For the purpose of determining the synchronous action of the electro-exposors while making a double series of exposures, the accuracy of the time intervals as recorded by the chronograph, and the duration of the shortest photographic exposures used in the investigation, the two batteries of portable cameras were placed side by side, and the exposors were each connected with the exposing motor by separate lengths of a hundred feet of cable. The two series of cameras were pointed to a rapidly revolving disc of five feet diameter. The surface of the disc was black, with narrow white lines radiating from the center to the edge like the spokes of a wheel. A microscopic examination of the two series of resulting negatives proved that no variation could be discovered in the synchronous action of ten of the duplicated series of exposures, and that in the remaining two a variation existed in the simultaneity of a few ten-thousandths of a second--a result sufficiently near to simultaneity for all ordinary photographic work. [Illustration: Fig. 9.] A reproduction of the chronographic record of one of these experiments is seen in figure 9. The first line records the revolution of the disc; the second the vibration of the tuning fork; and each group of three long double markings in the third line indicates a photographic exposure. The shortest exposures made at the University were--approximately--the one six-thousandth part of a second; such brief exposures are however for this class of investigation very rarely needed. Some horses galloping at full speed will, for a short distance, cover about fifty-six or fifty-eight feet of ground in a second of time; a full mile averaging perhaps a hundred seconds. At this speed, a foot recovering its loss of motion will be thrust forward with an occasional velocity of at least 120 lineal feet in a second of time. During the one one-thousandth part of a second the body of the horse will at this rate move forward about seven one-tenths of an inch, and a moving foot perhaps one and a half inches, not a very serious matter for the usual requirements of the amateur photographer. A knowledge of the duration of the exposures, however, was in this investigation of no value, and scarcely a matter of curiosity, the aim always being to give as long an exposure as the rapidity of the action would permit, with a due regard to the necessary sharpness of outline, and essential distinctness of detail. The power used for operating the magnets, through the exposing motor, was given from a lé Clanché battery of fifty-four cells, arranged in multiple arc of three series, each of eighteen cells. During the investigation at the University of Pennsylvania, more than a hundred thousand photographic exposures were made.