252 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. 27 



had do tiring effect on one standing in the aviator's ear. In fact, the vibration 

 in the first case resembled rather closely that of a motor vehicle supported on 

 wheels having metal tires, and in the second case a motor vehicle supported on 

 wheels having pneumatic tires. 



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 cm- 

 rent blowing across the latter to carry away the heat, it was necessary to con- 

 nect 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 warn- 

 ing- whatever, both propellers suddenly twisted off from the flanges by which 

 I hey were connected to the propeller shafts, thus leaving the engine entirely un- 

 loaded. 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 en- 

 gine, which had been running at about 850 R. P. M., immediately speeded up 

 lo an exceedingly high speed, which, while not exactly known, since the tachom- 

 eter only read to 2000 R. P. M., yet from the deflection produced on the tachom- 

 eter needle must have been considerably higher than this. Although the fly wheels, 

 which were .'>.'! 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, how- 

 ever, 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 lie 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 

 he entirely eliminated. 



In the tests of the engine working in the frame, both while mounted on the 

 car and also when suspended from the springs, a great amount of delay was 

 caused from the fact that the ball-bearings on the transmission and propeller 

 shafts frequently went to pieces. There were two reasons for this: In the first 

 place, although carefully selected balls were used, defective ones were continu- 

 ally encountered. Even a slight defect in a single ball resulted in its breaking 

 under the rather severe test to which they were subjected, and, as is well known, 

 the breaking of one hall in a ball-bearing usually results in the destruction of 

 the whole hearing, especially if the races are light. The second cause was that 



