IN THE OBSERVATORY OF GREENWICH. 
471 
other respects as nearly similar as possible. By this comparison, the correc- 
tion for nearly the full atmospheric pressure is ascertained; the proportional 
part of which is taken as the equivalent for the small residue of air which 
cannot be wholly withdrawn, and for the occasional small leakage of the appa- 
ratus during the experiments ; the pressure withinside the glasses being duly 
observed and registered by means of a syphon gauge placed by the pendulum. 
In regarding the correction for the last remaining inch of pressure as a propor- 
tional part, namely the thirtieth, of the correction for the whole atmosphere, I 
refer to the experiments related in a former paper in the Philosophical Trans- 
actions, in which it was shown that the corrections for a half atmosphere, 
and for a quarter atmosphere, bore corresponding proportions to the correction 
for the full atmosphere. 
The following is an abstract of two series of such experiments with the 
weight above. 
Reference to 
the details at 
the close of 
the paper. 
Therm. 
Vibrations. 
Reduction 
to mean 
temp. 57°. 
Vibrations at 57°. 
Barom. or Gauge. 
In the free air . 
Air withdrawn . 
{ i 
F 
O 
57.5 
59-6 
59-8 
86056.88 
86055.82 
86068.56 
+ 0.22 
+ 1.14 
+ 1.23 
86057.10-1 g 7 
86056.96 / 8b057-0d 
86069-79 
inch. 
{gili?} 29 ' 433 
1.045 
(Slider at 1.5) Differences 
12.76 
28.388 
In the free air . 
O 
56.05 
86055.95 
-0.41 
86055.54 
inch. 
29.376 
Air withdrawn . 
PtoS (mean) 
56.87 
86068.48 
-0.06 
86068.42 
1.572 
(Slider at 1 .633) 
Differences 
12.80 
27.804 
We have then, from these two series, the following differences : 
12.76 vibr. corresponding to 28.388 in. of air at 58.5 ; or 13.48 vibr. to 30 in. 
12.88 27-804 at 56.1 ; or 13.89. 
Whence we obtain the reduction, when the weight is above, in the proportion 
of 13.68 vibrations per diem for 30 inches of air at 57°-3. 
3 p 
MDCCCXXXI. 
