Page:Popular Science Monthly Volume 27.djvu/843

Rh them were Albania, Arenas el Rey, Albunuelas, Periana, Zaffaraya, and Venta de Zaffaraya. These places, situated above the center of the agitation, are scattered over a surface of which the principal dimension does not reach forty miles; but the movements of the ground extended far beyond this region, to Seville on the west, Cape Gadez on the east, and Molena de Aragon. The movements provoked phenomena of different kinds. Crevasses, several miles long and several yards wide, were opened at various places. From one of them, near Santa Cruz, exhaled fetid gases, having the odor of sulphureted hydrogen, and there burst from the same fissure a copious spring of sulphurous water with a temperature of about 90°; while at a short distance from this point the thermal springs of Alhama, that have been in use from antiquity, were heated to a higher temperature and acquired a sulphurous character.

The districts near the Sierra Nevada and its ramifying spurs have frequently been the center of subterranean commotions; and it is an important fact, not to be neglected, that the shocks have many times, as in the present case, been repeated for several weeks in succession.

Among the movements which the ground undergoes during earthquakes, vertical shocks of great energy may sometimes be felt. During the earthquake in Calabria in 1783, houses were thrown up into the air, as if by the explosion of a mine; and at Riobamba, in Colombia, in 1812, several persons were cast bodily upon a hill more than three hundred feet high. These motions are called succussions, or subsultory tremors. The most frequent movements, and generally the most extended, are the undulatory ones, which are propagated horizontally, like the waves which we can observe at any time on a liquid surface. Like those waves, they may, when continued for some minutes, cause a kind of sea-sickness. Sometimes the terrestrial undulations are so strong as to bend over trees till their limbs touch the ground. These two forms of tremor may be associated together, or they may succeed one another at very short intervals. Various instruments—seismographs or seismometers, analyzers, and pendulums—are used to determine their intensity, direction, and duration, and register their characters.

The intensity of the shocks is extremely variable. Sometimes they are hardly perceptible, or marked only by low rumblings; often they are so strong that works of masonry are overthrown by them. For this reason special modes of construction are employed in countries subject to earthquakes, as adapted to oppose the least resistance.

In the most usual undulatory movement, the agitation is naturally stronger at the top of buildings than at their base. Thus, in the theatre at Madrid, on the 25th of December last, the upper gallery was visibly shaken, while the parquette was unmoved. For the same reason, the motions are incomparably less sensible in the interior of mines than on top of the ground. M. Domeyko relates that he was