1881.] 



and rainfall to temperature in India. 



81 



Temperature of lower station, 



Pressure of lower station, ... 

 Barometric weight of inter- 

 vening air stratum, 



Pressure of upper station, ... 



HOSHANGAUAD AND PACH- 

 MARHI. 



May. 



92-7° 

 28-675" 

 2-340" 

 26-335" 



June. 



88-1° 

 28-584" 

 2-332" 

 26252" 



July. 



795° 

 28592" 

 2-367" 

 26-225" 



Roork.ee and Ciiakkata. 



May. 



87-7° 

 28 754" 

 5-545" 

 23-209" 



Juno. 



90-2° 

 28-615" 

 5-462" 

 23 153" 



July. 



84-6° 

 28-617" 

 5-499" 

 23-118" 



On the other hand, the months in which the temperature is lowest on 

 the plains, and the lowest stratum of air, on the average, most dense, are 

 December and January ; but at the level of the Himalayan hill-stations 

 Murree, Chakrata, Darjeeling, &c, the pressure in December is lower than, 

 in November, and in January still lower ; and at that of Leh it falls from 

 October to February, in which month, according to our present data, 

 occurs the absolute minimum pressure of the year. These facts seem to 

 point to the conclusion that, at some greater elevation, (perhaps at that of 

 the Karakoram plateau), the annual oscillation of pressure is probably 

 approximately the reverse of that which takes place on the plains of India, 

 the maximum occurring when, in the lower atmosphere, the summer mon- 

 soon is at its height ; and the minimum in January or February. 



In seeking the physical explanation of these changes, it may be postu- 

 lated at the outset, that the variations in the density of the atmosphere 

 with which we have to deal, whether those of the higher or lower strata, 

 are mainly due to variations of mean temperature ; to which, indeed, the 

 influence of variations in the quantity of vapour constituent, (regarded as 

 replacing dry air of the same tension), is of quite subordinate importance. 

 I have shown elsewhere, that the reduction of barometric weight, which a 

 column of air, 7,000 feet high, undergoes from January to July, over the 

 plains of Bengal, is due to the rise of temperature in the proportion of ^i, 

 to only T ^-, consequent on the replacement of dry air by vapour ; and the 

 relative importance of temperature may be shown more in detail and with 

 more direct reference to the present discussion, by the following compari- 

 son of the temperature and barometric anomalies, extracted from Tables XI 

 and VIII in the Report on the Meteorology of 1879. 



