594 MR. HUDSON’S HOURLY OBSERVATIONS ON THE BAROMETER. 
ference of only *00/ of an inch in mean results derived from these 360 simul- 
taneous observations ; and as the Royal Society’s Standard is placed at an ele- 
vation of forty-two feet above the Mountain Barometer, this small quantity by 
which it stands lower than the other, does not seem to indicate any of that 
undue depression of its mercurial column which ought to result from the in- 
sinuation of air into its vacuum. The mahogany pillar, also, which forms an 
intermediate portion of its scale, may be inferred, from the same simulta- 
neous comparison with the Mountain Barometer, which is furnished with a 
continuous brass scale, as well as from the circumstances of the dimensions of 
the pillar, the polished surface of its sides, the brass plate on its upper surface, 
and the careful insertion of its lower end into the cistern of the instrument, 
not to be subject to the same hygrometric influence as instruments of less 
guarded construction. I may add that a gentleman, who has been for some 
time extensively engaged in the prosecution of barometric levelling, deter- 
mined the elevation of this Standard Barometer above the level of the river, 
to within a very small extent of the estimated altitude, from the published 
observations only which had been made with it ; and Mr. Richardson, of the 
Royal Observatory at Greenwich, has informed me, that in an extensive ex- 
amination of barometrical observations which he was required, for particular 
astronomical reductions, to make, he found the published observations made 
with the Standard Barometer of the Royal Society to accord more accu- 
rately in their changes with the general result of those, made both in this 
country and on the Continent, which he had occasion to consult, than any 
of the other observations he made use of for that purpose. 
