We now come to a point at which, if we are to keep our pendulum vibrating, we must apply power to it, evenly, accurately and in small doses. In order to do this conveniently we must store up energy by raising a weight or winding a spring and allow the weight to fall or the spring to unwind very slowly, say in thirty hours or in eight days. This brings about the necessity of changing rotary motion to reciprocating motion, and the several devices for doing this are called “escapements” in horology, each being further designated by the names of their inventors, or by some peculiarity of the devices themselves; thus, the Graham is also called the dead beat escapement; Le Paute’s is the pin wheel; Dennison’s in its various forms is called the gravity; Hooke’s is known as the recoil; Brocot’s as the visible escapement, etc.
The Mechanical Elements.—We shall understand this subject more clearly, perhaps, if we first separate these mechanical devices into their component parts and consider them, not as parts of clocks, but as various forms of levers, which they really are. This is perhaps the best place to consider the levers we are using to transmit the energy to the pendulum, as at this point we shall find a greater variety of forms of the lever than in any other place in the clock, and we shall have less difficulty in understanding the methods of calculating for time and power by a thorough preliminary understanding of leverage and the peculiarities of angular or circular motion.
If we take a bar, A, [Fig. 21], and place under it a fulcrum, B, then by applying at C a given force, we shall be able to lift at D a weight whose amount will be governed by the relative distances of C and D from the fulcrum B. If the distance CB is four times that of BD, then a force of 10 pounds at C will lift 40 pounds at D, for one-fourth of the distance through which C moves, minus the power lost by friction. The reverse of this is also true; that is, it will take 40 pounds at D to exert a force of 10 pounds at C and the 10 pounds would be lifted four times as far as the 40 pound weight was depressed.
Fig. 21.
Fig. 22.
If instead of a weight we substitute other levers, [Fig. 22], the result would be the same, except that we should move the other levers until the ends which were in contact slipped apart.