Page:Life in Motion.djvu/82

 and the line appears as a bright shiny line on the screen. The wheeled car bearing the plate of glass is drawn up to the end of the long board, and this you observe is not level, but we may incline it by turning this screw which raises one end. A catch holds the car in position, and when the car is in position a strong spring is put on the stretch. When the catch is released the car runs down the incline, and at the same time the spring recoils, and, pulling on a lever, sends the car along with great velocity to the other end of the board. As it runs along we have an arrangement by which, when about midway in its course, the car breaks an electric circuit. We shall not use the "break" in the present experiment.

Now we shall show you a record of the movements of our chronograph worked by this tuning-fork, which is vibrating one hundred times per second. Mr. Brodie, you observe, brings the car up and fixes the catch. The spring is on the stretch. He now adjusts the marker of the chronograph on the glass plate and you hear the humming of the fork. All being ready, Mr. Brodie releases the catch, the car, carrying the glass plate, dashes across,