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III. Scales of Seismic Intensity. Bij Charles Davison, 

 Sc.D., F.G.S.* 



IN every country where earthquakes are studied, the want 

 of a simple scale of seismic intensity is felt, partly in 

 order to determine the relative intensity of different shocks, 

 but chiefly for the purpose of drawing isoseismal lines. The 

 use of accurately constructed seismographs cannot take the 

 place of such a scale ; for the intensity of the shock varies so 

 much even in one town, that they can hardly ever exist in 

 sufficient number for the purpose of drawing isoseismal lines. 

 The superiority of a simple scale on this point of view lies 

 entirely in the fact that by its means we can obtain a large 

 number of observations of the intensity from within a limited 

 area. 



Whatever the scale may be, it should satisfy the following 

 conditions : — 



1. The degrees of the scale should depend as far as possible 

 on the mechanical effects of the shock and not on personal 

 impressions, which may vary in different countries and with 

 different observers in the same country or with the same 

 observer at different times. 



2. Each degree of the scale should depend on one test 

 only, unless the exact equivalence of two tests has been 

 determined previously. 



3. The number and closeness of the degrees should be such 

 that the scale is equally serviceable for weak, for moderately 

 strong, and for destructive, earthquakes. 



(1) Comrie Scale. — Brit. Assoc. Rep. 1842, p. 93. 



The first attempt to devise a scale of seismic intensity was, 

 I believe, made on behalf of a British Association Committee 

 in 1841. A strong shock occurred on Oct. 23, 1839, having 

 its epicentre close to Comrie, and was followed by a large 

 number of after-shocks. Very great interest was aroused by 

 these earthquakes, and careful records were kept by 

 Mr. Macfarlane, postmaster of Comrie, who communicated 

 his notes to the British Association Committee, of which 

 Mr. David Milne (afterwards Milne-Home) was secretary. 

 In order to furnish a rough idea of the relative intensity of 

 these shocks as felt at Comrie, Mr. Macfarlane employed an 

 arbitrary scale, the intensity of the earthquake of 1839 being- 

 reckoned as 10. A similar scale was used to denote the 

 intensity of the accompanying sound. Both scales, no doubt, 

 served the purpose for which they were intended, but they 

 * Communicated by the Author. 



