664 THE POPULAR SCIENCE MONTHLY. 



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 invari- 

 able 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 em- 

 ployed 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, sucii 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 fo-ur 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 move- 

 ment 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 counter- 

 act the steel rod of a seconds pendulum. This is the pendulum em- 

 ployed 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 



