158 



DISCOVERY 



Now as to the origin of these regular vibrations 

 from sand grains : If we draw the point of a pen 

 vertically over the surface of a piece of glass, friction 

 puts a drag on it and tends to stop its progress. But 

 the point meets the increasing obstruction by finally 

 jumping over it, so that it progresses by means of a 

 series of regular " running jumps," so to speak, and 

 these give rise to the regular vibrations that produce 

 the squeaks emitted. 



If we increase the number of steel points we increase 

 the volume of sound. 



Now let us suppose that a number of sand grains, 

 complying with the requisite conditions, rub against 

 each other when in motion, and produce a number of 

 vibrations of equal length. They would then produce 

 a musical note. 



The quality and intensity of the notes emitted by 

 musical sands depend largely upon the shape and 

 composition of the receptacles in which they are 

 tested. A porcelain cup gives good results, but 

 when the same sample is similarly treated in a flower- 

 pot, a cardboard box, or a rubber vessel, it is prac- 

 tically mute. Pour it back into the porcelain cup 

 and it is as musical as ever ! We may assume that 

 these remarkable results are due to the fact that in 

 experimenting with small quantities of sand their 

 efforts to produce music need assistance. This they 

 obtain when the grains move freely against the sides 

 of glazed porcelain or smooth wooden vessels. They 

 possess greater mobility in such vessels. In the case 

 of a flower-pot, box, and rubber receptacle, these 

 encouraging factors are absent. Compression, too, 

 produces compaction, and therefore loss of coherency 

 in the sand. As the flower-pot is smaller at the bottom 

 than at the top, the sand is compressed when struck, 

 thus depriving the grains of the freedom to move 

 one over the other. 



Coherency is also destroyed by wetting. Sand that 

 is quite musical when dry may be rendered mute by 

 wetting. 



In regard to the plungers : Some are good for the 

 purpose required and others bad. A beech-wood 

 ninepin, a deal paint-brush handle, and a glass pestle, 

 are good. Rubber, cork, and lead are bad. I call 

 these positive and negative plungers. Some cylinders, 

 such as glass and metal, are positive, and some negative 

 plungers will give feeble results on a fresh patch of 

 musical sand in situ. 



A positive plunger may be rendered negative by 

 covering the striking end with rubber. A negative 

 plunger may sometimes be rendered positive by what 

 I call " coaxing." Thus, a negative cork will become 

 positive if we screw a porcelain knob into one end. 

 Also, a negative cork may produce positive results if 

 fixed in the end of a piece of one-inch iron piping. 



I have stated elsewhere that the pitch of the note 

 produced by a plunger is determined by its bulk and 

 the area of its striking surface. The striking surface 

 of a beech-wood plunger nearly an inch in diameter, 

 when plunged into the sand in a porcelain cup, emits 

 a note of a certain pitch. A smaller plunger, having 

 a striking surface of about half an inch in diameter, 

 gives a note of much higher pitch. 



With a rolling-pin and a large basin of musical 

 sand we can produce a deep note like the baying of a 

 large dog, and with a lead pencil a note like a shrill 

 whistle.' 



But it is important to observe that the pitch of a 

 note may be varied at will by increasing or decreasing 

 the bulk of the plunger. If we screw a brass knob on 

 to the upper end of a plunger, the pitch is considerably 

 lowered, though the area of the striking surface is 

 unaltered. 



I believe that the positive plungers \'ibrate in 

 sympathy with the vibrations produced by the sand 

 grains, and, acting as resonators and intensifiers, 

 are responsible for the greater part of the sound we 

 hear. The late Professor Poynting, who took con- 

 siderable interest in some of these experiments, thought 

 so too. 



Thus the pitch of a note does not depend only 

 upon the area of the striking surface, but rather upon 

 the size or bulk of the plunger, other things being 

 equal. 



A large plunger vibrates less rapidly than a small 

 one, and so produces lower notes. We may use a 

 plunger which, if vibrating, would always emit a 

 constant note. At one end we may have a large 

 surface, and at the other end a small one. But 

 whichever end is used for striking the sand, the 

 pitch of the note remains the same. A plunger 

 made to fit together in two parts acts in the same 

 way. 



Another curious fact is that if we take two positive 

 plungers, gi\'ing a low note and high note respectively, 

 and strike the sand with both simultaneously, no 

 note is produced. 



This is probably due to interference setting up 

 general confusion in the vibrations, and so destroying 

 the usual effect, which appears to suggest that it is 

 the plungers rather than the receptacles which act 

 as resonators. 



1 See Nature, July 15, 1909. 



Wirdcss Telegraphy and Telephony. By H. M. 



DowsETT. (Wireless Press, 9s.) 

 Practical Zoology. By Pkof. Marshall and Dk. Hirst. 



9th Edition, revised by Prof. F. W. Gamble, 



F.R.S. (John Murray, 12s. net.) 



