THE DYNAMOMETER-CHRONOGRAPH. 83 



evident tliat we reach the desired result of correct dynamometric measures taken 

 under all the essential circumstances of free flight, for in this case the propeller is 

 driving the model independently of any help from the turn-table, which latter 

 serves its purpose in carrying the attached Dynamometer. 



As a means of determining when the propeller is driving the model at a 

 speed just equal to that of the turn-table, let the whole apparatus on the end of 

 the arm be jilaced on a car which rolls on a nearly frictionless track at right 

 angles to the turn-table arm. Then, v.dien the turn-table is in rotation, let the 

 propeller of the model be driven by its motor with increasing speed until it 

 begins to move the model forward on the track. At this moment, that is, just 

 as the aerodrome begins to move forward relatively to the moving turn-table, 

 it is behaving in every respect with regard to the horizontal resistance (i. <?., 

 the resistance to advance), as if it Avere entirely free from the table, since it is 

 not moved by it, but is actually advancing faster than it, and it is subject in this 

 respect to no disturbing condition except the resistance of the air to the bulk of 

 the attached I)i/namometer. In another respect, however, it is far from being- 

 free from the table, so long as this helps to take pai't in the vertical resistance 

 which should be borne wholly by the air ; the aerodrome, in other words, will 

 not be behaving in every respect as if in free air, if it rests with any weight on 

 the track. The second necessary and sufficient condition is, then, that at the 

 same moment that the model begins to run forward with the car it should alse 

 begin to rise from it. This condition can be directly obtained by rotating the 

 turn-table at the soaring speed (previously determined) corresponding to any 

 given angle of the inclined plane. 



This conception of a method for attaining the manifold objects that I have 

 outlined was not carried out in the form of the track, which, although constructed, 

 was soon abandoned on account of the errors introduced by friction, etc., but in 

 the Component Recorder, whose freedom of motion about the vertical axis provides 

 the same opportunity for the propeller-driven model to run ahead of the turn- 

 table as is offered by the track. This instrument, therefore, a part of whose 

 functions have been described in the preceding chapter, has been used as a neces- 

 sary auxiliary apparatus to the Dynamometer-Chronograph, and this is an essential 

 part of the purpose for which it was originally devised. In naming the instru- 

 ment, however, only a part of its purpose and service could be included, or of 

 the mechanical difficulties that it surmounts indicated. 



The investigation of the velocity at which an inclined plane will sustain its 

 own weight in the air, and the determination of the end-thrust, or horizontal 

 resistance, that is experienced at this velocity, were made with the Recorder 

 independently of the Dynamometer, and have been presented in detail in chapter 



