330 PHILOSOPHICAL TRANSACTIONS. [aNNO 1/29. 



of summer 5 ; the mean heat of spring or autumn 3 ; and the least degree oi' 

 heat, at which vapours rise in winter, or the mean heat of winter, is 1. The 

 rarity of vapour proportionable to these four degrees of heat is 14000, 2058, 

 1235, and 823. The rarity of air is, in summer gOO, in spring or autumn 

 850, and in winter 800; the density of water, compared with the abovemen- 

 tioned densities, being inversely as one to the said forementioned four numbers. 

 The heights above the earth to which the vapours will rise, and at which they 

 will be in equilibrio, in air of the same density with themselves, will vary ac- 

 cording to the rarity of the vapour depending on the heat of the season. For 

 the vapour which is raised by the winter's heat, expressed by the number 2, 

 when the air's rarity is 800, will rise to, and settle at, a height of about the 

 6th of a mile, when the barometer is above 30 inches high. But if the heat 

 be greater then, the vapours will rise higher, and pretty much higher if the 

 sun shines, though in frosty weather, the barometer being then very high. If 

 the barometer falls, and thereby brings the place of equilibrium, for vapours 

 raised by the heat 2, nearer the earth, then also will the heat be increased, the 

 vapour more rarefied, and consequently the new place of equilibrium sufficiently 

 high. It is to be observed, that in winter, when the heat is only equal to 2, 

 the air is densest close to the earth, which has not any heat sufficient to rarefy 

 it near the ground, as happens in warm weather; therefore the vapour will 

 rise gradually in air whose density decreases continually from the earth up- 

 wards ; neither will the vapour be hindered of its full rise, by any condensa 

 tion from a greater cold of the ambient air, the air being then as cold next 

 to the ground, where the vapour begins to rise, as it is at any height from 

 the earth. 



The vapour which is raised by the heat of spring or autumn, expressed by 

 the number 3, will rise to the height of 3^ miles, when the barometer is at 30, 

 and the air's rarity is 850. But Uien, as the air is hotter nearer the ground 

 than at the height of half a mile or a mile, the vapour will condense as it rises; 

 and as the air, when the earth is heated, is rarer near the ground than at some 

 height from it, the place of equilibrium for vapour will, on these two accounts, 

 be brought much lower than otherwise it would be, as for example, to the 

 height of about a mile, which will agree with phaenomena. 



In summer, the two causes abovementioned increasing, the vapour raised by 

 the heat 5, whose place of equilibrium would be 5i miles high, if the vapour 

 after it began to rise was not condensed by cooling, and the air was densest 

 close to the earth, will settle at the height of about 1^ or 2 miles, which is also 

 agreeable to phasnomena. 



Lastly, as the density and rarity of the vapour is chiefly owing to its degree 

 of heat, and in a small measure to the increased or diminished pressure of the 



