As in these tests in the shop it was impossible to keep the engine cool by circulating its cooling water through the radiator, since there was no air current blowing across the latter to carry away the heat, it was necessary to connect an extra water tank in the cooling-water circuit. A tank holding about ten gallons was used, and this sufficed for about ten minutes before the water was raised to the boiling point. During one of these tests when the frame was supported from the springs, and while the engine was developing about fifty horse-power, without any warning whatever, both propellers suddenly twisted off from the flanges by which they were connected to the propeller shafts, thus leaving the engine entirely unloaded. The propellers both dropped quietly to the floor, making only about one or two turns in falling the distance of approximately 10 feet, and the engine, which had been running at about 850 R. P. M., immediately speeded up to an exceedingly high speed, which, while not exactly known, since the tachometer only read to 2000 R. P. M., yet from the deflection produced on the tachometer needle must have been considerably higher than this. Although the fly wheels, which were 33 inches in diameter, with the aluminum rims and wire spokes, had been exceedingly well made, yet it was not considered safe to run them at this speed, and the engine was immediately shut down. At the moment, however, that the engine had broken loose from its propellers and also momentarily jumped to this exceedingly high speed there was absolutely no vibration that could be noticed, the unloaded engine running as smoothly as an electric motor. This showed very clearly that the running balance of the engine was as near perfect as it would be possible to get it, except with a seven-cylinder engine, which is theoretically capable of more perfect balance. It was evident that what small vibration there was in the frame while the engine was developing its power was due almost entirely to the reverse torque, and, of course, could never be entirely eliminated.