2 H. F. Blanford — Influence of Indian Forests on the Rainfall. [No. 1, 



apparently attributes the great difference displayed by these two pro- 

 vinces, wholly or mainly, to the fact that, while the former is extensively 

 covered with forest, the latter, up to the Terai, is a broad sheet of field 

 cultivation. 



In this view I am unable to coincide. Without denying or even 

 questioning the effect of forests as one element of the result, the con- 

 clusion thus formulated seems to me far too sweeping. M. "Woeikoff 

 considers, and I think rightly, the action of forests in enhancing the 

 rainfall to be twofold. Firstly, they help to store water, by protecting 

 the soil, and to keep up a constant evaporation ; and secondly, by check- 

 ing and obstructing the movement of the wind, they prevent the evapo- 

 rated vapour being carried away, and tend to produce that calm state 

 of the atmosphere that is favourable to ascending currents and local 

 precipitation. But swamps, such as occupy large tracts of the Assam 

 valley, and the numerous broad river channels that intersect it, must 

 contribute a not unimportant quota to the vapour constituent of the 

 local atmosphere ; and the comparative stagnation of the air in the Assam 

 valleys and the exclusion of those dry westerly winds which play so im- 

 portant a part in the meteorology of the Gangetic plain are certainly 

 due, in far larger measure, to the fencing in of the Assam valley by the 

 Patkoi, Naga, Khasi, and Garo hills, and, as regards Upper Assam, to the 

 interception of westerly currents by the mid- valley obstruction of the 

 Mekhir hills, than to any retardation of wind movement that can be 

 effected by the forests. Furthermore, the action of the surrounding 

 hills in setting up a diurnal convection of the humid-atmosphere, and 

 its consequent dynamic cooling and precipitation, an action which also 

 takes place in the much less humid hill tracts of the peninsula, is a very 

 important item in the causes which contribute to produce the heavy- 

 spring rainfall of Assam ; a precipitation not very greatly inferior to that 

 of the summer monsoon. The other or passive effect of hills in enhan- 

 cing rainfall, namely, the forced ascent of horizontal air currents, is less 

 important in Upper Assam (the tract more particularly referred to by 

 M. Woeikoff), although exhibited by the southern face of the Khasi 

 hills, overlooking Sylhet, in a degree without parallel elsewhere in the 

 world. But to the other causes above specified must certainly be attri- 

 buted by far the larger part of that prevailing high humidity and copious 

 rainfall which foster the exuberant vegetation of the province ; render- 

 ing it, in the rich variety of its flora and its prolific insect life, compar- 

 able with the teeming productiveness of the Malay region. 



The difficulty so conspicuously illustrated in the foregoing example, 

 namely, of disentangling the combined effects of a number of causes all 

 favourable to increased rainfall or the reverse, is one which renders it 



