•cold, from those that bare such provision. 
Also simple pendulum, and detached pendu- 
lum, are terms sometimes used to denote such 
pendulums as are not connected with any 
clock, or clockwork. 
Pendulum, compound, in mechanics, is 
a pendulum whose rod is composed of two 
or more wires or bars of metal. These, by 
undergoing different degrees of expansion and 
contraction, when exposed to the same heat 
or cold, have the difference of expansion or 
contraction made to act in such a manner as 
to preserve constantly the same distance be- 
tween the point of suspension and centre of 
oscillation, although exposed to very diffe- 
rent and various degrees of heat or cold. 
There are a great variety of constructions 
for this purpose ; hut they may he all re- 
duced to the gridiron, the mercurial, and the 
lever pendulum. 
It may be just observed by the way, that 
the vulgar method of remedying the incon- 
venience arising irom the extension and con- 
traction of the rods of common pendulums, 
is by supplying the boh, or small ball, with a 
screw at the' lower end ; by which means the 
pendulum is at any time made longer or 
shorter, as the hall is screwed downwards or 
upwards, and thus the time of its vibration is 
kept continually the same. 
The gridiron pendulum was the invention 
of Mr. John Harrison, a very ingenious art- 
ist, and celebrated for his invention of the 
watch for finding the difference of longitude 
at sea, about the year 1725, and of several 
other timekeepers and watches since that 
time ; for all which he received the parlia- 
mentary reward of between 20,000 and 30,000 
pounds. It consists of five rods of steel, and 
four of brass, placed in an alternate order, 
the middle rod being of steel, by which the 
pendulum-ball is suspended; these rods of 
brass and steel, thus placed in an alternate 
order, are so connected with each other at 
their ends, that while the expansion of the 
steel rods lias a tendency to lengthen the 
pendulum, the expansion of the brass rods, 
acting upwards, tends to shorten it. And 
thus, when the length of the brass and steel 
rods is duly proportioned, their expansions 
and contractions will exactly balance and 
correct each other, and so preserve the pen- 
dulum invariably of the same length. The 
simplicity of this ingenious contrivance is 
much in its favour; aiid the difficulty of ad- 
justment seems the only objection to it. See 
Longitude. 
Mr. Harrison, in his first machine for mea- 
suring time at sea, applied this combination 
of wires of brass and steel, to prevent any 
alterations by heat or cold ; and in the ma- 
chines or docks he has made for this pur- 
pose, a like method of guarding against the 
irregularities arising from this cause is used. 
The mercurial pendulum was the invention 
of the ingenious Mr. Graham, in consequence 
of several experiments relating to the ma- 
terials of which pendulums might he formed, 
in 1715. Its rod is made of brass, and 
branched towards its lower end, so as to em- 
brace a cylindric glass vessel 13 or 14 inches 
long, and' about two inches diameter; which 
being filled about twelve inches deep with 
mercury, forms the weight or ball of the pen- 
dulum. If upon trial the expansion of the 
rod is found too great for that of the mer- 
cury, more mercury must be poured into the 
PENDULUM. 
vessel ; if the expansion of the mercury ex- 
ceeds that of the rod, so as to occasion the 
clock to go fast with heat, some mercury must 
be taken out of the vessel, so as to shorten 
the column. And thus may the expansion 
and contraction of the quicksilver in the glass 
be made exactly to balance the expansion 
and contraction of the pendulum-rod, so as 
to preserve the distance of the centre of os- 
cillation from the point of suspension invari- 
ably the same. 
Mr. Graham made a clock of this sort, and 
compared it with one of the best of the com- 
mon sort, for three years together; when 
he found the errors of his own but about one- 
eighth part of those of the latter. 
Mr. John Ellicott also, in the year 1738, 
constructed a pendulum on the same prin- 
ciple, hut differing from Mr. Graham’s in 
many particulars. The rod of Mr. Ellicott’s 
pendulum was composed of two bars only ; 
the one of brass, and the other of steel. It 
had two levers, each sustaining its half of the 
ball or weight ; with a spring under the lower 
part of the ball to relieve the levers from a 
considerable part of its weight, and so to 
render their motion more smooth and easy. 
The one lever in Mr. Graham’s construction 
was above the ball ; whereas both the levers 
in Mr. Ellicott’s w'ere within the ball, and 
each lever had an adjusting screw, to lengthen 
or shorten the lever, so as to render the ad- 
justment the more perfect. 
Notwithstanding the great ingenuity dis- 
played bv these very eminent artists on this 
construction, it must farther be observed, in 
the history of improvements of this nature, 
that Mr. Cumming, another eminent artist, 
lias given, in his Essays on the Principles of 
Clock and Watch Work, an ample descrip- 
tion, with plates, of a construction of a pen- 
dulum with levers, in which it seems he has 
united the properties of Mr. Graham’s and 
Mr. Ellicott’s, without being liable to any 
of the defects of either. The rod of this 
pendulum is composed of one flat bar of 
brass, and two of steel ; he uses three levers 
within the ball of the pendulum; and, among 
many other ingenious contrivances for the 
more accurate adjusting of this pendulum to 
mean time, it is provided with a small hall 
and screw below the principal ball or weight, 
one entire revolution of which on its screw 
will only alter the rate of the clock’s going 
one second per day ; and its circumference 
is divided into 30, one of which divisions will 
therefore alter its rate of going one second 
in a month. 
P e n d u lu M- clcck, is a clock having its 
motion regulated by the vibration of a pen- 
dulum. 
It is controverted between Galileo and 
Huygens, which of the two first applied the 
pendulum to a clock. 
After Huygens had discovered, that the 
vibration made in arcs of a cycloid, however 
unequal they might be in extent, were all 
equal in time; he soon perceived, that a 
pendulum applied to a clock, so as to make 
it describe arcs of a cycloid, would rectify 
the otherwise unavoidable irregularities of 
the motion of the clock; since, though the 
several causes of those irregularities should 
occasion the pendulum to make greater or 
smaller vibrations, yet, by virtue of the cy- 
cloid, it would still make them perfectly equal 
3/3 
in point of time ; and the motion of the clock 
governed by it, would therefore be preserved 
perfectly equable. But the diffic ulty w as, 
how to make the pendulum describe arcs of 
a cycloid; for naturally the pendulum, being 
tied to a fixed point, can only describe cir- 
cular arcs about it. 
Here Mr. Huygens contrived to fix the 
iron rod or wire, which bears the ball or 
weight at the top, to a silken thread, placed 
between two cycloidal cheeks, or two little 
arcs of a cycloid, made of metal. Hence the 
motion of vibration, applying successively 
from one of those arcs to the other, the thread, 
which is extremely flexible, easily assumes 
the figure of them, and by that means causes 
the hall or weight at the bottom to describe 
ajust cycloidal arc. 
This is doubtless one of the most ingeni- 
ous and useful inventions many ages have 
produced; by means of which it has been 
asserted there have been clocks that would 
not vary a single second in several days ; and 
the same invention also gave rise to the whole 
doctrine of involute and evolute curves, with 
the radius and degree of curvature, A c. 
It is true, the pendulum is still liable to 
its irregularities, how minute soever they 
may be. The silken thread by which it was 
suspended, shortens in moist weather, and 
lengthens in dry; by which means the length 
of the whole pendulum, and consequently 
the times of the vibrations, are somewnat 
varied. 
To obviate thisr 'inconvenience, M. De la 
Hire, instead of a silken thread, used a little 
line spring; which was not indeed subject 
to shorten and lengthen, from those causes; 
yet he found it grew stiller in cold weather, 
and then made its vibrations faster than in 
warm : to which also we may add its ex- 
S insion and contraction by heat and cold. 
e therefore had recourse to a stiff wire or 
rod, firm from one end to the other. In- 
deed, by this means he renounced the ad- 
vantages of the cycloid; but he found, as 
he says, by experience, that the vibrations 
in circular arcs are performed in times as 
equal, provided they are not of too great ex- 
tent, as those in cycloids. But the experi- 
ments of sir Jonas Moore, and others, have 
demonstrated the contrary. 
The ordinary causes ot the irregularities of 
pendulums Dr. Derham ascribes to the alter- 
ations in the gravity and temperature of 
the air, which increase and diminish the 
weight of the ball, and by that means make 
the vibrations greater and less; an accession 
of weight in the ball being found by experi- 
ment to accelerate the motion of the pen- 
dulum for a weight of six pounds added to- 
the ball, Dr. Derham found made his clock 
gain thirteen seconds every day; 
A general remedy against the inconveni- 
ences of pendulums, is to make them long; 
the ball heavy, and to vibrate but in small 
arcs. These are the usual means employed 
in England; the cycloidal cheeks being ge- 
nerally neglected. 
Pendulum-clocks resting against the same 
rail have been found to influence each other’s 
motion- See the Philos. Trans, numb. 453, 
sects. 5 and 6, where Mr. Ellicott has given, 
a curious and 1 exact account of this pheno- 
menon. 
Pendulum, royal, a name used among, 
us for a clock, whose pendulum swings 
