482 Pi^of. John Milne — On Earth Movements. 



turbance of eqnilibrmm, proving that slow but ceaseless changes are 

 for ever taking place beneath onr feet. However minute then these 

 earth-pulsations are, they deserve to be recorded by the cosmologist, 

 with the same care that the biologist bestows upon the movements 

 of living organisms, and we trust that Prof. Milne's efforts to awaken 

 an interest among the physicists and geologists of this country'' in 

 these researches may be crowned with success. — Edit. Geol. Mag. 



Earth-Movements. 



By Professor John Milne, F.G.S., 



Imperial College of Engineering, Tokio, Japan. 



EECENT investigations appear to have shown that terra finna 

 is a phrase indicating a condition of our solid earth which 

 scientifically has no existence. The crust of the earth is in a state 

 of constant movement. When these movements are sudden and 

 violent, we call them earthquakes ; transient shiverings, which 

 require the aid of instruments to make them visible, we call earth 

 tremors ; movements of longer period and greater amplitude, causing 

 large areas, like islands and continents, to rise and fall like rafts 

 upon the ocean, are called oscillations. To express the study of the 

 whole of these Earth-movements it would here be convenient to 

 have a special term. Seismology, or the study of earthquakes, is 

 principally indebted for its present scientific aspect to the labours 

 of Mr. Eobert Mallet, who, in his classical work on the Neapolitan 

 earthquake, and in his numerous other writings, showed the world 

 that earthquake phenomena, when properly questioned, could give 

 solutions to various problems respecting the physics of the globe, 

 which previously had not been thought of. In the methods em- 

 ployed by him, working tools were left for all succeeding seismo- 

 logists. Before his time earthquake literature was little more than 

 the records of strange occurrences and dreadful calamities ; but in 

 his hands seismology became a science. So much having been ac- 

 complished by an Englishman for the first and most conspicuous 

 branch of the study of Earth-movements, it ceiiainly devolves on 

 us vigorously to prosecute the remaining, and what are probably 

 the more important, branches of this study. 



We have learnt that earthquakes are the result of a complexity 

 of causes. Principal causes are usually so modified by secondary 

 ones that it is difficult and often impossible to assign definite reasons 

 for these terrible phenomena. Earthquakes are to our continents 

 what storms are to the ocean ; in both we have phenomena which 

 attract attention by their appalling nature. They are anomalies in 

 nature's operations which come upon us suddenly and are foretold 

 with difficulty. In the ocean, however, we have phenomena which 

 are more regular and universal in their working than the storms, 

 and which can be predicted with certainty. These are the tides. 

 In these motions of the earth's crust it is very probable we have 

 phenomena which are analogous to tides as well as storms, and 

 which, like the tides, are to be classed among the law-abiding pheno- 

 mena of nature's operations. These are tremors and oscillations. 



