342 
LIEUT.-COLONEL SYKES’S DISCUSSION OF METEOROLOGICAL 
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.” These discrepancies amounted 
on the 3rd and 14th of November, at the 19th hour. Got. mean time; — 
Standard . . . 82-4 Dry . . . 76-5 Wet bulb . . . 71-8 Diff. . . . 5-9 
Standard . . . 82-4 Dry . . . 75-4 Wet bulb . . . 69 3 Diff. ... 7-0 
Professor Orlebar therefore abandoned observing with the attached dry bulb. But 
supposing this cause of error to have been overlooked by other observers, the tension 
of vapour and the per-centage humidity would have been recorded by them greatly 
higher than the truth ; and if we apply this source of error to the mean monthly and 
annual results in the comparative table I have given ; — for instance, to the annual 
mean for Dodabetta, the 90 per cent, is reduced, for the first difference, to 64 per cent., 
and if the correction be made for 7°, by depression of the dry bulb below a standard 
owing to its proximity to the wet bulb, the 90 per cent, of moisture at Dodabetta is 
reduced to 60 per cent. Professor Orlebar says the dry bulb always stood below the 
standard (and he had determined that it was not owing to error in graduation of the 
thermometers), often to the extent of 2°, and even in the monsoon month of September 
I observe that on the 2nd it was 3°‘4 minus. Any amount of error in depression 
would necessarily affect the numerical determinations of the tension of vapour and 
degree of humidity; but supposing it not to exceed 2°, even this small depression 
would reduce the 90 per cent, of moisture in the air at Dodabetta to 80 per cent. 
Supposing therefore that the same error was not discovered at the other places of 
observation as was discovered in Bombay, there is necessarily some ground for the 
expression of my doubts, whether the air really did hold at the different stations the 
quantity of moisture represented by the figures I have elaborated. But Professor 
Orlebar observed another source of error, contingent upon the locality of the wet 
bulb apparatus, whether placed within doors or out of doors. To determine the 
amount of error he placed a wet bulb within the observatory, observing simultaneously 
with the wet bulb out of doors upon the meteorologic stand. This was done hourly 
for March, April, and to the 10th of May. The reading was almost always plus with 
the wet bulb inside ; on the 22nd of March, at 19th hour, to the extent of 3°'2, while 
at 18th hour it had been only 0‘2 plus ; but there were great irregularities in the read- 
ings, being plus or minus dependent apparently upon drafts of air within the obser- 
vatory, which would depress the wet bulb or raise it. Also the ‘‘atmosphere within 
the room would tend to keep up a reading at any time to whatever it had been at a 
time preceding,” and the latter. Professor Orlebar says, was the principal cause of 
the plus readings in-doors. Supposing this error of 3° to be applied as a correction 
to the reading of the annual means of the wet bulb at Bombay for 1843, the per- 
centage of moisture in the atmosphere would only be 65 instead of 76. The distin- 
guished experimental philosopher Regnault has pointed out the same sources of 
error. He placed the dry and wet bulb in the open air in the court of the College of 
