interval much may have happened outside of us in connection with each event, of which, however, we are unconscious.
Science, then, demands the accurate measurement of time, and no small fraction of time is too insignificant to be of importance. The twentieth, the hundredth, the thousandth of a second is to science as precious as an hour may be to many people. It may be said without irreverence that to science a day is as a thousand years, and a thousand years as one day. The methods of science for this purpose are founded on two sets of appliances: first, the use of instruments, like a tuning-fork, that are found to vibrate or move at a known rate, say one hundred or five hundred times per second; and, second, the application of the graphic method, by causing these instruments to record their movements on a rapidly moving surface, such as that of a cylinder travelling with great velocity. All such appliances are called chronographs, or time-writers. Let me illustrate to you the use of several of these ingenious appliances.
Here is a ball at the end of a long string suspended from the roof of the theatre. It con-
