This, together with the hairspring, is really the most important part of the watch; the timekeeping depends on it, and it serves the same purpose as the pendulum of the clock. When set vibrating, or revolving backward and forwards under the influence of its hairspring, it will move quite regularly, whether it is turning only a quarter of a turn each way or a turn and a half. As a matter of fact, there is in most watches an extremely small difference between times of the long and short vibrations, but this may be ignored for the present. The consequence of this equality in time of the vibrations of the balance is that, provided the mechanism of the watch is in fair order and capable of keeping the balance vibrating, the watch is sure to keep fairly good time, provided the balance and hairspring are themselves in perfect order. If the balance or spring is faulty, then the best possible mechanism in the rest of the watch will fail to make it go well. The essential points of a good balance are as follow : It must be in perfect poise - that is to say, its rim must have no heavy part, but balanced exactly, so that no matter in what position the watch is held, the balance will act the same. Then it must have fine and smooth pivots, working in well-fitting and smooth jewel holes and the endsof the pivots must rest on smooth endstones. The balance itself must revolve quite freely, not being nipped by the presence of the endstones on the pivots, but having just the least trifle of lift or endshake to ensure absolute freedom. The balance must not touch anything as it revolves or its freedom of motion will be impaired. For instance, the balance cross-arm sometimes touches the outer coils of the hairspring, and in such a case it cannot possibly keep time. The hairspring must be flat and true in its coils, not wobbling as the balance turns; and it must not touch anything. So the first thing to look at, on taking the watch out of its case, is the balance and hairspring. Give it a spin, and see if it revolves truly. Hold the watch up to the light and look at the pivots sideways as the balance revolves. If they are bent, a wobbling will be discernible. Try the endshake with the points of of a pair of fine tweezers, lifting the balance rim and letting it fall again to see what lift it has. When doing this do not look at the rim, as that may be bent, and so be deceptive, but watch the roller or the top pivot. Then the exact amount of lift can be seen. If the balance seems true and free, look at it's spring carefully. If that is also free, and seems perfect, the balance cock, may be removed, together with the attached balance and spring, and passed as in fair order. The Hands and Dial. Turn next to the hands and dial. The minute hand should be firm, and move stiffly when the hands are set. If it moves easily, it may drag as the watch goes and lag behind, making it appear that the watch loses, whereas the watch may be keeping perfect time all the while. The hour hand should be free-that is, it must have just a little lift, or endshake, under the minute hand, and a little sideplay between the teeth of the hour wheel which carries it. The seconds hand must be quite free, and not touch the dial at any point. Sometimes a seconds hand is free in one position, and just touches the dial in another, so see it make a complete revolution before passing it as correct. If the hands seem all right, take them off. Cutting nippers will remove them one by one. The dial feet pass through holes in the bottom plate of the watch frame, or " pillar plate," and are nipped tight by a small screw to each in the edge of the plate. Undo these dial screws a turn or two -they need not be taken right out-and the dial can be raised by the finger nail and removed. The underside of the movement can then be inspected. The hour wheel is lifted off, and then the minute wheel. The cannon pinion carries the minute hand on its square, and must be tight on the centre arbor on which it fits, turning stiffly. To remove it, grasp its square by cutting nippers and pull it off with a twisting motion.