'The Transactions of the Bombay Geographical Society. From January 1863 to December 1864. (Edited by the Secretary.) Volume XVII.' [‎191] (362/524)

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( 191 )
Art IX.— On a supposed effect of the Humidity derived from the
Cistern of the Wet Bulb Thermometer upon the reading of an
adjacent Dry Bulb. —By John Allan Broun, Esq., F.R.S.
[Read before the Society, January 21st, 1864.]
This effect Professor Orlebar deduced from his observations in 1845
(see Introduction to the volume of Bombay Observations for 1845, p.
Ixx.), and his conclusion, with some of the numbers upon which it
depended, were cited by Colonel Sykes in his paper on Indian Obser
vations, in the Transactions of the Royal Society of London (1850, p.
337), as an illustration of an additional source of error in attempting
to compute vapour pressures from the psychrometer. It was also cited
by other writers.
Professor Orlebar, in comparing readings of a standard thermometer
with those of the dry bulb thermometer (the latter on the same stand
with the wet bulb and its cistern), found differences amounting in one
case'to 11 *2 deg., and in four cases from 3 deg. to 7 deg. Faht. He
came in consequence to the following conclusion :
“ Although great allowance must evidently be made for error of obser
vation, yet these observations clearly show that the dry hulb was de
pressed below its proper height, and that this depression is greater as
the depression of the wet below the standaid is gieater* This seems
accountable only on the supposition that heat is extracted from the air
to form the shell of moisture round the wet bulb at a distance as far
off as the dry bulb. Hence I consider the observations of the dry bulb
as quite useless and have not published them.
Several years ago I had the curiosity to endeavour to discO\er the.
source of what I could not help believing to be an erroneous conclusion.
I found, upon examining the volume of Bombay Observations for 184o,
that the observations made according to the hours of Gottingen i ean
Time (as usual in the Magnetic Observatories) had probably also t le
corresponding hours of Bombay Mean Time noted. Professor Orlebar