t 30 ] 



IV. On a neglected Principle that may he employed in Earth- 

 quake Measurements, By John Perry and W. E. x^yrton, 

 Professors in the Imperial College of Engineering, Japan*. 



[Plate IV.J 



SPECULATIONS regarding the internal constitution of 

 the earth have interested philosophers for many years. 

 For a long time it was considered that our globe consisted of 

 a thin solid shell containing a fluid core : but Hopkins, who 

 was one of the first to investigate the subject on correct prin- 

 ciples, showed that this shell must be from 800 to 1000 miles 

 in thickness; and .still more recently Sir Wm. Thomson has 

 proved that the apparent absence of elastic tides in the earth's 

 surface leads to the conclusion that the average rigidity of the 

 earth is greater than that of glass, and possibly even greater 

 than that of steel. We do not on the present occasion propose 

 to consider whether the state in Avhich the internal part of the 

 earth exists is like any state of matter with which we are ac- 

 quainted ; but this is, of course, a subject well worthy of very 

 careful investigation. 



It is probable that the earth was once in a molten condition 

 and that it now is cooling ; so that the shrinking resulting 

 from this cooling must develop vast internal forces, producing 

 strains, or deformations, of great magnitude. Other powerful 

 forces, too, brought into existence by water being suddenly 

 changed into steam on entering a hot cavity, by the sudden 

 chemical combination of gases, or possibly by elastic tides in 

 the earth's substance produced by the joint attractions of the 

 sun and moon — all tend to cause disturbances and ruptures 

 which are brought vividly to our notice by volcanos and 

 earthquakes. 



An earthquake has been defined by Mr. Mallet as "the 

 transit of a wave, or waves, of elastic compression in any di- 

 rection, from vertically upwards to horizontally, in any azi- 

 muth, through the crust and surface of the earth, from any 

 centre of impulse or from more than one, and which may be 

 attended with sound and tidal waves dependent upon the im- 

 pulse and upon circumstances of position as to sea and land." 

 If we could only read the earthquake message rightly, we 

 should learn all about the deformation going on in the earth's 

 crust ; for there is no doubt that the nature of the stresses 

 and strains, and every condition of the rocks at the origin of 

 motion, all give their character to the earthquake vibrations. 

 It must be remembered, however, that the message before it 



* Communicated "by the Authors, having been read before the Asiatic 

 Society of Japan on the 23rd of May, 1877. 



