16 Heport on the Magnetic Survey. [No. 1. 



Bain. — Many instances have been observed showing the quantity 

 of rain to be sometimes of very local occurrence,* and its distribu- 

 tion as much modified by the configuration and topographical position 

 of the surface receiving the rains, as by the more general laws of the 

 movements of the atmosphere. 



Places on a steep declivity facing large plains are particularly 

 exposed to large quantities of rain, which exceeds, for instance, in 

 Cherra, the annual mean of 600 inches. 



It is very remarkable that at Cherra the proportion between the 

 rain during the day and the night is on an average like 2 to 3, very 

 often exceeding that proportion, but in the months after and before 

 the rainy season, the daily variation of the heights of the clouds is 

 quite different, the night being generally very clear and cloudless. 



The great quantity of dew in the tropics seemed an object worthy 

 of particular attention. 



We tried in the Khosia hills, and afterwards in central Assam, to 

 determine the quantity of dew ; the details of one series of experi- 

 ments are given in the following pages. I add that the absolute quan- 

 tity is much inferior to what the first appearance of the substances 

 exposed and the size of the drops made us expect, but a closer inspec- 

 tion explains it very well, by the distances of the drops from each 

 other ; a second experiment at Cherra gave for black wool 0.4 to 0.5 

 millimeters, one at Gowhatty 0.6, the quantity of water in the at- 

 mosphere decreasing with the mean daily temperature more rapidly 

 than the radiating power increases with the elevation above the 

 plains. 



Experiments for the determination of the quantity of dew and 

 the relative radiating power of different substances. 



Night at Cherrapunji from 23th to 29th of October, perfectly 

 clear, very small low cumuli, height not exceeding 3°, disappeared 

 after 10 p. m. 



Substances exposed at 7h. 30' p. m. Oct. 28th. Taken to the balance 

 at 10b. 10' a. m. Oct. 29th. 



The following substances were exposed. 



No. 1. — Empty paper box. 



No. 2. — Black wool (very fine black colour). 



* At Darjiling we had on the 12th of August, 1855, 1.15 inches in two hours, 

 and no rain fell at the military sauatarium not two miles distant. 



