288 Description of the country hetiveen [No. 10, new series. 



angles, and parallel to the bounding chains of hills. The hills 

 generally recede in height as we proceed westward, and are either 

 comparatively bare of jungle ; or covered with low jungle. 



Climate. 2. The climate of the plateau, which ave- 



rages 2,300 feet above the sea is away from the immediate vicinity 

 of the high hills, very dry and agreeable, the average range of the 

 thermometer in the beginning of the year being about 28, the 

 minimum being 50, and the maximum 78°. In the narrow valleys, 

 bounded by high hills, the thermometer sinks much lower than 

 mentioned above, and immediately under the Ghauts, the cold is 

 very severe. This is owing to the cold air, which at night rushes 

 down the sides of the hills, and fills the valleys, a phenomenon of 

 almost universal occurrence in all parts of the globe, and particu- 

 larly observable under mountain chains, as in the present case, 

 running N.andS. and whose slopes consequently receive the sun's 

 rays during only one half the day. It is well known that the at- 

 mosphere is but little heated by the direct action of the sun's rays, 

 and that it principally derives its heat by radiation from the earth's 

 surface, and that, moreover, it is when dry, an extremely bad con- 

 ductor of heat. On these hills covered with jungle, the ground is, 

 however, generally more or less moist and so therefore also the air 

 in its immediate vicinity, and thus the power of conduction being 

 added to that of radiation, these narrow valleys become filled with 

 air several degrees hotter than that of the atmosphere free of them. 

 When therefore that period of the day has arrived, at which the 

 earth ceases to receive heat, and commences to part with that re- 

 ceived, the strata of air close to the sides of the hills become heat- 

 ed (above the average prevailing temperature) much mero rapidly 

 than those more distant, the intensity of radiation being inversely 

 as the square of the distance from the radiating point. The por- 

 tion of the atmosphere then resting on the hill slopes becomes 

 gradually lighter, and would ascend but for the cooler and heavier 

 air, which rests above the hills, which descends by its superior 

 weight, and exerts a lateral pressure on the hotter air, the currents 

 of which (one from either side of the valley) gradually approach 

 one another, mingle and then ascend their upward passage being 

 facilitated by a partial vacuum, or quiescent state of the air medial 



