VOL. XXXVI.] PHILOSOPHICAL TRANSACTIONS. 325 



power of each other's attraction, and form such large drops as, being specifi- 

 cally heavier than any air, must fall in rain. For no gentle descent of a cloud, 

 but only an accelerated motion downwards, produces rain. 



Not however that the quick descent of a cloud is the only cause of rain: 

 for the shock from a flash of lightning, and the sudden return of the air, 

 after the vacuum made by the flash, will condense the floating vapour into 

 water ; and also the same cloud which, in the free air, might be carried hori- 

 zontally without being turned into rain, meeting with a high hill in its way, 

 will be condensed and fall in drops; especially if, in the day-time, it be 

 driven by the wind out of the sun-shine, against the shaded side of the 

 mountain. 



There is another opinion concerning the rise of vapours, namely, that 

 though water be specifically heavier than air, yet if its surface be increased 

 by very much diminishing the bulk of its particles, when once raised, it cannot 

 easily fall; because the weight of each particle diminishes as the cube of 

 its diameter, and the surface to which the air resists, only as the square 

 of the said diameter : that we see this in the dust in summer, and in 

 menstruums that sustain metals dissolved, which are specifically heavier than 

 the menstruums. 



But this will not explain the phenomenon ; because, though the increase of 

 surface, the weight remaining the same, will in a great measure retard the 

 descent of small bodies moving in the air, by its great resistance to so large a 

 surface ; it will for the same reason also hinder the ascent. For the rise of 

 dust is owing to the motion of animals feet in it, or to the wind : whereas 

 vapours rise in calm weather, as well as windy; neither do they, like the dust, 

 always fall to the ground when the wind ceases to blow. 



The third opinion, and which is most commonly received, is, that by the 

 action of the sun on the water, small particles of water are formed into hollow 

 spherules filled with an aura, or finer air highly rarefied, so as to become spe- 

 cifically lighter than common air, and consequently that they must rise in it 

 by hydrostatical laws. As for example, if a particle of water, as it becomes a 

 hollow sphere, be only increased 10 times in diameter, its bulk will be in- 

 creased 1000 times; therefore it will then be specifically lighter than common 

 air, whose specific gravity is to that of water, as I to 850; then if the density 

 of the aura, or spirit within the little shell, be supposed Q times less than that 

 of air, or as 50 to 850, that specific gravity of the shell and its contents, will 

 be to that of air, as goO to 1000; therefore such an aqueous bubble must rise 

 till it comes to an equilibrium in air, whose density is to the density of that 



