Given a moving body, with a velocity eight thousand eight hundred feet in a minute, what should be its elevation that it may fall eleven hundred feet in the transit of five-eighths of a mile?" He had not only to work up the parabola, comparatively simple, but he had to allow for the resistance of the air, on the supposition of a calm, according to the really admirable formulas of Robins and Coulomb, which were the best be had access to. Joslyn brought me, one day, a letter from Bill Todhunter, which shows how carefully he went into this intricate inquiry. Unfortunately for them all, it took possession of this spirited and accomplished young man. You see, he not only had the mathematical ability for the calculation of the fatal curve, but, as had been ordered without any effort of his, he was in precisely the situation of the whole world for trying in practice his own great experiment. At each of the two X X of Joslyn's map, the company had, as it happened, switches for repair-trains or wood-trains. Had it not, Bill Todhunter had ample power to make them. For the "experiment," all that was necessary was, that under the pretext of re-adjusting these switches, he should lay out that at the upper X so that it should run, on the exact grade which he required, to the western edge of the ravine, in a line which should be the direct continuation of the long, straight run with which the little map begins. An engine, then, running down that grade at the immense rapidity practicable there, would take the switch with its full speed, would fly the ravine at precisely the proper slopes, and, if the switch had been rightly aligned, would land on the similar switch at the lower X. It would come down exactly right on the track, as you sit precisely on a chair when you know exactly how high it is. "If." And why should it not be rightly aligned, if Bill Todhunter himself aligned it? This he was well disposed to do. He also would align the lower switch, that at the lower X, that it might receive into its willing embrace the engine on its arrival.