418 On Atmospheric Density and Pressure. 



ability in a body must arise not only from the closeness 

 with which its integrant parts are connected, but also from 

 the weight of these parts, and consequently the difficulty of 

 removing them ; so that, in other words, density, I think, 

 may be defined, that power resulting from the union of the 

 attraction of cohesion and gravitation. 



That the atmosphere is possessed of this power is well 

 known: — take two bodies of different - weights but equal 

 bulks, drop them from the same height, it will be found 

 that the differences of their velocity in descent will be di- 

 rectly as the differences of their weights, and as the dif- 

 ferences of the times of their descent, and that consequently 

 the spaces of atmosphere descended are directly as the squares 

 of the times, and as the squares of their velocity in falling. 

 To this it may be answered, that gravity is the sole cause, 

 and that the air has no effect whatever : however, that this 

 is not the case is well known by the old experiment of a 

 feather and guinea falling alike in the exhausted receiver of 

 an air-pump. Hence, were it not for the greater or less re- 

 sistance or impermeability of the atmosphere, the force of 

 gravitation would be equal in bodies; their absolute gravity 

 depends, more or less, on the density of the atmosphere, 

 not the latter on the former. 



Now -it is obvious that this power can only exist where 

 there are particles or molecules (see Boscovich) : it must 

 depend on the disposition of those particles, and even on 

 the particles of those particles to the ultimate one, viz., on 

 their size, their shape, form, &c., and their degree of 

 aggregation and consequent distance from each other. At- 

 mospheric air, therefore, must have its particles or mole- 

 cules, (though completely imperceptible to our organs of 

 ■sensation,) certainly, from its great permeability, its yielding 

 to the slightest impression, and affording little or no resist- 

 ance to the insinuation of bodies denser between the in- 

 terstices of its particles : this power in atmospheric air may 

 almost be said to be at its minimum. 



This permeability in atmospheric airarises, it is supposed, 

 from the very slight aggregation of its particles, and conse- 

 quently their small quantity and great distance from each 



other: — 



