" 



AMOUNT OF ATMOSPHERIC PRESSURE. 239 



but it must be boiled in it for some time, so that every 

 trace of air may be swept out by the vapour of mer- 

 cury which is formed when the latter is boiled. The 

 construction of a good barometer is difficult, and can only 

 be undertaken by skilful and experienced workmen. 



A kind of barometer which is much more portable 

 than the long tubes of the mercurial barometer, and 

 less liable to be broken, has recently come into use 

 under the name Aneroid-barometer. The essential part 

 of this barometer is an elastic metallic chamber, either 

 : in the form of a flat box or of a short tube, which has 

 been exhausted of its air and hermetically closed: if the 

 pressure of the air increases, the chamber is somewhat 

 compressed; if the pressure diminishes the chamber 

 expands, and these variations are transmitted by a 

 system of wheels and levers to a pointer, which travels 

 over a dial; the dial is graduated by comparison with a 

 mercurial barometer. 



The height of the mercurial column supported by 

 ; the atmospheric pressure being known, the weight can 

 be calculated which is equal to that pressure upon a 

 given surface. The pressure upon a square centimetre 

 is equal to the weight of a volume of mercury of 76 CC , 

 that is, it equals 76 x 13-6 = 1033 gr '6 or about l kgr . 



e body of a man has on an average a surface of 

 15,000 square centimetres: the pressure of the atmo- 

 sphere upon it is therefore 15,000 kgr , or nearly 15 tons. 

 Such an enormous pressure might seem impossible to be 

 borne, but it must be remembered that the pressures are 

 everywhere equal, but act. at opposite sides in contrary 

 directions and hence counterbalance one another. It 

 might also be supposed that the effect of this force would 



