1870.] On the Normal Rainfall of Bengal. 251 



spring rains, which as I have above remarked, are not felt in 

 Bombay, or indeed any where to the west of Nagpore. This will 

 be understood, if we consider what will be the source of the winds 

 that impinge upon the opposite coasts of the Peninsular in ac- 

 cordance with Buys Ballot's law, and as verified by observation. 

 On the east coast, the air comes from the south, less satura- 

 ted indeed, than that which brings the monsoon rains, since at this 

 period, it is not drawn in a steady current over so great an expanse 

 of ocean ; but containing a considerable quantity of vapour, which 

 it precipitates chiefly in the brief, but frequently violent storms 

 of which the Bengal ' North- Westers' are examples. On the West,*" 

 the air comes originally from the NW, that is, from the arid 

 region of Arabia, and the countries around the Persian Gulf, and 

 the expanse of sea traversed between these countries and the Indian 

 Peninsular is insufficient to charge it highly with vapour. 



The above explanation applies of course only to the Peninsular 

 of India, properly so called. In Eastern Bengal and Assam, heavy 

 rains begin in April, or shortly after the equinox. SW winds 

 now predominate, and precipitate their moisture abundantly on the 

 cool hilly but not very elevated region on which they immediately 

 impinge. Since the winds preserve their SW direction, they would 

 appear to flow towards the region of low barometric pressure which, 

 as Mr. B u c h a n ' s charts shew, prevails at this season over Tibet 

 and Western China, the Himalayan range terminating at the 94th 

 parallel of longitude, and ceasing, therefore, to present so great an 

 obstacle to the transfer of the air, as it does everywhere to the west- 

 ward. It may be observed that in April, the heavy rains are 

 restricted to latitudes north of the head of the Bay. At Akyab 

 and Sandoway there is little rain in this month, and heavy rains 

 begin with the strengthening of the monsoon, only a week or two 

 earlier than in Lower Bengal. 



The monsoon rains usually set in in Calcutta about the second or 



third week in June. At Darjiling, they are somewhat earlier, and 



, in Western Bengal, and the N. W. Provinces, a fortnight or three 



* See Board of Trade Wind Chart, No. IX for the north parts of the Indian 

 Ocean, and for Bombay the Magnetic and Meteorological observations of the 

 Bombay University. 



