MR. HUDSON’S HOURLY OBSERVATIONS ON THE BAROMETER. 577 
The following Table exhibits the relative levels of the surfaces of the fluids 
in the cisterns of the barometers. 
Above the 
bench-mark on 
Waterloo-bridge. 
Above the 
mean level of 
the Thames at 
Greenwich. 
Above the 
mean level of 
the Sea 
(presumed). 
Standard Barometer .. 
Water Barometer .... 
Mountain Barometer.. 
ft. in. 
83 2 \ 
42 11 
41 2 \ 
ft. in. 
94 9 
54 51 
52 9 2 
ft. in. 
95 0 
54 81 
53 0 
The Standard Barometer was made by Newman, and placed in its present 
sit uation on December 12, 1822 ; and, at the request of a Committee of the 
Royal Society, it was constructed with great care under the direction of Mr. 
Daniell, who has, in his Meteorological Essays, given a full account of the 
mode and principles of its construction*. Its peculiar advantages are, a tube 
of great diameter, a cistern of unusual extent of surface, and an apparatus for 
determining the height of the mercurial column, so delicate and perfect, that, 
with the unassisted eye, it may be determined, on successive trials, with a 
difference only in the ten-thousandths of an inch. The cistern is a cylinder 
of turned mahogany, with an internal diameter of 5*3 inches, and which termi¬ 
nates above, in a rectangular pillar of polished mahogany, encasing the tube, 
1^ inch wide, and 2^ inches deep, rising 25^ inches above the level of the 
mercury, and bearing on its upper surface, and firmly screwed into it, a metallic 
plate, on which rests the brass scale, with the divisions and vernier. The 
* I have been informed by Sir John Herschel, that the Royal Society’s barometer has been com¬ 
pared, intermediately, with almost every other standard barometer in Europe. A fine mountain baro¬ 
meter, belonging to him, and made by Mr. Troughton, having being compared with it, previously 
to his setting out on an extensive tour on the Continent, in which it accompanied him, was found to 
give on his return, as Mr. Henderson related to me, exactly the same difference as that obtained be¬ 
fore his leaving England, having been in the mean time the medium of comparison with a consider¬ 
able number of Continental instruments. At his suggestion, I have opened a permanent registry for 
these standard comparisons. This barometer, with which Sir John Hekschel had done me the honour 
of making some corresponding observations at Slough, is now entrusted to the care of Mr. Hender¬ 
son, the Astronomer Royal at the Cape of Good Hope, who has promised to undertake with me a 
series of observations to be made simultaneously in that Colony and in London. Mr. Dunlop, the 
Astronomer Royal at Paramatta, in New South Wales, and Mr. Forbes, now on a scientific tour in 
Italy and Greece, will each, I have reason to believe, be able to undertake with me similar corre¬ 
spondent observations. 
