Proceedings of the Asiatic Society. [July, 



fell diminished from a maximum along the line of sea coast to a 

 minimum at the foot of the Hills ; and again the rainfall diminish- 

 ed from another maximum at the first crest of the Hills to a small- 

 er amount further in upon the corresponding plateau. And it was 

 easy to see why this should be so ; the vapour-bearing air masses 

 continually parted with their freight as they went on, and so less 

 and less rain was the result, until some new element was introduced 

 into the conditions which had the effect of increasing the degree 

 of saturation. At the Hills this was brought about by a simple 

 mechanical process ; the advancing stream of air and vapour under 

 the influence of its own velocity, and the pressure from behind slid- 

 ed up the escarpment, and was thus with comparative suddenness 

 carried to an elevation, which at once gave rise to very great rari- 

 faction and consequent disappearance of heat. Also the low tem- 

 perature of mountain surfaces maintained by active radiation con- 

 duced to the same end. In this way, the charge of vapour in the 

 air brought up from below became excessive in relation to the heat 

 available for the purposes of maintaining it in a state of suspension 

 and large quantities were precipitated. He did not know whether 

 the great rainfall along the coast line had been ascertained to be a 

 true maximum or not. It would not be easy to make very accurate 

 observations on the amount of rainfall at sea : but he was prepared 

 to learn that this rainfall was really a maximum, and if so, he was 

 inclined to attribute it to a cause in some sense the reverse of that 

 which effected the saturation at the mountain top, i. e., an increase 

 in the quantity of vapour instead of a diminution of the tempera- 

 ture. Having regard to the fact that the delta was literally sown 

 with shallow tanks, khals, and pieces of water of every sort and 

 size, lie thought it very possible that the surface soil heated by a 

 tropical sun might, under these circumstances, yield a more plen- 

 tiful supply of vapour than the sea itself. In truth this arrangement 

 of land and water bore no slight resemblance to the general form of 

 contrivances for multiplying the heating surface for the generation 

 of steam in boilers. 



"With regard to the occurrence of a rise in the barometer at 

 Cherrapoonji immediately before a heavy fall of the monsoon 

 rains, this might be caused by increase in the vapour-flow from 



