Page:Popular Science Monthly Volume 30.djvu/667

Rh once in about three months; Venus, once in seven months; the Earth, once in a year; Mars, once in nearly two years; Jupiter, once in nearly twelve years; Saturn, once in about twenty-nine years; Uranus, once in eighty-four years; and Neptune, once in one hundred and sixty-five years. Besides giving the golden number, the dominical letter, and other interesting things, this clock gives the time when it is high tide at various points in Europe. A clock made by a Parisian consists merely of a glass dial, and two hands, which are balanced each with a ball on the other side of the center. These balls (Fig. 3) are only about an inch in diameter, and yet they contain all the machinery that turns the hands about. The back of the dial is a perfectly smooth surface. You may turn the hands round and round with your cane, and when you let them alone they will swing back and forth for a while, and then they will stop at exactly the right spot to show the true time. A clock in Brussels is so placed over a chimney (or pipe through which the air goes upward) that the draught keeps it wound up all the time. The most artistic clocks for mantels are, for the most part, made in Paris or Vienna. One variety has a tuning-fork for controlling the escapement. Another hides the working parts of the clock within a base that shows only the dial (Fig. 4). Upon this base stands a female figure holding a pendulum which vibrates without any cause that any one can see. But if the figure is taken off from its base it will be discovered that it rests upon a pivot which is connected with the escapement in such a way that it is swayed to and fro just a moment before the pendulum has reached the limit in the opposite direction. This sends the pendulum back again, just as you reverse the motion of a rocking-chair by leaning forward just before you have rocked back as far as you are going. Sometimes the female figure (Fig. 4) holds above her head a great ball, which is balanced by a pendulum that swings near her feet. The ball contains the clock, and inside of it is a small pendulum, controlled by a spring in such a way as to send the large pendulum back and forth after the manner of a rocking-chair. There are also clocks that are run by electricity instead of by weights.