ELEMENTS OF GENERAL SCIENCE 



FIG. 4. Weight of air 



Tlic jilolx-, \vlik-h was balanced after part of the air had 



been pumped from it, was found to be heavier when the 



air was again allowed to enter it 



may be pumped from a brass globe or glass flask which is 

 so constructed that it may be closed after the air has been 



removed. The 

 closed vessel may 

 then be placed 

 upon a balance 

 (see fig. 4) and 

 weighed. If the 



vessel is opened 



(thus allowing air 

 to enter it) and 

 again weighed, 

 any difference in 

 weight that is 

 found will be due 

 to the air which 

 has come into the 

 vessel. Suppos- 

 ing all the air was pumped out in the first place, the increase 

 in weight would be precisely the weight of the air the vessel 

 contains. Therefore we 

 may measure the vessel 

 used and calculate the 

 cubic contents, and thus 

 find out the weight of the 

 air per cubic foot or meter, 

 as shown by the experi- 

 ment. This result will 

 not be quite correct, since 

 not all the air can be re- 

 moved from the vessel by 

 the pump, and therefore 

 we include part of the air 

 in both weights. Is our result greater or less than the true 

 weight of air? 



FIG. 5. Air pressure 



When air is pumped from within the glass 



jar, the sheet of rubber tied over the upper. 



open end of the jar is forced downward by 



the weight, or pressure, of the outside air 



