NEW RESULTS IN ANIMAL MOVEMENTS. 445 



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 influ- 

 ence 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 mo- 

 tions 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 mem- 

 brane 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 cyl- 

 inder 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 



