146 



KNOWLEDGE. 



[AuGTJST 1, 1892. 



planet. However, those •who live in a region seldom visited 

 by earthquakes are apt to overlook the importance of the 

 subject. " Out of sight is out of mind," but as new 

 methods of investigating and recording earth-tremors, or 

 throbs, are invented, these things are brought more pro- 

 minently before us. Seismology, or the study of earth- 

 quakes, has lately been making great advances, and has 

 revealed slight movements, the existence of which was 

 previously unsuspected. 



In ^^ew of these additions to our knowledge of an im- 

 portant branch of natural science, we propose to say a few 

 words on earthquakes and earth-tremors of all kinds, 

 dividing the subject under three heads : (1) IVlmt they are, 

 (2) Wliat tJic;i (hi, and (3) Ihm' tlieij are caused. 



An earthquake has been defined by a high authority, 

 the late Mr. Louis Mallet, as a wave or series of waves, of 

 elastic compression, through the crust of the earth, in any 

 direction, and from any given "centre of impulse." To 

 understand this definition, think of what takes place when 

 a stone is thrown into a pool. A disturbance is made at 

 the place where the stone strikes the water ; that spot 

 corresponds to the " centre of impulse," the particles there 

 communicating the movement to those next them, and 

 these in their turn to others, and so on. In this way a 

 series of concentric waves is produced, which get fainter 

 and fainter, until finally they reach the edge of the pool. 

 This is very similar to what happens when a sub- 

 terranean disturbance gives a blow to the earth's crust, 

 and a series of earthquake waves is produced from some 

 seismic centre. But, in both cases, the waves really travel 

 in spherical shells. (See figure.) These waves, be it re- 



A, Centre of impulse ; 1, 2, 3, 4, 5, 6, Waves ; A C B, A D B, Angles 

 of emergence. 



membered, are due to wave-motion, like waves of sound, 

 and are by no means waves of translation. Each particle 

 of earth merely moves as the ears of corn move in a 

 field when they bend to the wind, and produce waves 

 which travel across the field before the wind. It is 

 clear that the undulatory movements due to an earth- 

 quake shock must strike the surface of the earth at 

 difl'erent angles, according to the distance of the seismic 

 centre. Thus, a person who might liappen at the time to 

 be standing on the spot B in a vertical line above such a 

 centre would feel an up and down movement, and a block 

 of stone lying near him might be thrown straight up in the 

 air ; but if the person were some miles away from this spot, 

 it is evident that the waves coming sideways would strike 

 him to the ground he stands on obliquely ; and " the angle 

 of emergence " becomes less and less the further we recede 

 from the spot lying just over such a centre. Now, it is 

 possible, by observing the effects of earthquakes on 

 buildings, to determine the direction in which the shock 

 arrived, and to calculate the " angle of emergence." This 

 is done chiefly by studying the cracks produced in buildings, 

 and makmg allowance for the circumstances of each case. 

 If, then, this angle can be ascertained for two places, and 

 the distance between them is known, a triangle is obtained, 

 the base of which is known, and the angles at the base ; 

 hence it is easy to calculate the depth of the centre of 

 disturbance. Such calculations have been made in 



several cases, and the results arrived at are of considerable 

 interest ; for they tell us that in no case is the seismic 

 centre at a greater depth than thirty miles. In some 

 cases the earthquake has been found to have originated 

 at a much less depth. If these results are trustworthy, as 

 there is reason to think they are, the conclusion is that 

 earthquake phenomena are not connected with the deeper- 

 seated portions of the mass of the globe, but with those 

 superficial portions commonly included in the earth's crust ; 

 and probably with the stratified series of rocks and their 

 associated volcanic and plutonic rocks, rather than with 

 the original mass which we believe to have solidified from 

 a molten and highly-heated state. 



Earthquake waves can be measured, and it is found that 

 they are quite small, haAing amplitude of perhaps only 

 a few inches. The crust of the earth vibrating in response 

 to a seismic blow may be compared to a big bell resounding 

 after its inner surface has been struck by the clapper. In 

 either case the amplitude of the vibrations is capable of 

 measurement, but the undulatory movements are not 

 visible as we look at a sounding bell, though a marble 

 suspended by a string and allowed to touch the bell's 

 rim would at once demonstrate their existence by 

 oscillating to and fro. The reboimding marble aptly 

 illustrates the case of a block of stone being hurled up into 

 the air by an earthquake wave. It is known that sound 

 travels with different velocities through different substances, 

 according to their compactness and elasticity. Hence we 

 need not be surprised to learn that earthquake shocks 

 have sopietimes been heard twice ; once through the solid 

 rock, and so up to the ear, and again through the air wliich 

 transmits the waves more slowly. Mr. Mallet made some 

 interesting experiments on the velocity of transmission of 

 waves due to a blow, through difl'erent substances. 



In air the mean velocity of sound-waves is 1138 feet per 

 second, but it varies with the atmospheric temperature and 

 pressure — in water 4692 feet, and in a bar of iron 11,040 

 feet per second. The movements of the gromid during an 

 earthquake are of a complicated character. In addition to 

 the two kinds of movement which have generally been 

 observed — namely, the upward shock, and the long 

 undulations, spreading in all directions, like marine waves 

 — most authorities have added a rotating or gyratory 

 movement. This causes a twisting of the ground, which 

 has not only been seen but felt. Humboldt says that in 

 Chili three great palm trees were seen to twist round one 

 another like willow-wands, after each had swept a small 

 space round its trunk. Pinnacles of buildings have likewise 

 been found to be twisted. The noise accompanying an 

 earthquake often resembles that of an explosion. Since 

 the velocity is afl'ected by the hardness of the rocks, it 

 follows tliat strata containing any hollows partly break and 

 check the waves, as stakes driven into a shore break the 

 force of sea- waves in a storm. Hence we find that the 

 early (rreeks and Romans dug wells to fortify some of their 

 cities, and prevent their complete destruction. In South 

 America the natives have long ago adopted the same plan. 

 Springs and natural underground passages for this reason 

 afford considerable protection to cities which are liable to 

 be visited by earthquakes. 



Much has of late years been learned regarding seismic 

 disturbances by taking observations which give the 

 direction of the wave or waves, its velocity (obtained from 

 the exact time at which it reached difl'erent places), and tlie 

 " angle of emergence " as previously explained. The 

 results are mapped out, and thus an " earthquake chart" 

 is made, somewhat resembling the "weather charts" 

 published daily in the Times newspaper. This done, it is 

 invariably found that the greatest destruction is eft'ected 



