228 The Realm of Nature CHAP. 



tion reached in the next second, the stroke is still more 

 oblique along a wider circle, and is more feeble on account 

 of loss of energy due to friction among the rock particles. 

 The distance of the centre of disturbance beneath the surface 

 may be calculated by observing the angle from which the 

 shock comes at different points and constructing a diagram 

 somewhat like the above. It appears from many observa- 

 tions recorded by Mallet and others that the depth of 

 origin rarely or never exceeds 35 miles. Although the 

 crust of the Earth is probably homogeneous at a consider- 

 able depth, it is very far from being so in its upper part, and 

 the earth-wave consequently travels at an unequal rate in 

 different directions as it nears the surface. A thick bed of 

 sand or loosely compacted and inelastic stones (S in Fig. 44) 

 greatly retards and may entirely absorb the wave by fric- 

 tion between the particles, so that no shock would be felt 

 on the surface, while houses built on the hard rock all round 

 would be shaken severely. On the other hand, a small 

 deposit of sand or alluvial soil occupying a shallow hollow 

 would be jarred by confused earth-waves from every side 

 and buildings on it damaged most severely. 



301. Earthquake Shocks. The area of the surface 

 shaken depends on the intensity of the original shock and the 

 nature of the Earth's crust at the place where it occurs. The 

 memorable earthquake that destroyed Lisbon in 1755 shook 

 a space four times as large as Europe, and probably made 

 the whole Earth tremble ; and that which damaged Charles- 

 town in 1886 was felt over 3,000,000 square miles, from 

 Cuba to Canada, and from Bermuda to the west of Missouri 

 State. By the use of delicate seismometers the dying 

 tremor of an earthquake-wave may be detected at a great 

 distance, beyond the limit of unaided observation. Thus 

 the tremor of an earthquake on the Italian Riviera in 1887 

 was distinctly recorded by instruments in Greenwich Obser- 

 vatory. The shaking of the Earth's crust throws down any 

 slenderly supported rock masses like perched blocks, natural 

 bridges, and earth pillars, and when such structures are 

 conspicuous features of the scenery the district may be 

 reckoned free from risk of serious shocks. Landslips, the 



