Drums are sometimes driven by clockwork, and tuning forks are also often kept vibrating by electricity, thus constituting very rapidly moving electric clocks. The arrangement is simple. An electro-magnet _E_ is put in the vicinity of the arm of the tuning fork. A small piece of wire from the arm is in contact with a piece of metal _Q_, from which a wire runs to the electro-magnet, thence to a battery, and from the battery to the tuning fork, through which the current runs to the wire _R_. When the fork vibrates the arm, being bent outwards, makes the wire _R_ touch _Q_. This at once causes the electro-magnet to give a small pull to the steel arm of the tuning fork, and thus assists the swing of the arm. The whole arrangement is exactly analogous to an electric clock, as may be seen by comparing Fig. 71 with Fig. 68. There is another method of measuring rapid intervals of time which also merits attention. It is to let a body drop at the commencement of the period of time to be measured, and mark how far it falls in the time, and then find the time from the equation given previously, S = 1/2 g t². It is practically done by letting a piece of smoked glass fall and making a small pointer make two dots upon it, one at the beginning, another at the end, of the time to be measured. An interesting adaptation of this method can serve as a basis of a curious toy. Take a crossbow, with a bolt with a spike on it; fix it firmly in a vice so that the barrel points at a spot _a_ on a wooden wall. On the spot _a_ hang a cardboard figure of a cat on to a nail so contrived that when an electro-magnet acts the nail is pulled aside, and the cat drops. Thus let _a_ be the cat, _b_ the loop by which it is hung over the nail _c_, that is fixed to another piece of iron furnished with a hinge at _c_, so that when the electric current is turned on the nail _c_ is withdrawn and the cat drops. Carry the wires from the electro-magnet and battery to the crossbow, and so arrange them that when the bolt leaves the muzzle one is pressed against the other, and contact made. Now here you have an apparatus such that exactly as the bolt leaves the crossbow, the cat drops. Now what will happen? [Illustration: FIG. 72.] When the bolt leaves the bow it is subject to two motions, one a motion of projection at a uniform pace in the direction of _b a_ from the bow to the target.