you listened to the beat of a clock in the same room with the telescope, and the mind was strained in the attempt to tell the exact second, or even the tenth of a second, when a star passed across a wire. In the new plan the clock may be out of sight and hearing, and all you have to do is to press an ivory key the moment the transit is made 5 and, in another room, through the intervention of the electric current, the record is made. A pen, held as it were in the hand of the clock, marks the precise moment on a sheet of paper, drawn by machinery under the pen at an equable rate. While the machinery is in operation, the pen is marking off seconds by making lateral offsets on a straight line—the space between any two offsets being a second of time. The effect of pressing the key is to make an additional mark. This mark indicates the precise moment when the observation was made. Suppose a copy is given to a school-boy in which to write during the course of a certain hour, and that while writing he receives a nudge from his neighbour which blots his copy, the moment when this happened can be afterwards ascertained by marking the part of the page where the blot is found—this being on the supposition that the boy wrote at an equable rate during the hour. Now, the pen held in the hand of the clock writes at an equable rate, and when the observer presses the ivory key, which is equivalent to the nudge, it records the event by an additional mark, the precise instant of which may be
