00 MOVEMENT 



in a fluid, the conditions of its equilibrium were modi- 

 fied in accordance with the velocity of movement, and 

 in accordance with the angle formed between the 

 axis of the lamina and the direction of movement. 

 We cannot now dwell upon the interpretation of these 

 experiments, which ought to be studied in a methodical 

 manner.* It is only necessary to indicate how chrono- 

 photography may assist in researches of this kind. 



Applications of Chronophotography to Hydrodynamics. 

 — The study of the movements of fluids is very difficult, 

 and can only be accomplished by resorting to par- 

 ticular methods. Thus Savart illuminated a fluid vein 

 by an electric spark, and observed the changes in shape 

 of the drops of fluid, as well as the distances traversed 

 by them. 



Mr. Boys, applying instantaneous photography to 

 this study, obtained excellent results, in which the 

 appearance of the fluid vein was reproduced by means 

 of very short flashes of the electric light. In a moving 

 mass of liquid, extremely complex phenomena occur ; 

 changes of surface shape, and intrinsic molecular dis- 

 placements. The phenomena can be represented in 

 the form of chronophotographs. 



Let us consider the conditions represented in Fig. 50, 

 where the water is contained in a tank with glass sides, 

 and is illuminated from below by sunlight reflected by 

 a mirror situated beneath the tank, and on a level with 

 the ground. If the water is perfectly clear, the sun- 

 light is transmitted through it without any escape in 

 the direction of the photographic apparatus, except 

 from that part of the surface which wets the side of 

 the glass near the observer. In this situation capillary 

 attraction causes the formation of a concave meniscus 

 which extends all along the side of the glass. The 

 light which traverses the water suffers total reflection 



* See The Flight of Birds, chap xix. Paris, G. Masson. 1890. 



