February i i, 1892] 



NA TURE 



347 



discovery. To quote his own words : " The field is wide i foot square. A line AB, f inch in length should then be 

 and completely unexplored, and at every step a new truth j drawn near the centre and a circle described about it, 



is gleaned, a novel fact observed." 



WA VE- MOTION MODEL. 



AS a teacher of Physics I have always experienced 

 considerable difficulty in giving to elementary 

 students of Sound a clear conception of the motion of 

 the air in organ-pipes when sounding. In Weinhold's 

 " Physics " a method is shown in which a series of 

 sinuous lines, drawn on a sheet of paper, exhibit this 

 motion when drawn across a narrow slit, but the difficulty 

 attending the drawing of these lines has, I imagine, pre- 



half of which should then be divided as shown into 12 



equal parts. Perpendiculars should then be dropped 

 upon the line AB, which is thus divided in harmonical 

 progression in the points i, 2, 3 . . . . 13. With the 



eluded its general adoption for class purposes. It struck 

 me that it ought to be possible to draw a series of 

 eccentric circles upon a disk in such a way that, when 

 rotated, the motion of the intercepted lines, as seen 

 through a narrow radial slit, should correctly represent 

 this motion. This, of course, is done for progressive 

 waves by Crova's disk. After spending some thought 

 upon the matter I succeeded in producing such a disk, 

 a copy of which I inclose. It has given such satisfaction 

 that I have been advised by several scientific friends to 

 send a description of the method to you for publication, 

 for the benefit of teachers and students generally. 



In the following description I have given the dimen- 

 sions which I myself employ in describing these disks, 

 but they can of course be varied at will : — 



A piece of stout cardboard should be taken about a 

 NO. 1163, VOL. 45] 



points I, 2, 3 . . . . 13, 12, II, 10, 9, 8, 7 successively as 

 centres, a series of circles should then be drawn be- 

 ginning with a radius of i:|^ inch, and increasing it each 

 time by ^f,^ inch. The last circle therefore, described 

 with the point 7 as centre, has a radius of \% inch. The 

 two circles described with the point 7 as centre, since 

 they represent nodes, should be drawn rather thicker than 

 the others to distinguish them. 



The disk is now complete. It should be cut circular 

 in shape and mounted to rotate upon a pin struck through 

 the point 7. If it now be examined by means of a narrow 

 radial slit extending across the marked portion of the disk, 

 the short lines intercepted will, by their pendulum-like 

 motions, represent the motion of the air particles in a 

 closed organ pipe giving its first overtone. When the 

 slit is shortened so as to show only the portion of the 



