4 EARTHQUAKES. 



it would appear that the ground on which we dwell is 

 incessantly in a state of tremulous motion. 



A further subject of investigation which is before the 

 seismologist is the experimental verification of the exist- 

 ence of what may be called ' earth-pulsations.' These 

 are motions which mathematical physicists affirmed the 

 existence of. but which, in consequence of the slowness of 

 their period, have hitherto escaped observation. 



The oscillations, or slow changes in the relative posi- 

 tions of land and sea, might also be included ; but this 

 has already been taken up as a separate branch of geology. 



These four classes of movements are no doubt inter- 

 dependent, and seismology in the widest sense might 

 conveniently be employed to include them all. In suc- 

 ceeding chapters we wdll endeavour to indicate how far 

 the first three of these branches have been prosecuted, 

 and to point out that which remains to be accomplished. 

 It is difficult, however, to form a just estimate of the 

 amount of seismological work which has been done, in con- 

 sequence of the scattered and uncertain nature of many 

 of the records. Seismology, as a science, originated late, 

 chiefly owing to the facts that centres of civilisation are 

 seldom in the most disturbed regions, and that earthquake- 

 shaken countries are widely separated from each other. 



As every portion of the habitable globe appears to 

 have been shaken more or less by earthquakes, and as 

 these phenomena are so terrible in their nature, we can 

 readily understand why seismological literature is ex- 

 tensive. In the annals of almost every country which has 

 a written history, references are made to seismic disturb- 

 ances. 



An idea of the attention which earthquakes have re- 

 ceived may be gathered from the fact that Professor Alexis 

 Perrey, of Dijon, who has published some sixty memoirs 



