THE PHONOGRAPH^ OR VOICE-RECORDER. 279 



When we take a circular plate of glass, clamped at the 

 middle, and touching one part of its edge with the finger, 

 draw the bow across a point of the edge hah a quadrant from 

 the finger, we see the sand arrange itself along two diameters 

 intersecting at right angles. If the bow is drawn at a point 

 one-third a quadrant from the finger-clamped point, we get 

 a six-pointed star. If the bow is drawn at a point a fourth 

 of a quadrant from the finger-clamped point, we get an eight- 

 pointed star. And so we can get the sand to arrange itself 

 into a star of any even number of points ; that is, we can get 

 a star of four, six, eight, ten, twelve, etc., points, but not of 

 three, five, seven, etc. 



In these cases the centre of the plate or disc has been 

 fixed. If, instead, the plate or disc be fixed by a clip at the 

 edge, or clamped elsewhere than at the centre, we find the 

 sand arranging itself into other forms, in which the centre 

 may or may not appear ; that is, the centre may or may not 

 be nodal, according to circumstances. 



A curious effect is produced if very fine powder be strewn 

 along with the sand over the plate. For it is found that the 

 dust gathers, not where the nodes or places of no vibration 

 lie, but where the motion is greatest Faraday assigns as the 

 cause of this peculiarity the circumstance that " the ligh f 

 powder is entangled by the little whirlwinds of air produced 

 by the vibrations of the plate ; it cannot escape from the 

 little cyclones, though the heavier sand particles are readily 

 driven through them ; when, therefore, the motion ceases, 

 the light powder settles down in heaps at the places where 

 the vibration was a maximum." In proof of this theory we 

 have the fact that " in vacuo no such effect is produced ; all 

 powders light and heavy move to the nodal lines." (Tyndall 

 on "Sound.") 



Now if we consider the meaning of such results as these, 

 we shall begin to recognize the perplexing but also instructive 

 character of the evidence derived from the telephone, and 

 applied to the construction of the phonograph. It appears 

 that when a disc is vibrating under such special conditions 



