DYNAMICAL GEOLOGY. 283 



Whitney's observations in California, by Wynne's in the Punjab, 

 and by Heim's in Switzerland. 



Perrey's long historical catalogue of earthquakes was 

 intended in the first instance to determine how far earth- 

 tremors had been encouraged by the particular times of the 

 day, or seasons of the year, or by the disposition of the earth 

 with reference to other heavenly bodies. The results are not 

 altogether satisfactory, for although they prove greater 

 frequency of earth-tremors in winter and autumn than in 

 other seasons, no definite law can be induced. Neither do 

 the statistics give any confirmation of the idea that the 

 occurrence of earthquakes may have some connection with 

 meteorological conditions. On the other hand, they led 

 Professor Perrey to conclude that an explanation of earthquakes 

 might be found in the varying attraction of the moon at its 

 different phases. 



He supposes the earth's crust to be as uneven on its inner 

 concave surface towards the nucleus as upon its outer surface ; 

 that under the attraction of the moon the hot nucleus swells 

 upward in wave-like form and presses against the weakest 

 parts of the crust, with the result that the terrene impulse is 

 transmitted through the crust as an earthquake. 



Dr. Rudolf Falb in 1869 independently formulated a theory 

 of earthquakes similar in character, but more fully elaborated 

 than that of Professor Perrey. Dr. Falb connects high tidal 

 waves of the earth's magma with the attractions exerted upon 

 the earth by the sun, the moon, and other .heavenly bodies, 

 and he therefore thinks it possible to foretell from astronomical 

 calculations "critical" days or periods on which violent 

 seismic disturbances will take place. A general connection 

 between solar and lunar attraction and the occurrence of 

 earthquakes is accepted by a considerable number of 

 astronomers and geologists, amongst others, by J. Schmidt, 

 C. F. Naumann, Von Lasaulx, Pilar, and others. But several 

 authors have disputed Dr. Falb's theory. One main conten- 

 tion is the uncertainty regarding the actual condition of the 

 earth's nucleus ; many physicists and geologists now believe 

 that the nucleus is practically solid, and that molten rock- 

 magma can only be present under certain definite conditions 

 of depth and pressure, and is necessarily of limited distribution 

 in the earth's mass. 



Friedrich Hoffmann had distinguished different kinds of 



