254 On the Normal Rainfall of Bengal. [No. 3, 



being 560 ins. Teria Ghat, immediately at the foot of the escarpment, 

 at an elevation of 130 feet, would doubtless shew an amount inter- 

 mediate between the two. I have already noticed the influence of 

 the hills in increasing the rainfall on the plains to windward, and 

 it is easy to see that such an effect must be produced wherever (as in 

 this case) a steep escarpment directly faces the prevalent vapour- 

 bearing wind. 



This effect is two -fold, direct and indirect ; direct since, as a 

 physical obstacle, it must cause a piling up so to speak of the lower 

 and more saturated strata of the atmosphere, and force them to 

 an elevation at which their temperature falls below the dew point, 

 causing precipitation ; and indirect, since the vast quantities of 

 water discharged from the hills and spreading themselves over the 

 plain, present an extensive evaporating surface which may extend 

 far beyond the region of the former influence. Such is the case in 

 Silhet. In the rains, the whole region traversed by the SW winds 

 in their passage from the Bay of Bengal, is covered with broad 

 flooded rivers, innumerable creeks and extensive jheels which occupy 

 the whole intervening space, with the exception of the river, banks 

 and the small elevations on which are built the villages. At this 

 season, the whole country may be not inaptly described as an 

 •expanse of water. The atmosphere is, therefore, kept in a state 

 -constantly bordering on saturation, and to this fact, and not solely 

 to the direct or (so to speak) mechanical effect of the hills, must be 

 attributed the high rainfall of Eastern Bengal. The following list 

 «of stations, all on the plains, and within 70 feet of sea level, with 

 their distance from the hills, and their annual rainfall will shew the 

 •combined effects of the two causes above noticed. 



Distance from Annual Rain- 

 the hills. fall. 



Dacca, 100 miles. 7523 inches. 



Bogra, , 60 „ 91-07 „ 



Mymensingh, 30 „ 108-03 „ 



Silhet, 20 „ 149-76 „. 



I now turn to the corresponding facts presented by the Himalayan 

 and Sub-Himalayan stations. 



