-1859] WRITINGS OP JOSEPH HENRY. 269 



or pressure of the atmosphere, while the upward pressure on 

 the lower side, due to the repulsion of the atoms, is desig- 

 nated indiscriminately by the terms elasticity, elastic pressure, 

 ■elastic force, and simply the tensio7i of the air. 



The force analogous to the latter (in the case of vapor) is 

 more generally known by the name of tension, though it is 

 sometimes called elastic pressure. In the foregoing experi- 

 ment, if the pressure of the superincumbent air is increased, 

 the exterior surface of the paper will assume a concave form, 

 the atoms of the inclosed air will be pressed nearer together, 

 and their repulsive energy will be increased by the approxi- 

 mation of the atoms, and thus a new equilibrium will take 

 place. If conversely the column of air above the tumbler is 

 diminished in weight, the surface of the paper will assume 

 a convex form, because the atoms within the tumbler being 

 pressed with less force will separate to a greater distance, and 

 the repulsion will be reduced by their separation, until a new 

 equilibrium is attained between the pressure without and 

 the repulsion within. In this case, variations of the elastic 

 force or tension of the air within the tumbler become an 

 exact measure of the pressure of the exterior column, pro- 

 vided the temperature remains the same; and it is upon 

 this principle that the barometer called aneroid is con- 

 structed. It consists practically of a flat flask of thin metal, 

 tilled with air and hermetically sealed by means of solder ; 

 the motion of the sides of this flask, precisely analogous to 

 that of the paper closing the mouth of the tumbler, is com- 

 municated by means of lever and wheel work to a hand, 

 which indicates the variations of the tension of the inclosed 

 air and consequently of the weight of the atmosphere. 



Now if the aqueous vapor formed a separate or entirely 

 independent atmosphere around the earth, the variations in 

 its pressure would be accurately measured by the variation 

 of its tension or elastic pressure at the surface ; but since the 

 vapor, on account of the resistance of the air with which it 

 is entangled, is not uniformly distributed, its tension at 

 the surface cannot give a true measure of its whole pressure. 

 It is true that as a whole the weight of the atmosphere is in- 



