276 ANNUAL OF SCIENTIFIC DISCOVERY. 



sinks with tremendous noise and violence, often taken for an earth- 

 quake shock. " At the latter end of last century, one or more of the 

 great vertical and impost stones of Stonehenge suddenly fell down ; 

 the concussion produced a wave, which was transmitted around in 

 every direction, like that upon a pool of water into which a pebble 

 has been dropped, and the shock felt in all the neighbouring hamlets 

 was so great, that for some time, until the cause was discovered, it 

 was thought to have been an earthquake, as in fact it was, though not 

 produced by the usual causes. So, also, when the great powder mag- 

 azine was blown up near Corunna, at the conclusion of Sir John 

 Moore's retreat, the ground rocked sensibly for miles away, and the 

 wave was felt at a distance before the sound of the explosion was 

 heard." 



It is generally known that an earthquake is frequently accompanied 

 by a disturbance of the ocean, which at times does great mischief at 

 places far from the centre of the shock ; more so where the land slopes 

 gradually to the water than where it is precipitous. "It is remarka- 

 ble," says Darwin, " that while Talcahuano and Callao, both situ- 

 ated at the head of large shallow bays, have suffered severely during 

 every earthquake from great waves. Valparaiso, seated close to the 

 edge of profoundly deep water, has never been overwhelmed, though 

 so often shaken by the severest shocks." Many readers will remem- 

 ber that the great earthquake at Lisbon was followed by a huge wave, 

 which came rushing in from the sea some time afterwards, and fear- 

 fully aggravated the previous alarm and destruction. The focus of 

 the shock was forty miles from land, and the wave was forty feet in 

 height ; it swept three thousand persons off the quay, to which they 

 had betaken themselves to be out of the way of falling buildings. An 

 attempt has been made to account for the effect produced, by suppos- 

 ing that the falling in of a vast cavity in the ocean bed far away from 

 the coast caused a sudden recession of water on the shore ; or that, the 

 whole mass of dry land being bodily elevated by the shock, and let 

 down again, it w r ould appear as though the sea had retreated and 

 come in again ; or that these effects were referhble to an upheaval of 

 the bottom of the sea. None of these views satisfy the newly ad- 

 vanced theory. According to Mr. Mallet, the original impulse given 

 to the bed of the sea acts simultaneously upon the earth, the sea, and 

 the atmosphere, originating at the same instant, and transmitting one 

 or more waves through each. " The earth-wave moves with an im- 

 mense velocity, probably not less than 10,000 feet per second, in hard 

 stratified rock, and perhaps little short of this in the less dense strata." 

 But while the earth-wave travels at this rate, sound moves through 

 water at about 4,700 feet per second, so that a double sound will be 

 heard from the sea after the land sound. Yet at times the waves of 

 sound are absent while the others are present. In such cases it is 

 supposed that no fracture of the earth's crust takes place, but merely 

 a bending or flexure, which might naturally occur without the con- 

 cussions that accompany actual breaking. Differences or anomalies in 

 the times of shocks becoming audible are accounted for by the differ- 

 ence of strata through which they travel. The earth-wave varies in 



