

LIBRARY 



Barometric Pressur^c^N 



^AS5^ 



dry air from above during the middle of the aa.y^ — lis the semi- 

 diurnal curve of vapour tension at sea, near land, due, as Buchan 

 says, to an inter-mixture with the air forming the sea-breeze of 

 descending thin and drier air filaments from higher levels? These 

 points are noted here not with the idea of formulating a theory, but 

 rather for the purpose of strengthening a criticism. Let me illustrate 

 by citing a representative experiment from a series which I hope to 

 discuss at length at some future time : — 



There is nothing exceptional about these quantities of 

 evaporation ; they are quite typical of the series. The experiments 

 were taken in a particular way for a particular purpose, and they shew 

 that there are easy circumstances under which evaporation is not 

 controlled by either the humidity of the air alone, the temperature of 

 the water alone, or the rate of change of vapour tension 

 between one level and another in the space above the 

 water. For the rate of evaporation here shewn is greatest 

 after sunset when on the whole the dew point is at its maximum, 

 the relative humidity is rising to its maximum, and the temperature is 

 falling ; and, moreover, the average rate of evaporation for the three 

 hours, 8 p.m. — ii p.m., is actually greater (often it is much greater) 

 than during the three hours ii a.m. — 2 p.m. Now, we know that 

 evaporation proceeds most rapidly over the land areas during the heat 

 of the day : my experiments suggest that it is not at all impossible that 

 evaporation over the oceans may actually be greater at night. 

 That is to say, during a great portion of the dav there may be two 

 meridians on the earth's surface somewhere about half a rotation 

 apart at which evaporation is going on more rapidly than it is at 

 intervening parts : in other words, a kind of semi-diurnal oscillation 

 of the rate of evaporation shewing its greatest effect when the sun is 

 on the meridian of some continental area. I do not by any means 

 assert that such a process as this, if it exist, is going to settle the 

 question of the diurnal variation of barometric pressure ; but I do 

 assert that until we know more of the physical processes involved in 

 the behaviour of water vapour we shall not be in a position to solve 

 the fundamental problem. 



