essential element of the chronoscopes, or, more accurately speaking, of the chronometers, used to measure the velocities of projectiles, and to solve such problems as the rates of progress of the nervous influence and of the muscular wave.
To make any tuning-fork a chronoscope, it is only necessary to know the number of vibrations which the fork makes in a second at a known temperature. This number is determined to the last degree of precision by the following method, devised by the author of this article: A break-circuit clock is placed in the primary or battery circuit of an induction-coil; while one terminal wire of the secondary coil is connected with a metallic cylinder covered with smoked paper, the other terminal wire is led to the tuning-fork, which traces its vibrations, by means of a delicate metallic point, on the paper-covered cylinder. At each second the break-circuit clock sends a spark from the point attached to the vibrating fork, through the smoked paper to the metallic cylinder. It is evident that, on counting the number of sinuosities made by the vibrating fork between two contiguous spark-holes, we have the number of vibrations per second made by the fork. After the above determination has been made, the tuning-fork becomes the most accurate and uniformly rated chronometer yet devised by men of science. But the time-recording tuning-fork is only one part of the apparatus required in the study of physiological motions. We must also be in possession of some contrivance which can be readily applied to an organ, the durations and varied velocities of whose motions we would study, and this contrivance must make a graphic record of these motions alongside of the time-record drawn by the tuning-fork. To Marey we are indebted for many effective recording instruments, but the apparatus which he has most extensively used, and which is admirably adapted to the study of the motions in some of the vital functions and in locomotion, consists of a small drum of shallow depth, one of whose ends is covered with an elastic membrane. The interior of this drum is connected with the interior of a similar drum by a rubber tube of very small internal diameter. The membrane of one of these drums presses against the surface whose motions we would study. A delicate lever rests on the membrane of the other drum, and the end of this lever is armed with a delicate point which touches a revolving cylinder covered with smoked paper. On this cylinder the tuning-fork also simultaneously traces its time-record. Now, as both drums, and the tube which connects them, form an air-tight space, it follows that any depression, given to the membrane which touches the moving surface, will compress the air in this drum, in the connecting tube, and in the drum which carries the delicate lever. The membrane of the latter drum will move outward and cause the pointed lever to move and make its trace on the revolving cylinder. Of course an elevation of the membrane in the first drum will cause a depression in the membrane of the second drum, accompanied by a
