The recording of time is now usually made by the electric method, which may be here described briefly, though many are probably familiar with it. The principle is the following, subject to various modifications in the particular form of apparatus: A line is described by a pen made to move uniformly over the paper by means of clock-work. That this line may be indefinitely prolonged without retracing, it is usual to make it a spiral round a horizontal cylinder, which revolves, say, once a minute, while the marking-pen (otherwise stationary) moves slowly from one end of the cylinder to the other, perhaps requiring several hours for the complete passage. The pen making this line is held in its place by the action of an electro-magnet pulling against a spring; the circuit through this magnet is broken every second by the escapement of a clock or chronometer; the magnet then for an instant ceases to act, and the spring pulls the pen aside, making a break in the line at regular intervals corresponding to every second of time. The same interruption of the circuit can also be made by an observer provided with a key like those used by telegraph operators, and the time of his observation thus registered on the chronograph, as the instrument is called. For identification of the clock-mark preceding his observation, mechanical arrangements can easily be devised, by which the first second in each minute shall be omitted, the circuit not being broken; so that it will be known what second of every minute each mark corresponds to; and the fraction of the second elapsed from this clock-mark to his own can easily be estimated by the eye, or measured more carefully. The reading of the record is, of course, facilitated by having the cylinder revolve once a minute, so that all the clock-marks answering to any particular second (as the twenty-third, for example, of each minute) will come in the same horizontal row; and the marks are not made on the cylinder itself, but on a sheet of paper fastened round it, which can be detached when filled. Instruments of this character were used at Shelbyville, and also at the border stations near the edge of the path of the shadow, but inside of it, one of which was at Falmouth, about thirty miles south of Cincinnati, the other at Oakland, near the Mammoth Cave. The observations of time were especially important at these places, since, as will readily be seen, the length of time required for a circular or elliptical shadow to pass a point near its edge will vary very rapidly for a slight change in the size of the shadow, or a slight shifting of its path toward or from the point selected. Even rough observations, merely of the duration of the eclipse, made at two such stations on opposite sides of the central line, suffice to determine with great accuracy the dimensions and precise track of the shadow, and thus give the elements of the moon's motion.