418 
MR. BAILY ON THE CORRECTION OF 
The two planes (B and b) not here enumerated, lie between the other pairs ; 
as will be best seen in the preceding figure. The vibrations on the planes A 
and a , are rendered synchronous, or nearly so ; and also on the planes C and c. 
The length between each synchronous pair of planes is, as nearly as possible, 
equal to the standard yard. 
This completes the list of pendulums hitherto proposed or adopted for the 
purpose of any physical inquiry, and it embraces almost every variety that has 
been suggested. I took advantage however of the favourable opportunity that 
was presented for trying the effect of the pressure of the atmosphere on a few 
clock pendulums. In these cases the pendulum was suspended by a spring, in 
the same manner as when it is attached to the clock. I shall not stop to in- 
quire whether the arcs, on these occasions, diminished in a geometric ratio ; 
because as the experiments were carried on nearly under the same circum- 
stances in each case, the comparative results will be but little affected by such 
a consideration. 
No. 39 is a mercurial pendulum, such as is now generally attached to astro- 
nomical clocks. The pendulum actually employed by me on this occasion, 
was one that Mr. Hardy was about to attach to an excellent clock which he 
had just made for His Royal Highness the President of this Society; and is 
the first that has ever been submitted to so rigid a test. It is constructed in 
the usual manner, and similar to one described by me on a former occasion *, 
except that the rod and sides of the stirrup are half an inch wide, which I con- 
sider an improvement. The whole is rivetted together in a very firm manner, 
and finished in a very superior style. The height of the mercury in the glass 
cylinder, when I swung it, was 6’8 inches. The vibrations were made, as I 
have already observed, on its own spring, and not on a knife edge. The weight 
of the mercury was 82960 grains, the weight of the glass cylinder was 6463 
grains, and the weight of the steel parts was 13565 grains. The specific gra- 
vity of the glass I found to be 3‘300 ; and I have assumed that of mercury to 
be 13\586, and of steel to be 7'800 : the vibrating specific gravity, therefore, of 
the mass, deduced agreeably to the formula (2), I find to be 1 0*59 1 . 
No. 40 is another clock pendulum formed of a cylindrical rod of deal, about 
■ ■ 1 1 1 s of an inch in diameter, passing (at its lower end) through a cylinder of lead 
* Memoirs of the Astronomical Society of London, vol. i. p. 409. 
