576 MR. HUDSON’S HOURLY OBSERVATIONS ON THE BAROMETER. 
by the strict inductive intimations only of the results themselves, and with¬ 
out reference to any particular theory or current hypothesis. 
I have now the honour of laying before the Society the first portion of these 
hourly observations, amounting to about three thousand in number, and made 
in the months of April, May, June and July of 1831, and in those of January 
and February of 1832. The Standard Barometer of the Society has been ob¬ 
served for about sixteen or eighteen hours during the day, through a period of 
seventy-five days ; and also at every hour through the whole twenty-four hours 
for thirty days ; the Water Barometer every hour, day and night, for fifteen 
days; and the Mountain Barometer also every hour, day and night, for the 
same period. In making these observations, no pains have been spared to en¬ 
sure their accuracy ; and I was enabled to extend the series through the whole 
twenty-four hours, with three barometers for fifteen days, and afterwards with 
one barometer for the same period, through the assistance of Mrs. Hudson, 
who supplied my place as the observer for six hours of the night during these 
thirty days, and whose estimation in registering the instruments was found, 
on every comparison, to accord exactly with my own. 
The Standard Barometer is fixed in the upper library, the Water Barometer 
within the public staircase, and the Mountain Barometer in the entrance-hall, 
of the Royal Society. Mr. Bevan, of Leighton Bussard, was, in 1827, re¬ 
quested by a Committee of the Royal Society, of which he was also appointed 
a member, to determine the levels of the barometers then in the possession of 
the Society, above a fixed mark on Waterloo-bridge. From Mr. Sevan’s 
report on that occasion, and from the additional information with which he 
had subsequently the kindness to furnish me on my application to him, I am 
enabled to lay before the Society the relative altitudes of the three barometers 
employed in my observations. 
Mr. Bevan adopted, as his bench-mark, the base of the columns of Waterloo- 
bridge, which base line, at that time, agreed nearly with the highest tide-line 
observed in the river, and was eleven feet six inches above the estimated mean 
level of the surface of the Thames at Greenwich. The presumed mean level 
above the sea at Sheerness was at the same time determined, from theoretical 
considerations, by the late Dr. Young ; and with an accuracy which, I am in¬ 
formed, has been confirmed in a remarkable manner by actual measurement. 
