'^ I 



260 



NATURE 



[August 25, 1910 



prove, therefore, how it is that the " head " is destroyed, 

 and explain why the powder issues from an orifice at a 

 uniform rate. 



Lord Rayleigh applied this principle to a very interest- 

 ing device, which he used here some years ago, for the 

 purpose of slowly rotating a smoked disc. \ weight stood 

 upon a sand column contained in a glass tube. Its down- 

 ward motion as the column lowered, due to escape of 

 powder from a nozzle at the end, served to operate a 

 train of wheels. The question arises, however, as to 

 whether such a motion is quite uniform. In other words, 

 does the sand move regularly in the tube? Experiments 



indicate that it is very difficult to obtain an absolutely 

 uniform motion by this means. Friction appears to be 

 the controlling factor. A tube, oiled upon its inner 

 surface, is now filled. On freeing the nozzle, you see that 

 the sand moves out by slow regular jerks. Certain curious 

 rattling sounds, emitted occasionally by the column 

 descending in a glass tube, also drew attention to the 

 intermittent motion of the grains. 



It seemed reasonable to hope, therefore, that this might 

 be made sufficiently rapid and regular to give rise to a 

 musical note. 



Now many strange noises have been heard in the neigh- 

 bourhood of large sand masses when surface layers have 

 been disturbed by someone walking over them ; and there 

 are curious shrieking sands — rarely met with upon the 

 coast. ■ 



Thanks to the great kindness of Mr. Carus-Wilson, 

 whose work in this direction is so well known, I am able 

 to exhibit a remarkable specimen of sand from the Isle 

 of Eigg, in the Hebrides. When a plunger strikes down 

 upon the grains contained in a suitable cup, you hear a 



^^^ 



>>>■ 

 ^^> 



-.\bnonnal Piling. 



i'H-. 9.— Normal Piling. 



piercing musical sound. Mr. Carus-Wilson attributes this 

 to the friction between the particles, the effect being pro- 

 duced in much the same manner as that which results 

 from gently rubbing an agate style upon glass. He has 

 discovered musical sand in Poole Harbour, as well as at 

 other places. 



The essential conditions for the production of this sound 

 are : — 



(i) That the grains be nearly of the same size and 

 rounded. 



NO. 2130, VOL. 84] 



(2) That they be clean and free from adherent fine dust. 



(3) That the vessel in which they are struck have 

 sloping sides, and be made of a suitable material. 



But to return to the question of obtaining musical sounds 

 from ordinary sand. 



There stands, fixed to the wall, a large glass-fronted 

 section of a tube. It is filled with alternate bands of 

 white and black sand, the latter being about one-sixth as 

 deep as the former. .An outlet is provided at the bottom. 

 This arrangement enables the motion of the different por- 

 tions of the sand column to be observed while the powder 

 issues from the orifice. 



On freeing the nozzle, we see that the centre of the 

 lowest black band immediately falls, and that, as the 

 sand continues to escape, successive bands become similarly 

 deformed. It is clear that the grains from the central 

 part of the column are moving rapidly downward, and, 

 since no eddies can form in the remainder, the whole 

 becomes divided into a core of moving particles and a 

 large surrounding mass of dead sand (Fig. ii). 



The diminished density of the axial region releases the 

 lateral pressure upon the sides of the tube, and the upper 

 part of the column suddenly slips until the grains again 

 pack and seize as before. 



Now if sand of a suitable fineness be slowly passed in 

 this manner through a glass tube of correct dimensions, a 

 musical note may be produced. 



The tube should be about i inch in diameter, and filled 

 with sand resembling that found in the Charlton pits. 

 The length of the one now ready is 3 feet. When the flow 

 begins, a curious 

 rattling sound is 

 heard, w h i c h 

 finally changes to 

 a distinct musical 

 note. It may b^' 

 varied slightly, 

 say to the extent 

 of a whole tone 

 or so, by gripping 

 a part of the tube 

 while the sand 

 pours out. The 

 two upper dark 

 bands (Fig. n) 

 have not beconT- 

 deformed, except 

 slightly at their 

 ends, owing to 

 friction between 

 the sand and tubn. 

 It is essential for 

 the production ol 

 musical sounds 

 that the ratio of the length of a column to its diameter 

 be such that the upper portion moves downward without 

 central deformation. In order to explain the cause of the 

 sound, we must therefore consider the motion of this 

 more or less compact bodv of particles. 



Now, if the lower half of the tube be filled with mercury 

 and the rest with well-packed sand, the regular lowering 

 of the liquid causes the granular piston apparently to 

 stietch until its extension is about 2 per cent, of its 

 original length. It is not until that point is reached that 

 the upper layers begin to move downward. The particles, 

 however, are no longer normally piled. A further slight 

 movement of the lower layers causes the upper ones to 

 follow and to overrun a little (owing to their momentum). 

 Therefore, even if the mercury is adjusted to pour out 

 uniformly from the orifice, the upper part of the sand 

 column moves downward with an intermittent motion, 

 analogous, in fact, to that of a weight drawn over a 

 rough surface by an elastic string. It is also clear that, 

 within wide limits, the motion of the upper layers may 

 b>" independent of, or completely out of phase with, that 

 of the lower ones, and still produce a musical note. 



The glass wall of the tube is thrown into violent vibra- 

 tion bythe intermittent rise and fall of the lateral pressure 

 upon it, so that damping the barrel raises the pitch of 

 I he note. The greater part of the sound is due, however, 

 •n the direct action of the sand column upon the air above 

 it, for even a tight wrapping of tape but slightly affects its 



