bulb, and the bulb, in its turn, from the inclosed ther- 

 mometrical fluid, until an equilibrium is established by 

 the influx of heat from the air, counterbalancing the 

 dispersion of it by evaporation. The heat of the air is 

 therefore imparted to the ball, merely by absorption ; 

 and *et limits to the progress of refrigeration, by 

 the increasing rate at which it flows into the cooling 

 surface, as the temperature of that surface recedes from 

 the temperature of the air. This view of the subject is 

 equally applicable to vaporization, whether the process 

 be carried on in the open air, or under an exhausted 

 receiver; and it is finely illustrated by the result of an 

 experiment which we have described under the article 

 EVAPORATION, p. S20, and which demonstrates that the 

 caloric necessary for the formation of vapour is derived 

 almost entirely from the water itself, and scarcely at 

 all from the contiguous bodies. In short, we cannot 

 perceive how Mr Leslie's hypothesis can afford an ex- 

 planation of the great reduction of temperature which 

 is produced by evaporation under an exhausted recei- 

 ver, where, according to his opinion, there is no solvent 

 present to convert the water into vapour. He speaks, 

 indeed, of air having its scale of watery solution ex- 

 tended by rarefaction ; but this is merely a gratuitous 

 accommodation of fact to theory, and does not at all 

 explain why the greatest solution takes place, when 

 the supposed solvent is most deficient in quantity, or 

 entirely excluded. At the same time, it must be ad- 

 mitted, that the results which he has deduced from the 

 theoretical view he has taken of his hygrometer, accord 

 remarkably well with the quantity of moisture in the 

 air, a* determined by actual experiment. 



70. In a preceding part of this article, ( 52.) we en- Confireu- 

 dearoured to shew that the point of deposition for any tioo of the 

 place must, in general, coincide nearly with the mini- g^^ 

 mum temperature for the *eaon. The truth of this opi- |(OJ "," t ^ 

 nion is amply confirmed by applying our formula, for p^t^and 

 Leslie'* hygrometer, to the very accurate observations the mini- 

 made with that instrument by Mr Gordon, to which we mum um- 

 formerly adverted. The point of deposition assigned by P*'ture. 

 the formula, will be found in the following Table to 

 correspond almost exactly with the mean minimum tem- 

 perature for each month, by actual observation. The 

 coincidence is greatest, as might be expected, when 



the temperature of the season is declining, and least 

 liable to sudden changes. The barometrical pressure 

 was introduced into the formula. 



