63 PHYSIOGRAPHY. [chap. 



Since air possesses weight, it necessarily presses upon any 

 object exposed to its influence. The atmosphere forms an 

 ocean of air bathing the entire earth ; and, on the floor of 

 this ocean, man, in common with all terrestrial beings, has 

 his dwelling. Everything around us on the earth's surface 

 must therefore bear the pressure of the air above, just as 

 anything on the bed of the ocean is "pressed upon by the 

 superincumbent water. The depth, or rather the height of 

 this aerial sea has never been determined, but there are 

 reasons for believing that the atmosphere extends to at 

 least 50 miles upwards from the earth's surface. Hence 

 it is clear that all terrestrial objects must be subjected to 

 enormous pressure. The roof of a house, for example, has 

 to bear the pressure of a column of air resting upon its 

 surface and extending upwards to the limit of the atmo- 

 sphere. Now, it is found that our atmosphere exerts 

 a pressure of nearly 15 lbs. (1473 lbs.) on every exposed 

 square inch of surface. The roof is consequently pressed 

 upon by a weight of many tons. Yet the most delicate 

 structure may be freely exposed to the atmosphere witliout 

 the slightest danger of being crushed. This arises from 

 the fact \hz.\. fluids'^ transmit pressure in a manner entirely 

 different from that in which it is transmitted by solid 

 bodies. A solid presses downwards only, but a fluid 

 presses equally in all directions, upwards as well as dovn- 

 wards. The air in a room, for instance, presses on the 

 ceiling not less than on the floor ; and on each of the 

 walls not less than on the ceiling. Under ordinary con- 

 ditions, therefore, the atmosphere has no power to crusli, 

 because its pressure downwards is exactly neutralised by 



1 Fluid, from fliio, I flow ; a tenii embracing both liquids and gases 

 or vapours, since the particles of both classes of bodies flow freely over 

 each other. 



