Phenomena of Wave-Motion by means of Flexible Cords* 263 



distinguish any particular rope when two or more are used 

 together, it is well to cover the ropes with differently coloured 

 fabrics, say red, blue, and green. A few other accessories are 

 necessary, which will be described as they are required. 



By means of this apparatus waves may be produced which 

 move slowly enough to be readily examined by the eye. The 

 chief phenomena of wave-motion which can be shown are as 

 follows : — 



1. Transmission of a Wave. — One end of a rope, a few feet 

 longer than the board, is fixed to a hook at the end of the board. 

 The free end of the rope is then taken in the hand, and, the 

 the rope being quite slack, a sudden up-and-down movement of 

 the hand is made. A protuberance is thus formed which moves 

 very slowly along the rope, presenting the appearance shown in 

 Plate V. fig. 1*. 



A single up-and-down movement produces a wave consisting 

 of a crest only, the trough being suppressed by the board ; if, 

 however, with the rope very slack, the hand be moved up and 

 down very quickly and energetically, a series of waves, consisting 

 of both crest and trough, are produced (fig. 2). 



2. Amplitude and Wave-length. — Waves having any length, 

 from 1 to 6 or 7 feet, and amplitudes of similar dimensions, are 

 easily produced by properly controlling the rapidity and energy 

 of the motion of the hand. 



3. Decrease of Intensity with Distance, — This is illustrated by 

 a succession of waves produced by the well-timed motion of the 

 hand (fig. 2). The actual decrease of amplitude in this case is, 

 of course, due to the loss of energy by friction, and not to lateral 

 spreading. 



4. Relation of Velocity to Elasticity. — Two similar ropes, one 

 covered with red and the other with blue, are laid side by side 

 along the board and fastened to hooks at one end. The free 

 ends of the ropes are held in the hand, with the finger between 

 them, and, care being taken that they are equally loose, the hand 

 is moved up and down as usual. The result is that a wave of 

 the same height and length is produced on each rope, and the 

 two waves travel side by side to the ends of the ropes. The ex- 

 periment is repeated with one rope somewhat tighter than the 

 other, when the wave on the tighter rope is observed to travel 

 faster than that on the looser one (fig. 3). On continuing to 

 tighten the rope the velocity of the wave is more and more in- 

 creased, and may be caused to reach the end of the rope a whole 

 length or more before its fellow. 



5. Relation of Velocity to Density. — To exhibit this relation a 



* The length of the board in the figure.? is drawn to a much smaller 

 scale than the other parts. 



