348 On the Modifications oj" Clouds, and 



" On the Evaporation of IVater below 212". 

 " I havefreq\ienlly tried the evaporation at all the tem- 

 peratures below 212:' it would be tedious to enter into de- 

 tail of all the experiments, but shall give the results at some 

 remarkable points. In all the high temperatures I used the 

 vessel above mentioned*, keeping a thermometer in it, by 

 which I could secure a constant heat, or at least keep it 

 oscillating within narrow limits. 



'' The evaporation from water of ISO" was from 18 to 2-2 

 grains per minute, according to circumstances ; or about 

 one-half of that at 212°. • 



" At 164° it was about one-third of the quantity at the 

 boiling temperature, or from 10 to Ifi grains per minute. 



" At 152^ it was only one-fourth of that at boiling, or 

 from 8 to 1 2 grains, according to circumstances. 



" The temperature of 144"aftbrds one-tifth of the effect at 

 boiling; 138° t>;ave one-sixth, &c. 



" Having previously to these experiments determined thft 

 force of aqueous vapour at all the temperatures under 212°, 

 I was naturally led to examine whether the quanlity of water 

 evaporated in a given time bore any proportion to the force 

 of vapour of the same temperature, and was agreeably sur- 

 prised to find that they exactly corresponded m every part of 

 the thermometric scale: thus the forces of vapour at 212°, 

 180°, 161°, 152°, 144°, and 138°, are equal to 30, 13, 10, 

 7 1, 6, and h inches of mercury respectively ; and the grains of 

 water evaporated per minute in those temperatures were 30, 

 15, 10, 7t, 6, and 5, also; or numbers proportional to these. 

 Indeed it should be so from the established law of mecha- 

 nics, that all effects are proportional to the causes producing 

 them. The atmosphere, it should seem, obstructs the dif- 

 fusion of vapour, which would otherwise be almost instan- 

 taiieous, as in vacuo: but this obstruction is overcome in 

 proportion to the force of the vapour. The obstruction, 

 however, cannot arise from the weight of the atmosphere, 

 as has till now been supposed ; for then it would effectually 

 prevent any vapour from arising under 212°: but it is caused 

 by the ?;« /;/t'r/;V^ of the particles of air, and is similar to 

 that which a stream of water meets with in descending 

 amoncst pebbles. 



" The theoi7 of evaporation being thus manifested from 



''* This refers to experiments on the evaporation of water at zn" i^pr 

 which sec the Essay. 



experiments 



