HUMPHREYS: DIURNAL VARIATIONS 373 



The inertia of rising and of falling masses of atmosphere; the 

 formation and evaporation of dew ; the pressure due to radiation— 

 "light pressure;" atmospheric tides and a number of other things 

 have been advocated as actual and sufficient causes of the daily 

 variations of the barometer. But a simple numerical test shows 

 that nearly every one of the supposed causes is wholly inadequate 

 to produce the observed pressure changes. 



There are however three phenomena which, in the author's 

 opinion, cooperate in such manner as to produce the barometric 

 waves in question. 



1 . Overflow of the atmosphere from the regions where it is warmest 

 and most expanded towards those where it is coldest and most con- 

 tracted. The exact hour at which the atmosphere, top to bottom, 

 of any given locality averages warmest and therefore, as a whole, 

 is most expanded depends upon a variety of circumstances, but in 

 general it is not very far from 4 o'clock in the afternoon. Hence 

 at about this time, because of the overflow that the expansion pro- 

 duces, the amount of air overhead, counting from sea level, must 

 be least; and therefore at this same hour a sea-level barometer 

 must have its lowest average reading. Calculations, too long and 

 tedious to include here, based on the average daily change in 

 temperature and on the viscocity of the atmosphere, indicate 

 that the atmospheric overflow resulting from the known thermal 

 changes may fully account for the afternoon or 4 o'clock baromet- 

 ric minimum. 



2. Interference by vertical convection with free horizontal flow. 

 It is well known that in general the velocity of the wind in- 

 creases with increase of elevation, and therefore that the free 

 flow of the atmosphere must be disturbed to some extent by that 

 vertical interchange of its parts which local temperature varia- 

 tions always induce. 



We will consider this point a little closer: Let the mass of air 

 m be near the ground and have the horizontal velocity v, and let 

 the larger mass M be at a higher elevation and have, in the same 

 direction, the greater velocity V. If now these two masses of 

 air should mingle in such a manner as to be free from all disturb- 

 ance, except their own mutual interference, the resulting final 



