332 HISTORY OF CHRONOPHOTOGEAPHy. 



however varied, and also how the molecules themselves will move at 

 the different points throughout the mass. 



Motions of the air. — An analogous arrangement makes it possible to 

 render visible by means of smoke certain fillets of air in the midst of 

 a regular current. We ascertain in that case by chronophotography 

 the changes of direction and of velocity of this current when it meets 

 obstacles of different forms. 



In a large canal having walls of plate glass and before a dark back- 

 ground a draft of air is created by means of a ventilator. In order 

 to regulate the current it is filtered through a very fine silk gauze. At 

 the top of the canal, we set free, by means of a series of little tubes, 

 fillets of smoke which descend parallel to one another like the three 

 cords of a lyre. Now, if we place in the interior of the canal obsta- 

 cles of different forms, we immediately see the threads bend on these 

 (obstacles, slide over them, and form behind them eddies of varied 

 forms. Figs. 27, 28, and 29 (PI. V) show the same experiment under 

 different conditions. In fig. 27 a magnesium flash light illuminates 

 the phenomenon for a very short time. We see how the fillets of 

 air lick the plane, slide on it, and form backwater behind it. Fig. 

 28 shows the same phenomenon with chronophotographic indica- 

 tions. The series of little tubes which bring the smoke are made to 

 vibrate ten times a second, so that the smoke no longer appears in 

 rectilinear fillets but as sinusoidal undulations, more or less elongated 

 at each point according to the velocity of the current. The motion 

 slows up upon approaching an obstacle and is accelerated at the sides 

 of the obstacle. It will be remarked that the conceptions of time and 

 space which are peculiar to chronophotography are brought together in 

 this experiment. Finally, in fig. 29 the chronography is suppressed. 

 The illumination is no longer instantaneous, but is produced by the com- 

 bustion of magnesium 

 ribl)on, so that a sort of 

 mean state of the current 

 is recorded. 



Resistance of the air to 

 flying apparatus. — One 

 of the applications of the 

 previous experiments is 

 to make comprehensible 

 the action of the air on 

 apparatus of different 

 forms which move in this fluid. Fig. 30 shows more directly the effects 

 of this resistance. It shows how a little paper soaring model left to 

 fall vertically behaves and how it receives from the resistance of the air 

 changes of direction and of velocity which are faithfully represented. 

 Yihrations (f cords. — These motions are easily seen upon bright 

 cords vibrating before a dark backtrround. Our learned fellow-aca- 



.-"^^^^^T 



Fig. 30. 



