[62 SMITHSONIAN CONTBIBUTIONS TO KNOWLEDGE Vol.. -7 



1 ion at the moment of launching the aerodrome being much less than that of a 

 freely falling body. Since so many factors enter into the problem no confidence 

 was felt in calculations as to what the rate of acceleration would be. It was, 

 therefore, decided to determine it experimentally at the same time that tests 

 were made on the car to determine what spring- tension would be necessary to 

 enable the aerodrome and car to acquire soaring speed by the time they readied 

 the end of the track. 



It was obviously impossible to make this initial test with the aerodrome 

 mounted on the launching car, as the aerodrome would certainly wreck both it- 

 self and the car were it allowed to remain fastened when the ear was stopped 

 at the end of the track. It was, therefore, decided to make the tests by mount- 

 ing on the car boards which would have a head resistance equal to that of the 

 aerodrome. In order to minimize as much as possible the Mow due to the 

 car striking the buffers at the end of the track, the car had been made as 

 light as possible. On this account it was felt to be unwise to risk adding to it 

 a weight of 850 pounds to represent the aerodrome, and supplying an addi- 

 tional spring tension to represent the thrust of the propellers, as the total effect 

 of the added weight and the added pull would certainly completely demolish the 

 car. By calculation it was found that the omission of the 850 pounds weight of 

 the aerodrome and the spring tension to represent the thrust of the propellers 

 would practically counterbalance each other; and that if sufficient spring tension 

 were provided to cause the ear, with the light hoards representing the head re- 

 sistance of the aerodrome, to reach the soaring speed by the time it arrived at 

 the end of the track, it would be safe to assume that this spring tension would 

 be sufficient for use in launching the aerodrome. 



The method of measuring the final speed of the launching car for the mod- 

 els consisted in fastening a strip of smoked paper to the launching car in such 

 a position that it was drawn past a stylus fastened to the end of a vibrating 

 tuning fork placed at the end of the track. This had proved perfectly success- 

 ful, but it gave a record merely of the final speed attained by the car at the mo 

 iim'hI of launching the aerodrome. Tn the case of the large aerodrome it was de- 

 sirable to have a record of the speed of the car during the first few feet, and 

 also at several other points in its travel down the launching track, and the more 

 numerous these points the better. Short strips of copper were accordingly 



placed every twelve inches along the length of the track, and these were con- 

 nected by a wire to one terminal of a small electric battery. Mounted on the car, 

 in such a way that it would he drawn across these contact strips, was a copper 

 brush arranged to make continuous contact with another wire stretched along 

 the track-, this second wire being connected to the other terminal of the electric 

 battery and having in its circuit the magnet which actuated a pen on a chrono- 

 graph. Since the rate of revolution of the chronograph barrel was known, the 



