

PRESSURE OF THE ATMOSPHERE. 229 



It is the pressure of the atmosphere which causes the 

 water to remain in the vessel, in apparent opposition 

 to the action of gravity. Atmospheric air has weight, 

 and, like a liquid, it exerts, in virtue of its weight, a 

 certain pressure upon all bodies immersed in it. Now, 

 although the specific gravity of air is very small, the 

 pressure which the air exerts is very considerable, 

 because the atmosphere reaches to an immense height, 

 or, in other words, because the surface of the solid 

 earth is the bottom of an ocean of air which has an 

 enormous depth. 



The pressure of a gas agrees also in this particular 

 with that of a liquid, that at any point the pressure is 

 equal in all directions; a surface turned upwards is 

 pressed downwards with the same pressure with which 

 that surface is pressed upwards when it is directed 

 downwards :. this is the case with the pressure upon 

 the surface of the paper. The paper prevents the 

 escape of water at one point and the entrance of air 

 s at another, which would happen if the surface of the 

 water were not perfectly horizontal ; but it is impossible 

 to maintain that surface horizontal without the paper. 

 That the paper is not, in this case, a kind of lid 

 used for confining the water in the vessel, may be 

 easily proved by tying a piece of ' bobbin-net,' having 

 'its meshes as wide as shown in fig. 157, or even a 

 I little wider, over ajar of about 0*5 or 1 litre capacity. 

 The jar may then be filled with liquid and emptied 

 again just as if no net were tied over it; but if it is- 

 quite filled with water, covered with a plate,, and 

 inverted so as to keep the mouth as horizontal as 

 possible, the plate may be removed, and the water will 



