40 EXPERIMENTS IN AERODYNAMICS. 



inaccuracies of obsei'vation. The general result presented by both the falling 

 and soaring planes is that when the double pairs of planes are placed 4 inches 

 apart, or more, they do not interfere with each other, and the sustaining power 

 is, therefore, sensibly double that of the single pair of planes ; but when placed 

 2 inches apart, there is a very perceptible diminution of sustaining power shown 

 in the higher velocity required for support and in the greater rapidity of fall. 

 Manifestly, however, this result can hold good only above some minimum 

 velocity of translation, and, in general, we may say that the closeness with 

 Avhich the planes can be set without producing any diminution of sustaining 

 efficiency is a function of the velocity of translation, so that the higher the velocity, 

 the greater the proximity. It was desired, therefore, to ascertain the minimum 

 velocity for which the preceding conclusion holds good, namely, that planes 4 

 inches wide do not suffer any loss of sustaining power if placed one above the 

 other and 4 inches apart. Experiments with these double pairs of planes were, 

 therefore, continued on August 22, 23, and 24 for the purpose of getting these 

 data. The same planes were used and were placed at the same distance apart, 

 viz., 2, 4, and 6 inches, and a set of experiments was also made with the single 

 pair. Previous to these experiments at high speeds the Plane- Dropper was 

 stiffened in order better to preserve its verticality under strong wind pressures, 

 and precaution was taken to observe how closely this condition was maintained. 

 The new observations were somewhat different from the early ones, and consisted 

 in measuring the time of fall of the double planes — i. e., one over the other when 

 set at different angles ranging from — 7° to + 7° at three different velocities, viz., 

 23.5, 13.0, and 6.5 meters per second. For every setting the brass frame was 

 turned on its pivot through an angle of 180°, so as to present first one side then 

 the opposite as the advancing face. The two positions are designated by A and 

 B in the accompanying Tables, IX, X, and XI, which contain 125 separate 

 observations at the above-named different velocities, angles, and settings. 



