♦20 0?h Atmospheric Density and Pressure, 



other bodies, were it not for a self-impelling self-existing 

 force in the atmosphere. For instance, were the experiment 

 tried under a fluid still heavier than air, but lighter than 

 water (oil for example), in a vacuum, Quere, Will not the 

 •water remain level inside and outside? 



Hence we must argue, that pressure is distinct from den- 

 sity, self-existing in the atmosphere, and in that alone; 

 nor even do they always act directly of each other, particu- 

 larly in the upper regions of the atmosphere. (See Bouguer's 

 Travels &c. in the Andes.) However, although they do 

 not appear to be one and the same power, yet they seem 

 intimately connected, and to act directly with each other, 

 and what affects the one affects the other. 



Galilei was the iirst discoverer of atmospheric pressure, 

 by observing that fluids rose to a certain height in a vacuum, 

 which had formerly been accounted for by the old idea of 

 W Nature abhorring a vacat^m,*' 



Torricelli, his pupil, completed the discovery*, observing 

 that the atmosphere pressed equally on all bodies' at the 

 earth's surface, and in proportion to their densities; viz., 

 more on a fluid than a solid, and more on a gas than either: 

 and as fluids are of different densities, and the only bodies 

 susceptible of being acted on by this pressure, from the 

 slight cohesion and consequent motion of their parts, he 

 natural! v conceived that the rarer the fluid the higher it 

 would rise in a vacuum, and vice i>et\sa. 



Thus, he first found that water, at the medium tempera- 

 ture of the atmosphere, would ascend about 32 feet, con- 

 sidering this the rarest fluid, which height or column of 32 

 feet was consequently an equipoise to a column as high as 

 the atmosphere: — calculating the comparative densities of 

 fluids, he conceived that one twice as dense as water would 

 ascend half as high, and so on, the density of the fluid being 

 inversely as the height of its ascent. Then, considering the 

 comparative densitv of water, the rarest, with that of mer- 

 cury, the densest fluid known, he found it to be 14*1 ; con- 



• After Torricelli, Pascal brought this discovery to still further perfection, 

 and made many important additions, particularly by his celebrated experi- 

 ments on the Puy dc Dome. 



sequent!/, 



