EARTHQUAKE MEASUREMENT AT MIYAKO. 163 



As typical specimens, are given in figs. 1, 2, and 3, PI. 

 XVII, the seismographic diagrams of three earthquakes, Nos. 2, 

 3, and 11. Further, in figs. 4, 5, 6, 7 and 8 are given, for the 

 sake of reference, maps illustrating the areas of disturbance of 

 seven earthquakes, Nos. 3 and 20, 8, 11, 10, and 1 and 2. 



3. In the following description of the seisn^ographic dia- 

 grams, the terms preliminary tremor, principal portion and end 

 portion have been employed to designate three successive stages 

 of an earthquake motion, defined as follows : 



The preliminary tremor of an earthquake denotes the in- 

 troductory movements of very small amplitude, their period 

 being generally short. 



The principal portion denotes the most active part of an 

 earthquake, which follows the preliminary tremor and which 

 consists of movements of larger amplitude. 



The e7id-portion denotes the feeble finishing part of an 

 earthquake, which follows the j^rincipal portion. 



It is here to be noted that the limits of the principal 

 portion are i^ometimes very well marked, but in other cases 

 more or less indefinite. 



Again the terms, ripples and sloiv undulations, have been 

 used to denote respectively those quick earthquake vibrations, 

 whose periods are mere fractions of a second, and those com- 

 paratively gentle movements, whose periods are mucli longer 

 than those of the former. 



The abbreviations used in this paper are as follows : — 



H.M. denotes Horizontal Motion. 

 V.M. „ Vertical 

 2a ,, Kange of 



