1909.] OUTLOOK OF SEISMIC GEOLOGY. 293 



for they would appear to have an extremely small amplitude of 

 vibration, are a fore-runner of most heavy earthquakes. These 

 fore-tremors should not be confused with the preliminary tremors in 

 the record of the distant seismograph, for they are of such small 

 amplitude that they would probably not be registered by any instru- 

 ments today constructed, except perhaps within the affected district 

 itself. Our best evidence that such fore-tremors exist is furnished 

 by the behavior of certain of the lower animals. In the opinion of 

 the writer, such a body of evidence has now accumulated, that it 

 can no longer be waved aside. Just as the sense of smell is so much 

 more highly developed in the dog, for example, than it is in man, 

 so there seems no valid reason for doubting that the detection of 

 small motions by the lower animals may be by as much superior to 

 the human sensibility. Dr. Omori has expressed his belief that 

 seismographs will yet be made sufficiently sensitive to record these 

 microscopic tremors. Just as a block tested in our experiments 

 assumes very large deformations as it approaches rupture, so the 

 earth structure may behave during a period which is as much longer 

 in proportion as the time of augmenting the stresses exceeds that in 

 our experiments. Judging from the recorded behavior of animals, 

 it would not be surprising if the period during which warning may 

 be possible on this basis, should prove to be a large fraction of a day, 

 or even longer. If measurable deformation does occur as a result 

 of the accumulated stresses long before the limit is reached, it may 

 be possible in the case of those earthquakes particularly which 

 result in horizontal shearing movements, to determine by frequent 

 measurement of the distances which separate properly placed monu- 

 ments, the approach of the strain limit. It is a subject which is at 

 least worthy of investigation. 



Since the days of Perrey, who devoted his life to an attempt to 

 find a connection between earthquakes and lunar conditions, there 

 have been those who have sought to connect seismic and volcanic 

 disturbances with periods of special gravitational stress due to luni- 

 solar phases. The most recent advocate of such a connection, is 

 Perret,*^ who is so convinced that he has found the secret behind 



*° " Some Conditions Affecting Volcanic Eruptions," Science, Vol. 29, 

 1908, pp. 277-287. 



