88 MOVEMENT 



with compasses, and compared with the earlier ones, an 

 appreciable remission in velocity will be noticed. In 

 the same way, if the rate of fall be measured, its degree 

 of acceleration will be found to have diminished under 

 the influences of aerial resistance. We have even 

 noticed in another experiment that if an object is 

 allowed to fall vertically and then thrown in a hori- 

 zontal direction from the same altitude, the duration of 

 the vertical fall is not the same in the two cases. But 

 in the second the resistance of the air has a greater 

 retarding effect. 



This experimental result struck Captain Uchard, 

 who was present at the Physiological Station when the 

 experiment was tried. Applying this know ledge, which 

 he believed to be new, to the question of the motion 

 of artillery projectiles, he found by calculation that 

 the resistance offered by the air to their descent was 

 quite different, according as they were simply let fall 

 in a vertical direction, or were provided with an initial 

 velocity.* 



Resistance of the Air to Surfaces variously inclined. — 

 The constant attempts that have been made to con- 

 struct flying machines prove that a complete know- 

 ledge of the action of the air on inclined planes, 

 travelling at different velocities, and at various angles, 

 is essential for success. Clever experimentalists have 

 succeeded in constructing small, light machines which, 

 when let go in the air, glide about, something after the 

 manner of a soaring bird. The eye can hardly follow 

 the evolutions, as they are complicated, sinuous, and 

 combined with an ever-changing inclination of the axis 

 of the system. A sheet of Bristol board folded length- 

 ways so as to form an obtuse angle, elevated at the 

 back and pointed in front, was weighted by means of a 



* A. Uchard, Remarks on the Laws of Resistance of the Air. Paris, 

 Berger-Levrault. 1 1 92. 



