Gravity Clock
This is a large glass clock dial, with a stud fixed in the center on which revolve two hands, as shown in Fig. 155, without any visible power to operate them. Hung in a jeweler's window so that it can be inspected from both sides without anyone discovering the source of power, it forms a great attraction to the curious and so becomes a durable and valuable advertisement. Take a plate of glass two feet square and lay out and gild a clock dial upon it, avoiding all ornament, in order to give the observer as little to see and as much to guess as is possible; cement, drill or otherwise fasten a stud in the center of the dial, projecting from the rear side so as to give facility for adjustment and certain exhibitions, which will be mentioned later. This stud must be perfectly hard and very finely polished in order to reduce the friction, which is considerable. The hands should be made of cedar, perfectly dry pine, or some other extremely light wood, left about a half inch thick, so that they can be nicely counterbalanced with lead, and will appear to the observer to be merely wooden hands. The circular discs at the inner extremities of the hands are hollowed out to receive two watch movements, and the boxes are closed with a cover fitting closely enough so that it cannot be perceived by ordinary inspection. Each side of the hands is perfectly jeweled with large English fusee jewels, so as to revolve on the stud with as little friction as possible. The two watch movements must be regulated to run as closely together as possible and to keep exact time. Two half circles of lead are attached to the movements in such a way that their rotation in the hollow discs will change the center of gravity of the hands and so cause them to rotate on the stud of the dial. The lead half-circle for the minute hand is attached to the minute arbor of its movement and that of the hour hand to the hour pipe on its movement. If the wooden hands are nicely gilded it will add to the deception, as the disc for the movement may then be made of tin and be much smaller and more symmetrical. When finished, the clock is hung in the window, suspended by chains from holes bored in the corners. We will suppose that you have the clock finished and running nicely, and that the time shown is 2:20; take hold of the hands, bring them together and send them twirling around the dial; when they stop they will show the correct time, say 2:21. Suppose the hands show 9:45 or 2:45, bring both hands to 12 or 6 and they will immediately assume the correct position. Take off the minute hand and lay it on the bench for five or six minutes; put it on again, give it a twirl, and it will stop at the correct time. Various other tricks will suggest themselves for the astonishment and mystification of the jeweler's patrons, and considerable benefit can be derived from the curiosity of an excited town.