372 Reviews — Prof. John Milne — Earthquakes. 



The chapters on the ' Distribution in Time ' of earthquakes contain 

 much information and some valuable tables. Extremely interesting 

 is an attempt to exhibit graphically the variations of seismic energy 

 for a particular district, that of Kioto, during twelve centuries. It 

 is curious to find not a continuous decrease but a general diminution 

 interrupted by a temporary increase four centuries back. Not much 

 faith, however, can be placed in any conclusions based on mere 

 records of remarkable shocks. We find surely the temperature of 

 a day easier to estimate than the intensity of a shock. Yet how 

 uncertain are we still as to any real alterations in English climate 

 during a like succession of ages. 



Far better founded are the discussions as to the relations of 

 earthquake frequency, intensity, or time of occurrence, with seasons, 

 tides, planets, or barometer. Even here, however, negative answers 

 are disappointingly prevalent, and the solitary result to emerge is 

 that their number is greater in winter. 



More valuable and far more trustworthy conclusions are obtained 

 under the heading, ' Distribution in Space.' The author's somewhat 

 novel sport of ' Earthquake hunting' (Chap. X.) has already brought 

 to light several facts of the first rank in importance. Such are, the 

 existence of groups with more or less definite boundaries and separate 

 centres of radiation (see p. 191) ; the significant coincidence of much- 

 shaken regions with lines where continents dip steeply into oceans ; 

 and the action of mountain ranges in barring the progress of a 

 shock. More information on this last now celebrated observation is 

 very much to be desired. Some mountain countries (as Switzerland) 

 are often shaken. Do these shocks originate in the mountains, and 

 if so do they extend into the plains ? or does each region keep its 

 products to itself? Is there Free Trade in earthquakes there, or 

 Protection ? If delicate instruments like Professor Milne's were 

 judiciously placed and regularly observed, we think it probable that 

 much information might be obtained on points of this nature. 



Descriptions of these beautiful instruments form a very interesting 

 portion of the book. The account is, however, brief. It enables the 

 reader to see the action of the machines, but does not go very deeply 

 into either fundamental principles, or practical difficulties. For 

 these a student must refer to the various original memoirs. The 

 principle which underlies all these machines alike is that the surface 

 of the earth moves, and that to obtain a true record of its motion, 

 something must be provided which remains unaffected and does not 

 move with the earth. A freely suspended pendulum fails because 

 a motion of its point of attachment soon creates motions of the bob. 

 A slab on rolling spheres fails because impulses to the slab by 

 friction from the balls cannot be avoided. And whereas a beginner 

 would expect a moving point to write records on a plate, the reader 

 will learn that the writing point is arranged to be motionless, while 

 it is the recording plate which moves with the earth. The difficulty 

 of recording vertical motions by any device is so great that one 

 almost disbelieves any measure of success can be obtained. 

 Ingenious means for overcoming these difficulties are described, 



