CHAPTER VII. 



THE DYNAMOMETER-CHRONOGRAPH. 



Having determined by means of the Component- Recorder the resistance that 

 must be overcome in moving a material plane horizontally through the air at 

 different speeds, the next step of my investigation has consisted in devising means 

 for measuring the power that must be put out by a motor in doing this useful 

 work ; for, by any form of aerial pi-opulsion, the useful work that can be derived 

 from the motor is only a percentage, either large or small, of that which is 

 expended. It becomes important, therefore, to determine the ratio between the 

 propelling force obtained, and the amount of power that must be expended in any 

 given case. 



In devising the following apparatus I have confined my attention to aerial 

 propellers for reasons of present convenience, and not because I think them the 

 only practicable method of propulsion, although they are undoubtedly a most 

 important one. 



If we consider the actual circumstances of such experiments, where the motor 

 under investigation is mounted at the extremity of the large turn-table arm and 

 is in motion, frequently at a rate of over a mile a minute, and that the end of 

 this slender arm is 30 feet from any solid support where an observer might be 

 stationed, it will be seen that the need of noting at every moment the action of 

 apparatus, which under such circumstances is inaccessible, imposes a difficult 

 mechanical problem. After frying and dismissing other plans, it became evident 

 that a purely automatic registry must be devised which would do nearly all that 

 could be supposed to be done in the actually impracticable case of an observer 

 who should be stationed at the outer end of the whirling arm beside the apparatus, 

 which we may suppose for illustration to be an aerodrome moved by a propeller. 

 The registering instrument for the purposes desired must indicate at every 

 moment both the power expended on the supposed aerodrome to make it sustain 

 itself in flight, and also the portion of that power which is utilized in end-thrust 

 on the propeller shaft, driving the model forward at such a rate as to maintain 

 soaring flight, under the same circumstances as if it were relieved from all 

 constraint and actually flying free in a horizontal course in the air. For this 

 purpose a peculiar kind of dynamometer had to be devised, which, after much 

 labor over mechanical difficulties, finally became completely efficient in the form 



(75) 



