16 On C. S. Lyman's new form of Wave -apparatus. 



successively, each in its turn. The relative position of the cranks 

 of each horizontal series is such that the directions of any two, in 



regular order, differ by the same fraction of a whole revolution 

 that the distance between their axes is of a whole wave-length. 

 Thus in the apparatus the wave-length is supposed to be divided 

 into eight equal parts ; and hence the common difference between 

 the directions of adjacent crank-arms is one-eighth of a circle, 

 as shown in the figure. The cranks in each vertical set have 

 their positions always alike. The number of cranks, whether 

 taken horizontally or vertically, is arbitrary — a matter of conve- 

 nience in construction. The synchronous revolution of the 

 cranks is effected by means of any suitable mechanism — such as 

 equal toothed wheels on the several axes, with alternate idle 

 wheels connecting them ; or equal rag-wheels, with endless chain 

 or metallic ribbon ; or equal cranks, with a rigid connecting 

 frame or plate. The first method is used in the original machine, 

 the third in the model for the Patent-Office, the second and third 

 in the larger and smaller sizes respectively for the market. 



The crank-pins represent as many liquid particles, the circles 

 on the background their orbits. The transverse wires represent 

 continuous lines of particles, which at rest would be horizontal, 

 and be represented by the lines on the background drawn just 

 below the centres of the orbits — the upper one of these being 

 the snrface-line, the lower a line of particles one-ninth of a 

 wave's length down. The upright wires represent lines of par- 

 ticles which at rest would be vertical. Every point in these mo- 

 ving lines describes its own distinct orbit. 



The apparatus is constructed to a scale, and so represents a 

 wave of given length, height, and period ; but equally represents 

 also a wave of any other length and proportionate height, though 

 of period proper to its length, according to the law of that rela- 

 tion, as stated further on. In the original instrument, for ex- 

 ample, the wave-length is 36 inches, height from trough to 

 crest 4 inches, and period for that length 076 second ; but it 



