Chap. XX.] FLOW OF LIQUIDS T PI ROUGH Ti^ES. 2IJ 



action of its own elastic force. This elastic reaction 

 acts upon the fluid within the tube, pressing upon it, 

 and the increased pressure is thus passed on to a 

 succeeding part of the tube, which dilates, and then 

 recovers itself, by its recovery transferring the in- 

 creased pressure still farther, and so it is propelled 

 onwards. A wave is in this way propagated along the 

 tube. Now this propagation of a wave is to be dis- 

 tinguished from the passage of the fluid. The onward 

 movement of the molecules of the fluid, which forms 

 the current, is in the direction of the axis of the tube, in 

 a straight line, and is a movement of translation ; but 



O ' ' 



the wave movement is one across this path, and is a 

 movement of oscillation, due to the molecules deserting 

 the straight line. In a rigid tube, as has been seen, 

 only the movement of progression exists. In an 

 elastic tube, with no current, the wave movement may 

 exist alone. In an elastic tube open at the end, not 

 only can both co-exist, but they may co-exist in 

 different directions. Thus the wave may pass in the 

 same direction as the current, in which case it is called 

 positive ; but it may travel in the opposite direction to 

 the movement of progression, and is then called a 

 negative wave. The characters of wave movements 

 have been very elaborately studied, by means of the 

 graphic method, by Professor Marey. He has adapted 

 the tambour, described on page 185, to obtain a register 

 of the movements. The tambour is contained in a 

 rectangular frame, the membranous side, which is 

 turned downwards, having attached to it the one half 

 of a piece of split tube. The other half rests on the 

 bottom of the frame. The elastic tube is made to pass 

 over the lower half, and then the tambour is lowered 

 by a screw, so that the tube is grasped by the upper 

 half, so as to be surrounded by the piece of tube. Any 

 movement, even the slightest, will affect the upper 

 portion, which, being attached to the membrane of the 



