48 EAKTHQUAKES. 



in which they are produced, we should expect to find 

 waves of compression and extension. 



3. A truly spherical cavity is suddenly formed by the 

 explosion of steam in the midst of an elastic medium. 

 In this case all the waves will be those of compression, 

 each particle moving backward and forward along a radius. 



Should the cavity, instead of being truly spherical, be 

 irregular, it is evident that, in addition to the normal 

 vibration of compression, transverse waves of distortion 

 will be more or less pronounced, depending upon the 

 nature of the cavity. 



The combination of these two sets of vibrations may 

 cause a point in the earth to move in a circle, an ellipse, 

 the form of a figure eight, and in other curves similar to 

 these, which are produced by apparatus designed to show 

 the combination of harmonic motion. From these ex- 

 amples it will be seen that we have therefore to consider 

 two kinds of vibrations — one produced by compression or 

 the alteration of volume, and the other produced by an 

 alteration in shape. 



Now the resistance which a body offers, either to a 

 change in its volume or in its shape, is called its elasticity, 

 and the law which governs the backward and forward motion 

 of a particle under the influence of this elasticity may be 

 expressed as follows : 



If T be the time of vibration, or the time taken by a 

 particle to make one complete backward and forward swing, 

 D the density of the material of which this particle forms a 

 part, and E the proper modulus of elasticity of the mate- 

 rial, then, 



From this formula, t = 2 tt ^ 5 , we see that the 



