When the tooth _b_ has done its work by pressing on the driving face, and thus driving the anchor over, say, to the left, then the tooth on the opposite side falls on the sliding face of the other pallet. This being an arc of a circle, has no effect in driving the anchor one way or the other; hence the pendulum is free to swing to the left as far as it likes and return when it feels inclined, always with the exception of a little friction of the tooth on the faces of the pallets, but when it returns and begins to move towards the right, the tooth slides back along the face of the pallet till the pendulum is almost at the middle of its swing; then an impulse is given by the pressure of the tooth upon the inclined plane _a´ b´_. As soon, however, as the tooth leaves _b´_, another tooth on the other side at once engages the sliding face _b c_ of the other pallet, and so the motion goes on. This beautiful escapement is at present used for astronomical clocks; the pallets are made of agate or sapphire, and therefore do not grind away the teeth of the wheel perceptibly, and the loss by friction on the sliding surfaces is exceedingly small. There are several other ways even better than this for securing a free pendulum movement. We have now to return to our clock. The centre arbor moves round once in an hour, and carries the minute hand. In order to provide an hour hand, which shall turn round once in twelve hours, we fasten a cogwheel and tube _N_ on to the minute hand arbor by means of a small spring, which keeps it rather tight, but allows it to slip if turned round hard (see Fig. 45). This spring is a little bent plate slipped in behind the cogwheel on which its ends rest; its centre presses on a shoulder on the minute hand arbor; it is a sort of small carriage spring. The cogwheel _n_ has thirty teeth. This cogwheel engages another cogwheel _o_ with thirty teeth, on a separate arbor, which carries a third cogwheel, _p_, with six teeth, and this again engages a fourth cogwheel, _q_, with seventy-two teeth, mounted on a tube which slips over the tube to which the cogwheel _a_ is attached. It is now easy to see that for each turn of the minute hand arbor the arbor _p_ makes one turn, and for each turn of the arbor _p_ the cogwheel _d_, makes one-twelfth of a turn. From which it follows that for each turn of the minute hand arbor the cogwheel _d_ with its tube, or, as it is sometimes called, its “slieve,” makes one-twelfth of a turn, and thus makes a hand fastened to it show one hour for every complete turn of the minute hand. The minute hand is attached to the tube or slieve which carries the cogwheel _N_. The hour hand is attached to the tube or slieve which carries the cogwheel _Q_, and one goes twelve times as slowly as the other.