1876.] Inequality of the two Semidiurnal Oscillations. 323 



that this has not been generally recognized, I attribute to the fact that the 

 consequences of the theory, as a purely physical problem, have never yet 

 been traced out and verified by such a mass of facts as Mr. Buchan is now 

 bringing together. So long as the ivliole phenomenon is not satisfactorily 

 accounted for, some doubt may reasonably attach to the explanation offered 

 of one only of its elements. 



My own attention was first drawn to the subject of the explanation 

 which I am about to give, by a paper of Mr. F. Chambers in the Philosophi- 

 cal Transactions for 1873, in which that gentleman showed as the result of 

 an analysis of the diurnal variation of the winds at Bombay, that one 

 element of this variation is a double rotation of the wind direction ; of such 

 a character, that the southerly components attain their maximum value at 

 the epoch of the most rapid semi-diurnal rise of pressure, the easterly 

 components at the epoch of maximum, the northerly with the most rapid 

 fall, and the westerly with the epoch of minimum. On these facts Mr. 

 Chambers based a suggested explanation of the barometric tides, regarding 

 them as a phenomenon of static pressure ; and assumed (as now appears, on 

 insufficient grounds) that the phenomenon in the northern hemisphere is 

 generally of the same type as at Bombay. There was indeed one feature 

 in his explanation, which it seems difficult to reconcile with mechanical 

 laws ; since he supposed air to flow from both east and west towards a region 

 where the pressure has already risen above the mean, and by its accumulation 

 to produce a maximum of static pressure. But apart from this, the disco- 

 very was an important one ; and, since it clearly shewed that a regular hori- 

 zontal transfer of air corresponded to the oscillations of pressure, it held out 

 a promise that further steps in the same path might clear up what appeared 

 to be anomalous, and possibly lead to a complete explanation of the diurnal 

 oscillation. 



Some time before this paper reached me, the Rev. M. Lafont had 

 placed in my hands four years' traces of a Secchi anemograph, erected on 

 St. Xavier's College, Calcutta ; and these having been measured off, tabu- 



with a greater velocity than the sound wave, and it is probably much less ; since the 

 action being slow and prolonged, the heat developed by the compression must be in 

 part dissipated. To explain the observed phenomenon on this hypothesis, the retar- 

 dation must however be such, that the unrelieved excess of pressure at the ground 

 surface, must be equal to that generated by from half to three quarters of an hour's 

 action of the sun. 



This would require us to assume a much lower average temperature for the higher 

 strata than results from Pouillet's calculation, and also that a certain diurnal oscilla- 

 tion of temperature affects the atmosphere to a greater height then has been usually 

 assumed. But this hypothesis is free from most of the objections to be lugcd against 

 those of the authors quoted. — Note added January 20th, 1877. 



