178 H. F. Blanford — IneqxmVdii of semi-diurnal Barometric Tides. [AuorsT, 



curve of the barometric oscillation, instead of leaving a single curve of one 

 oscillation, a very irregular curve resulted, in which the double oscillation 

 Avas still a very prominent feature. This, it was suggested, was a local pecu- 

 liarity owing to the alternation of the sea and land breezes ; but it was 

 speedily discovered that so far from being exceptional it was the general 

 rule in all parts of India, and that the hypothesis of Dove and Sabine 

 could in no way be made to suffice for the facts. 



Another view had been put forward independently by Broun of Tre- 

 vandi'um and Lament of Munich, and had received support from 

 Mr. Homstein of Vienna. This is that the element of the double oscilla- 

 tion is an effect of either the Solar magnetism or electricity, and Mr. 

 Hornstein had demonstrated that, in certain respects, the phenomenon 

 shews a periodicity coi'responding to the frequency of sun-spots and 

 auroras, and also of the period of the sun's rotation on his axis. 

 Beyond, however, such coincidences, which seem to establish no more 

 than that the phenomenon varies with certain solar phenomena and others 

 which are known to vary with them, there appears to be little ground to 

 assign the tides to magnetic rather than to thermal agency. 



Meanwhile Espy, Davies and Kreil had, as it ajjpears, independently of 

 each other, drawn attention to one necessary consequence of the diurnal 

 heating of the atmosphere, which had escaped the attention of Kaemtz, 

 Dove and Sabine ; and which, whether affording a complete or only a par- 

 tial exj)lanation of the oscillation, must cause a double diurnal oscillation 

 such as is to be accovmted for. This is the increase of atmospheric pres- 

 sm-e produced by the exjianding atmosphere in the forenoon, and that again 

 produced by its contraction in the evening. It follows from elementary 

 mechanical laws, that a mass of air resting on the ground and expanding, 

 must exercise presstxre in excess of that due to its weight ; that this pressure 

 will increase as the rate of expansion increases, will be constant when the 

 rate of expansion is constant, and will fall as that rate decreases. Thus 

 will arise an oscillation of j)ressure, similar to, and about coincident with the 

 morning oscillation. As a jjartial verification of this coincidence, Mr. 

 Blanford stated that he had found, on comparing the Calcutta diurnal 

 curve of pressure with that of temj^erature, that the instant of the morn- 

 ing maximum of the former falls less than half an horn* later than the in- 

 stant of most rapid rise of the latter near the ground surface. 



In the evening, the contraction of the atmosphere in consequence of its 

 cooling, will necessarily produce an increase of pressure, arising from the 

 subsidence of the contracting atmosphere, and this seems a not impro- 

 bable explanation of the evening maximum. It a2:)peared to be somewhat 

 inexplicable that this suggested explanation has not received more attention 

 at the hands of physicists. As put forward by Davies and Kreill it pi-esents 

 some weak jjoints, but these are not essential. 



