PENDULUM. 
centre of oscillation ; so that by this means 
its distance from the point of suspension 
will be shortened, and therefore the pendu- 
lum on this account will vibrate quicker ; 
wherefore, if the circumstances of the tube 
and mercury are skilfully adjusted, the 
time of the clock might, by this means, for 
a tong course of time, continue the isame, 
without any sensible gain or loss. This was 
the invention of Mr. Graham, in the year 
1721, who made a clock of this sort, and 
compared it with one of the best of the 
common sort for three years together, and 
found the errors of the former but about 
one-eighth part of the latter. 
Mr. Graham also made a pendulum con- 
sisting of three bars, one of steel between 
two of brass, and the steel bar acted upon 
a lever, so as to raise the pendulum, when 
lengthened by heat, and to let it down, 
when shortened by cold j but he found this 
clock liable to sudden starts and Jerks in its 
motion. 
The ingenious Mr. Ellicott, in the Trans- 
actions of the Royal Society, describes a 
pendulum of his invention, composed of 
brass and iron, with the method of applying 
it, so as to avoid the many jerks to which 
the machine might be liable. 
But besides the irregularities arising from 
heat and cold, pendulum clocks are liable to 
others from friction and foulness; to obviate 
which, Mr. Harrison has several excellent 
contrivances, whereby his clocks are almost 
entirely free from friction, and never need 
to be cleaned. See Longitude. 
The gridiron pendulum is a contrivance 
for the same purpose. Instead of one rod, 
this pendulum is composed of any conve- 
nient odd number of rods, as five, seven, or 
nine; being so connected, that the effect 
of one set of them counteracts that of the 
other set ; and therefore, if they are proper- 
ly adjusted to each other, the centres of 
suspension and oscillation will always be 
equidistant. Fig. 11 represents a gridiron 
pendulum composed of nine rods, steel and 
brass alternately. The two outer rods, 
A B, CD, which are of steel, are fastened 
to the cross pieces AC, B D by means of 
pins. The next two rods, EF, GH, are 
of brass, and are fastened to the lower bar 
B D, and to the second upper bar E G. 
The two following rods are of steel, and 
are fastened to the cross bars E G and I K. 
The two rods adjacent to the central rod 
being of brass, are fastened to the cross 
pieces I K and L M ; and the central rod, 
to which the ball of the pendulum is attach- 
ed, is suspended from the cross piece L M, 
and passes freely through a perforation in 
each of the cross bars IK, B D. From 
this disposition of the rods, it is evident that, 
by the expansion of the extreme rods, the 
cross piece B D, and the two rods attached 
to it, will descend: but since these rods 
are expanded by the same heat, the cross 
piece E G will consequently be raised, and 
therefore also the two next rods ; but be 
cause these rods are also expanded, the 
cross bar I K will descend : and by the 
expansion of the two next rods, the piece 
L M will be raised a quantity sufficient to 
counteract the expansion of the central 
rod. Whence it is obvious, that the effect 
of the steel rods is to increase the length 
of the pendulum in hot weather, and to 
diminish it in cold weather, and that the 
brass rods have a contrary effect upon the 
pendulum. The effect of the brass rods 
must, however, be equivalent not only to 
that of the steel rods, but also to the part 
above the frame and spring, which con- 
nects it with the cock, and to that part 
between the lower part of the frame and 
the centre of the ball. 
Another excellent contrivance for the 
same purpose is described in a French au- 
thor on clock-making. It was used in the 
north of England by an ingenious artist 
about fifty years ago. This invention is as 
follows : a bar of the same metal with the 
rod of the pendulum, and of the same di- 
mensions, is placed against the back-part 
of the clock-case : from the top of this a part 
projacts, to which the upper part of the 
pendulum is connected by two fine pliable 
chains or silken strings, which just below 
pass between two plates of brass, whose 
lower edges will always terminate the length 
of the pendulum at the upper end. These 
plates are supported on a pedestal fixed 
to the back of the case. The bar rests 
upon an immoveable base at the lower part 
of the case ; and is inserted into a groove, 
by which means it is always retained in the 
same position. From this construction, it 
is evident that the extension or contraction 
of this bar, and of the rod of the pendulum, 
will be equal, and in contrary directions. 
For suppose the rod of the pendulum to 
be expanded any given quantity by heatj 
then, as the lower end of the bar rests upon 
a fixed point, the bar will be expanded up- 
wards, and raise the upper end of the pen- 
dulum just as much as its length was in- 
creased ; and hence its length below the 
plates will be the same as before. Of this 
