325 
HOROLOGY. 
clocks and watches ever fince their firft appearance in Eu¬ 
rope, is extremely Ample; and its mode of operation is too 
obvious to need much explanation. In fig.51, PlateVIII. 
XY reprefents a horizontal axis, to which the pendulum 
P is attached by a flender rod, or otherwife. This axis 
has two leaves (pallets) C and D,-attached, one near each 
end, and not in the fame plane, but lb that when the 
pendulum hangs perpendicularly, and at reft, the piece 
C inclines a few degrees to the right hand, and D as much 
to the left. They commonly make an angle ot from 70 
to 90 degrees. AFB reprefents awheel turning round 
on a perpendicular axisEO, in the .order of the letters 
A FEB. The teeth of this wheel are cut into the form 
of the teeth of a law, leaning forward, in the direction of 
the motion of the rim. As they fomewhat relemble the 
points of an old-falhioned royal diadem, this wheel has 
got the name of the crown wheel. In watches it is often 
called the balance-wheel, or fcapement-wkeel. The number 
of the teeth is generally odd, 9, 11, 13, 15, or 17; fo 
that, when one of them B is preiTing on a pallet D, the 
eppofite pallet C is in the fpace between two teeth A and 
I. The figure reprefents the pendulum at the extremity 
of its excurfion to the right hand, the tooth A having 
Juft efcaped from the pallet C, and the tooth B having 
juft dropped on the pallet D. It is plain, that as the pen¬ 
dulum now moves over to the left, in the arch P G, the 
tooth B continues to prel's on the pallet D, and thus acce¬ 
lerates the pendulum, both during its delcent along the 
arch PH, and its al'cent along the arch HG. It is no.lefis 
evident, that when the pallet D, by turning round the 
axis XY, raifes its point above the plane of the wheel, 
the tooth B efcapes from it, and I drops on the pallet C, 
which is now nearly perpendicular. I preffes C to the 
light, and accelerates the motion of tire pendulum along 
the arch GP. Nothing can be more obvious than this 
adion of the wheel in maintaining the vibrations of the 
pendulum. We can eafiiy perceive, aifo, that when the 
pendulum is hanging perpendicularly in the line XII, the 
tooth B, by preiiing on the pallet D, will force the pen¬ 
dulum a little way to the left of the perpendicular, and 
will force it lb much the further as the pendulum is 
lighter; and, if it be fufficiently light, it will be forced fo 
far from the perpendicular, that the' tooth B will efcape, 
and then I will catch on C, and force the penduh 1 back 
to P, where the whole operation will be repeated. 
The fame effed wlii be produced in a more remarkable 
degree, if the rod of the pendulum be continued through 
the axis XY, and a ball Q put on the other end to ba¬ 
lance P. And, indeed, this is the contrivance which was 
firft applied to clocks all over Europe, before the applica¬ 
tion of the pendulum. They were balance-clocks. The 
force of the wheel was of a certain magnitude, and there¬ 
fore able, during its adion on a pallet, to communicate a 
certain quantity_of motion and velocity to the balls of 
the balance. When the tooth B efcapes from the pallet 
D, the balls are then moving with a certain velocity and 
momentum. In this condition, the balance is checked by 
the tooth I catching on the pallet C. But it is not in- 
llantly flopped; it continues its motion a little to the left, 
and the pallet C forces the tooth I a little backward. But 
it cannot force it fo far as to efcape over the top of the 
tooth I; becaufe all the momentum of the balance was 
generated by the force of the tooth B ; and the tooth I is 
equally powerful. Befides, when I catches on C, and <1 
continues its motion to the left, its lower point applies 
to the face of the. tooth. I, which now acts, on the balance 
by a long and powerful lever, and foon ftops its further 
motion in that direction; and now, continuing to prefs 
on C, it .urges the balance in the opposite direction. Thus 
we fee that in a fcapement.of this kind the motion of the 
wheel mull be very hobbling and unequal, making a great- 
ftep forward, and a ihort llep backward, at every beat. 
This has occafioned the contrivance to get the name of 
the recoiling Jcapancnt, or the fcapement of recoil. This ig- 
the fcapement kill in ufe for common pocket-watches. 
In this fcapement the vibrations are quicker than if 
the balance or pendulum vibrated freely: for the recoil 
fhortens the afcending part of the vibration, by contract¬ 
ing the extent of the arc, and the re-aftion of the wheel 
accelerates the defcending part of the vibration. In this 
fcapement, too, if the maintaining power be increafed, the 
vibrations will be performed in larger arcs, but in lefs 
time: becaufe the greater preflure of the crown-wheel on 
the pallet will caufe the balance to vibrate through larger 
arcs; and the time will be lefs increafed on this account 
than it will be diminifhed by the acceleration that preflure 
gives to the balance and the diminution of the time of 
recoil. 
The preceding fcapement not being well adapted to 
fuch vibrations as are performed through arcs of a few- 
degrees only, another conftrudion has been made which 
has been in co.nftant ufe for about a century in clocks, with 
a long pendulum beating feconds. In fig. 52, AB repre¬ 
fents a vertical fcapement-wheel called the fwing-wheel, 
having thirty teeth. C D reprefents a pair of pallets con¬ 
nected together, and moveable in conjunction with the 
pendulum on the centre or axis F. One tooth of the 
wheel, as fhown in the figure, refts on the inclined furface 
of the inner part of the pallet C; on which its difpofition 
to Aide tends to throw the point of the pallet further from 
the centre of the wheel, and confequently affifts the vibra¬ 
tion in that direction. While the pallet C moves out¬ 
wards, and the wheel advances, the point of the pallet D 
of courfe approaches towards the centre in the opening 
between the two neareft teeth ; and, whan the ading tooth 
of the wheel (lips off, or efcapes from the pallet C, ano¬ 
ther tooth on the oppofite fide immediately falls on the 
exterior inclined face of D, and by a fimilar operation 
tends to pufh that pallet from the centre. The returning- 
vibration is thus affifted by the wheel, while the pallet C 
moves towards the centre, and receives the fucceeding 
tooth of the wheel, after the efcape from the point of D. 
Thus may the alternation be conceived to go on without 
limit. 
In this fcapement, as well as the former, the vibrating 
part is conftantly under the influence of the maintaining 
power, except during the interval of the drop, or actual 
efcape of the wheel from one pallet to the other. One 
principal recommendation of this fcapement feerns to have 
been the facility with which it affords an index for fe¬ 
conds in the face of the dock. Though the pendulum,, 
according to this conftrudion, is conftantly connected 
with the maintaining power in a clock, yet the variations 
of that power have not the fame mifchievous efted as in 
a watch, becaufe the momentum of the pendulum, com¬ 
pared with the impulfe of the maintaining power, is pro- 
digioufly greater in the former of thefe inftruments. A 
very confiderable change in the maintaining power of a 
clock with a long pendulum will only caufe a variation. 
of a. few feconds in the daily rate. 
2-. Mr. Graham’s fcapement, already fpoken of, was a 
confiderable improvement upon that juft defcribed. He 
took off part of the Hope furtheft from the points of the 
pallets; and inftead of that part he formed a circular or 
cylindrical face, having its axis in.the centre of motion. 
Pallets of this kind: are fhown at the lower part of fig. 52, 
at E and G, having H- for their centre or. axis. A tooth 
of the wheel is feen reftirrg upon the circular inner furface 
of the pallet G, which therefore is not affeded by the 
wheel, excepting fo far as its motion, arifing from any 
other caufe, may be affected by the fridion of the tooth; 
and this refiftance is exceedingly minute, not amounting 
to one-eighth of the preflure on the arch. Nay, we think 
it appears from the experiments of Coulomb, that, in the 
cafe of fuch minute preflures on a furface covered with, 
oil, there is no fenfible-retardation analogous to that pro¬ 
duced by fridion, and that, what retardation we obferve 
arifes entirely from the damminefs of the oil:. If the vi¬ 
bration of the pendulum be fuppofed to carry G outwards,, 
the Hope furface will be brought to the point.of.the tooth, , 
whicbn 
