PNEUMATICS. . 69 



ceeds 3J inches, is not greater than one ninth part of 

 the total pressure, which is 30 inches of mercury. 



Specific Gravity. 



One hundred cubic inches of atmospheric air 

 weigh 30J grains. 



The specific gravity of atmospheric air being con- 

 sidered as 1, it is about 828 times lighter than the 

 same bulk of water ; but with respect to steam it is 

 as 1 to 0,6235. 



Evaporation and Temperature. 



The reduction of temperature by the evaporation 

 of moisture is obviously a measure of the hygrometric 

 state of the atmosphere, for if the air be saturated 

 with moisture, it produces no evaporation. 



A Vacuum. 



Whether there be any such thing in nature as an 

 absolute vacuum ; or whether the universe be com- 

 pletely full, and be an absolute plenum, are points 

 that have been controverted by philosophers in all 

 ages. 



That the motion of the planets and comets may be 

 regular and permanent, it appears necessary that the 

 celestial spaces be devoid of all matter, except the 

 passing rays of light. 



The quantity of matter in any given space may be 

 diminished by rarefaction, and why not ad infinitum? 

 The solid ultimate particles of body have all the 

 same density, being incapable of further condensa- 

 tion, and are only rarefiable by means of their pores. 



Sir Isaac Newton observed that a thermometer 

 suspended in vacuo, and in that state removed to a 

 warm or cold room, receives the heat or cold, rises 

 or falls, almost as soon as another in the open air. 

 By what medium is the heat or cold conveyed ? 



