291. Arnold’s chronometer or free escapement, sometimes used in watches. A spring, A, is fixed or screwed against the plate of the watch at b. To the under side of this spring is attached a small stop, d, against which rest successively the teeth of the escape-wheel, B; and on the top of spring is fixed a stud, i, holding a lighter and more flexible spring which passes under a hook, k, at the extremity of A, so that it is free on being depressed, but in rising would lift A. On the axis of the balance is a small stud, a, which touches the thin spring at each oscillation of balance-wheel. When the movement is in the direction shown by the arrow, the stud depresses the spring in passing, but on returning raises it and the spring, A, and stop, d, and thus allows one tooth of escape-wheel to pass, letting them fall immediately to arrest the next. At the same time that this tooth escapes another strikes against the side of the notch, g, and restores to balance-wheel the force lost during a vibration. It will be understood that only at one point is the free movement of balance opposed during an oscillation.