. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. 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 for
. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. 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. F
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