Page:Popular Science Monthly Volume 29.djvu/328

314 are the worst. The progress of the wave may be interrupted by the interposition of a mountain-range or a hill, in which case we have behind the barrier the phenomenon called an earthquake-shadow; it may be cut off by a deep ditch, as a canal; and in certain parts of South America there appear to exist tracts of ground which are practically exempt from the shocks, while the whole country around is violently shaken. It would seem as if the shock passed beneath such a district as water passes beneath a bridge; and for this reason such tracts have been christened "bridges." In the Syrian earthquake of 1837, neighboring villages, and even neighboring houses, suffered differently. In one case a house was entirely destroyed, while in the next house nothing was felt. In Japan, at a place called Choshi, about fifty-five miles east of the capital, earthquakes are seldom felt, although the surrounding districts may be severely shaken. At this place a large basaltic boss rises in the midst of alluvial strata. The immunity of the district from earthquakes has probably given rise to the myth of the Kanam rock, which is a stone supposed to rest upon the head of a monstrous cat-fish, whose writhings cause the shakings so often felt.

Possibly something may be done in arranging the surroundings of buildings to ward off the destructive effects of earthquakes. The Temple of Diana, at Ephesus, was built on the edge of a marsh for this object. Pliny says that the Capitol of Rome was saved by the Catacombs. Elisée Reclus says that the Romans and Hellenes found out that caverns, wells, and quarries retarded the disturbance of the earth, and protected edifices in their neighborhood. The Tower of Capua was saved by its numerous wells. Vivenzis asserts that in building the Capitol the Romans sank wells to weaken the effects of terrestrial oscillations; and Humboldt relates the same of the inhabitants of San Domingo. Quito is said to receive protection from the numerous canons in the neighborhood, while Lactacunga, fifteen miles distant, has often been destroyed. Similarly, it is extremely probable that many portions of Tokio have from time to time been protected more or less from the severe shocks of earthquakes by the numerous moats and deep canals which intersect the city.

Various causes have been assigned for the production of earthquakes, and, although they may all singly or in combination contribute to the effect, we must conclude, after considering the whole subject, that the primary cause is endogenous to our earth, and that exogenous causes, like the attraction of the sun and moon, and barometric fluctuations, play but a small part in the actual production of the phenomena, their greatest effect being to cause a slight preponderance in the number of earthquakes at particular seasons. The majority of earthquakes are due to explosive efforts at volcanic foci. The greater number of these explosions take place beneath the sea, and are probably due to the admission of water through fissures to the heated rocks