SIS 
HOROLOGY, 
reft the compenfation, the central rod of lead mult be 
left fo long that we may.be fure the compenfation is in 
excels. The quantity of error may then be found by the 
freezing apparatus, and how much it is requiiite to cut 
from the rod may be calculated with the greateft exaftnefs. 
Ward's Compenfation Pendulum is fimilar in principle to the 
above. It coniilts of two fiat rods of iron or fteel, about 
half an inch wide, and an eighth of an inch thick, with 
a rod of zink interpofed between them nearly a quarter of 
an inch thick. The corners of the iron rods are bevelled off, 
that they may meet with lefs refiftance from the air; and it 
likewife gives them a much lighter appearance. Thefe three 
rods.are kept together by means of three or four fcrews 
which pafs through two of them, and lodge in the third, be- 
fides another fcrew called the adjujlingfcrew. The fteel rods 
have a Ihoulder, one at top the other at bottom, turned at 
right angles, which fupports the zink rod, and is fupported 
by it. It is necefl’ary to have leveral holes for the adjufting 
fcrew, in order to make, an equal compenfation; i'o that, 
by removing the fcrew one hole, it will produce an alter¬ 
ation in .the going of the clock of about a quarter of a 
, fecond per day with a change of thirty degrees of Fahren¬ 
heit’s thermometer. “ Now it is evident, (fays the in¬ 
ventor,) that, if any degree of heat or cold be applied to 
this compound rod, the one of zink expands and con- 
trafts as much as the two iron ones together; the diftance 
from the point of fufpenlion to the centre of ofcillation 
mull remain the fame.” In May 1807, the filver medal 
of the Society of Arts was voted to Mr. Ward, (of Bland- 
ford, Dorfet,) for the communication of this pendulum. 
Lever Pendulums. From all that appears concerning this 
conftruftion of a pendulum, we are inclined to believe 
that the idea of making the difference of the expanfion of 
different metals operate by means of a lever, originated 
with Mr. Graham, who in the year 1737 conftrufted a 
pendulum, having its rod compofed of one bar of Heel 
between two of brafs, which afted upon the Ihort end of 
a lever, to the other end of which, the bail or weight of 
the pendulum was fufpended. This pendulum however 
was, upon trial, found to move by jerks; and therefore 
laid alide by the inventor, to make way for the mercurial 
pendulum, juft mentioned. 
Mr. Short informs us,, in the Philof. Tranf. vol. xlvii. 
art. 88, that a Mr. Frotheringham, a quaker, in Lincoln- 
fhire, caufed a pendulum of this kind to be made: it con- 
lifted of two bars, one of brafs, and the other of fteel, 
faltened together by fcrews, with levers to raife or let 
down the bulb; above which thefe levers were placed. 
. M. Caffini too, in the Hiltory of the Royal Academy of 
Sciences at Paris, for 1741, deferibes two forts of pendu¬ 
lums for clocks, compounded of bars of brafs and fteel, 
and in which he applies a lever to raife or let down the 
bulb of the pendulum, by the expanfion or contraftion of 
the bar of brafs. 
Mr. John Ellicott alfo, in the year 1738, conftrufted a 
pendulum on the fame principle, but differing from Mr. 
Graham’s in many particulars. The rod of Mr. Ellicolt's 
Pendulum was compofed of tvvo bars only; the one of brafs, 
and the other of fteel. It had two levers, each fuftaining 
its half of the ball or weight; with a fpring under the 
lower part of the ball to relieve the levers from a confi- 
derable part of its weight, and fo to render their motion 
more Imooth and eafy. The one lever in Mr. Graham’s 
conftruftion was above the ball: whereas both the levers 
in Mr. Ellicott’s were within the ball; and each lever had 
an adjufting fcrew, to lengthen or fhorten the lever, fo as 
to render the adjuftment the more perfeft. See Cumming, 
p. 97, where Mr. Ellicott’s methods of conftruftion are 
deferibed, and illuftrated by figures. 
Notwithftanding the great ingenuity difplayed by thefe 
very eminent artifts on this conftruftion, it muft farther 
be obferved, that Mr. Cumming has given an ample de- 
feription, with plates, of a conftruftion of a pendulum 
with levers, in which it feems he has united the properties 
.of Mr. Graham’s and Mr. Eliicott’s, without being 
liable to any of the defefts of either. The rod of Cum- 
ming's Pendulum is compofed of one fiat bar of brafs, and 
two of fteel; he ufes three levers within the ball of the 
pendulum; and, among many other ingenious contrivances 
for the more accurate adjufting of this pendulum to mean 
time, it is provided with a fmall ball and lerevv below the 
principal ball or weight, one entire revolution of which 
on its fcrew will only alter the rate of the clock’s going 
one fecond per day; and its circumference is divided into 
30, one of which divifion's will therefore alter its rate of 
going one fecond in a month. 
Another excellent contrivance for the fame purpofe is 
deferibed by M.Thiout, a French author on clock-making. 
It was ufed in the north of England by an ingenious artift 
about forty years ago. This invention is as follows: A 
baivof the lame metal with the rod of the penduimn, and 
of the fame dimenfions, is placed againft the back-part 
of the clock-cafe : from the top of this a part projefts, 
to which the upper part of the pendulum is connefted by 
two fine pliable chains or filken fixings, which juft below 
pafs between two plates of brafs, whofe lower edges will 
always terminate the length of the pendulum at .the upper 
end. Thefe plates are fupported on a pedeftal fixed to 
the back of the cafe. The bar refts upon an immoveable 
bafe at the lower part of the cafe; and is inferted into a 
groove, by which means it is always retained in the fame 
pofition. From this conftruftion, it is evident that the 
extenfion or contraftion of this bar, and of the rod of the 
pendulum, will be equal, and in contrary direftions. For, 
fuppofe the rod of the pendulum to be expanded any given 
quantity by heat; then, as^ the lower end of the bar refts 
upon a fixed point, the bar will be expanded upwards, 
and raife the upper end of the pendulum juft as much as 
its length was increafed; and hence its length below the 
plates will be the fame as before. 
Of this pendulum, fomewhat improved by Mr. Crofth- 
waite, watch and clock-maker, Dublin, we have the fol¬ 
lowing defeription in the Tranfaftions of the Royal Irifii 
Academy, 1788.—“A and B, (fig. 35.) are two rods of 
fteel forged out of the fame bar, at the fame time, of the 
fame temper, and in every refpeft fimilar. On the top of 
B is formed a gibbet C; this rod is firmly fupported by a 
fteel bracket D, fixed on a large piece of marble E, firmly 
fet into the wall F F, and having liberty to move freely 
upwards between crofs ftaples of brafs, 1,2, 3, 4, which 
touch only in a point in front, and rear (the ftaples having 
been carefully formed for that purpofe) ; to the other rod 
is firmly fixed by its centre the bob G, of 24 pounds 
weight, although it Ihould in ftriftnefs be a little below 
it. This pendulum is fufpended by a Ihort fteel fpring on 
the gibbet at C ; all which is entirely independent of the 
clock. To the back of the clock-plate I, are firmly ferewed 
two cheeks nearly cycloidal at K, exactly in a line with 
the centre of the verge L. The maintaining power is ap¬ 
plied by a cylindrical fteel-ftud, in the ufual way of regu¬ 
lators, at M. Now, it is very evident, that any expanfion 
or contraftion that takes place in either of thefe exactly 
fimilar rods, is inftantly counterafted by the other; where¬ 
as in all compenfation-pendulums compofed of different ma¬ 
terials, however juft calculation may feem to be, that can 
never be the cafe, as not only different metals, but alfo 
different bats of the fame metal that are not manufactured 
at the fame timq, and exactly in the fame manner, are 
Found by a good pyrometer to differ materially in their 
degrees of expanfion and contraftion, a very final 1 change 
afiefting one and not the other.” 
In the lame paper of the Irifii Tranfaftions, Mr. C’rofth- 
waite deferibes another pendulum of his invention; and 
alfo a very ingenious contrivance by which the maintain¬ 
ing power is applied immediately from the fcapement- 
wheel to the axis of the pendulum, by means of a notch 
cut into the pendulum-rod itfelf, but deeper than its axis, 
in which the fcapement-wheel is made to aft exaftly in. 
the line of the centres of magnitude, ofcillation, fufpeh- 
fion, motion, and gravity. 
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