428 Holden — Earthquake-Intensity in San Francisco. 



reported as to allow an intensity on the scale, to be assigned 

 with, certainty. In San Francisco, 417 shocks in all have been 

 recorded. Of these, 200 were accurately described. 



TJie Rossi-Forel Scale. 



I. Microseismic shock — recorded by a single seismograph, or by seismographs 

 of the same model, but not putting seismographs of different patterns in motion ; 

 reported by experienced observers only. 



II. Shock recorded by several seismographs of different patterns ; reported by 

 a small number of persons at rest. 



III. Shock reported by a number of persons at rest ; duration or direction noted. 



IV. Shock reported by persons in motion ; shaking of movable objects, doors 

 and windows, cracking of ceilings. 



V. Shock felt generally by every one ; furniture shaken ; some bells rung. 



VI. General awakening of sleepers; general ringing of bells; swinging of 

 chandeliers; stopping of clocks; visible swaying of trees; some persons run out 

 of buildings. 



VII. Overturning of loose objects ; fall of plaster ; striking of church bells ; 

 general fright, 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 ; overturning of rocks ; fissures in the surface of the earth ; 

 mountain slides. 



Determination of the mechanical equivalent of each degree on the 



Rossi-Forel scale. 



It is necessary to determine the value of each degree on the 

 Rossi-Forel scale in terms of some natural units. This it is 

 impossible to do with exactness, owing to the nature of the 

 subject, and it is somewhat difficult to get results sufficiently 

 exact to be used in practice. 



Referring to the Rossi-Forel scale, we find that degrees I, 

 II, III correspond to the feelings of the observer — to his sen- 

 sations. The rest of the scale (IY-X) refers chiefly to the 

 effects of the shock in producing motion upon inanimate mat- 

 ter. The problem is to get some kind of a common unit of a 

 mechanical sort, and to express the various degrees of the scale 

 in terms of this unit. There is no question as to what unit to 

 employ. The researches of the Japanese seismologists have 

 abundantly shown that the destruction of buildings, etc., is 

 proportional to the acceleration produced by the earthquake 

 shock itself in a mass connected with the earth's surface. 



The earthquake motion is a wave-motion, and although it is 

 not simple harmonic, it is necessary to assume it to be such to 

 obtain a basis' for computation. We assume then a = ampli- 

 tude of the largest wave ; T = period of the largest wave ; 



Y = -=- = velocity of the impulse given by the shock ; I = 



V 2 a 



— = 47T 2 . ™ = intensity of the shock, defined mechanically 

 a jl 



