34 OF CLOUDS. CHAP. 1. 10. 



taneous formation of nimbi more than once. 

 Of this I shall say more hereafter, when I shall 



" At 152 7 it was only one fourth of that at boiling, or from 

 8 to 12 grains, according to circumstances. 



" The temperature of 144 affords one fifth of the effect at 

 boiling; 138 gave one sixth, &c. 



" Having previously to these experiments determined the 

 force of aqueous vapour at all the temperatures under 212\ I 

 was naturally led to examine whether the quantity of water 

 evaporated in a given time bore any proportion to the force of 

 vapour of the same temperature, and was agreeably surprised 

 to find that they exactly corresponded in every part of the 

 thermometric scale; thus the forces of vapour at 212, 180, 

 164, 152, 144, and 138, are equal to 30, 15, 10, 7|, 6, 

 and 5 inches of mercury respectively: and the grains of water 

 evaporated per minute in those temperatures were 30, 15, 10, 

 7f, 6, and 5, also; or numbers proportional to these. Indeed 

 it should be so from the established law of mechanics, that all 

 effects are proportional to the causes producing them. The 

 atmosphere, it should seem, obstructs the diffusion of vapour, 

 which would otherwise be almost instantaneous, as in vacua; 

 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 vis inertice of the particles 

 of air, and is similar to that which a stream of water meets 

 with in descending among pebbles. 



" The theory of evaporation being thus manifested from 

 experiments in high temperatures, I found that if it was to be 

 verified by experiments in low temperatures regard must be 



