OBSERVATIONS TAKEN IN INDIA. 
377 
dras ; and the passage of the sun twice over both places does not derange the curve. 
The anomalies of the annual mean temperatures of Madras, Bombay, Calcutta and 
Aden, not diminishing with the increase in the latitude of the respective places, are 
pointed out, and numerous instances are given of the very great power of the slant- 
ing rays of the sun beyond the tropic. As is the case with the barometric, so do the 
heat tables indicate that the annual and daily ranges of the thermometer diminish 
with the elevation of the place of observation above the sea-level, the elevated table- 
land of the Deccan however being an exception to this rule. At Mahabuleshwur, at 
4500 feet, the temperature of the air was never below 45° with a, maximum and 
minimum thermometer ; and at Dodabetta the temperature of the air was never 
below 38°'5 ; nevertheless at both places ice and hoar-frost were frequently found 
on the ground at sunrise, resulting from the separate or conjoined effects of radia- 
tion and evaporation. I have already stated that I do not attach much value to the 
readings of the wet bulb, owing to the various sources of error in the instrument 
itself, and to the manifest sources of error in the existing theoretical formulae for 
giving a numerical value to its readings to fix the tension of vapour in the atmo- 
sphere for the determination of the dew-point. No doubt the dew-points obtained 
by means of the wet bulb have a certain relation to the truth ; and in some favour- 
ably concurring conditions may be as proximate to the truth as dew-points would 
be, obtained by direct means ; but the elaborate experiments of Regnault show 
that in a calm in the open air or in a chamber, the wet bulb surrounds itself with 
its own humid atmosphere, and with a breeze blowing upon it the temperature of 
the wet bulb falls or alternates as the wind blows more or less rapidly upon it. 
Moreover, Professor Orlebar points out a source of error to which no doubt all 
the observations in India were more or less subject, namely, the proximity of the 
dry bulb to the wet bulb, and the cold from the latter in consequence depressing the 
temperature of the dry bulb, two, three, or more degrees below the temperature of 
the air, necessarily producing fallacious results. Making allowance for the defects of 
Daniell’s hygrometer, the dew-points obtained by its means are infinitely more 
worthy of confidence than those obtained by means of the wet bulb. On the whole, 
I would not venture to say more with respect to normal conditions of moisture in 
India, than that the air of the sea-coast has always a much greater fraction of satu- 
ration than the lands of the interior, and that the elevated plateau of the Deccan is 
periodically subject to very great degrees of dryness. 
The rain-tables are so extensive that it will only be necessary to point out some 
unexpected phenomena connected with the distribution of rain. It is found both on 
the sea-coasts and on the table-lands of the Deccan, that within very limited areas, 
the differences in the fall of rain may be very great. With nine rain-gauges employed 
in the small island of Bombay in the months of June and July, in the monsoon of 
1849, the quantity collected in the different gauges ranged in July from 46 inches to 
102 inches, and in June from 19 inches to 46 inches. At Sattarah, with three rain- 
3 c 
MDCCCL. 
