Barometric Pressure. 25 



take the average " pressure of dry air " over large areas, so as to 

 eliminate local irregularities, it is still certain that the resulting 

 diurnal variation of this pressure of dry air is not much, if any, 

 simpler than the diurnal variation of the barometer. All this is pretty 

 certain, and yet, to my mind, it by no means follows, as some 

 meteorologists have, perhaps, too hastily assumed, that the semi- 

 diurnal wave of pressure is not mainly due (indirectly though it may 

 be) to the presence of aqueous vapour. We have to begin with the 

 fact already mentioned that in the interior of great continental areas, 

 where the air is necessarily dry, the night minimum becomes very 

 small, whereas in maritime situations it is great. Again, the annual 

 averages of vapour tension for each hour of the day at land stations 

 give a curve with a strongly-pronounced double maximum and 

 minimum. Also in such a place as Kimberley if we resolve both the 

 diurnal variation of pressure, and of vapour tension, into their 

 harmonic constituents, the epochs of the second, third, and fourth 

 harmonic terms are almost the same in each. And it is also a curious 

 fact that if from the total barometric pressure we subtract one-third 

 of the observed vapour tension for each hour the resulting differences 

 fall upon a curve which contains practically no third or fourth term. 

 Now, it has been argued that the resemblance between the annual 

 curves of the diurnal variation of the barometer and of the vapour 

 tension is purely fortuitous since it is not found in the monthly curves. 

 Such an argument does not seem to me to be quite sound. For 

 although the diurnal curves of barometric pressure and vapour tension 

 do differ from each other month by month, yet such variation as 

 there is is in the same direction. For instance, if we draw winter, 

 summer, and annual curves of the diurnal variation of barometric 

 pressure and clew point as observed at Kimberley, we find that the 

 winter deviations of both are positive before noon and negative after, 

 as compared with the annual curves of each, and vice versa in 

 summer. 



Now, a moment's consideration will shew that this is exactly 

 what it should be in all places where the relative humidity is not 

 great, if the observed variations of barometric pressure indicate any 

 movements at all of the air. For, supposing at any instant we have 

 in a given space a barometric pressure of 26 -140 inches, and that the 

 vapour tension is i % of this, i.e., -2614 inch. Suppose further for 

 any reason an indraught of air of the same composition, not 

 necessarily lateral, raising the barometric pressure pretty quickly to 

 26 -240 inches. Is it not clear that the vapour tension must have 

 increased in the same time to -2624 inch? Therefore I say that all 

 variations of barometric pressure that depend solely upon air 

 movements must give rise to similar variations in the vapour tension. 

 But it is truly said that over the ocean, or at great altitudes, the 

 semi-diurnal curve of vapour tension does not exist, and that the 

 observed curve is to all appearance a temperature curve. No doubt ! 

 But, then, it must be borne in mind that in all places where the 

 relative humidity is high movements of air are as likely as not 

 accompanied by changes of state in its contained water vapour. 



