IOWA ACADEMY OP SCIENCES. 35 



hook in the base of the model. The other end is tied to 

 an elastic band, and this in turn is fastened to another 

 hook in the base. The string being short enough to be 

 under considerable tension from the stretching of the 

 elastic, acts as a brake upon the grooved wheel and allows 

 it to turn only against more or less friction. The eccentrics 

 are separated by washers or rings W, of sufficient thickness 

 to prevent the brads and staples from touching the faces 

 of the eccentrics opposite. On turning the shaft by the 

 crank at one end, the first eccentric, being fast on the 

 shaft, will after turning 20 degrees, or until one of its pins 

 P, engages the stop T, on the second eccentric, cause this 

 to turn with it until it in its turn carries with it the third. 

 Finally the last eccentric will be set revolving, and will 

 carry with it the grooved wheel, against the friction of the 

 string. This brake prevents irregularity in the motion of 

 the last eccentrics on the shaft. The eccentrics now 

 revolve together, but each one is 20 degrees earlier in phase 

 than the one just ahead. As there are 27 of them they 

 assume an arrangement like a screw of long pitch whose 

 thread makes a turn and a half from one end to the other. 

 The rods which carry the 27 discs D, are so spaced that 

 each rests on a corresponding eccentric, and transversely 

 to the shaft. As these revolve, the free ends of the rods, 

 with the discs, describe vertical simple harmonic motions 

 differing in phase by successive intervals of 20 degrees. 

 The effect of these motions is to produce a sinusoidal wave 

 motion of the discs, which may be taken to represent 

 individual, equidistant particles of a medium transmitting 

 a wave by transverse oscillations. The motion may be 

 stopped and started at any instant or made as rapid or as 

 slow as may be washed. The particles may be brought into 

 a straight line, representing the medium at rest, by revers- 

 ing the motion a turn and a half. The model is especially 

 useful in elementary instruction, and represents in a plain 

 way the mechanism of this class of wave motion, without 

 departing seriously from theoretical exactness. Its novelty 

 consists chiefly in the application of the loose eccentrics 

 on a shaft. Any phase difference, and any amplitude, may 



