Page:Illustrations of Indian Botany, Vol. 1.djvu/204

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ILLUSTRATIONS OF INDIAN BOTANY.

Taking the means of the tables of Mr. Reeves and of Mr. Davis, the mean annual tem- perature of Canton will be very nearly 70 : the mean of the four hottest months in the year 82-2, and of the four coldest months 54.

The mean annual heat of Sadiya, in upper Assam, will be 67-2.

The mean temperature of the four hottest months will be 80, and of the four coldest months 57-8.

Canton. Sadiya. Mean annual temperature, 70 67-2

Mean temperature of 4 hottest months, 82-2 80

coldest months, 54 57-8

The greatest degree of cold which, so far as I know, has occurred at Sadiya, was in January, 1837, when Major White observed it on one occasion at 6 A, M. to be so low as 32 0. On another occasion it fell to 37 ; 42° is not uncommon. The highest range during the hot months was in 1836, 92°. It is singular that Captain Jenkins had never observed it so high as this at •Gowahatti, (90 being its highest range at that place) although considering its latitude and situation, the contrary would be supposed to take place. In upper Assam, January is the coldest month of the year ; August and September the hottest and most oppressive."

Were we to assume the climate of Assam as the most suitable for rearing this shrub, we might find it difficult indeed to find a parallel one in southern India, but this cannot be necessary as it is next to impossible that similar climates can be common to the various and widely separated countries where it is cultivated to so vast an extent.

Mr. CcClelland describes Assam as an extensive deluvial valley surrounded on all sides by vast mountains ; those on the north extending backwards to the Himalayas which in this portion of the range are covered with almost perpetual snow ; and travesed its whole length by a vast river, fed by 5 or 6 others, the least of which nearly equals the largest river in England. A valley so situated has its climate modified in a very remarkable degree by local causes — Thus owing to the proximity of the snowy mountains a cool current of air blows from the north-east at all seasons. That meeting the south-west winds from the plains of Bengal in the months of March, April and May, is productive of frequent showers by which the air is kept cool and moist during the season of heat and draught, again

" Throughout the cold season, dense vapours arise from the Bramaputra about day-light, and continue to increase until 8 A. M. when they begin slowly to ascend. They are then drifted before the N. E. wind, which from the diminished heat of the valley now amounts only to a gentle movement, the direction of which is modified by the action of the sun's rays on the upper stratum of mist, causing a more or less powerful dissipation, and exciting a movement 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 by noon, their broken masses lingev- on the northern face of the Naga mountains, receiving daily fresh accumulations, 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."

These mists are attributed to the inequality of temperature existing between the river and surrounding air; that of the former amounting to about 15° higher than the latter, hence the vapour rising from it, becomes partially condensed and has the effect of raising and equalizing