ISOSEISMALS: DISTRIBUTION OF APPARENT INTENSITY. 161 
at the surface. In attempting this task, certain conditions which militate against the 
accuracy of the results and others which affect their interpretation should be stated. 
In the first place, the scale upon which the gradation of intensity is indicated, that 
known as the Rossi-Forel scale, is more or less arbitrary. At the outset of the inquiry, 
the Commission revised and simplified this scale somewhat, with the object of adapting 
it for general use, and its present form, as amended by the Commission, is as follows: 
I. Perceptible, only by delicate instruments. 
Il. Very slight, shocks noticed by few persons at rest. 
Ill. Slight shock, of which duration and direction were noted by a number of persons. 
IV. Moderate shock, reported by persons in motion; shaking of movable objects; 
cracking of ceilings. 
V. Smart shock, generally felt; furniture shaken; some clocks stopt; some sleepers 
awakened. 
VI. Severe shock, general awakening of sleepers; stopping of clocks; some window 
glass broken. 
VIL. Violent shock, overturning of loose objects; falling of plaster; striking of church 
bells; some chimneys fall. 
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 surface of earth; mountain slides. 
It is apparent that the scale leaves room for wide variation in the personal equation. 
Different reporters interpret the same experiences and the same phenomena differently. 
It was also found that in the periphery of the region affected, where the earth waves were 
of slow period, pendent objects and liquids were more sensitive indicators of earth move- 
ment than direct perception by individuals, altho the latter is placed first in the scale. 
Prof. G. D. Louderback, who reported upon the intensity of the shock in the region 
east of the Sierra Nevada, makes the following pertinent comment upon this point: 
In the towns along the east base of the Sierra Nevada and within 25 or 30 miles of the 
base, the shock was distinctly felt, movable objects were seen to swing and heard to bump or 
rattle, and a very small number of persons were awakened. Farther east the most notable 
feature of the reports is that wherever the effects of the earthquake were made evident, the 
physical signs, such as the swinging of suspended objects, etc., were described almost to the 
exclusion of direct physiological effects. This is apparently at variance with the principle 
upon which the Rossi-Forel scale is founded, as the first three grades of intensity are based 
on feeling, the visible disturbance of objects not beginning till grade IV is reached. Perhaps 
the most important physical sign reported is the disturbance of smooth water surfaces. In 
five instances, at three different localities, ditch tenders or irrigators noticed an agitation of 
quiet water surfaces, and that water lightly splashed against the sides, as if from low waves, 
or as in a vessel of water when it is slightly tilted. - As the morning was clear and entirely 
without wind, it imprest them as peculiar, and the matter was reported when they went to 
breakfast. The suggestion of one that something peculiar had happened, and of another 
that it was an earthquake, was each in its place the incitement of sallies of wit at the ex- 
pense of the reporter. News of the California earthquake reached these places several 
hours afterwards and the time was then found to agree as closely as determinable with the 
phenomena of the morning. In each of the cases, however, it was reported that no shock 
was felt. It is suggested that with moderately long waves such surfaces might prove very 
sensitive indicators of intensities down to the lowest degree on the scale. 
The movement of liquids in vessels, ponds, lakes, or streams, is not included in the scale, 
altho numerous reports were made of such movement and estimates as to the intensity 
of the shock were based thereon. The stoppage of clocks appears to be a very uncertain 
criterion of intensity. In the 6th, 7th, 8th, and 9th degrees of the scale, wherein damage 
to buildings is relied upon for an estimate of the intensity, two important factors tend to 
vitiate the conclusions arrived at as to the comparative intensity. These are (1) the 
great variability of the character of the structures and (2) the character of the ground 
upon which they are built. The scale was probably designed originally for regions where 
brick and masonry structures prevail, while in California wooden structures are by far 
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