APPLICATIONS TO MECHANICS 85 



would have to be performed in a closed space protected 

 from every current of air. If during the night-time 

 a vertical beam from an electric lantern were allowed 

 to illuminate the under surface of various-shaped 

 bodies as they fell through such a space, chrono- 

 photographic images representing the various stages 

 of the fall could be taken on a plate at successive 

 intervals of time.* 



But in the open air the least breath of wind disturbs 

 the progress of the moving body, and if the fall is only 

 a short one the resistance of the air has not time 

 to make the velocity uniform, and more especially is 

 this the case if the object is a very light one. In the 

 experiment referred to on page 53, an indiarubber ball 

 of 30 grammes weight and 11 centimetres in diameter 

 was the object which was allowed to fall. After a 

 descent of two metres the diminution in acceleration 

 hardly manifested itself. But this diminution would 

 have been very obvious in the case of a small and light 

 air-ball. 



* The Machinery Hall at the Paris Exhibition of 1889 would have 

 lent itself admirably to experiments of this kind. The objects might 

 have been allowed to fall from the dome of this immense building 

 into a beam of light, and side by side with this beam a chain of 

 incandescent lamps would have done excellently as a scale of 

 distance ; and, further, a chronometric dial with a bright needle 

 might have s*rved to register the time. Experiments carried out in 

 this manner would have been very interesting from the point of view 

 of aarial locomotion — they would have controlled and amplified the 

 beautiful researches which are now being conducted by our colleague 

 and friend Cailletet and by M. Colardeau at the Eift'el Tower. One 

 could calculate the resistance of the air for any particular velocity by 

 allowing an object to fall until, as shown on the photographic plate, 

 the fall became uniform, because then the resistance of the air would 

 equal the weight of the falling object. 



Now, in a series of experiments, by letting the same object fall 

 through the air, weighted with ever-increasing ballast, so that the 

 weight increased as the following progression — 1, 2, 3, etc., it could be 

 seen at what velocity the fall became uniform. Since the resistance 

 of the air is always equal to the weight, one could thus calculate for 

 an object of any particular shape the law of aerial resistance for 

 different degrees of velocity. 



