A fork with arms about six or seven inches long will make about fifty or sixty vibrations in a second. How are we to record those vibrations, and how keep the tuning fork vibrating? [Illustration: FIG. 69.] A train of wheels is almost an impossibility, not perhaps so impossible as might be supposed, but still very difficult. So a different method is adopted. A little wire projects from one tuning fork arm. A piece of glazed paper is gently smoked by means of a wax taper, and is stretched round a well-made brass drum. The tuning fork is then put so that the little wire just touches the paper. The tuning fork is then made to vibrate by a blow, and while it is vibrating the drum is revolved. Thus a wavy line is formed on the drum by the wire on the tuning fork. If the tuning fork made fifty complete vibrations to and fro in a second there would be one hundred such indentations, fifty to the right and fifty to the left, and by these the time can be measured as you would measure a length upon a rule. [Illustration: FIG. 70.] If an arm _a b_ be fitted to move sideways when a little string _c d_ is pulled, and be also provided with a small wire, so as to touch the drum, then it also will trace a straight line on the drum as the wire lightly scratches away the thin coating of smoke. Now, if it is suddenly jerked and flips back, then a little indentation will be made in the line, and if when we are to measure a rapid lapse of time a jerk is given at the beginning, and another jerk at the end of it, we should get a diagram like that in the adjoining figure, where _a_ is the trace of the tuning fork, _b_ that of the indicating arm. The time which has elapsed between the jerk which produced the indentation _c_ and that which produced the indentation _d_ will be about three and three-quarter double indentations of the tuning fork line, thus indicating three and three-quarter fiftieths of a second. It is easy to see how delicate this means of measurement can be made. With small tuning forks we can easily measure times to a thousandth part of a second, and much less if desired. The jerk may be given by electricity if it is wished. When the current is joined a little electro-magnet pulls a bit of iron and gives a pull to the string. So extremely rapid is the flight of electricity that no appreciable time is lost in its transit through the wires, so that the impulse may be given from a distance. Thus we may arrange that when a cannon ball leaves a gun an electric impulse shall be given. When it reaches and hits a target another electric impulse is given. These make nicks in the tracing line on the drum from which we can easily compute the time that has elapsed between the leaving of the mouth of the gun and the arrival of the shot at its destination. [Illustration: FIG. 71.] Such an apparatus is used in modern gunnery experiments. It is an elaborate one, but is based on the principle above described.