A PENDULUM FOR THE REDUCTION TO A VACUUM. 
413 
No. 21 is a solid copper cylindrical rod, 0'41 inch in diameter, and 58'8 
inches long-. This pendulum was invented by Mr. Troughton, and was made 
by him about 16 years ago, when the Commissioners were appointed by 
Government for determining the length of the seconds pendulum. It would 
take up some time to describe the mode in which this pendulum was originally 
intended to be mounted and swung ; and would be irrelevant to the present 
subject : but, as great part of the apparatus could be dispensed with, on the 
present occasion (the results being comparative only), I shall merely state 
that I at first attempted to swing it by suspending it, at one end, with a piece 
of steel wire, drawn close up to the cylinder mentioned in No. 8 and 9. But 
I found the discordancies (to which I shall afterwards have occasion to allude,) 
so enormous, that I was obliged to abandon this mode : and I ultimately fast- 
ened it, by means of an adjusting screw, to the knife edge used in the pre- 
ceding experiments. As I had no means of determining the specific gravity 
of the rod, I have assumed it as equal to that of the copper bar No. 27 ; viz. 
8‘629 : its weight is 16810 grains. 
No. 22 is Rater’s invariable brass pendulum. Several pendulums of this 
kind have been made for our own, and for other Governments, and for public 
bodies ; and all from the same model, which is that described by Captain 
Rater in the Philosophical Transactions for 1819, page 341. I have two now 
in my possession (numbered 10 and 11,) belonging to the Admiralty; and are 
those that were taken out by the late lamented Captain Foster, in his voyage 
of experiment. They are formed of a bar of brass 1*8 inch wide, and rather 
less than T ^th of an inch thick. At the top of this bar is a knee piece, also 
of brass about three tenths of an inch thick, to which a steel knife edge is 
firmly screwed : and, at about 40^ inches from this knife edge, is fastened a 
flat circular bob of solid brass, about 6 inches in diameter, and l^inch thick, 
but tapered at the edge. Below this bob the bar is reduced to about T 7 oths of 
an inch in width, and is continued about 16^ inches: thus forming what is 
called a tail piece, — a most unnecessary and inconvenient appendage ; since 
the arc of vibration, which this tail piece was intended to indicate, can be as 
readily observed by means of the edge of the bar above the bob. As my 
vacuum apparatus was not sufficiently large to receive the bob of this pen- 
dulum, I shall deduce the results from the experiments made by Captain 
3 H 
MDCCCXXXII. 
