MICHIGAN ACADEMY OF SCIENCE. 55 



amplitude — and according as they take place the more rapidly. The sur- 

 face of a violin bow slips over the strings of a violin, and the elastic air me- 

 dium transmits the movement as waves of sound, which will be the louder 

 the more rosin is upon the bow — the greater the friction — the farther it is 

 moved over the string and the more rapidly. The motion of adjustment at 

 the margin of blocks produces a fault accompanied by vibrations which are 

 transmitted as earthquake waves by the elastic rock medium. These waves 

 will be of at least two types, one yielding successive shocks which cause 

 damage, the other relatively cjuick and feeble and perceived as sounds 

 only. Experience has taught that the slips upon earthquake faults are 

 accomplished within a few seconds at most, and often within the fraction of 

 a second; and, further, that a definite relation appears to exist between the 

 size of the faults produced and the intensity of the successional earthquake 

 shocks. 



Shocks must be transmitted from every fissure upon which a slip has 

 taken place. The distance to which these waves are carried is much less 

 than has generally been supposed for the reason that the cumulative effect 

 of the slipping on many planes often widely separated from each other, has 

 been erroneously traced to a disturbance supposed to emanate from a single 

 focus near the center of the affected district. It appears that the waves 

 travel with the least loss of intensity along the fissure planes themselves, 

 but in directions at right angles to these fissures their intensity is rapidly 

 dissipated, so that at relative short distances they are impotent against well 

 built structures. Thus may be explained the mysterious and repeated im- 

 munity of certain villages from earthciuake damage even though situated in 

 the heart of an earthquake district; as well as the hitherto equally unaccount- 

 able shocks wdiich have been felt in villages located far outside the so-called 

 destructive zone of the great earthquakes. In far too many instances wholly 

 reliable reports of so-called "freak" shocks which have been felt at great dis- 

 tances from an earthquake "centrum" have been wholly disregarded be- 

 cause in conflict with an accepted explanation. 



An oft observed result of earthquake disturbance is the rotation upon 

 their bases of the higher blocks in heavy monuments. A recent illustration 

 has been furnished by the twisting of Queen Victoria's statue in the square 

 at Kingston during the recent Jamaican earthquake. It has long been realized 

 that heavy shocks have reached the same point of an earthquake district 

 from different directions. This has often been illustrated by the throwing 

 of objects first in one direction and later in another. The late Professor 

 Sikiya of the University of Tokyo prepared with much care a twisted wire 

 model which recorded in its changing direction the sequence of the shocks 

 and the exact direction of each for the Japanese earthquake of January 15, 

 1897. Three conqjlicated snarls of wire were necessary to record in this man- 

 ner the variation in direction of shocks arriving at a single point for an earth- 

 quake which lasted about a minute. These models, which have hitherto 

 been given no satisfactory explanation, we may now interpret as due to the 

 shocks which have reached the station from tlie numerous fissm'e planes of 

 the district upon which the movement has occurred. »Such waves should 

 reach the station at different times, in different surface directions or azimuths, 

 and with different angles of elevation. Should two or more shocks reach 

 the station at the same instant from different directions, the result must be a 

 rotatory movement, such as would explain the long and gradual curves in 

 the wire as well as the twisted monuments. 



I am painfully conscious that it has been possible to touch but lightly 



