Snell and Powell's Wave Machines
 The 1860 apparatus catalogue of Edward S. Ritchie of Boston describes the apparatus at the left as     "Snell's Improved Powell's Wave Instrument, for showing the Undulations of Light, in Plane, Elliptical and Circular Polarization. The frame is of mahogany, 24 inches long by 30 inches in height; twenty four white balls are supported upon slender steel rods, to which motion is communicated by an equal number of eccentrics placed upon a shaft within the frame, the balls being arranged to give two entire waves. By raising or depressing the sliding frame, which is sustained by springs, the balls may be made to move either in straight lines, ellipses or circles ...\$35.00"    This example is on display at the National Museum of American History of the Smithsonian Institution in Washington, D.C.
 Ebenezer Strong Snell (1801-1876) was a central figure in science education in New England. He was one of the three graduates of Amherst College in its first class of 1822, having transferred from Williams. After receiving an M.A. from Amherst in 1825, he taught mathematics and physics at Amherst until his death. Snell had a considerable flair for designing and constructing apparatus; the 1852/1870 list of apparatus at Amherst that he drew up contains numerous references to apparatus he built and used. Indeed, his lecture notes are often just lists of apparatus to be used for demonstrations, and the rest of the class followed from the demonstrations of the phenomena.
 The ultimate Powell and Snell wave machine is the Universal Wave Motion Apparatus made by the L. E. Knott Apparatus Co. of Boston, priced in the 1916 catalogue at \$65.00.     The form of the waves that are displayed is a function of the position of the sliding frame. When it is down, as shown in the picture, turning the crank will cause the balls on the top to demonstrate transverse waves and the balls on the top show longitudinal waves. Slide the frame up and the upper balls will trace out the circular or elliptical motions characteristic of particles on the surface of a water wave.     This apparatus is in regular use at the College of Wooster in Ohio.
 The mechanism of the Wooster machine can be seen below. The eccentric cams are circled by thin bands to which the vertical rods are attached.    At the right the machine is shown with the sliding fame pushed down, showing the helical arrangement of the spheres that illustrates the passage of a circularly polarized wave.

 This unmarked Snell and Powell wave machine is in regular use in lecture demonstrations at the University of Texas in Austin. The mechanism can be seen in the closeup below.