A STUDY OF RECENT EARTHQUAKES. 821 



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 di- 

 mension 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 phe- 

 nomena 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 hy- 

 drogen, 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 earth- 

 quakes, vertical shocks of great energy may sometimes be felt. Dur- 

 ing 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 Co- 

 lombia, 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 un- 

 dulations 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 pendu- 

 lums 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 



