338 HISTOKY OF CHKONOPHOTUGRAPHY. 



threads, which the current moved along; while sometimes he explored 

 the direction of the air movements b}^ means of little gas flames, which 

 he applied at diflerent points of the l)odies that were in the tube. But 

 the method which gave him the best results was that of Schlieren, 

 which consists of rendering visible the movements of ver}' small streams 

 of air by changing the index of refraction, which is done by sending a 

 current of hot air into a colder current. The small streams or threads, 

 which are warmed, then show either clearer or darker than the sur- 

 rounding air, and the magnesium flash light permits us to photograph 

 the phenomenon. 



Mr. Mach's experiments have given results quite conformable to 

 those which 1 obtained in the movements of li(piids under similar cir- 

 cumstances. So, for instance, on meeting the bodies the air current 

 divides and re-forms behind them without producing many whirlpools, 

 and when the plane is inclined under different angles and solids of 

 diflerent forms these disturb the air as if it were water. Mr. Mach 

 measured the speed of his air currents by means of an anemometer, 

 regulating the indications of the instrument by an acoustic method 

 devised by his father, Prof. E. Mach. The vibration caused by a 

 Koenig flame introduced into the air current gives the appearance of 

 a cluster of little clouds, which move on while keeping their respec- 

 tive distance, and as the latter correspond to known intervals of time 

 they enable one to measure the speed of the current. 



Mr. Mach noticed a lack of fixity in the direction of air currents, 

 which showed continual oscillations, and he attributes these movements 

 to changes in the aerodynamic pressure. 



These studies were not known to me when I presented to the Acad- 

 emy the result of experiments where I had studied the action of differ- 

 ent bodies in an air current placed in conditions identical to those 

 which I had studied with the liquid currents. To follow the move- 

 ments of the air, 1 used smoke threads, which, drawn along with the 

 air by the action of ventilators, entered with it and at the same speed 

 into the glass tube. The air and smoke were filtrated through fine- 

 meshed cloth and advanced parallel to each other in the interior of the 

 tube as long as the current met no obstacle. 



These experiments, like those of Mr. Mach, have shown that at the 

 rates emplo^^ed air and liquids behave in substantially the same way. 



At this time Mr. Bertin, an engineer of the Navy, brought me into 

 correspondence with Mr. Hele-Shaw, of Liverpool, who had been pur- 

 suing similar experiments in closed chambers for several years. The 

 clear images given by photographing colored glycerin threads showed 

 how the incompressibility of liquids affect eddies in an inextensible 

 space, while the eddies always occur in different degrees behind bodies 

 immersed in an air current, or even in a liquid current if it is moving 

 in an open tube. 



