The lever has a spring _K E_ fastened to it at the point _K_. This spring is very delicate. If the lever _C D_ is turned so that the little projection _M_ on it strikes the spring _E_ from left to right, then, as the spring rests on the lever, the whole lever is pushed over, and the teeth of the scape wheel set free. At that instant, however, the escapement is so arranged that the arm _C D_ is just opposite the tooth _D_ of the scape wheel, so that the scape wheel, instead of running away, leaps with its tooth _D_ on to the lever _C D_ and swings the balance wheel round. The balance wheel is free to twist as much as it pleases, but the moment it has twisted so much that the projection _M_ passes the spring _E_, then the lever _G E_ flies back to its place, and the scape wheel is again checked. Meanwhile the balance wheel flies round till at last it is brought to rest by the balance spring. It then recoils and sets out on its return path. This time, however, the projection _M_ merely flips aside the spring _E_ and the balance wheel goes back, till again it is brought to rest and returns. As soon as the lever comes opposite _D_ the projection _M_ then again hits the spring _E_, and releases the catch at _F_, and another tooth of the scape wheel goes by. There then you have a completely free escapement, and consequently an accurate one. Many watches are made with these escapements, but they are more expensive than those in common use. There is but little remaining in a watch that is not in a clock, for the wheel-trains and general arrangements are very similar. It is possible to apply the chronometer’s detached escapement to a clock. This was done by several clock-makers in the eighteenth and early part of the nineteenth century. One method of doing it is as follows: _A_ is a block of metal fitted to the bottom of the pendulum, _B_ a light lever pivoted on it. _C_ is the scape wheel, with four teeth; _D_ a tooth of the scape wheel, which hops on to the projection of the pendulum the moment that the impact of the point _E_ of the lever _B E_ has pushed aside the lever _G F_, and thus released the scape wheel. The advantage is that it is a very easy escapement to make. But it is in reality a detached (that is to say, a completely free) chronometer escapement, as can easily be seen. [Illustration: FIG. 67.] Turret-clocks are open to considerable disadvantages, for the wind blowing on the hands gives rise to considerable pressure, so that the clocks are sometimes urging the hands against the wind, sometimes are being helped by the wind. And this inequality of driving force makes the pendulum at some times make a bigger arc of swing than at others. But we saw above that though difference of arc of swing ought to make no difference in the time of swing of the pendulum, yet this was only strictly true if the arc of swing were a cycloid.