74 HALL AND MOTORA :
made very true, to serve as the track for a truck from which was suspended a little platform to carry weights. To this was attached a long horizontal band running about the drum of a kymograph, which we used as a motor on account of its ap- proximately uniform rate of motion, changes in the latter being found, by careful measurement, so small within the times we used that they could be disregarded. The contact of the knife edge, on which the balance was pivoted, with its support, the center of the pivots of the wheels on the truck, and the application of the force by means of the band, were all on the same level, and by this means the effects of traction on the free oscillation of the bal- ance were so slight that sudden reversals of the direction of motion, which could be brought about instantly at any time by a key described in a pre- vious communication [Mind, No. XL., page 557], did not sensibly affect it. The car, which, after careful experiments with flowing sand (which sug- gests how irregular the best hour-glasses must have been), was found to be much more reliable, may thus travel along the entire length of the beam, and bearing any weight placed on its platform, at any rate in which the drum can be set in motion, and a pointer which it carries may be made to pass over the divisions of the millimetre-scale on the track to the beat of a metronome. Certain suitable veloci- ties and weights with the rate of increment of pressure per second were carefully predetermined. Under one end of the beam was a metalic button, any size of which could be used, which was covered with rubber to eliminate temperature sensations — a matter which, where the contact of such an arrangement is for so long times, must be con-
