ON THE EARTHQUAKE AND VOLCAMC rHEJsOMEXA OF JAPAA'. 177 



In the case of tlie first three of these examples, their hodograpliic 

 character may be due to the fact that observations were made in epifocal 

 areas, within which disturbances radiating from a centrum were recorded ; 

 but in the last example this character has been lost, because most of the 

 times which were noted probably refer to the arrival of a surface disturb- 

 ance capable of being felt, and which might have been recorded by ordinary 

 seismograplis. 



These latter records are therefore such that we could not expect them 

 to conform with the hypothesis under consideration, and until a number 

 of stations separated by long distances are provided with instruments 

 capable of recording minute tremors which may go through the earth. 

 Until these have been established, it would seem that the confirmation of 

 the attractive theory put forward by Dr. Schmidt must lemain in abeyance. 



3. A Suggestion that there are three Classes of Jfovement. 



The last hypothesis is one that takes into consideration three classes of 

 movements wliich immediately round an epicentre are hopelessly confused. 

 These are the truly elastic disturbances which from a focus reach the 

 surface of the earth along rectilinear or curvilinear paths, forced displace- 

 ments, and quasi-elastic waves, causing tumultuous movements in the 

 centre of a megistoseismic area, and long undulatory elastic-gravity waves 

 which are propagated over the surface of the earth. 



The escape of energy is most pronounced along the paths of least 

 resistance, that is round the seismic vertical to an epifocal area, and then 

 radially over the surface of the globe. The rate of propagation of the 

 surface waves seems to be about "J or 3 km. per second, and it may be 

 fairly constant. The minute tremors which have been observed at 

 stations 6,000 miles distant from their originating cause, if they travelled 

 through the superficial crust of the earth they did so at a rate of perhaps 

 12 km. per second, while if they were created on the passage, their 

 velocity, which is increased, becomes more abnormal. Assuming that they 

 came as condensational waves tln'oiigh the earth, then their velocity 

 is reduced to 8 or 10 km. per second, a quantity which, as suggested 

 by Dr. E. von Rebeur-Paschwitz, may possibly throw new light upon the- 

 nature of materials constituting the interior of our earth. At present 

 the facts bearing upon this latter question are both few and imperfect. To 

 confirm or dispel the important conclusions indicated by the few facts at 

 our disposal, it would seem desirable that investigations should be extended 

 in such a manner that the results obtained by different observers would be 

 comparable. With a set of stations situated round the globe at intervals 

 of 1.^)° or 30° apart, provided with instruments similar in character,, 

 similarly installed and similarly worked, which are capable of recording 

 not simply small changes in level but also minute vibrations, we might 

 easily extend our present knowledge, not simply respecting the propaga- 

 tion of surface undulations, but possibly of motion transmitted through 

 the rigid globe. An indication of the latter phenomenon would be an 

 enormous increase in the apparent velocity of a disturbance as it approached 

 the antipodes of its origin, while the concentration of energy in such a 

 region would suggest internal refraction. 



< )ther phenomena which might be recorded would be the diurnal and 

 longer period wanderings of the instruments, local eai'thquakes and eartli 

 tremors. The latter, although important in themselves, because they so 

 1895. N 



