scape-wheel except to unlock it. Absolute uniformity can be secured by this device, as the variations of the clock's power are not felt by the pendulum.
We come naturally now to the problem of maintaining an invariable length of the pendulum in spite of atmospheric changes. There is no substance known which does not expand in the case of a rise in temperature, and vice versa. It has been found, however, that white deal wood varies, with the grain, but very little, and hence it is employed in many of the better class of clocks, as being better than a cheap and imperfectly constructed compensating pendulum. Still, this does not give full satisfaction, as other changes, such as that from moist to dry, do affect it in a degree; and hence pendulums have been devised in which the variation of one metal is counteracted by the variation of another in the opposite direction.
The most common form in which we see such pendulums is the so called "gridiron," which takes advantage of the greater sensitiveness of brass than of steel to changes in temperature. It is made with nine bars of brass and steel alternately arranged, the total length of brass employed being to the steel inversely as the two metals are affected by changes of temperature. It is constructed so that the brass lifts the bob in case of a rise in temperature as much as the steel lets it down. To illustrate the principle of it, imagine a simple pendulum rod of steel; to the bottom of this fix a rod of brass slightly shorter than the steel one, letting it extend upward parallel with it; let a second steel rod now be affixed to the upper end of this brass one, also parallel to the others, and to the lower end of this attach the bob. We have now a gridiron pendulum, but one in which the amount of brass is not sufficient to counteract the changes in the steel. Before it will do this, we must make one more journey up with a brass rod and down with a steel one, affixing on this the bob. To construct such a pendulum it is found necessary to duplicate the first four rods; hence the nine that we always see. The genuine "gridiron" is a pretty good clock, but it is so often spurious that this kind of clock is going out of favor.
A third common device is the mercury pendulum, consisting of one or more cylinders filled with mercury to such depth that the movement of the highly sensitive mercury in the bob will counteract that of the entire rod. This is readily understood by viewing the center of the mercury as the center of oscillation (which it is very nearly), and imagining that the temperature rises. Of course, this center is carried upward half as much as the surface, and so great is the variation in the case of mercury that a vessel of it about six inches deep will counteract the steel rod of a seconds pendulum. This is the pendulum employed for fine astronomical clocks, and all jewelers who can afford them have them for regulators. The only objection to this pendulum is that the mercury, owing to its mass, is not affected by a change of
