EARTHQUAKE MOTION DEDUCED FROM OBSERVATION. 87 



of motion in the solid rocks beneath the surface as 

 compared with the extent of motion on the surface, the 

 disturbances are passed by unnoticed, or else the dis- 

 turbance is, at a distance from its origin, practically 

 confined to the surface. 



Velocity of Propagation of an Earthquake. — Although . 

 many have written upon earthquakes and have endea- 

 voured to give to us the velocity with which they were 

 propagated, the subject is one about which we have as 

 yet but little exact information. 



The importance of this branch of investigation is 

 undoubtedly great. By knowing the velocity with which 

 an earthquake has travelled in various directions we are 

 assisted in determining the locality of its origin ; we may 

 possibly make important deductions respecting the 

 nature of the medium through which it has passed; 

 perhaps also we may learn something regarding the 

 intensity of the disturbance which created the earth- 

 quake. In the Eeport of the British Association for 

 1851 Mallet gives the table on next page, in which are 

 placed together the approximate rates of transit of shocks 

 of several earthquakes which he discusses. Some of 

 these, it will be observed, are records of disturbances 

 which must have passed through or across the bed of the 

 ocean. 



In Mallet's British Association Eeport for 1858, he 

 gives data compiled by Mr. David Milne ' respecting the 

 Lisbon earthquakes of 1755 and 1761, from which data 

 the tables of velocities (p. 89) have been calculated, 

 omitting those which Mr. Mallet has marked as un- 

 certain. 



The distances are marked in degrees of seventy 



* See Edinburgh Phil, Trans., vol. zzzi. 



