CHRON'OGRAPH (Lat. chronographus, from Gk. xpovoypdcpos, ronogra phos. from xp6vos, chronos, time -ypdfpetv, gra phci P, to write). A term applied to instruments measur ing very short of time as well as to in struments recording such measures. Chrono graph watches.or pocket chronographs. will meas ure interval: of time down to fifths of a second, for use at horse races and other occasion: where a seconds watch is not exactly suited. They have an ordinary !nick-train lever movement. carrying hands which move over :t dial. One of these is a seeonds hand, very peeuliarly made. The second: hand is double, consisting of two di.tinet one superpo.N1 on the other. As the double seconds hand revolves, it is possible to stop one of its component halyes by touching a spring at the side of time case. A record having thus been made of the exact fraction of a second marked by position of the stopped hand, all ot licr pressure of the spring makes it Hy back to its former position on the moving component. The instrument is then ready for another obser vation. There are numerous modifications of this form of instrument; they are sometime: called stop-watches. or split-second tly-back watches.
The astronomical chronograph is used to re cord permanently the exact instant of tune when certain astronomieal observations are made. Such a time record is of course extremely impor tant in almost all forms of astronomical work. As usually constructed, the instrument consists of a metal eylinder or drum, upon which is wound a sheet of paper to receive the record.
The drum is connected with a clockwork appara tus which makes it turn on its axis once each minute. At the same time, a fountain pen, or some other form of marking instrument. is pressed against the paper so that a line is traced around the drum as it turns. The pen is attached to an eleetro-magnet. which in turn is moved very slowly by the clockwork along the axis of the drum. The result of these motions of the drum and pen is to trace a continuous spiral line on the paper around the drum. The eleetro-magnet is connected with the astronomer's standard clock by wires, and once every second an electric sigma] is sent automatically by the cluck into the magnet. This results in a short interruption or break in the line traced by the poll. Such a break, then, marks the beginning of each second on the time-record. When the as tronomer wishes to record the exact time of an observation, he has but to tap a telegraphic key held in his hand, and a signal similar to the clock signals will reach the ehronographic electro magnet. The result is a break in the record line, similar to those due to the clock. It is then merely necessary to measure the position of the observer's break with reference to the clock breaks, to fix the exact fraction of a second cor responding to the observation. There is little difficulty in thus measuring a chronographic rec ord to the twentieth part of a second of time. See CunoxoscorE.