1S25.3 Barometrical Measurement of Heights, 47 



continues unaltered. On restoring the dry air to its original 

 volume, increased in proportion to the diminution of that of the 

 aqueous fluid, by turning inwards the screw U, if we now increase 

 the compressing weight in proportion to the augmentation of 

 elasticity derived from the introduced vapour, we shall find, on 

 liberating the weight, that it will remain precisely where we had 

 clamped it : the additional pressure compensates the additional 

 elasticity, and the original volume, or height, remains unaltered. 



(33.) Varying the experiment, if we double the pressure now 

 sustained by the mixture, the volume will be found to be de- 

 creased more than one-half. As the space, or volume, diminishes, 

 the vapour existing therein hquefies and is precipitated. This 

 is continued until the elasticity of the air, augmenting as its 

 volume diminishes, added to that of the residual vapour, still at 

 its initial force, equal together the doubled pressure. 



(34.) From the former of the two experiments, we learn that 

 the same quantity of vapour, and of the same force, is formed in 

 a given space, uniformly of a given temperature, whether con- 

 taining dry air, or being a perfect vacuum ; for if we mix equal 

 quantities of aeriform fluids of different elasticities, they may be 

 compressed into the space occupied by one by submitting them 

 collectively to a pressure equal to the sum of their elasticities. 



(35.) From the other experiments, it is evident that when any 

 space, being a vacuum, or containing dry air of any elasticity, 

 is fully saturated with aqueous vapour in contact with water or 

 not, we cannot increase, or otherwise alter the force of the 

 vapour so long as any exists by augmenting the pressure, or 

 otherwise partially butnot entirely diminishing the space. 



(36.) It is also obvious that the maximum of the force of the 

 vapour existing in dry air will be determined by the temperature 

 of the latter ; so that a volume of vapour generated at a temper- 

 ature of 50° F. could not possibly exist in its aeriform state in 

 an equal volume of dry air of the temperature of 49° F. A por- 

 tion of it would be precipitated, and the elasticity b6 reduced 

 from 0*4 in. to 0*388 in. The air would now hold its maximum 

 quantity of vapour, or be what is termed completely saturated. 



(37.) Saturated air supporting any pressure being exposed,' 

 when out of contact with water, to any other superior temperature, 

 or having its pressure diminished j will hsive its volume increased, 

 and its density diminished, in the same ratio as dry air. In 

 , both cases, as well as when the supply of water is cut off* before 

 the volume of air has acquired the whole of the quantity of 

 vapour it is capable of containing, the air is said to be but par- 

 tially saturated, or only to a certain degree. 



Of the Hygrometer. 

 (38.) A polished surface of glass, brass, 8cc. inferior in the 



