ON PNEUMATIC ZQUHIBRIUM, S,7l 



be raised, in the ordinary circumstances of the atmosphere, to tlie lieight of 

 nearly 30 inches. But supposing the magnitude of the tube such, that the 

 portion of air must expand to twice its natural bulk, before the mercury ac- 

 quired a height sufficient to counterpoise it, this height would be 15 inches 

 only. For it appears to be a general law of all elastic fluids, that their pres- 

 sure on any given surface is diminished exactly in the same proportion as 

 their bulk is increased. If, therefore, the column of mercury in the vacuum 

 of the air pump were 60 inches high, the air would be reduced to half its 

 natural bulk ; and for the same reason, the pressure of a column of 30 inches 

 of mercury in the open air will reduce any portion of air to half its bulk, since 

 the natural pressure of the atmosphere, which is equal to that of about 30 

 inches of mercury, is doubled by the addition of an equal pressure. In the 

 same manner the density of the air in a diving bell is doubled at the dej)th of 

 34 feet below the surface of the water, and tripled at the depth of 68 feet. 

 This law was discovered by Dr. Hooke; he found, however, that when a very 

 great pressure had been applied, so that the density became many times 

 greater than in the natural state, the elasticity appeared to be somewhat less 

 increased than the density ; but this exception to the general law has not 

 been confirmed by later and more accurate experiments. 



Not only the common air of the atmosphere, and other permanently elastic 

 gases, but also steams and vapoMs of all kinds, appear to be equally subject 

 to thisuniversallaw: they must, however, be examined at temperatures suffici- 

 ent to preserve them in a state of elasticity ; for example, if we wished to deter- 

 mine the force of steam twice as dense as that which is usually produced, we 

 should be obliged to employ a heat 30 or 40 degrees above that of boiling water:" 

 we should then find that steam of such a density as to support, when confin- 

 ed in a dry vessel, the pressure of a column of 30 inches of mercury, would 

 be reduced to half its bulk by the pressure of a column of 60 inches. But if 

 we increased the pressure much beyond this, the steam would be converted 

 into water, and the experiment would be at an end. 



That the air which surrounds us is subjected to the power of gra- 

 vitation, and possesses weight, may be shown by weighing a vessel which 

 has been exhausted by means of the air pump, and then allowing the air to 

 enter, and weighing it a second time. In this manner we may ascertain the 



