630 



NA TURE 



[Oct. 26, 1 8*2 



the rise and fall of tides. Sudden alterations in baro- 

 metrical pressure may possibly produce earthquakes of 

 large amplitude and slow period, similar to those here 

 referred to, which hitherto have been passed by unnoticed. 

 The following are examples of the various records which 

 have been referred to. 



The diagram (Fig. 5) is a tracing from a photograph of 

 the east and west component of the earthquake of March 

 11, 1882, as recorded in Tokio. The regularly marked 

 intervals represent seconds of time. For about 12 seconds 

 before the shock there was a rapid tremulous motion. It 

 will be observed that the westward motion of the shock 

 is less in amplitude, and has been performed more slowly 

 than the eastward motion. The origin of the shock was 

 to the S.S.E. After the shock, which had a maximum 

 amplitude of about 3'5 mm., the motion died out irregu- 

 larly. Altogether the earthquake lasted about \\ minutes. 

 Prof. Ewing, who recorded this same disturbance at a 

 station about a mile distant, recorded a maximum motion 

 of 6 mm., and the duration of the disturbance could be 

 traced over a period of about 4^ minutes. 



Fig. 6 represents the normal motion of the ground pro- 

 duced by exploding about 2 lbs. of dynamite in a bore 

 hole about 8 feet deep. The distance at which the seis- 

 mograph was placed from the explosion was 100 feet. 

 The intervals represent half seconds of time. The up- 

 ward movement indicates motion inwards towards the 

 origin of the disturbance. 



The three diagrams (Figs. 7, 8, 9) are diagrams of 

 actual earthquake motion, as drawn by a pendulum seis- 

 mograph on stationary smoked glass plates. 



Fig. 7 an earthquake at Chiba (16 miles E. of Tokio), 

 1 1.49.0 p.m., February 16, 1882. Here the motion has 

 been simply in one direction, S. 35 W. Its extent is 

 about "9 mm. 



Fig. S an earthquake at Chiba, December 23, 1SS1. 

 Here the motion has been in at least two directions, 

 N. and S., and N. 60° W. The maximum amplitude is 

 about 1 mm. 



Fig. 9 an earthquake at Tokio, 4.15.0 p.m., March 8, 

 1882. Here there has been motion in several directions. 

 The maximum amplitude is about 2 - 2 mm. 



Another class of seismic experiments which, although 

 they are by no means sufficiently complete have yielded 

 good results, are those in which time observations have 

 formed the important features. One result of these ex- 

 periments, in addition to telling us the side and locality 

 from which earthquakes have come, has been to show 

 that the direction in which the ground has vibrated has 

 sometimes been at right angles to the direction in which 

 the disturbance was being propagated. The chief results, 

 however, have been with regard to the velocity of propa- 

 gation. These may be epitomised as follows : — 



1. Different earthquakes, although they have travelled 

 across the same district, have done so with different 

 velocities. 



2. The greater the initial disturbance the greater the 

 velocity of propagation. 



3. The same disturbance is propagated with a decreas- 

 ing velocity. 



These results it may be remarked have received direct 

 confirmation, both for normal and transverse motions, in 

 experiments made by exploding charges of dynamite. 



Another point which has received considerable atten- 

 tion has been the production of what are apparently earth 

 currents at the time of an artificial disturbance. 



A problem of local interest which has been worked at 

 for some years has been the localisation of the origins of 

 the shocks which from time to time disturb the eastern 

 shores of Japan. The result of these labours has shown 

 that the greater number of shocks have originated beneath 

 the sea, off a coast which shows clear evidence of recent 

 and rapid elevation. 



A phenomenon which has clearly been illustrated in 



these investigations has been the very rapid manner in 

 which heavy mountain ranges have completely prevented 

 the spread of a disturbance. 



By placing a large number of similar seismographs on 

 the hills and in the valleys of a limited area, it has been 

 quantitatively demonstrated that we may have two locali- 

 ties within a quarter of a mile of each other, one of which 

 will experience at least double the amount of disturbance 

 as the other. In some localities the hills appears to be 

 the most affected, at others the plains are the troubled 

 regions. ' 



Having before us the diagram of an earthquake, and 

 knowing its origin relatively to the locality where it was 

 drawn, by comparing this with the diagram produced by 

 the explosion, say, of 5 lbs. of dynamite at the distance 

 of 100 feet, we are now enabled to calculate in pounds of 

 dynamite or other units a value for the impulse which 

 created the earthquake. 



Not only have earthquakes been investigated, but with 

 the help of microphones and apparatus similar to that 

 employed by the Brothers Darwin at Cambridge, a good 

 deal of attention has been given to the recording of earth- 

 tremors. The results of these investigations have not as 

 yet been sufficient to enable us to form any general laws 

 such as those which have been formulated by Prof. Rossi 

 and other workers in the Italian Peninsula. 



A utilitarian branch of seismology has been a study of 

 the effects which have been produced upon buildings. 

 Walls with openings in them, which run parallel to the 

 direction in which there is the greatest motion, appear to 

 be more cracked than those at right angles to such direc- 

 tions. At the time of an earthquake existing cracks in a 

 building have been found to open and shut. Records of 

 these motions have been obtained by means of specially 

 contrived indicators placed across the cracks. Other 

 cracks which had been marked and dated at their extre- 

 mities appear to have extended. The effects of placing 

 brick chimneys with wooden houses, giving too steep a 

 pitch to a roof, causing archways over doors and windows to 

 meet their abutments at sharp angles, have been objects 

 of observation. The difference in the effects produced 

 upon buildings like those of the Japanese, which simply 

 rest upon the surface of the ground, and those which by 

 means of foundations are firmly attached to the soil, have 

 yielded instructive lessons. In short it would seem that 

 for earthquake countries the rules and formulae used by 

 engineers and architects require considerable modifica- 

 tion. In England the principal elements which enter 

 into consideration, are stresses and strains produced by 

 gravity acting vertically. In an earthquake country we 

 have in addition sudden stresses and strains arising from 

 forces applied more or less horizontally. 



After the lessons placed before us by the ruins which 

 earthquakes have produced in various portions of the 

 globe, should we undertake any great engineering work 

 in an earthquake country, as, for instance, the Panama 

 Canal, without first having carefully considered how 

 best to avoid the evils arising from the sudden acqui- 

 sition of momentum consequent on seismic disturbances, 

 is for shareholders in these undertakings a financial 

 suicide. Because earthquakes are strong the usual method 

 to meet them is by strong construction. Still very much 

 more than this may be done. And if we cannot prevent 

 the destructive effects of earthquakes, observation in Japan 

 has shown that we can at least mitigate them. This is 

 testified by the modification in the style of buildings now 

 adopted in Japan by all who suffered by the earthquake 

 of February 22, 1880. , __ K J 



The observation of earthquakes in Japan has therefore 

 led to results which are utilitarian as well as scientinc. 

 The description which has here been given of the worK 

 which is going on in that country is short and lmpertect, | 

 many branches of seismological investigation which have| 

 been taken up has not even been referred to. 



