316 Mr. W. S. Jcvons on the Semidiurnal 



equal to half an inch of mercury, the weight or barometric pres- 

 sure of the atmosphere will not be increased by this amount until 

 the whole column of the atmosphere is saturated, or, more cor- 

 rectly speaking, until the independent aqueous atmosphere is 

 fully established by slow diffusion through the gaseous one. 



5. A contiued body of air evidently differs in its mechanical 

 conditions from the atmosphere, in the fact that the aqueous 

 vapour reacts against the upper confining surface of the former, 

 and is thus enabled instantaneously to exert a similar pressure 

 on all sides ; but the atmosphere is uncontined except by its own 

 weight, and affords no fixed fulcrum. If air were impermeable 

 to aqueous vapour, none could possibly rise except at a tempera- 

 ture exceeding the boiling-point, when the rising vapour would 

 become able to lift the atmosphere in mass. But the air is per- 

 n\eable ; and at a temperature of 60°, for instance, water, although 

 totally unable to lift the atir.osphere by its elastic force, projects 

 aqueous vapour into it and causes it gradually to ascend with a 

 force equal to half an inch depth of mercury. The elastic force 

 of the aqueous vapour is therefore exei'ted against the ubstructive 

 power of the air. Ultimately, when the permeation of the one 

 through the other is complete, the aqueous vapour will exert 

 pressure only against itself, and will be so far independent of 

 the air. The atmosphere will then, but not till then, have in- 

 creased in total weight or barometric pressure, by the amount of 

 the elastic force of vapoiu- indicated by the hygrometer at the 

 surface of the earth. 



6. The above rather tedious but incontrovertible reasoning 

 amounts to a rednctioad ahsurdum of the hypothesis that aqueous 

 vapour adds its independent elastic force to the pressure of the 

 gaseous atmosphei'e, a hypothesis which can only have arisen 

 from a confusion of terms. The actual weight of the atmosphei'e 

 is what occasions its pressure as measured by the barometer. 

 If we wish thei'efore to separate the aqueous and gaseous pres- 

 sures, we must separate (not the elastic force of vajiour at the 

 surface, but what is very different) the actual iceiglU of aqueous 

 vapour contained in the whole column of the atmosphere from the 

 united actual weight of nitrogen, oxygen, and carbonic acid. 



7. This being clearly understood and allowed, it is almost 

 needless to say that the observation of a hygrometer at the sur- 

 face of the earth cannot inform us how much aqueous vapour is 

 diffused through the lighter strata of air. The balloon observa- 

 tions of the British Association sufficiently prove this ; for in the 

 ascent of November 10, 1852, Mr. Welsh determined that within 

 an elevation of 22,930 feet the fluctuations of the humidity were 

 numerous, " there having been no fewer than fo\n- or perhaps 

 five different strata of vapour." It is further almost certain, as 



