TRANSACTIONS OF THE SECTIONS. 
571 
the bottom in a fine point, and having the usual scale affixed 
to the top. This rod is movable by a screw, so that the lower 
point can always be brought in contact with the surface of the 
mercury in the cistern, while the scale at the upper end of the 
rod marks, with the utmost precision, the exact distance be¬ 
tween the upper and lower mercurial surfaces, without the ne¬ 
cessity of further adjustment or calculation. The distances on 
the scale are set off from a standard formerly belonging to Mr. 
Cavendish, and which is presumed to be identical with the old 
national standard. This, however, has not yet been actually 
verified by experiment. There may , therefore, exist a slight 
constant error, affecting all the following results, from this 
cause, and also from another, pointed out in the memoir, and 
amounting to about -f‘008 inch. 
The standard thermometer referred to in these experiments 
belongs to Mr. Daniell, and is that described by Capt. Sabine, 
in his work on the Pendulum (page 188). The bore of the 
tube of this instrument has been repeatedly examined, and found 
to be uniform between 82° and 212°. 
The weights employed are of platina, and adjusted with the 
greatest care by Mr. Robinson, from a Troy pound, expressly 
verified for the purpose by Capt. Kater with the national stand¬ 
ard. The measures were determined from the weights in grains 
of water at 62°, of which 252,458 are presumed to be equal to 
a cubic inch. The balance employed was made by Robinson, 
and devoted expressly to the purpose. It is mounted with a 
counterpoise of glass, as nearly as possible of the same size and 
weight as the balloon in which the air is weighed, by which all 
errors from buoyancy, &c. are completely obviated. 
The air to be weighed was first passed through lime wa¬ 
ter, into a large bell glass receiver, where it was permitted 
to remain for six or eight hours, with the view of separating 
the carbonic acid present. One portion of it was then intro¬ 
duced into a similar smaller apparatus filled with the strongest 
sulphuric acid, while another portion was conveyed into a simi¬ 
lar apparatus filled with distilled water. With these two fluids 
the different portions of air were permitted to remain in contact 
for at least twelve hours, with the view in the one instance of 
separating the whole of the water present, and in the other of 
saturating it with that fluid. A known quantity of air in each 
state, as determined by a very simple gasometer, was then in¬ 
troduced into the weighing balloon, and its weight carefully de¬ 
termined, with all the necessary precautions. In weighing air 
at 82°, an apparatus on the same principle was employed, but 
