23 



Nagpur plateau, and ascending that, they go on at the higher 

 level, while the sea breezes from the Korth of the Bengal Ocean 

 continue to blow over the plains, and thus by creating eddies, 

 often produce those local storms which are known there as Nor- 

 Westers. And now we reach the period of greatest rainfall, 

 between Jane and October. During this season the whole of the 

 N.W. provinces and the Panjab and down to Bombay on the West 

 coast show a fairly regular set of contours, not increasing in value 

 Northwards in lines running from N.W. to S.E. as in the first 

 diagram, but in lines increasing Southwards in lines whose general 

 direction is N.E. to S.W. 



The rainfall in this season shews a more or less steady 

 increase from 5 inches at the foot of the Afghanistan mountains, 

 to 70 at Calcutta, and 100 to the East of Calcutta. 



On the Western coast of the peninsula thei'e is a break in the 

 uniformity of the progression, for we find the greatest rainfall on 

 the Western coast, where the rainfall is over 100 inches. This 

 is due to the interception of the winds by the lofty range of 

 mountains running down that side of India close to the coast, 

 called the Western Ghauts. Meteorological observations establish 

 the general law that however vapour-laden the air may be, 

 however near it may be to the point of saturation, it does not 

 cause a precipitation of rain so long as its general current preserves 

 a horizontal movement, unless on its way it is exposed to con- 

 siderable change of temperature. The wind observations at 

 Calcutta shew that it is not when the monsoon is blowing steadily 

 that rain is most probable, but when it is deflected by some local 

 irregularity, the greater the deflection the more probable will be 

 a fall of rain. 



Any cause, therefore, which sets up an ascending movement, 

 or any deflection at an angle to the plane of motion, such as a hill 

 range, a coast line, or the connection of local currents round the 

 borders of a barometric depression, causes dynamic cooling, and 

 clouds and rain follow, the amount of rain precipitated depending 

 chiefly upon (a.) the initial state of humidity in the air, (^.) the 

 sine of the angle of the upward deflexion al movement. 



Change of temperature was alluded to as an element in the 

 precipitation of rain from vapour-laden currents of air, and it is 

 evident that it must be so, for cold air cannot hold water in 

 suspension to the same degree as warm air. Starting from the 

 vast ocean space in the torrid zone the air drinks in the abundant 

 vapour and is cari'ied to cooler places where saturation becomes 

 complete because of the lower temperature. If the temperature 

 falls still lower, the superfluous vapour is given off as mist, and 

 ultimately, as the moisture becomes too heavy to be sustained by 

 the air, it falls in the shape of rain. 



r 



