170 | ASTRONOMY. 
rotations while the other (the hour hand) makes but one. There is often a 
third index (the second hand) which makes one rotation in a minute. In 
the better clocks the second hand springs from one second to another, thus 
showing each one separately ; and as in astronomical clocks minute divisions 
of time are desirable, in these the division of seconds has sometimes been 
brought to thirds. 
As the motive power can never act uniformly, every clock requires a 
regulator, which may compensate for the irregularities of the power. The 
whole wheelwork is consequently in such connexion with a single wheel— 
the escapement, that when the latter is checked the motion of the whole 
stops. This escapement, in the pendulum clock, is connected with the 
pendulum, which vibrates either whole or half seconds; the motion ceases, 
therefore, between every swing of the pendulum. It is thus seen that the 
proper motion of the clock depends upon the accurate length of the pen- 
dulum, and that as pendulums swing in proportion to their lengths, a clock 
may be regulated by lengthening or shortening the pendulum. The pen- 
dulum itself, however, needs regulating; for, being lengthened by heat and 
shortened by cold, the correctness of the clock’s motion is impaired. As it 
is not possible always to determine this variation of length, and the 
variation is often very sudden, it cannot be provided for by any manual 
regulation. To meet this difficulty Harrison invented a compensation 
pendulum which regulates itself. In this pendulum, rods of brass and steel) 
alternate in such a manner, that the elongation of the steel rods, and conse- 
quently of the pendulum, is counteracted by that of the brass rods, which in 
this manner shorten the pendulum as much as it is lengthened by the steel 
rods. Its length thus remains unchanged in all temperatures. Graham’s 
mercurial pendulum is intended to accomplish the same end. (For further 
details see the article, Compensation Pendulum, under the head of 
Physics.) 
In the second kind of regulators, all the wheels are connected with the 
balance wheel by means of the escapement, so that this produces the 
necessary check to the motion. Of escapements there are various forms, 
all, however, being in connexion with a balance, which is a flat wheel, on 
whose pallets the scape wheel catches, endeavoring to move it forwards. 
The pallets are so fixed on the verge of the balance wheel, that the scape 
wheel must leave them free after a certain time, and then the spiral spring 
acts upon the balance, bringing it back to the former position to meet a new 
tooth of the escapement. It will be readily seen that the quicker or slower 
motion of a clock will depend upon the time in which the balance makes its 
movenient, and that this time depends upon the length or shortness of the 
spiral spring. There is for this reason a regulator on the watch whose 
motion alters the length of the spring. 
As in the pendulum isochronism of oscillation is effected by the principle 
of compensation, so in the balance there must also be a compensation, since 
both it and the spiral spring change their dimensions, and consequently their 
times of vibration, with change of temperature. Compensation is brought 
about in the balance by the bending of thermometric metal springs, steel 
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