90 WEIGHT OF THE AIR. 



so as to close it, inverted in a vessel of mercury so as to im- 

 merse its open end, it will be found on removing the finger 

 that the mercury in the tube will descend through a certain 

 space, leaving a vacuum at the top of the tube, but resting 

 with its upper surface at a height of about 29 or 30 inches 

 above the surface of the mercury in the vessel. It thus 

 appears that the atmospheric pressure, acting on the sur- 

 face of the mercury in the vessel, and transmitted (Art. 8)> 

 supports the column of mercury in the tube, and hence that 

 the weight of the mercurial column is exactly equal to the 

 weight of the atmospheric column standing on an area 

 equal to that of the internal section of the tube. The 

 weight of this column of mercury then is an exact measure 

 of the atmospheric pressure, or of the elastic force of the 

 atmosphere at any instant. 



42. Weight of the Air. This may be directly proved 

 by weighing a flask filled with air, and afterwards weighing 

 it when the air has been withdrawn by means of an air- 

 pump ; the difference of the weights is the weight of the 

 air contained by the flask. 



The opinion was long held that air was without weight, 

 or rather, it never occurred to any of the philosophers who 

 preceded Galileo to attribute any influence in natural phe- 

 nomena to the weight of the air. The fact that air has 

 weight escapes common observation in consequence of its 

 extreme levity compared with solids and liquids, and espe- 

 cially in consequence of its being the medium by which we 

 are continually surrounded. The experiment of weighing 

 air was performed successfully for the first time in 1650, by 

 Otto Guericke, the inventor of the air-pump.* 



By means of the weight of air we may account for the 

 fact of atmospheric pressure. The earth is surrounded by 

 a quantity of air, the height of which is limited (see Art. 



* Deschanel's Natural Philosophy, Part I., p. HI. 



