Page:Popular Science Monthly Volume 26.djvu/679

Rh devices which have been found to do this with anything like accuracy, the pendulum and the balance-wheel. Postponing, for the present, the discussion of the balance-wheel, we have before us the pendulum.

That the pendulum might be used for the measurement of time is a discovery which dates back, at least, to the time of Galileo. A great many, even among well-educated people, suppose that that philosopher discovered that a pendulum of a given length will always oscillate in the same time. This he could not have discovered, for it is not true. The correct statement of the law he discovered is, that a pendulum will always oscillate in the same time through equal arcs, but not through any arc. It has been found that if the curve in which a pendulum swings were a portion of a cycloid instead of a circle, and the pendulum were simple, that is, consisting of a bob or ball suspended by means of a thread imagined to have no weight, its oscillations would be in equal times through any arc. To accomplish this, clocks were at one time made with pendulums which were suspended between cycloidal cheeks, and were thus conformed, in swinging, to the cycloidal curve. But this was soon abandoned, as it was found that it was impossible to construct such cheeks without variation or imperfection sufficient to make a greater error in the pendulum than it would have if allowed to swing on a circular arc. For a short distance the cycloidal curve corresponds quite closely to the circle. Therefore, by adjusting the pendulum to swing but a short distance, it was found to be possible to secure substantial uniformity. This is the plan now universally adopted.

If the arc of vibration is increased, the clock will lose time. Experience with the common house clock would seem to contradict this, for every one has noted that when the clock is first wound it will gain time, and then that it will lose as it runs down, and, seemingly, this is due to the difference in the swing of the pendulum. The explanation, however, is to be found in the fact that in case the pendulum swings farther it is shortened by the curving of the spring by which it is suspended, and also by an effect which the longer swing is found to have upon the escapement, quickening its time. Any ordinary house-clock would keep far better time if the weight of its bob were considerably increased, as this would do much toward equalizing its swing. Ordinarily the weight of the bob is about three ounces; if the clock is properly put in beat it will carry a bob weighing as many pounds, and all spring-clocks would be greatly improved as time-keepers by such a change. If this is true, some one may ask, Why is it not made by the manufacturers? The answer is, that any firm who should put such clocks on the market, superior time-keepers as they would really be, would soon find themselves getting the reputation of making a clock that would not run, and all because the public generally would not have the skill or the patience to adjust the beat properly. Let a servant, for example, take such a clock to her room