MECHANICS 79 



because the lower strata have to bear all the weight of the 

 strata of air above them ; and the aeriform bodies being 

 exceedingly compressible, the lower strata are much the 

 denser, and density rapidly diminishes upwards. 



The body of an ordinary man has to sustain a pressure 

 of about 33,600 pounds or 15 tons ; but we are no more 

 inconvenienced thereby than is a fish by the pressure 

 of the ocean, and for the same reason.* The property 

 which both water and aeriform substances possess, of 

 pressing equally in all directions, serves no less to sustain 

 than to oppress. Thus the most delicate glass or other 

 vessel is enabled, without the slightest injury, to sustain 

 atmospheric pressure. If, however, by any contrivance 

 the air within such a vessel be removed, then the vessel 

 will be immediately crushed. 



There are machines to remove air from a vessel, 

 as well as to force more into it such as an exhaust- 

 ing syringe (for the former process) and a condensing 

 syringe (for the latter purpose). By means of the con- 

 densing syringe, air may be added to what was already con- 

 tained, e.g., in a copper flask of 100 cubic inches capacity. 

 Thereupon the flask will be found to have increased in 

 weight. If, on the contrary, the air is removed from 

 such a flask by means of an exhausting syringe, it will 

 weigh 31 grains less, so that the weight of an ordinary 

 100 cubic inches of air must be 31 grains. 



Another instrument for this purpose is called an air- 

 pump, and by it air can be removed from the interior of 

 a large and strong glass-vessel called a " receiver." If 

 before it is emptied (so far as it can be emptied) of air, 

 a delicate glass-vessel, perfectly closed and containing 

 air, be put within it, then, as the air is removed from 



* See ante, p. 70. 



