1890.] Relations between the Hills and Plains of Northern India. 43 



amount of compression necessary to counterbalance almost exactly the 

 effect of diminishing temperature on the pressure. In the open Gani^e- 

 tic plain, more especially near the hills, it may be accompanied by sliglit 

 horizontal movements, but they are generally too small to be measured 

 by an anemometer. Hence the adjustment of pressure takes place in 

 the cold weather during the day time chiefly by convection currents and 

 partly by expansional movement of the lower strata and partly by hori- 

 zontal motion from west to east or from the area of later to earlier solar 

 action during the day ; and during the night, almost solely by vertical 

 movement accompanying or producing compression. 



Through such a nearly motionless atmosphere the heat radiated 

 from the earth's surface will pass readily. The chief proportion of the 

 small absorption which occurs will be in the lowest strata. Hence the 

 upper strata which receive little heat and give out little by radiation 

 will have their temperature very slightly affected by this cause. Also 

 if the compression of the lower strata be effected by the expansion of the 

 upper strata, these strata will be slightly cooled, whilst the compression 

 of the lower strata will cause a slight increase of temperature, but these 

 changes can be shown to be so small as not to affect the temperature at the 

 utmost more than 1° or 2°. The most important action, however, occurs 

 in the lowest strata. The earth is cooled rapidly by radiation from its 

 surface into space, and in the vast level plains of Northern India, the air 

 remains quiescent or stagnant over it and hence cools down rapidly. (The 

 cooling of the lowest strata probably takes place chiefly by conduction 

 and to some extent by convection currents extending to a comparatively 

 small height, determined partly by height of vegetation, trees, houses, 

 &c.) The chief fact, however, remains that the cooling occurs in a 

 stagnant or quiescent stratum near the earth's surface, and hence goes on 

 continuously during the night, and produces a very large accumulated 

 decrease of temperature. 



This action is, however, chiefly confined to the lowest strata and 

 above these the fall of temperature will be almost solely due to conduc- 

 tion (a slow process in air) and hence be small in amount. Also, as 

 the lower strata are compressed and the upper strata expand, there will 

 be some level at which at each instant there is neither compression nor 

 expansion. Whether this will alter much in position during the night 

 can only be conjectured, bat it appears on the whole most probable that 

 it will not. The total fall of temperature during the night will hence 

 decrease rapidly in amount with elevation and at some elevation become 

 practically constant where it will be due almost entirely to slight 

 cooling by radiation and by expansion and to a very slight extent by con- 

 duction and probably not exceed 2° or 3° in amount. 



