Page:Popular Science Monthly Volume 26.djvu/681

Rh Assuming that the power for an accurate clock must be a weight, we are ready to pass to the application of this power to the propelling of the pendulum, save this one consideration, that, unless there is some special provision, the clock will not advance while it is in process of winding. This provision is made in fine clocks by means of what is termed a retaining-click—an ingenious contrivance which brings the power of a small spring to bear while the key withdraws the power of the weight. This device is also found in the English—that is, the fusee-watch. Other watches need no such contrivance, for, as one end of the spring is fast upon the winding-post and the other upon the outside of the barrel, winding tends rather to stimulate and not to stop its going. The same is true of marine clocks. But common pendulum-clocks have the outer end of the spring attached to the frame of the clock, and hence the application of the key takes off the power, and the scape-wheel does not advance while the clock is in process of winding.

Supposing ourselves now possessed with a uniform power, uniformly applied to the scape-wheel, the problem arises as to the mode of communicating the power of the scape-wheel to the pendulum, in such a way as to sustain its beat, but not affect the time of it. Without going minutely into the discussion of the escapement, it may be remarked that there are two current forms of it. One of them is termed the "recoil" escapement, and its peculiarity is that it at no time arrests wholly the power of the scape-wheel—that is, it recoils by its own action after a tooth has passed one side of the verge, and returns toward the other beat. This is the escapement of the common kitchen-clock, and the chief objection to it is that, according to no definite law, the swing of the pendulum is made more rapid when the power of the spring is increased, as by winding.

A better escapement by far, and the one used in fine clocks, is the "dead" escapement, of which the characteristic is that, after it makes one beat, the pallet must be thrown off by the return of the pendulum before the scape-wheel can again apply its power. An example of this may be found in the ordinary marine clocks as well as in most watches, in which the lever is at rest after the balance has been thrown in one direction, until the return of the balance again trips it, and then its power is applied in the opposite direction.

This is found to be a satisfactory escapement for fine clocks which are not to be disturbed by any outside influences, but, for tower-clocks, which are affected by the wind, still another form is employed called the "gravity," or "remontoire" escapement, the principle of which is that the power of the clock merely lifts a small weight which is then unlocked by the swing of the pendulum and falls upon it, applying the uniform amount of its weight to propel the pendulum. There are, of course, two such small weights lifted alternately at each beat of the pendulum. The pendulum, therefore, has nothing to do with the