The water-clock just described led easily and directly to the weight-clock. Clockmakers in the Middle Ages for centuries tried with more or less success to make clocks that would run by means of weights. In 1370, Henry De Vick, a German, succeeded in solving the problem. De Vick was brought to Paris to make a clock for the tower of the king's palace, and he made one that has become famous. In a somewhat improved form it can still be seen in Paris in the Palais de Justice. Let us remove the face of this celebrated timepiece and take a look at its works (Fig. 8). It had a striking part, and a timekeeping part, each distinct from the other. The figure shows only the timekeeping part. The weight (A), of 500 pounds, is wound up by a crank (the key) at _P_. _O_ is the hour-hand. If _A_ is allowed to descend, you can easily see how the whole system of wheels will be moved--and that very rapidly. But if something does not prevent, _A_ will descend faster and faster, the hour-hand will run faster and faster and the clock will run down at once. If the clock is to run at a uniform rate and for any length of time, the power of the weight must escape gradually. In the clepsydra (Fig. 1) the descent of the weight was controlled by the size of the stream of flowing water. De Vick invented a substitute for the stream of flowing water. Fasten your attention upon the workings of the saw-toothed wheel _II_ and the upright post _K_, which moves on the pivots _l_ and _k_, and you may learn what he did. Fixed to the upper part of the post _K_ is a beam or balance _LL_, at the ends of which are two small weights _m_ and _m_, and projecting from the post in different directions are two pallets or lips _i_ and _h_. Now, as the top of the wheel _II_ turns toward you, one of its teeth catches the pallet _i_ and turns the post _K_ a part of the way round _toward_ you. Just as the tooth _escapes_ from _i_ a tooth at the bottom of _II_ (moving from you) catches the pallet _h_ and checks the revolving post and turns it _from_ you. Thus as _II_ turns, it gives a to-and-fro motion to the post _K_ and, consequently, a to-and-fro motion to the balance _LL_. _II_ is called the _escapement_ because the power of the descending weight gradually _escapes_ from its teeth. In the clepsydra the trickling of _water_ regulated the descent of the weight; in De Vick's clock the trickling of _power_ or _force_ from the escapement regulated the descent of the weight. The invention of this escapement is the greatest event in the history of the clock. The king was much pleased with De Vick's invention. He gave the clockmaker three shillings a day, and allowed him to sleep in the clock tower; a scanty reward indeed for one who had done so much for the world, for De Vick's invention led rapidly to the excellent timepieces of to-day, to both our watches and our clocks. After the appearance of the weight-clock, the water-clock gradually fell into disuse, and all the ingenuity of the clockmaker was