length of the pendulum vibrating seconds. 63 
vacuo, it is necessary to apply to it a correction for the 
buoyancy of the atmosphere. 
For this correction, the weight of the pendulum, compared 
with that of air, at the time of observation, must be known. 
The pendulum being composed of different kinds of brass, 
the specific gravity of each part was carefully determined, 
and from thence the specific gravity of the whole mass. 
Part of the pendulum. 
Weight in 
air. 
Specific gravity. 
3 weights (cast brass) 
4 knee pieces (cast brass) 
Bar ( plate brass ) 
lb. 
3,14 
3> l 3 
3 S° 
8,417 
7,816 
8,532 
From the above data, the specific gravity of the pendulum 
is 8,469 ; or the weight of the pendulum compared with 
water is as 8,469 to 1. 
It has been determined by Sir George Shuckburgh 
(Phil. Trans, for 1777) that water is 836 times heavier than 
air, when the thermometer is at 53 0 , and the barometer at 
29,27 inches. But the specific gravity of air varies directly 
as the height of the barometer, and inversely as its expansion, 
which is known to be ^|^th part of its bulk for each degree 
of Fahrenheit; consequently, for any other state of the 
barometer and thermometer, the number 836, will vary in- 
versely as the height of the barometer, and directly ^l^-th part 
for each degree of the thermometer above 53 0 . 
Thus the specific gravity of water, compared with that of 
air, may be known for the temperature and altitude of the 
barometer at the time of observation ; and multiplying this 
