36 IOWA ACADEMY OF SCIENCES. 



be provided for by varyinj^ the proportions of the parts. 

 It is desirable in practice to include somewhat more than 

 one wave length. This model includes one and a half. 

 The front of the model is painted black so as to set off, 

 with sufficient contrast, the discs, which are white. 



A MODEL ILLUSTRATING THE LONGITUDINAL OR SOUND WAVE 



This model is about 13 inches wide by 36 inches long. At 

 its back is a five inch board E, Fig. 5, set on edge and 

 extending nearly the whole length of the model. Driven 

 into the upper edge of this board at intervals of one inch 

 are 27 straight, smooth wires, or small rods, R, two inches 

 long, inclined to the left, say, at about 45 degrees from the 

 vertical. Another 5 inch board M stands on edge about 

 the middle line of the model. To the edge of this board 

 are fastened thin strips, inclined, so as to form 27 slots 

 leaning to the right about 45 degrees. In a third board F 

 standing on edge at the front of the model is a horizontal 

 slot 30 inches long, 5 inches from the base. A small rod or 

 stout wire E I, Fig. 5, bent so as to have an eye or loop E 

 at one end and carrying a half inch disc at the other end 

 I, passes through the horizontal slot in F, and through an 

 inclined slot B, while the loop E encircles an inclined rod 

 K. A light spiral spring G fastened to the side of the board 

 so as to have the same angle of inclination as R, produces 

 gentle tension on the rod against or toward the upper 

 edge of the board. There are 27 rods like EI. It will be 

 seen that if a rod be grasped between E and B and raised, 

 the loop E will swing toward the left as it rises along R 

 while its middle point will move to the right in moving up 

 in the slot B, which leans to the right. I will therefore 

 move toward the right, as viewed from the front. On letting 

 the rod drop, motions in the opposite direction will take 

 place at E and B; and I will move back toward the left. 

 If now a point on the rod between E and B describe simple 

 harmonic motion up and down, I will describe the same kind 

 of motion horizontally. The up and down motion of the 27 

 rods is produced by a shaft H with 27 eccentrics K under 

 the rods and between the upright boards E and M. The 



