drawn together, and a small bit of platinum wire fixed to one of them is removed from contact with a platinum surface, so as to break the circuit. On the circuit being thus broken, the electro-magnet ceases to act; the limbs of the fork, by their elasticity, spring back to their original position; and thus the platinum wire is again brought into contact with the platinum surface. Thus again the circuit is completed and the action is repeated. In this way, the marker of the chronograph vibrates in unison with the fork, and you see, when I bring it into contact with the cylinder, a beautiful series of little waves is described, each little wave representing the one-hundreth of a second. Let us take a rough illustration to show the value of recording time in this way. Suppose I draw my hand, bearing a pencil, quickly from left to right and then back again. We wish to know the time occupied by that movement. I set the cylinder in rapid motion, the time is registered by the chronograph, and I now make the wished-for movement, bringing my pencil point into contact with the blackened surface. Here is the result. We have only to count the number of little waves made by the
