pulse to the pendulum; the spring at the end of the detent immediately locks the wheel again, and the pendulum swings on freely to the left. When the pendulum swings to the right, the light spring at the end of the detent lets it pass without unlocking the wheel. The right-hand pallet is only intended to catch the wheel in case of accident and forms no essential part of the escapement. Thus, it will be seen, the pendulum is quite free except during a part of every alternate second, when it releases the escapement and receives an impulse; the seconds-hand, attached to the escape-wheel, moves only once every two seconds.
The most important source of error in the running of a fine clock is the change in the length of the pendulum due to change of temperature. Two methods suggest themselves of eliminating this error: 1. To put the clock where it will not be subject to changes of temperature. 2. To counteract the effect of changes of temperature. To this end various kinds of pendulums have been devised, notably the mercurial and gridiron forms, which are known under the general name of "compensating pendulums." At Greenwich the two methods are combined to insure complete success. The clock is placed in the magnetic basement of the observatory, where the temperature is as nearly uniform as possible, and apparatus is provided to annul the effect of any change of temperature which might occur.
Tests made with a mercurial pendulum disclosed the fact that the steel rod responded more quickly than the mercury to a change of temperature, and that consequently