Parr Sucr ii] EARTHQUAKES. 269 
during the earthquake of 1692 were built upon solid white limestone, 
while the parts built on sand were shaken to pieces. 
It has been observed that an earthquake shock will pass under 
- a limited area without disturbing it, while the region all round has 
been affected, as if there were there some superficial stratum pro- 
tected from the earth-wave. Humboldt cited a case where miners 
were driven up from below-ground by earthquake shocks not 
_ perceptible at the surface, aud on the other hand, an instance where 
they experienced no sensation of an earthquake which shook the 
surface with considerable violence. Such facts bring impressively 
before the mind the extent to which the course of the earth-wave 
must be modified by geological structure. In some instances the 
shock extends outwards from a common centre, so that a series of 
concentric circles may be drawn round the focus, each of which 
will denote a certain approximately uniform intensity of shock 
(“coseismic lines” of Mallet), this intensity of course diminishing 
with distance from the focus. The Calabrian earthquake of 1857 
and that of Central Hurope in 1872 may be taken in illustration 
of this central type. In other cases, however, the earthquake travels 
chiefly along a certain band or zone without advancing far from it 
laterally. ‘This type of linear earthquake is exemplified by the 
frequent shocks which traverse Chili, Peru and Ecuador, between 
the line of the Andes and the Pacific Coast. ; 
Extent of country affected—rThe area shaken by an earth- 
quake varies with the intensity of the shock, from a mere local tract 
where a slight tremor has been experienced, up to such catastrophes 
as that of Lisbon in 1755, which, besides convulsing the Portuguese 
coasts, extended into the north of Africa on the one hand and to 
Scandinavia on the other, and was even felt as far as the east of 
North America. Humboldt computed that the area shaken by this 
ereat earthquake was four times greater than that of the whole of 
Europe. The South American earthquakes are remarkable for the 
great distances to which their effects extend in a linear direction. 
Thus the strip of country in Peru and Ecuador severely shaken 
by the earthquake of 1868, had a length of 2000 miles. 
Depth of source.—Over the centre of origin the shock is felt 
as a vertical up-and-down movement (Seismic vertical of Mallet), 
Receding from it in any direction this shock is felt as an undulatory 
movement and comes up more and more obliquely. The angle of 
emergence, as Mallet showed, may be obtained by taking the mean 
of observations of the rents and displacements of walls and buildings. 
In Fig. 65, for example, the wall there represented has been rent 
by an earthquake which emerged to the surface in the path marked 
by the arrow. 
_ By observations of this nature Mr. Mallet has shown how it may be 
possible to estimate approximately the depth of origin of an earth- 
quake. Let Fig. 66, for example, represent a portion of the earth’s 
_? “© Cosmos,” Art. Larthquakes. 
