EARTHQUAKE MOTION DISCUSSED THEORETICALLY. 51 



felt at a point distant from the origin of the shock would 

 be a backward and forward motion in the direction to and 

 from the origin, and then, a short interval afterwards, a 

 motion transversal, or at right angles to this, would be 

 experienced. 



From the mathematical theory of vibratory motions it 

 is possible to calculate the velocity with which a distur- 

 bance is propagated. As the result of these investigations 

 it has been shown that normal vibrations travel more 

 quickly than transverse vibrations. 



Deductions from experiments on small specimens are, 

 however, invalidated by the fact that the specimens used 

 for experiments are, of course, nearly homogeneous, whilst 

 the earthquake passes through a mass which is hetero- 

 geneous and more or less fissured. Mallet, by experiments 

 'on the compressibility of solid cubes of these rocks, 

 obtained the mean modulus of elasticity,' with the result 

 that 'nearly seven- eighths of the full velocity of wave- 

 transit due to the material, if solid and continuous, is lost 

 by reason of the heterogeneity and discontinuity of the 

 rocky masses as they are found piled together in nature.' 

 The full velocities of wave-transit, as calculated by Mallet 

 from a theorem given by Poisson, were — 



For slate and quartz transverse to lamination, 9,691 feet per second. 

 „ „ in line of lamina ion, 5,415 „ „ 



This more rapid transmission in a direction transverse 

 to the lamination, Mr. Mallet observes, may be more than 

 counterbalanced by the discontinuity of the mass trans- 

 verse to the same direction. 



The Intensity of an Earthquake. — The intensity of 

 an earthquake is best estimated by the intensity of the 

 forces which are brought to bear on bodies placed on the 

 earth's surface. These forces are evidently proportional 



