Oscillation of the Barometer. 315 



Thus far the aqueous vapour appears to be quite iudepeudent 

 of the air ; but it should be remarked that whereas water, when 

 introduced into a vacuum, instantly exerts its elastic pressure on 

 all sides, and instantly diffuses itself thi-ough the whole space, it 

 does not, in the presence of air, add its independent pressure with 

 equal instantaneity, but, on the contrary, only slowly diffuses itself, 

 or makes its way among the particles of air ; and the more 

 condensed the air within the vessel, the more slowly will the 

 aqueous vapour thus diffuse itself. 



4. Now a confined vessel full of air does not truthfully repre- 

 sent the atmosphere, which has no upper confining surface. The 

 pressure of the atmosjjhere is only the effect of gravity ; and at 

 any given place the elastic force which it exhibits is only due to 

 the superincumbent weight of air. If the earth's atmosphere 

 were suddenly removed, the water on its surface would evaporate 

 with excessive rapidity, and we should soon have a purely aqueous 

 atmosphere, of which the pressure would be closely limited by 

 the existing temperature. But now, 1 ask, if the atmosphere 

 were supposed to be suddenly rendered perfectly free from 

 aqueous vapour, would the evaporation from the water on the 

 earth's surface be so rapid as almost instantly to produce an 

 aqueous atmosphere coextensive with the gaseous one? And, 

 secondly, would the pressure of the gaseous atmosphere be in- 

 stantly increased by the amount of the elastic force of vapour 

 due to the temperature at the surface of the earth ? To both 

 tliese questions the answer must be — No, only very gradually. 

 It need hardly be said that aqueous vapour is impeded in its 

 motion by the ])resence of air, and that the one only diffuses 

 slowly through the other. It would be a very long time before 

 the atmosphere could become completely saturated, just as a 

 certain length of time is required for a confined body of air to 

 become so. 



But the pressure also of the atmosphere could only increase 

 in proportion as the aqueous vapour became diffused through it, 

 because gravity is the only cause of such pressure, and the mere 

 fact of water lying on the surface of the earth cannot increase 

 the weight of what lies above. The iveight of the atmosphere can- 

 not be greater than the weight of its compmoif.s, of which aqueous 

 vapour is one. The lowest strata of the atmosphere will, indeed, 

 S)on become saturated with aqueous vapour rising from the moist 

 surface of the earth ; that is to say, the hygrometer will there 

 indicate aqueous vapour of the greatest elastic force possible at 

 the existing temperature ; but the real pressure of the atmosphere 

 will only be increased by the actual weight of water raised into 

 it by evaporation. Although, within four feet or within a mile of 

 the earth's surface, the hygrometer indicates an aqueous pressure 



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