On Atmospheric Density and Pressure. 4t<J 



rjther : — however, this may not be the case ; \t (the atmo- 

 sphere) may contain many particles, and these particles 

 closely approximated together by a strong cohesive force ; 

 but this permeability may arise from their extreme minute- 

 ness and want of gravity, from their spherical form, (their 

 angles being blunted off by friction,) and their thus sliding 

 through, among, and under each other, yielding to, and 

 being impelled by every motion communicated. Indeed the 

 latter hypothesis, I think, seems the more probable. How- 

 ever, from whatever cause it is, a degree of tenuity or 

 permeability arises which scarcely any other body possesses^ 

 and which may be estimated by the velocity with which 

 heat, light and sound travel through it. 



Atmospheric density is generally confounded with atmo- 

 spheric pressure, though distinct from each other; the one 

 is a property it possesses in common with all other bodies, 

 the other is peculiar to itself alone. In the one there ap- 

 pears an inherent and self-existent direction of the particles, 

 and this direction seems positive and determinate ; in the 

 other there appears no direction but what is given by exter- 

 nal and accidental causes, and therefore quite vague and 

 indeterminate, and only what is possessed by all matter. 

 Atmospheric pressure may be shown in various ways, per- 

 haps one of the simplest is thus :-— invert over a bason of 

 water, a tumbler previously exhausted of its air; the water 

 of the bason will ascend in the tumbler much higher (ac- 

 cording to the dimensions of its column) than its level in 

 the basort, — being pressed down by the external atmosphere^ 

 its particles are forced to cohere closer together, until the 

 force of pressure is withdrawn ; they then recede from each 

 other, the interstices between them are'increased, and thus 

 taking up a greater space, they consequently ascend, being 

 prevented by the sides of the tumbler from expanding la- 

 terally. 



Now surely it is inconceivable trfat a body weighing only 

 the eight or nine hundredth part that of another, could pos- 

 sess the power of raising it up, and consequently forcing it 

 to assume a contrary direction to that gravitating one which 

 it, though in so alight a degree, possesses in common with 



D d 2 other 



