3 H THE POPULAR SCIENCE MONTHLY. 



tions 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 prac- 

 tically exempt from the shocks, while the whole country around is vio- 

 lently 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 dif- 

 ferently. 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 Cata- 

 combs. Elisee 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 terres- 

 trial 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 earth- 

 quakes, 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 fluc- 

 tuations, play but a small part in the actual production of the phe- 

 nomena, 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 proba- 

 bly due to the admission of water through fissures to the heated rocks 



