MODERN STUDIES OF EARTHQUAKES. 365 



of the shocks. It has cast much doubt on speculations as to the 

 seasons in which earthquakes are more or less frequent; and it has 

 demonstrated the inadequacy of former methods of determining the 

 central focus. It has furthermore brought us much that is new. 

 First is the momentous fact that the earth's crust is never at rest ; that 

 it undergoes a multitude of very diversified movements besides those 

 of the earthquake. Thus a periodical swelling, a flood wave, is pro- 

 duced by the attraction of the moon ; and other heavings are induced 

 by the daily and annual course of the sun's heat. But such move- 

 ments and other similar ones do not come within the scope of this 

 article. 



Real earthquakes, or movements that originate in the depths of 

 the earth, also appear in very different forms. First are the directly 

 perceptible shocks, from the powerful ones that create great dis- 

 turbances to the merely local ones often hardly remarked. Of the 

 immediate workings of these shocks, microscopic instruments have 

 taught us nothing essentially new. But very many macroscopic 

 movements, often continuing for several hours, but which are not 

 felt, have been revealed, that have been shown in many instances to 

 be distant effects of other strong earthquakes; effects which are some- 

 times propagated over the whole surface of the earth. There is, 

 furthermore, another series of movements, only partly explained as 

 yet, of a peculiar sort: first, small, quickly passing disturbances, 

 which appear in the photographic reproductions of the curves as 

 larger or smaller knots, and which are regarded with great probability 

 as distant effects of minor seismic movements most likely imper- 

 ceptible anywhere. They can not be local earthquakes, for they give 

 entirely different curves. There also appear, with considerable regu- 

 larity, at certain seasons of the year, very slow movements of the 

 ground, called pulsations; and finally the multitude of vibrations 

 called tremors, which assume various forms. Sometimes they come 

 as forerunners, accompaniments, or followers in close association with 

 those great disturbances that originate in distant earthquakes; some- 

 times as shocks of minute intensity in separate groups, which it has 

 not yet been possible to account for; and in other cases they are 

 traced to the shaking of the ground by the wind. It is hardly neces- 

 sary to observe that the seismic apparatus should be most carefully 

 guarded against disturbance by the movements of trade, wagons, etc., 

 so that the problem shall not be complicated by them. 



The theory of the nature of earthquake shocks, their transmission 

 and their velocity, has been set in a new light by the labors of Au- 

 gustus Smith, of Stuttgart. From some calculations of their velocity 

 made by G. von Nebeur, it is found that the earthquake of April 17, 

 1889, in Tokio, Japan, was perceived in Potsdam, Prussia, nine thou- 



