I 



EARTHQUAKE MOTION. 333 



N.N.E. line, it would appear infinite; on the E.S.E. line, 30 miles a 

 minute; on intermediate courses, intermediate velocities. The apparent 

 velocity of the wave at the surface will be greatest near the point 

 vertically situated over the disturbance. 1 



Mr. Mallet, 2 by careful experiments on the sands at Kingstown, 

 and in the granite rocks at Dalkey, obtained velocities of wave transit 

 as under : 



Feet per Second. Miles per Hour. 



Loose sand, Killiney . . . 825 .... 9^ 



Solid granite, Dalkey . . . 1,306 . . . . 15 

 Fissured granite, do. ... 1,665 J 9 



The velocity at the surface may be in these cases somewhat less 

 than that of sound in air, but is far less than was expected by Mr. 

 Mallet from his knowledge of the elasticity of stony media. To judge 

 by experiments on their elasticity, we might expect the sound wave 

 to travel 



Feet in a Second. | Feet in a Second. 



In air . . . 1,140 j In primary limestone . 6,696 



,, water . . . 4,700 i ,, carboniferous limestone 7,075 



,, lias .... 3,640 | ,, hard slate . . . 12,757 

 ,, coal sandstone . . 5,248 I ,, granite perhaps still higher rate. 

 oolite. . . . 5,723 ! 



Professor Milne finds that in Japan different earthquakes travel 

 with different velocities, and that the velocity of a wave decreases as 

 it travels. In 1881, earthquakes between Tokio and Yokohama moved 

 at rates per second of 4500 feet, 3900 feet, 1900 feet, 1400 feet, 2454 

 feet, and 2200 feet. 



Depths and Origin of Earthquakes. According to Mallet, the 

 depth of the point where the blow or concussion which is the origin 

 of earthquake movements take place may be as far down below the sur- 

 face as the versed sine of the arc cut off by the extreme points of the 

 space subject to tremor. In some cases this passes very deeply into 

 the earth. 



Origin of Earthquakes. Taking into account all the phenomena 

 of earthquakes, Mallet admits for consideration the following modes 

 of origin of the impulse : 3 



(i.) The operation of steam extricated by cooling from the spheroidal state. 



(2.) Evolution of steam through fissures, and its irregular condensation under 

 pressure of sea- water. 



(3.) Recoil from volcanic explosions. 



(4. ) Great fractures and dislocations in the earth's crust, suddenly produced 

 by pressure or contraction in any direction. 



Earthquake Motion. The actual movement of the ground, says 

 Professor Milne, is sometimes 8 millimetres, but often under i milli- 

 metre ; there are seldom more than two or three vibrations a second. 

 The motion of the ground towards the origin of the disturbance is 

 usually much greater than the motion away from it. 



Professor Ewing of Tokio finds that in almost every instance the 



1 Hopkins, Brit. Assoc. Reports, 1847. 2 Mallet, Rep. to Brit. Assoc. 1851-52. 

 3 Reports of Brit. Assoc., 1850. 



