OBSERVATIONS TAKEN IN INDIA. 
341 
to attract much vapour, nevertheless the greatest quantity of rain does not fall at 
Dodabetta. The mean of the hourly observations once a month, and the mean of 
the two observations daily throughout the year, giving respectively 87 per cent, and 
90 per cent, of moisture in the air at Dodabetta, observation and expectation thus go 
pretty well together, and the records might be satisfactory, were there not doubts 
about the manipulations with the wet bulb. The hygroinetric observations in the 
Deccan with Daniells hygrometer, which were taken by myself thrice daily, are 
quite in accord with the feelings and with the expectations of the observer, except- 
ing for the month of March, in which the feelings indicate the air to be quite as dry 
as in the months of February or April, but which the hygrometer indicates to be 
6 per cent, nearer to saturation than in either February or April. At Aden, in the 
latitude of Madras, on the arid coast of Arabia, where little rain falls, the mean 
monthly and annual per-centage of moisture appears unexpectedly high ; the lowest 
per-centage (60f) is in November, and the next lowest 68^ and 65^ respectively in 
August and September. The maximum per-centage was 77^ in March, and the 
mean for the year 71- These results have a certain relation to the phenomena at 
Madras, which is destitute of a S.W. monsoon. The mean maximum tension of 
vapour was *902 in May at Aden. 
The mean monthly and annual results at the several stations have no doubt a cer- 
tain relation to truth, as it is seen that the per-centages of moisture or fractions of 
saturation in the atmosphere in the different months of the year have an increasing 
or diminishing amount as the several months approximate to, or recede from, the 
monsoon months of the year; this is sufficiently shown at Madras, where the mon- 
soon months are the dry months at Bombay and in the Deccan. Nevertheless the 
amount of moisture in the atmosphere, deduced from the observations of the wet 
bulb, is so very great compared with the amount determined by the direct method 
by Daniell’s hygrometer (itself an imperfect instrument) in the Deccan, and at some 
of the stations is so little in accord with personal recollections and experience, that I 
cannot refrain from suspecting some error of observation, some mismanagement in 
the manipulations, or a fallacy in the formula by which the dew-point is deduced 
from the temperature of the wet bulb. The first cause of error that struck me was 
that arising from the proximity of the dry to the wet bulb, as noticed by Professor 
Orlebar in his Report of Meteorological Observations taken at the Bombay Obser- 
vatory in 1846. He had a stand erected out of doors, 6 feet high, and with a thatched 
roof, and every precaution was taken to guard off radiation by layers of cotton and 
tow upon a board under the roof upon which the meteorologic instruments were 
placed ; there was lateral access for the air all round. He soon found that the dry 
bulb, in the neighbourhood of the wet bulb, was almost always depressed below the 
neighbouring standard thermometer, and that the depression of the dry bulb was 
greater as the depression of the wet bulb below the standard was greater. Professor 
Orlebar explains this in the following words : — “This seems accountable only on the 
