NALURE 
41 
THURSDAY, JUNE 17, 1886 
EARTHQUAKES AND OTHER EARTH- 
MOVEMENTS 
Earthquakes and other Earth-Movements. 
“International Scientific Series,” Vol. LVI. 
Kegan Paul, Trench, and Co., 1886.) 
“THE object of this work is “to give a systematic 
account of various earth-movements”: these are 
classified as (1) Earth-quakes, (2) -Tremors, (3) -Pulsa- 
By J. Milne. 
(London : 
tions, and (4) -Oscillations, which are severally defined | 
as (1) sudden or violent, (2) minute, (3) slow, (4) secular 
movements of the ground. The earthquakes occupy 
305 pp., whilst only 43 pp. are given up to the other three | 
motions. After an introduction follows a description 
(22 pp.) of about twenty different kinds of instruments for | 
recording earth-movements (seismo-scopes, -meters, and 
-graphs), beginning with an ancient Chinese seismoscope 
(A.D. 136). The construction of a proper seismograph 
which shall record period, amplitude, and direction of | 
movements is difficult, the inertia of the moving parts | 
masking the earth-movement. A set of tipping columns 
seems to be the simplest seismoscope, and some form of 
pendulum the simplest seismograph: these can be made 
“‘astatic,” so as to retain any deflected position. Of 
recorders of motion a smoked-glass plate seems the 
simplest. The “Gray and Milne seismograph” is an 
elaborate instrument, recording continuously and simul- 
taneously the times, and also the three rectangular 
components, of any displacement. 
The explanation of earthquake propagation by waves 
of elastic compression and distortion, shadowed forth by 
Dr. T. Young, is fully discussed and illustrated for the 
general reader : the former are compared to sound-waves 
and the latter to light-waves; unlike these, however, the 
direct waves travel the quicker. It appears that about 
seven-eighths of the speed of wave-transit through 
homogeneous rock is lost in actual rocks as found 
tm situ. 
Experiments on artificial earthquakes have—as might | 
Only three sets appear to have | 
be expected—been few. 
been published. Those by Mr. Mallet (published in 1851) 
were based on explosions of large masses of powder up 
to 12,000lbs. Those by General Abbot in America (pub- 
lished in 1876) were based on explosions of from 70 to 
1880, were based on the fall of a ton weight through from 
10 to 35 feet, and on explosions of 1 to 2 lbs. of dynamite. 
The general conclusions are that the wave-speed is 
quicker for direct than for transverse vibrations, is quicker 
through hard than through soft rock, increases with 
increase of the shock, and decreases with distance. It is 
clear that opportunities for experiment on the effect of 
explosions such as the above must often occur, and that a 
small expenditure on seismographs alone is needed to 
embrace the opportunities. 
There appears to be no known limit to the frequency 
and duration of earthquakes. Shocks otcur continuously 
VOL. XXXIV.—No. 868 
| rupture. 
in the Andes (p.246), and have lasted for weeks in New Zea- 
| land at the rate of 1000 a day (p.72). Continuous shaking 
has been /e/¢in Japan during 44 minutes (p. 73). Recorded 
vibration-periods vary from two-tenths of a second to one 
second, and the amplitude may amount to 1 foot without 
Some calculations are given of velocity of 
motion of the ground derived from fall of buildings and 
projection of copings, caps, &c,: these are open to large 
error from omission of resistance to fracture and of fric- 
tion. The conclusions as to the rate of earthquake propa- 
gation agree with those derived from the artificial earth- 
quakes quoted above. 
The most practically useful chapter is that on effects on 
buildings (a subject recently discussed at the Institution 
of Civil Engineers), as to which the most important con- 
clusions are—(r) local knowledge alone will guide to the 
safest site; thus hills, valleys, plains, hard strata, soft 
strata, &c., are each safe in some, unsafe in other coun- 
tries. (2) If the direction of shocks be definite, place the 
blank walls parallel, and the pierced walls perpendicular 
thereto. (3) Openings at different levels inthe same wall 
to be échelonned. (4) Avoid flat arches, or place wood 
lintels over them. (5) Avoid heavy copings, caps, &c., 
and tall chimneys. (5) Use roofs of low pitch. (7) Struc- 
tures of different vibration-period should not be con- 
nected. 
Structures of bamboo and timber are seldom injured 
by earthquake-shock directly ; earthquakes are only 
indirectly destructive to them through fires (caused by 
the upsetting of lamps) and floods (which sometimes 
follow). 
The earthquake-effects on land are the opening of 
cracks and fissures, with occasional discharge of water, 
mud, &c., landslips, and disturbances of lakes, rivers, &c. ; 
| also upheavals and depressions of whole tracts : these are 
—with the exception of the last—seldom extensive. The 
chief effect on the ocean is the raising of a great sea- 
wave, sometimes very large, e.g. 60 feet high at Lisbon 
(1761), 80 feet at Callao (1724), 210 feet at Lupatka 
(1737). These waves are often more destructive on land 
than the actual shocks ; the influx is usually preceded by 
an outflow, which in fact acts as a warning. One of the 
most remarkable effects is the distance to which these 
waves are propagated as “ great waves,” e.g. right across 
the Pacific. Thus most large earthquakes on the east or 
west coasts of the Pacific produce waves which are 
recorded on the opposite coast about twenty-four hours 
after. From the recorded time of transit of these waves 
| and the known distance the average depth of the ocean 
400 Ibs. of dynamite. Those by Mr. Milne in Japan, since | 
(supposed uniform) can be calculated by Russell’s formula 
(v? = gh); the calculated depths are generally less than 
the soundings (which err necessarily in excess). 
As to source of earthquakes eight methods are given 
for finding the “epicentrum” or sz7face-origin, and four 
for finding the “centrum” or actual origin, on various 
assumptions: e.g. radiation from a point (viz. the epi- 
centrum), uniform propagation, homogeneous strata, &c. 
Results depending on such doubtful assumptions can only 
be very rude approximations. 
As to distribution over the world, it appears probable 
that all parts of the world have been shaken at various 
times, but that in the historic period the regions most 
H 
