pair of gears, for there is always a certain amount of back-lash between the teeth of the gears, and the effect of this back-lash is still further intensified when the driven mechanism is turned over by hand in order to start the engine, as this takes up the back-lash on one side of the gears, and the moment the engine starts permits a free movement until it suddenly takes up the back-lash and strikes the other side of the gear teeth with a blow. The effect of this sudden starting of the engine proved most disastrous to the thrust-recording devices, and, although they were considerably strengthened, it was found after a short time that in order to make them strong enough to withstand the shock of the sudden starting of the engine it would be necessary to make them inordinately heavy. It was therefore decided to abandon all attempts to incorporate the thrust-recording device on this quarter-size model, but it was hoped to install it later on one of the steam-driven models, where the engine starts so slowly that there would be no need for excessive strength in it. The engine for the quarter model when reconstructed with stationary instead of rotating cylinders was found in the shop tests referred to above to develop when working at its best between 1-1/2 and 2 horse-power, as measured by the absorption dynamometers. However, it was impossible to maintain this power steadily for more than 30 seconds. In the first place, the same difficulties (heretofore described) that were met with in securing a suitable carburetor for the experimental engine were experienced at the same time in the development of the small engine. In the second place, as the engine had no cooling apparatus of any kind, it was found that it could not be tested in the shop for more than 30 seconds owing to premature explosions. It was hoped, however, that by having everything ready for a flight before starting the engine, it might be possible to launch the aerodrome before the cylinders began to heat seriously, and that the greatly increased cooling effect due to the motion of the aerodrome through the air would permit the engine to develop sufficient power to secure a flight that would show whether or not the balancing was correct, as the final disposition of some of the accessories on the large aerodrome could not be so well settled until it was known just how the calculated balancing of this new model corresponded with the actual balancing necessary for flight.