1S37.] 



the Assam Tea Plant* 



441 



in the general mass towards the side on which this action is taking 

 place. The whole of the vapours are thus attracted towards the south, 

 where unless entirely dispersed hy noon, their broken masses linger on 

 the northern face of the Naga mountains, receiving daily fresh accu- 

 mulations, until they are precipitated in heavy rain, seldom however 

 before they have served as an impenetrable canopy to this side of the 

 valley for several weeks. 



This tendency of the mists to occupy the south side of the valley, is 

 an interesting point if considered with reference to what I have already 

 stated regarding the absence of the tea plant on the northern side. 



The plains on the northern side of the Bramaputra, may indeed be 

 considered generally to enjoy two hours more sunshine daily, during 

 the cold season than those on the south, a circumstance which is of it- 

 self calculated to influence vegetation, and cause a difference in this 

 respect between the two sides of the valley. 



With regard to the cause of these mists, it would be easy to say that 

 they arise from the moisture of the forests ; but if this were all, they 

 should appear after sun-set when the heat of the day is first with- 

 drawn ; the loss of temperature would then cause a condensation in the 

 lower atmosphere, which is not the case in Assam ; nor do the mists 

 first make their appearance in the forests, but on the river, and on such 

 parts of it as are shallowest and most languid in current. 



I attended as closely as I could without neglecting other equally im- 

 portant matters, to these peculiar phenomena, and the only conclusion 

 I could come to from the observation of simple facts was, that they 

 arose from the difference of temperature between the water of the Bra- 

 maputra and the air, which amounted to a mean difference of 15° Fahr. 

 during the month of December. Throughout that month, in our pro- 

 gress up the river from Gowahatti to Suddya, I found the water at 

 sun-rise vary from 55° to 56° Fahr. and the temperature of the air at 

 the same time, from 38° to 42° Fahr. This difference causes the water 

 to give off more vapour than the air can hold suspended in an invisible 

 form ; partial condensation immediately ensues, and the sensible va- 

 pour is then seen curling over every portion of the river, just as steam 

 presents itself to view ascending from the surface of heated water. 



Those high altitudes by which Assam is surrounded, occasion a ra- 

 pid abstraction of heat during the nights of the cold season, in which 

 the waters and the air participate unequally, as conductors of differ- 

 ent powers. The latter consequently retains a higher mean tempera- 

 ture than the former, and assists during the cold season to check ex- 

 cessive diurnal variations. On the other hand, in the hot season, when 



