IOWA ACADEMY OF SCIENCES. 37 



shaft, eccentrics, washers, brake, etc. are exactly like the 

 driving mechanism detailed in the description of the model 

 for the transvers wave motion. The length of the eccen- 

 trics K upon the shaft H is greater in this model than in 

 the other, chiefly because it is desirable to separate the 

 discs I farther than the discs in the former model described. 



The limit of motion of each eccentric being 20 degrees 

 with respect to the one next to it, the harmonic motions 

 of the discs I will differ by intervals of 20 degrees difference 

 of phase. The model therefore represents a longitudinal 

 or sound wave, and includes a wave length and a half. 

 But the phase differences could be made different by 

 choosing different angles between the pins in the eccen- 

 trics. The degree of compression and rarefaction in the 

 wave will depend on the "throw" of the eccentrics and 

 other proportions of the model. The chief usefulness of 

 this model, as of the other, consists in the simplicity of the 

 mechanism, and the perfect control which the operator 

 has over the motions. 



The models can be made with very meager shop facilities. 

 Anyone who sets about making them will easily apply 

 improvements in the devices. The contrivance of the 

 loose eccentrics on a shaft is probably susceptible of being 

 adapted to other illustrations of wave motion. 



A MODEL FOR COMPOUNDING SIMPLE HARMONIC MOTIONS. 



In teaching the properties of simple harmonic motion it 

 is desirable to show in an elementary manner how two 

 such motions, when compounded, will produce the beauti- 

 ful figures shown by the method of Lissajous. But when 

 the tuning forks are used the actual tracing of curves can- 

 not be watched. The resultant is all that can be shown. 

 In addition to the tuning forks of Lissajous and various 

 contrivances, employing pendulums, the stereopticon, and 

 the like, a contrivance is useful which will trace the figures 

 so slowly that their production may be watched by a whole 

 class, and which may be stopped, and started again at any 

 point to take up the tracing where it was stopped, without 

 spoiling the continuity or regularity of the curve. 



