Much time was also lost in the effort to construct an apparatus by which a record could be obtained of the power actually used in propelling the aerodrome. Various methods were in use by which the thrust of the propellers could be more or less satisfactorily measured while the aerodrome was at rest, but it was desired to know just how much power the aerodrome consumed while in actual free flight. Such a record it was hoped to obtain from a device incorporated in the propeller shafts. This thrust-measuring device consisted essentially of a propeller shaft made in two sections, one section telescoping the other for a short distance. On the section of the shaft to which the propeller was attached there was mounted a drum, having in its circumference two long slots diametrically opposite. To the other section of the shaft a disc was fastened with two diametrically opposite rollers mounted on its periphery, which fitted the slots in the drum of the other section. A compression spring was interposed between the disc and the drum, and the outside of the drum was so arranged that a strip of paper could be wound around and fastened to it which would serve as a chronograph sheet. A pencil was fastened to the frame, and, since the drum was connected to the section of the shaft to which the propeller was attached and which therefore moved to and from the frame under the action of the propeller thrust, a record of the actual thrust of the propeller at any particular moment could be obtained by simply pressing the pencil up against the paper on the drum and calculating the thrust from the calibration of the compression spring. Since the thrust would naturally be greater when the propellers were revolving in a moored condition, during the few moments after the engine was started up and before the aerodrome was launched, it was necessary to provide means for having the pencil point held away from the chronograph sheet until the aerodrome was launched, and then have the point come to bear on the sheet. This was accomplished by having the point held off by a small trigger arrangement which was to be released just at the moment that the aerodrome left the launching car. A set of propeller shafts embodying this thrust-recording device was constructed, but when they were actually tested on the aerodrome many difficulties were encountered which had not been anticipated. In the first place the gasoline engine for the model was started up (or “cranked over”) by turning the propellers by hand. A gasoline engine never starts slowly, and on account of this suddenness of starting causes a very great strain in any [p230] shafting by which it is connected to any driven mechanism. The inertia of the driven mechanism, even though it be apparently small, becomes a most serious matter when an attempt is made to start up very suddenly. This effect is very much intensified if the driven mechanism is connected to the engine through even one