COMMON THINGS AIR. 



they exert, acts equally in all possible directions. Thus, if any 

 body be let down into the sea, the weight of the water, which is 

 above it, will press equally on its top, bottom, and sides. It is 

 very easy to demonstrate this by a simple experiment. 



Let several empty bottles be carefully corked, and being loaded 

 with weights so as to sink in the water, the neck of one being 

 presented upwards, that of another downwards, another horizontal, 

 and the others oblique in various degrees, it will be found that 

 when they have been sunk to a certain depth, the corks will 

 be all forced into the bottles by the pressure of the surrounding 

 water, with which the bottles will be immediately filled, and 

 this will take place equally, and at the same time, with all the 

 bottles, in whatever directions the corks may be presented to the 

 water. 



It is evident, therefore, that the pressure produced by the weight 

 of the incumbent column of water at any given depth is equally 

 propagated in all directions, and that a body, a fish for example, 

 or the body of a diver, sustains that pressure, not downwards 

 only, or on the upper surface of the body as might be at first 

 imagined, but equally on the under surface, the sides, and, in 

 a word, on every part of the body in contact with the water. 



Now this equal transmission or propagation of pressure in all 

 directions, is not an exclusive property of water, but is common 

 to all substances whatever in the fluid state. Air possesses 

 fluidity in even a greater degree, if possible, than water, being 

 more freely mobile, and air accordingly transmits freely and 

 without diminution in all directions whatever any pressure 

 which it receives. The stratum of air in which we live is under 

 the pressure, as has just been stated, of the incumbent column 

 of air extending iipwards to the limits of the atmosphere, this 

 pressure amounting to 15 Ibs. on each square inch. A body, 

 therefore, exposed to the contact of this air is subject at all parts 

 of its surface, upper, under, and lateral, to this pressure ; and the 

 total amount of the pressure by which it is affected will be expressed 

 in pounds weight by the number obtained by multiplying the 

 number of square inches in its entire surface by 15. 



The body of a man of average size has a surface of about 2000 

 square inches. The total pressure which it sustains from the 

 surrounding air is therefore 15 X 2000, or 30000 Ibs., or nearly 

 fourteen tons ! 



9. It may seem wonderful that a force so enormous, acting on 

 all parts of the surface of the body, should not crush it and 

 actually destroy its delicately constructed organs. This, however, 

 is prevented by the perfect equilibrium of pressure outwards and 

 inwards, produced by the property of fluids just explained, in 

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