THE EARTH'S INTERIOR— ADAMS 



259 



One variety travels along the surface and is responsible for the damage 

 caused by large earthquakes. More important for the present purpose 

 are the two varieties that pass through the body of the earth. One 

 of these "through-waves" consists of longitudinal vibrations, analogous 

 to ordinary sound waves in air; the other consists of transverse vibra- 

 tions, more nearly akin to light waves. Their formation is in accord 

 with the conclusion from the theory of elasticity that any disturbance 

 in an elastic isotropic material should give rise to the two kinds of 

 vibrations, traveling with velocities depending only on the density 

 and elastic constants of the material at each point. Earthquake 

 waves, which have passed through the earth, are recorded by the 



200 400 600 800 1000 1200 1400 1600 1800 2000 2200 



Depth From Surface in Kilometers. 



Figure 1.— Velocity-depth curve (after Repetti). Such curves supply direct evidence concerning the 



nature of the earth's interior. 



delicate instruments of the seismologist, who is able to distinguish 

 the several types of waves and to tell with high precision their time of 

 arrival at various stations. From the observations we may construct 

 for each kind of wave a "travel time" -distance curve, which shows for 

 any distance the time required for the vibrations to pass from the earth- 

 quake center to the recording instrument. The mathematician is 

 now called upon. After subjecting the curve to a remarkable and 

 intricate mathematical analysis he finds the shape of the path along 

 which the wave travels — it tarns out to be curved — and finds also the 

 velocity at each point of its path, that is, the velocity at the various 

 depths to which the wave has penetrated in its journey through the 

 earth from focus to station. The recent careful determinations of 

 Repetti ^ are shown in figure 1. The particular value of this velocity- 



» Repetti, Wm. C, Dissertation, St. Louis University. Printed in Manila, 1930. 



