242 SCIENCE PROGRESS 



yet begun to pass through the streets but the crowd was so 

 great within the churches that nearly three or four thousand 

 persons were crushed by the falling of the roofs." 



The suddenness of onset of the shock is a second factor of 

 considerable importance. Almost invariably the shock is pre- 

 ceded by a deep rumbling sound accompanied by a faint tremor 

 which may last five or more seconds before the vibrations attain 

 a destructive strength ; the same sound precedes both weak 

 and strong shocks and at first affords no certain warning of 

 the disaster but in earthquake countries it is one that is always 

 heeded. " If it had happened in the middle of the night," wrote 

 Darwin of the Concepcion earthquake of 1835, " the greater 

 number of the inhabitants . . . must have perished, instead of 

 less than a hundred ; as it was, the invariable practice of running 

 out of doors at the first trembling of the ground alone saved 

 them. In Concepcion each house or row of houses stood by itself, 

 a heap or line of ruins." To the same cause may be attributed 

 the comparatively small loss of life in such earthquakes as those 

 which destroyed Cumana in 1797 and Port Royal in 1692. 



In many earthquakes, however, the warning given by the 

 earthquake sound is too brief to be of service. This was 

 the case, even with those who were awake, at Dharmsala 

 in 1905 and at Messina in 1908. In the Ischian earthquake 

 of 1883 sound and preliminary tremor were both absent within 

 the central district. So suddenly and with such intense violence 

 did the shock begin that survivors at Casamicciola found them- 

 selves beneath the ruins of their houses before they realised 

 that an earthquake had occurred. 



The death-rate of an earthquake is often increased by the 

 rapid succession of strong after-shocks. In the central district 

 every great earthquake is followed by almost incessant tremors 

 among which stronger shocks are interspersed. The Riviera 

 earthquake of 1887 occurred at about 6.20 a.m. At 6.29 there 

 followed a second shock and at 8.51 a third of intermediate 

 strength. To these two shocks are attributed one-quarter of 

 the total amount of damage and also the small number of 

 wounded, many of those who lay buried in the ruins having 

 been killed by the subsequent overthrow of the shattered walls. 

 In this earthquake the number of persons wounded was only 

 72 per cent, of the number killed. The Neapolitan earthquake 

 of 1857 was succeeded after about an hour by another strong 



