Although many variables enter into the personal equation of the driver of an automobile, this paper concerns principally his reaction-time. The tests described had for their objects the determining of (a) the average time that elapses between the hearing of a signal, such, for example, as the shot of a pistol, and the applying of the brake; (b) the relation between the reaction-time and the variability of the individual; and (c) the effect on reaction-time of such factors as the speed of driving, training, age, sex, race, and general intelligence.The reaction-time was determined by two pistols mounted on the under-side of the running-board of an automobile and pointed toward the ground, the first being fired by the experimenter when the car had reached the desired speed, the second, by the person under test in making the initial motion of operating the brake-pedal. The shells used, being loaded with red lead, made bright spots on the road, the distance between which could be measured accurately. The ratio of this distance, measured in feet, to the speed of the car, in feet per second, gave the reaction-time. The subjects of the test included 36 students from George Washington University, including 10 female students; 11 colored students from Howard University; and 10 taxicab drivers. Each person was tested at speeds of 10, 15, 20, 25, and 29½ m.p.h. An average reaction-time for the total number of 285 runs was found to be 0.54 sec. Variability was determined by subtracting the shortest reaction-time of each person from the longest and dividing the difference by two. When these results were plotted against the reaction-time of the various persons, the surprisingly high correlation factor, 0.822, was obtained.The conclusions reached were that the reaction-time (a) is not appreciably affected by the speed of driving, (b) may be reduced by training, (c) is not affected by age or sex, and (d) is related to general intelligence. The number of data at hand was insufficient to show what, if any, is the influence of race.Thus we find among children those who are slow and sure, slow and erratic, quick and sure, quick and erratic, and no one seriously expects these tendencies to be altered any more than he expects the leopard to change his spots. These permanent peculiarities we call the personal equation.-Seashore.