face A is pressed upward against it : this force is the weight of the column of 

 mercury in the leg B A above the level of F together with the height of the 

 atmosphere pressing on the top G of the column. Let a horizontal line he 

 drawn from the surface F', to the leg B A, and let the column G H be meas- 

 ured ; its height will be found to be accurately 30 inches, and its weight is, 

 therefore, equal to the atmospheric pressure. The force with which F is 

 pressed upward is, therefore, equal to twice the atmospheric pressure, or to 

 double the force with which F, in fig. 7, was pressed upward. Hence it np- 

 pears that the elasticity of the air confined in the space D F, fig. 8, is double 

 its former elasticity when filling the space D F', fig. 7. Thus, when the air is 

 compressed into half its volume its elasticity is doubled. 



In like manner, if mercury be poured into the tube A until the air included 

 in the shorter leg is reduced to a third of its bulk, the compressing force will 

 be found to be three times the atmospheric pressure, and so on. 



That the elasticity of the air which surrounds us is equal to the weight of 

 the incumbent atmosphere, has been proved incidentally in the preceding ex- 

 periment. Indeed, this is a proposition the truth of which must appear evi- 

 dent upon the slightest consideration, and which is manifested by innumerable 

 familiar effects. If the elastic force of the air around us were less than the 

 weight of the incumbent atmosphere, it would yield and suffer itself to be com- 

 pressed until it acquired an elastic force equal to that weight. If it were 

 greater in amount than the weight of the incumbent atmosphere, it would over- 

 come that weight, and would press the atmosphere upward until, by expand- 

 ing, its elasticity were reduced to equality with the weight of the atmosphere, 

 and these effects are continually going forward. 



The incumbent atmosphere is subject to continual fluctuations in weight, as 

 will hereafter be proved, and the lowest stratum of air which surrounds us is 

 continually undergoing corresponding contractions and expansions, ever ac- 

 commodating its elasticity to the pressure which it sustains. Also this stra- 

 tum of air is itself subject to changes of elasticity from vicissitudes of tempera- 

 ture proceeding from the earth to which it is contiguous. These changes pro- 

 duce a necessity for expansion and contraction in it, even while the weight of 

 the incumbent atmosphere remains unchanged ; but the full development of 

 this last consideration belongs to the theory of heat rather than to our present 

 subject. 



An open vessel which is commonly said to be empty, is, in fact, filled with 

 air ; and when any solid or liquid is placed in it, so much of the air is ex- 

 pelled as occupied the space into which the solid or liquid entered. If such a 

 vessel be closed by a lid or stopper, the pressure of the external atmosphere 

 will act upon every part of the exterior surface with an intensity proportionate 

 to its weight. The air which is enclosed in the vessel will, however, act on 

 the interior surface with an intensity proportionate to its elasticity. Accord- 

 ing to what has already been explained, this elasticity is equal to the pressure ; 

 and, therefore, there is a force tending to press the sides of the vessel outward 

 exactly equal to the pressure acting on the exterior surface, and tending to 

 press them inward. These two forces neutralize each other, and the ve.?t is 

 circumstanced exactly as if neither of them acted upon it. 



When the operation and properties of some pneumatical instruments have 

 been explained, we shall have occasion to notice many other effects of the 

 elasticity of air. 



