October, 1905 ] 



KNOWLEDGE & SCIENTIFIC NEWS. 



245 



divide the great oceanic and terroceanic basins of the 

 earth's surface. 



(3.) Earthquakes may become visible at any point 

 on the earth's surface, but the greater effects are con- 

 fined to those areas in the vicinity of the lines of vol- 

 canic activity. 



Prof. G. Darwin also' prepared an "earthquake map," 

 which shows (see Fig.) a broad band completely en- 

 circling the world, with which area earthquakes were 

 both frequent and severe. 



This band, as will be seen, encloses the following 

 countries : — Southern Europe, the Mediterranean area, 

 Asia Minor, Syria, Persia, Northern India, China, 

 lapan, across the Pacific to Central America and the 

 West Indies, then through the Atlantic to the Azores, 

 Teneriffe, Portugal, Spain, and North-West Africa. 



There are other Seismic bands, such as those of the 

 Andes and the Malay Archipelago, but these may be re- 

 garded as siiort ofl'shoots of the "great seismic band ;" 

 these latter, it will be noticed, are at right angles to the 

 line of general disturbance. 



If we review the list of earthquake shocks that have 

 taken place within recent years, we shall see how they 

 confine themselves to areas that come within the black 

 band marked out on the map. 



1868. — Peru and Ecuador. When four cities were 



destroyed. 

 1875. — San Jose (Colombia). Earthquake so sudden 

 that the people had no time tO' escape, so 

 that many thousands of lives were lost. 

 1903. — Turkestan. Similar disaster to that of San 



Jose. 

 1904. — Macedonia. 

 1905. — Albania. 

 1905. — Northern India, Lahore. Terrible loss of 



life. 

 It is known to everybody, that when a shock occurs 

 in any particular locality, it develops an "earth wave," 

 which traverses a greater or less portion of the globe 

 according to the magnitude of the original disturb- 

 ance. In fact, it is this wave which, in most cases, 

 produces the terrible effects about which we read. 



This wave travels easiest and quickest through solic 

 rock. It has been calculated that it traverses granite 

 at the rate of 1,665 feet per second, which is very much 

 greater than the velocity with which sound travels. 



Through shattered rock it goes at the rate of 1,306 

 feet per second, through slate 1,089 f^*^* P^f" second, 

 and through wet sand with a velocity of 825 feet per 

 second. The deeper the rocks are, the quicker does 

 this wave get through them. 



Of course, it travels very much slower through water. 

 Observations on the velocity of the sea wave, which 

 invariably accompanies the earth wave, show that 

 it depends upon the depth of the water through which 

 it has to pass, going much quicker through deep 

 water than through shallow. When it is remembered 

 that this sea wave is some twenty feet in height, and 

 its velocity may attain six miles a minute, it can be 

 readily imagined that a sea coast with deep water in its 

 immediate vicinity is not a desirable locality during an 

 earthquake. 



A very curious anomaly has Ix'en obser\ed in the 

 study of this subject, namely, that the region immedi- 

 ately above the centre of the disturbance suffers only 

 very slightly from its effect. The seismoJogical term 

 for this area is the "Epicentrum." 



Suppose the centre of the disturbance were twelve 

 miles below the epicentrum (measured perpendicu- 

 larly), then the region which would be most seriously 



affected would be twelve miles distant in any direction 

 from the epicentrum on the surface. 



Of course, the whole region is affected, but the 

 serious effects are not apparent until a region is ap- 

 proached which is as far away from the epicentrum as 

 the latter is from the internal centre. 



It has been estimated that the origin of an earth 

 quake very rarely occurs at a distance of more than 

 30 geographical miles below the surface, but, as has 

 been stated before, the earth wave has practically no 

 limit. 



The surface effects of an earthquake are more de- 

 structive when it traverses soft rocks, because the 

 cracks that are produced at the surface are kept open 

 for a longer time, and allow the soil to slip, and the 

 buildings to subside ; whilst in the harder rocks, fis- 

 sures are formed which are narrower, and will close 

 more quickly, causing, thereby, far less displacement. 

 Perhaps the greatest destruction is caused when the 

 waves travel from compact rocks to loose and soft ones. 

 In these cases complex reflections and reverberations 

 of the shocks ensue, producing the shivering of the 

 surface of the land, which, of all disturbances, is the 

 most to be feared, and the worst to be experienced. 

 This is undoubtedly what has happened in Calabria, 

 where the surface rocks are soft and loose, while the 

 lower ones are hard and compact. 



If the angle of emergence of the wave is small, the 

 difficulty which the wave experiences in passing from a 

 compact rock to an overlying soft rock is such tha" a 

 very small shock is felt. This is of especial importance 

 and interest to England, since our country is so far re- 

 moved from the areas of intense seismic activity that 

 the angle of emergence is ahvays low, added to which 

 the surface of the land is composed mostly of soft rocks; 

 and this is the reason that, when a severe earthquake 

 takes place in Europe or North-West Africa, it is usu- 

 ally felt in .Scotland, where the surface is almost uni- 

 formly of solid rock ; the same shock being scarcely 

 perceptible in England. 



Should the reader then live in fear of being swallowed 

 up by the earth, and should he be fortunate enough 10 

 be able to choose anv part of the world for his abode, 

 he would have to consider (i) his proximity tO' either 

 active or extinct volcano, (2) his proximity to lant' 

 bounded by shores with ai high gradient, (3) the nature 

 of the strata beneath the surface, and, lastly, the 

 distance from any other earthquake region, having a 

 proper regard as to whether there was a relatively 

 small depth of soft rock on a bed of granite, or other 

 solid substratum. 



The Birkbeck College, Bream's Buildings, Chan- 

 cery Lane. — This Institution, which has now completed 

 82 years of educational work, will commence the new 

 session on Monday, October 2nd, when the Right Hen. 

 .Sir Edward Fry will give the Opening Address, at 7.30 

 p.m. The Day and Evening Courses of study comprise 

 the various branches of Natural Science (Chemi.'-try, 

 Physics, Botany, Zoology, Geology, etc.). Mathe- 

 matics, Latin, Greek, Modern Languages, Economics, 

 Law, Logic, and Commercial Subjects. Courses con- 

 ducted by recognised teachers of the University pro- 

 vide for the l^xaminations of the Lhiiversity of Lon- 

 don, in liu- h'aculties of Arts, Science, Co'mmerce and 

 Law. I'hc icport for the last session shows that during 

 the year 84 students passed sO'me University Examina- 

 tions, while a large number gained successes at various 

 public examinations. Special classes prepare for the 

 Conjoint Board and Ci\il Service Examinations. 



