the air. There is a less quantity of aqueous vapour in a given quantity
of air in January than in May, but the difference of temperature
makes the drying power of the air in the latter month superior to that
in the former. I find by a rough calculation that the weight of aque-
ous vapour in a cubic foot of air varies from 3-3 grains in January to
10.3 in August. Comparing the driest month at Dehli with the driest
in Calcutta, I find the ratio as 5 to 3 nearly : the dampest month is
nearly the same at both places, and the mean of the year as 5 to 4
nearly : I should have expected a much greater difference.
/ D l • 28S
I have added a table of the factor y 5-84 — s ) for finding the dew point from observations of the temperature of the air and that of the wet thermometer ; also the table of the force of vapour which I have used in my reductions : should these be thought worth printing, it will be as well to give an example of their use to prevent misconcep- tion. Suppose the temperature of the air 100°, that of the wet ther- mometer 70° .•. D = 30°, the barometer standing at 27*0 inch j required the dew point and comparative tension?
For wet thermometer 70° and D = 30° Table IV. gives -402 For barometer 27.0 inch, deduct "402 X '1= "040 Force of vapour at 70° 362 •723 Dew point 49°.9 — force of vapour answering thereto = "361 log. = 9'55751 Force of vapour at temp, of air 100° = 1'874 colog. = 972723 Comparative tension = 0.193 = 928474
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Table I. Barometer reduced to 32°, and Temperature of the external Air observed at Gurgaon. Monthly Elevation of Year and 10 A. M. 4 I . M. Mean. Dally difference Gurgaon Month. range of Bar. from mean. above Cal- Bar. Temp. Bar. Temp. Bar. Temp. in.
in.
in. O in. in. Feet. May, 1829. 28.832 100.6 28.712 107.6 28.772 104.1 .120 — .182 868 June, .... .813 96.1 .714 100.5 .763 98.3 .099 — .191 841 July, .717 84.0 .646 87.0 .682 85.5 .071 — .272 855 August,... . .791 83.9 .711 85.8 .751 84.8 .080 — .203 803 September, .882 86.6 .798 91.7 .840 89.2 .084 — .114 807 October, . . 29.034 84.5 .955 91.1 .995 87.8 .079 — .041 808 November, .223 72.7 29.143 79.1 29.183 75.9 .080 + .229 750 December, .255 62.4 .158 71.3 .207 66.8 .079 +.253 771 Jan. 1830. .230 66.0 .140 74.9 .185 70.5 .090 + .231 854 February, .153 69.8 .067 76.0 .110 72.9 .086 + .156 813 March, . . .052 77.3 28.963 83.4 .008 80.4 .089 + .054 814 April, .... •• •• •• •• -• •• •• •• •• 28.954 .089 817
