58 C. Davison — Bntish Earthquakes. 



In many cases, however, this evidence has been supplemented by- 

 inquiries made in the several districts or by letters addressed to the 

 local newspapers asking for further observations. The accounts 

 I have thus received, being generally replies to a few definite lead- 

 ing questions, are often of considerable value ; and I should like to 

 take this opportunity of again thanking the ladies and gentlemen 

 who have in this way rendered me such courteous and serviceable 

 assistance. 



In the description of each earthquake, the heading contains its 

 name and date, the name being taken from that of the district 

 principally affected by the shock. The time of occurrence is given 

 in Greenwich mean time, the hours being numbered from to 24. 

 The intensity is determined by means of the Rossi-Forel scale, a 

 translation of which is given below. This scale is very generally 

 adopted by Italian and Swiss seismologists, and, though rough and 

 undoubtedly variable to a slight extent, is well suited to the nature 

 of the evidence at our disposal, the range of variability of any degree 

 of the scale being probably less than the limits of error of ordinary 

 observations. 



Bossi-Forel Scale of Intensity} 



I. Recorded by a single seismograph, or by some seismographs of the same model, 

 but not by several seismographs of different kinds ; the shock felt by an experienced 

 observer. 



II. Eecorded by seismographs of different kinds ; felt by a small number of 

 persons at rest 



III. Felt by several persons at rest ; strong enough for the duration or the 

 direction to be appreciable. 



IV. Felt by persons in motion ; disturbance of moveable objects, doors, -windows, 

 cracking of ceilings. 



V. Felt generally by every one ; disturbance of furniture and beds, ringing of 

 some bells. 



VI. General awakening of those asleep ; general ringing of bells, oscillation of 

 chandeliers, stopping of clocks ; visible disturbance of trees and shrubs. Some 

 startled persons leave their dwellings. 



VII. Overthrow of moveable objects, fall of plaster, ringing of church bells, 

 general panic, without damage to buildings. 



VIII. Fall of chimneys, cracks in the walls of buildings. 



IX. Partial or total destruction of some buildings. 



X. Great disasters, ruins, disturbance of strata, fissures in the earth's crust, rock- 

 falls from mountains. 



The disturbed area is defined as that at every point of which the 

 earthquake-shock is felt. The form and position of its boundary 

 must therefore depend on the practice of observers and on their 

 power to detect a feeble shock ; but, as a knowledge of" this boundary 

 can possess but little value when it does not represent an isoseismal 

 line, I have endeavoured to draw it on the earthquake maps so as to 

 pass as nearly as possible through places where the intensity was 

 the same. 



The epicentrum of an earthquake is the projection of the seismic 

 focus or centrum on the surface of the earth. To determine its 

 position, three methods have been proposed, depending respectively 



1 M. S. de Eossi. Bull. Vulc. ital., anno. iv. (1873), pp. 39-40; F. A. Forel, 

 Arch, des Sc. phys. et nat., 3me per. t. xi. pp. 148-149. 



