324 H. F. Blanford — On the Physical Explanation of the [No. 4, 



lated, and reduced, I was interested to find that the diurnal wind variation 

 at Calcutta showed the double diurnal oscillation quite as distinctly, and 

 relatively even more prominently than that of Bombay. But one important 

 difference presented itself. The north and south elements of the oscillation, 

 while agreeing in epoch with those of Bombay, were reversed in direction ; 

 and, taken together with the latter, showed a tendency to a cyclonic circula- 

 tion of the atmosphere around the peninsula during falling pressure, and 

 an anticyclonic circulation with rising pressure. Moreover, the east and 

 west components agreed almost exactly, in epoch, with the north and south 

 components ; the result being a movement of air from the north-west with 

 falling pressure, and from the south-east with rising pressure. These facts, 

 taken in conjunction with the positions of Bombay and Calcutta on ojyposite 

 sides of the peninsula, seemed to point to the differential conditions of land 

 and water as being probably concerned in the phenomenon. Another and not 

 less important fact connecting the winds with the diurnal oscillation of the 

 barometer appeared at the same time. When the wind variation was 

 analysed by Bessel's method, there appeared an east and west oscillation of 

 considerable magnitude, corresponding in epoch with the barometric 

 inequality expressed by the first periodical term of the barometric formula. 

 This was easily distinguished from the oscillation of the sea and land winds, 

 since the latter are nearly north and south at Calcutta. At Bombay where 

 the sea and land breezes are -nearly east and west, such an oscillation would 

 be undistinguishable, even if it really existed. 



The east and west oscillation of diurnal period indicates an outflow 

 of air to the eastward during the day time, an inflow from the east during 

 the night ; and the former phase of it evidently corresponds to the hot 

 winds of the Gangetic plain and Northern India and, indeed, to the day 

 winds of the dry months of the greater part of India. They blow towards 

 the sea from the eastward, only in the western portion of the Dekhan, 

 Mysore, &c. Tbis system of day winds consists of an outflow of ah' from 

 the peninsular towards the sea on both coasts, the westerly direction greatly 

 predominating. 



The next step in the enquiry was to ascertain what general cause would 

 operate to produce this efflux and influx of air ; and the obvious suggestion 

 was that it must consist in the differential action of the sun's heat on dry 

 air and water. 



Let V be any volume of dry air at pressure P and absolute temperature 

 T and let t units of heat be communicated to it, raising its temperature 

 from T to T+ 1, while the volume remains constant. The pressure will 

 be increased thereby from P to P+j? wherein — 



P 



= P (^±J_ 1 \ = P _i (1) 



