¥ ON THE FACTS OF EARTHQUAKE PHENOMENA. 31 
is enormous; thus, in the great Lisbon earthquake of November 1755, an 
area of the earth’s crust more than four times the surface of all Europe was 
shaken. 
It was felt inthe Alps, on the shores of Sweden, in the West Indies, on the 
- Jakes of Canada, in Ireland, in Thuringia and in Northern Germany ; at 
Toepliz (where the hot springs ran dry), and at the Lesser Antilles, the usual 
tide of two feet or so was one of twenty feet. 
Thus, taking the area shaken at 3300 miles long and 2700 miles wide, which 
is equal to 7,500,000 square miles, and supposing the motion only extended 
to an average depth of twenty miles, there must have been 150 millions cubic 
miles of solid matter put in motion, a mass which conveys to the imagination 
some notion of the enormous power of the originating impulses. Yet let it 
be remembered that the whole of this mass was never in motion at once, but 
merely a comparatively small crest or wall of its particles put in motion, 
which transferred their moving force again to those beyond. 
- The earthquake in Syria in 1759, extended, says Sir C. Lyell, over a space 
of ten thousand square leagues, and for three months continuously this vast 
area was shaken. 
Hamilton thinks the main force of the great Calabrian earthquake was 
- comprised within a circle of 44 miles diameter, or 1520 miles area, but that 
its shocks were felt throughout a circle of 144 miles diameter or over an area 
_ of country of 16,286 square miles. 
M. Place (Quart. Journ. vol. xvii.) says of the earthquake of 1820, the 
| principal force was exerted in a circle of about 50 miles diameter, the centre 
- alittle N.E. of Valparaiso ; persons N. of that felt the shocks trom the S.W., 
_ those to the S. of it from the N.E. The earthquake was felt from Copiabo 
_ inthe north to Valdivia in the south, distant 900 miles, and convulsed not 
Jess than 100,000 square miles. 
| Sometimes however the area shaken even by a very violent shock is ex- 
_ tremely limited; thus the city of Coquimbo was destroyed in great part by a 
_ shoek in 1820, which produced no alarm and did no mischief in any other 
| part of the country, according to M. Place (Quart. Journ. vol. xvii.). The 
_ shock here probably in every case is a vertical one, from directly beneath, 
_ and at a small depth as regards centre of impulse. 
15th. The shock or earth-wave is a true undulation of the solid 
crust of the earth. 
_ “The sand in the streets of Port Royal rose like waves of a troubled sea,” 
says the recounter of the great Jamaica earthquake of 1692. 
ο΄ Theamount of undulations, and the rapidity with which these succeed each 
other, differ, but the great mass of earthquake observers concur in descri- 
bing a distinct undulation of the surface of the ground. In the greatest 
_ shocks this undulation has been often visible to the eye, as in the great 
_ Jamaica earthquake, where the passage of the wave was said to be rendered 
_ visible by the opening and immediate closing in again of fissures (this how- 
ever needs confirmation). It is indirectly rendered evident by the tops of 
᾿ς trees bending over first to one side and then to the opposite, and by various 
_ other motions described as communicated to solids and liquids. 
᾿ς Whenever the undulation of the surface has been described as most distinct, 
the direction of the shock has been also described by being nearly horizontal ; 
_ where, on the other hand, the shock has been felt as coming up from beneath, 
_ the undulation of the surface has escaped observation or not existed. 
_ In the smaller shocks, whatever their direction may have been, the undu- 
_ lation of the surface has not been observed, that is to say, was not directly 
observable ; but it has been inferred from observations made as to the oscilla- 
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