376 HUMPHREYS: DIURNAL VARIATIONS 



so with the forced forenoon maximum since, so far as the inter- 

 ference or damming effect is concerned, it depends upon a flow 

 or circulation of the atmosphere, parallel roughly to the equator. 



It remains now to account for the night 10 o'clock maximum 

 and 4 o'clock minimum. 



3. Natural or free vibration of the atmosphere as a whole. This 

 subject has been discussed by several mathematical physicists of 

 great eminence. The latest and most complete of these discus- 

 sions, and the one to which those interested in this phase of the 

 barometric problem are especially referred, is by Lamb, 1 who 

 concludes : 



Without pressing too far conclusions based on the hypothesis of an 

 atmosphere uniform over the earth, and approximately in convective 

 equilibrium, we may, I think, at least assert the existence of a free oscilla- 

 tion of the earth's atmosphere, of " semi-diurnal" type, with a period 

 not very different from, but probably somewhat less than, 12 mean 

 solar hours. 



Hence any cause of pressure change, having a semidiurnal 

 period, or harmonic of this, would, if of sufficient magnitude 

 and proper phase, account for the twelve-hour barometric curve. 

 Such a cause, many think, may be found in the irregular daily 

 march of temperature, since the curve expressing this march is 

 more or less approximately resolvable into a diurnal and semi- 

 diurnal sine curve. But the resolution is not perfect and besides 

 there is no obvious cause for a temperature increase by night, 

 and hence the reality of the semidiurnal component in the tem- 

 perature curve is equally doubtful. 



All that is needed, apparently, to give the semidiurnal pressure 

 curve is a pressure impulse of the same period, twelve hours, as 

 that of the free vibration of the atmosphere as a whole. And 

 this, it seems, is furnished by the forced forenoon barometric max- 

 imum, followed, six hours later, at the same place, by the forced 

 afternoon barometric minumum. In other words, taken together 

 the forenoon and afternoon forced disturbances appear to occur 

 with the proper time interval necessary to set up and maintain 

 the twelve-hour free vibrations of the atmosphere. 



'Proc. Roy. Soc, A, 84: 551. 1911. 



