Page:Astronomy for Everybody.djvu/108

The pen is so connected with an electric current passing through the clock, and through a key held by the observer, that every beat of the clock and every pressure of the key by the observer makes a notch in the trace left by the pen. When the observer sees that a star has reached the thread of his instrument he presses the key, and the position of the notch thus made in the pen-trace between two notches made by the clock gives the moment at which the key was pressed.

The astronomer's clock must be of the highest attainable perfection, running for a whole day or more without a deviation of one tenth of a second. With a common house clock, the change in the length of the pendulum produced by changes of temperature between the day and night would cause deviations of several seconds. Hence in the astronomical clock these changes must be neutralised. This is done by making the pendulum of such a combination of different materials that the unequal expansions of the latter shall neutralise each other. The most common combination is that of a steel rod bearing at its lower end a steel or glass jar of quicksilver, which serves as the bob of the pendulum. Then, when the temperature rises, the upward expansion of the quicksilver compensates the downward expansion of the steel.