Sound 



Space 



SCIENTIFIC SIDE-LIGHTS 



636 



in countries where the earth has been fre- 

 quently exposed to shocks. A striking and 

 unparalleled instance of uninterrupted sub- 

 terranean noise, unaccompanied by any trace 

 of an earthquake, is the phenomenon known 

 in the Mexican elevated plateaux by the 

 name of the " roaring and the subterranean 

 thunder " (bramidos y truenos subterraneosj 

 of Guanajuato. This celebrated and rich 

 mountain city lies far removed from any 

 active volcano. The noise began about mid- 

 night on the 9th of January, 1784, and con- 

 tinued for a month. I have been enabled 

 to give a circumstantial description of it 

 from the report of many witnesses, and 

 from the documents of the municipality, of 

 which I was allowed to make use. From 

 the 13th to the 16th of January it seemed 

 to the inhabitants as if heavy clouds lay 

 beneath their feet, from which issued al- 

 ternate slow rolling sounds and short, quick 

 claps of thunder. The noise abated as gradu- 

 ally as it had begun. It was limited to a 

 small space, and was not heard in a basaltic 

 district at the distance of a few miles. Al- 

 most all the inhabitants, in terror, left the 

 city, in which large masses of silver ingots 

 were stored; but the most courageous, and 

 those more accustomed to subterranean thun- 

 der, soon returned, in order to drive off the 

 bands of robbers who had attempted to 

 possess themselves of the treasures of the 

 city. Neither on the surface of the earth, 

 nor in mines 1,600 feet in depth, was the 

 slightest shock to be perceived. No similar 

 noise had ever before been heard on the 

 elevated table-land of Mexico, nor has this 

 terrific phenomenon since occurred there. 

 HUMBOLDT Cosmos, vol. i, p. 209. (H., 

 1897.) 



3152. SOUND LAGGING AFTER 

 SIGHT Ice-cliffs Seem to Fall in Silence 

 Rlow Reverberations, as of Distant Battle. 

 " The [Turner] glacier expands on entering 

 the water, as is the habit of all glaciers 

 of clear ice when unconfined, and ends in 

 magnificent ice - cliffs some two miles in 

 length. The water dashing against the bases 

 of the cliffs dissolves them away, and the 

 tide tends to raise and lower the expanded 

 ice-foot. The result of these agencies and 

 of the onward flow of the ice itself is to 

 cause huge masses, sometimes reaching from 

 summit to base of the cliffs, to topple over 

 into the sea with a tremendous crash. Owing 

 to the distance of the glacier from Haenke 

 Island, we could see the ice fall long before 

 the roar it caused reached our ears; the 

 cliffs separated and huge masses seemed to 

 sink into the sea without a sound ; the spray 

 thrdwn up as the blue pinnacles disappeared 

 ascended like gleaming rockets, sometimes 

 as high as the tops of the cliffs, and then 

 fell back in silent cataracts of foam. Then 

 a noise as of a cannonade came booming 

 across the waters and echoing from cliff 

 to cliff. The roar of the glacier continues 

 all day when the air is warm and the 



sun is bright, and is most pronounced when 

 the summer days are finest. Sometimes 

 roar succeeds roar like artillery fire, and 

 the salutes were answered, gun for gun, by 

 the great Hubbard glacier, which pours its 

 flood of ice into the fiord a few miles north- 

 east of where Turner glacier terminates." 

 RUSSELL Glaciers of North America, ch. 6, 

 p. 93. (G. & Co., 1897.) 



3153. SOUND MAY QUENCH SOUND 



Interference of Sound - waves Beats, 

 What They Are. The most familiar illus- 

 tration of the interference of sound-waves 

 is furnished by the beats produced by two 

 musical sounds slightly out of unison. When 

 two tuning-forks in perfect unison are agi- 

 tated together the two sounds flow without 

 roughness, as if they were but one. But 

 by attaching with wax to one of the forks 

 a little weight, we cause it to vibrate more 

 slowly than its neighbor. Suppose that one 

 of them performs 101 vibrations in the time 

 required by the other to perform 100, and 

 suppose that at starting the condensations 

 and rarefactions of both forks coincide. 

 At the 101st vibration of the quickest fork 

 they will again coincide, that fork at this 

 point having gained one whole vibration, or 

 one whole wave-length upon the other. But 

 a little reflection will make it clear that, 

 at the 50th vibration, the two forks are in 

 opposition; here the one tends to produce, 

 a condensation where the other tends to pro- 

 duce a rarefaction; by the united action of 

 the two forks, therefore, the sound is 

 quenched, and we have a pause of silence. 

 This occurs where one fork has gained half 

 a wave-length upon the other. At the 101st 

 vibration, as already stated, we have coin- 

 cidence, and therefore augmented sound; at 

 the 150th vibration we have again a quench- 

 ing of the sound. Here the one fork is three 

 half-waves in advance of the other. In gen- 

 eral terms, the waves conspire when the one 

 series is an even number of half-wave 

 lengths, and they destroy each other when 

 the one series is an odd number of half-wave 

 lengths in advance of the other. With two 

 forks so circumstanced, we obtain those in- 

 termittent shocks of sound, separated by 

 pauses of silence, to which we give the name 

 of beats. By a suitable arrangement, more- 

 over, it is possible to make one sound wholly 

 extinguish another. Along four distinct 

 lines, for example, the vibrations of the 

 two prongs of a tuning-fork completely blot 

 each other out. TYNDALL Lectures on Light, 

 lect. 2, p. 61. (A., 1898.) 



3154. SOUND, MINGLED WAVES OF 



Discriminating Power of the Ear. You 

 must conceive the air of a concert-hall or 

 ballroom as traversed in every direction, 

 and not merely on the surface, by a varie- 

 gated throng of intersecting wave-systems. 

 From the mouths of the male singers pro- 

 ceed waves of six to twelve feet in length; 

 from the lips of the songstresses dart shorter 



