HOROLOGY. 
Chronometer. In the present, we shall 
chiefly attend to the means by which the 
train of wheel-work is made to make a num- 
ber of successive advances, all so very 
nearly equalin the measurement of time, that 
a surprising degree of precision is obtained 
in ascertaining the intended object. 
The machines which, for centuries, have 
been commonly used to measure time, con- 
sist of a movement, or train of wheels, 
drawn by a weight or spring, and a regula- 
tor, the object of which is to keep the mo- 
tion of the train within the required degree 
of uniformity. A continual rotatory mo- 
tion, which constantly tends to accelerate, 
is thus corrected by means of an alternate 
motion; while the power which carries 
round the movement, restores also, to the 
regulator, the action lost by friction and 
other causes. The mechanism, by which 
the two principal parts act on one another, 
is called the escapement ; and this most ad- 
mirable contrivance may be reckoned the 
distinguishing characteristic of the modem 
art of time-piece making. 
One of the most ancient escapements is 
that which is now applied in almost all com- 
mon pocket watches. It is represented in 
fig. 1. Plate Horology, and is best suited 
to the long vibrations of the balance, which 
was invented earlier than the pendulum. A B 
denotes the rim of a contrate wheel, called 
the crown wheel, having its teeth pointed 
and sloped on one side only, so that the 
points advance before any other part of the 
teeth, during the motion. C and D are two 
pallets or flaps, proceeding downwards 
from the verge E F. The pallets are nearly 
at right angles to each other ; and when the 
balance F G, fixed to the verge, is at rest, 
the pallets remain inclined to the plane of 
the wheel, in an angle of about forty-five 
degrees ; but when it is made to vibrate, 
one of the pallets is brought nearer to the 
perpendicular position, while the other be- 
comes more nearly parallel. The wheel 
must be supposed to have one of its teeth 
resting against a pallet, by virtue of the 
maintaining power. This tooth will slip off 
or escape, as the pallet rises towards the 
horizontal position, at which instanPa tooth 
on the opposite side of the wheel will strike 
against the other pallet which is down. 
The returning vibration, by raising this last 
pallet, will suffer that tooth to escape, and 
another tooth will apply itself to the first- 
mentioned pallet. By this alternation, the 
crown-wheel will advance the quantity of 
half a tooth each vibration, and the balance 
or pendulum will be prevented from com- 
ing to rest, because the impulse of the teeth 
against the pallets will be equal to the re- 
sistances front friction and the re-action of 
the air. 
The common escapement here described, 
was well known to Leonardo de Vinci, who 
describes an instrument acting by an es- 
capement of this kind, similar, as he says, 
to the verge of the balance in watches, 
which he does not seem to mention as a 
new thing : he died about 1513. The iso- 
chronisin of the pendulum was known to 
Galileo in 1600, who, before his death, 
namely, about 1633, proposed to apply it to 
clocks. The actual application by Huy- 
gens was made before 1658, when he pub- 
lished his “ Horologium Oscillatorium.’’ 
He applied it by means of the common es- 
capement already in use with the balance, 
and still retained in our table-clocks. Sanc- 
torius had made the same application about 
forty years before that time, as appears by 
his “ Commentarii in Avicennam,” (quest. 
56), printed in 1625, in which several in- 
struments are described as having been pub- 
licly exhibited and explained to his auditors 
at his lectures in Padua, for thirteen years 
previous to that time. 
This escapement not being adapted to 
such vibrations as are performed through 
arcs of a- few degrees only, another con- 
struction has been made, which has been in 
constant use in clocks for this century past, 
with a long pendulum beating seconds. 
(Fig. 2.), A B represents a vertical wheel, 
called the swing wheel, having thirty teeth. 
C D represents a pair of pallets connected 
together, and moveable, in conjunction with 
the pendulum, on the centre of axis F. 
One tooth of the wheel, in the present po- 
sition, rests on the inclined surface of the 
inner part of the pallet C, upon which its 
disposition to slide tends to throw the point 
of the pallet further from the centre of the 
wheel, and consequently assists the vibra- 
tion in that direction. While the pallet C 
moves outwards, and the wheel advances, 
the point of the pallet D, of course, ap- 
proaches towards the centre, in the open- 
ing between the two nearest teeth ; and 
when the acting tooth of the wheel slips off, 
or escapes from the pallet C, another tooth 
on the opposite side immediately falls on 
the exterior inclined face of D, and by a si- 
milar operation, tends to push that pallet 
from the centre. The returning vibration 
is thus assisted by the wheel, while the pal- 
let C moves towards the centre, and re- 
