264 Mr. W. S. Davis on a Method of Illustrating 



loaded rope is required. That now used has strung upon it a 

 number of rings of lead cut from a leaden water-pipe; these 

 are placed about 6 inches apart, and are covered with india- 

 rubber bands to prevent their making unpleasant noise. The 

 loaded and an unloaded rope are laid on the board side by side, 

 and fixed at one end. Then, taking care the tension is equal 

 in the two ropes, waves are simultaneously generated on them, 

 as before described. It is then observed that the wave on the 

 loaded rope lags considerably behind the other (fig. 4). By suffi- 

 ciently tightening the loaded rope the velocity of its waves may 

 be made equal to, or even greater than that of the waves of the 

 unloaded rope. This may be used to explain why the velocity 

 of sound in water is greater than in the much less dense medium, 

 air. 



6. Transmission of Waves from one Medium to another of dif- 

 ferent Density. — The loaded cord is attached end to end to one 

 much lighter than itself; the united cords are laid on the board 

 with the splice at about the middle of its length. Then, fasten- 

 ing the end of the lighter cord, waves are generated on the 

 heavier one. These waves pass onwards to the lighter cord, on 

 reaching which they acquire greater amplitude, velocity, and 

 length (fig. 5). If the heavier cord be fixed and waves be. gene- 

 rated on the lighter one, the reverse changes to those just stated 

 occur on the waves reaching the heavier cord. It is an interest- 

 ing experiment to transmit waves along a succession of three or 

 more cords alternately heavy and light. With three cords joined 

 end to end, the middle one being heavier than the others, a good 

 illustration is produced of the changes of velocity, length, and 

 amplitude which setherial waves undergo in parsing perpendi- 

 cularly through a medium with parallel faces. 



7. Separation of a Wave into two or more smaller Waves. — A 

 single cord extending half the length of the board is joined to a 

 double one extending the other half. Waves are transmitted 

 from the single cord to the double one ; on reaching the latter 

 each wave divides in two, one wave traversing one part of the 

 double cord, and the other wave the other part. By giving each 

 part of the double cord a different tension, the velocity of the 

 waves will be different in each (fig. 6). The waves on the double 

 cord may be made to move in planes at right angles to each 

 other by the use of proper guides, thus furnishing an illustration 

 of some of the phenomena of double refraction. 



8. Superposition and Interference. — The same arrangement is 

 used as in 7, but the waves are transmitted from the double 

 cord to the single one. With equal tension in each part of the 

 double cord, the waves simultaneously produced on each part run 

 side by side until they enter the single cord, when they are su- 



