The temperature of the earth was taken by sinking a brass tube a yard long in the soil.
A thermometer with the bulb blackened affords the only means the traveller can generally compass, of measuring the power of the sun's rays. It should be screened or put in a blackened box, or laid on black wool.
A good Photometer being still a desideratum, I had recourse to the old wedge of coloured glass, of an uniform neutral tint, the distance between whose extremes, or between transparency and total opacity, was one foot. A moveable arm carrying a brass plate with a slit and a vernier, enables the observer to read off at the vanishing point of the sun's limb, to one five-hundredth of an inch. I generally took the mean of five readings as one, and the mean of five of these again I regarded as one observation; but I place little dependence upon the results. The causes of error are quite obvious. As far as the effects of the sun's light on vegetation are concerned, I am inclined to think that it is of more importance to register the number of hours or rather of parts of each hour, that the sun shines, and its clearness during the time. To secure valuable results this should be done repeatedly, and the strength of the rays by the black-bulb thermometer registered at each hour. The few actinometer observations will be found in another part of the Appendix.
The dew-point has been calculated from the wet-bulb, by Dr. Apjohn's formula, or, where the depression of the barometer is considerable, by that as modified by Colonel Boileau.* [Journal of Asiatic Society, No. 147 (1844), p.135.] The saturation-point was obtained by dividing the tension at the dew-point by that at the ordinary temperature, and the weight of vapour, by Daniell's formula.
The following summary of meteorological observations is alluded to at vol. i., chapter i.
I.—Table-land of Birbhoom and Behar, from Taldanga to Dunwah.
Average elevation 1,135 feet.
It is evident from these observations, that compared with Calcutta, the dryness of the atmosphere is the most remarkable feature of this table-land, the temperature not being high; and to this, combined with the sterility of the soil over a great part of the surface, must be attributed the want of a vigorous vegetation. Though so favourably exposed to the influence of nocturnal radiation, the amount of the latter is small. The maximum depression of a thermometer laid on grass never exceeded 10 degrees, and averaged 7 degrees; whereas the average depression of the dew-point at the same hour amounted to 25 degrees in the morning. Of course no dew was deposited even in the clearest star-light night.
February 1848.
Hour Sunrise 9 a.m. 3 p.m. 9 p.m.
TEMPERATURE
Mean 56.6 70.1 75.5 61.7
Max. 65.2 77.0 81.7 66.2
Min. 46.3 61.2 65.2 55.5
Range 18.9 15.8 16.5 10.7
WET-BULB
Mean 48.2 53.7 55.3 49.3
Max. Depression 12.5 19.3 22.5 20.5
Min. Depression 6.0 14.3 16.7 9.0
Elasticity of Vapour .276 .264 .248 .248