XU 



ill tilt' soil arc caiisrd by alti'riiatii)iis nf wot wcatiicr witii druiiglit, and l>v tiie 

 absorption and radiation of solar heat. Tiieso and porhaps other causes cond)iiu' 

 to produce an incessant yielding of" the earth's crust, so considerable and at the 

 same time so irregular that the attempt to distinguish in it a periodic part 

 directly due to lunar attraction has been abandoned as hopeless. 



Although tiie mt)vements just described are due to the elasticity of the eai-th, 

 it would be a misnomer to call them earth(juakes. The far more rapid tremors 

 caused by a sudden mechanical disturbance are easily distinguishable from them, 

 and rc()uire wholly ditlerent apjiliances for their detection and measurement. 



After freely applying the restrictions which have been indicated, we are left 

 with an inunensc number of disturbances to which the name earthquake is 

 practically applicable. Their magnitudes vary within the widest jMissible 

 limits, from tlie scaicely perceptible movements familiar to all residents in an 

 eartlujuake country, to the convulsions which have destroyed cities and changed 

 the face of a continent. From these, in some form, no part of the earth's surface 

 is entirely exemj)t, and in certain favourable districts they occur with almost 

 daily fre(|ueucy. 



Earth(jnake Measurement, the subject of the present paper, consists in 

 determining as fully and ex.ictly as ])ossible the character of the motions which 

 make up an eartlnjuake. If this determination could be pushed back so far as 

 to include the initial motions of the portions first disturbed, it would involve 

 discovery of the originating im])ulse. It cannot be said that this result has 

 hitherto been achieved, excejit in a very jiaitial degree; and our kiiowli'dge of 

 the origin of earthijuakes consists cliieflv of deductions as to what may be expected 

 to result from the earth's gradual ajiproach to a state of th<'rnial and mechanical 

 equilibrium, along with inferences fi-om what geology tells aVjout ancient and 

 contem]ioraiy disturbances of the crust. This department of Seismology lies 

 outside the sc()pe of the ju'csent paper. It has bet'U developed in a masterly 

 manner by Hopkins in his Ileport to the British Association on the Geological 

 Theories o( Klevation and Eartli(|uakes.* In the same jiaper he has applied the 

 theory of waves in an elastic solid to the case of terrestrial disturbances. A brief 

 restatement of the latter jiart of the theory of earthquakes will not be out of 

 jilace here, since it both teaches the earthquake observer what to look for and 

 guides him in the interpretation of his results. It will accordingly be found in 

 Chapter I ; after which instruments for eartluiuake measurement will be described, 

 and the results of actual observations be stated and examined. 



» Biitish .Vssocialion Kciidit for 1847, pP- 83-9-J. 



