EARTHQUAKES. 7 
The first theory as to the cause of earthquakes which attained 
to any importance is that of the Rev. J. Michell, who published 
it in 1760. By this theory earth-waves are supposed to be 
generated by movements of the liquid interior of the earth. 
“ They are,” says the author, “ like folds in a shaken carpet.” Few 
geologists, if any, now accept this theory, for it is founded on 
erroneous ideas as to the nature of the interior of the earth. It 
has sometimes been advanced that the cause of the shock is the 
sudden flashing into steam of superheated water as it cooled 
down. Messrs Volger and Morh think that some of the smaller 
earthquakes may be accounted for by the falling in of the roofs 
of caves, which have been formed by rock salt or limestone, 
having been dissolved and removed by water. The late Mr 
Poulett Scrope supposed that they are due to the sudden rupture 
of rocks, caused by contraction or expansion. Mr Mallet, our 
greatest authority on the subject, thinks that this could only give 
rise to weak shocks, and that the larger shocks are due to the 
crushing of rocks by the cooling and contraction of the earth’s 
crust. 
A study of the geographical distribution of earthquakes shows 
that they are, to a large extent, connected with volcanic eruptions. 
- Those countries whichcontain active volcanoes are far more subject 
to earthquakes than any others, although the most violent earth- 
quakes arenotinthe immediate neighbourhood of thevolcaniccone, 
but several degrees away from it. There are, however, many coun- 
tries in which earthquakes occur, and which do not contain any 
volcanoes. Such are the plains of Holland and Prussia, of the 
Indus and Cutch. Indeed, no district in the world appears to be 
altogether exempt ; even in Egypt earthquakes were felt in Janu- 
ary, 1740. The existence of extinct volcanoes does not appear 
to increase the number of earthquakes in a district. 
The times at which earthquakes occur are very irregular, and 
subject to great variations. During the Roman Empire, Antioch 
was shaken almost every year. Then followed a long period of 
repose lasting nearly 300 years, and since that time the shocks 
have been as violent as ever. Earthquakes occur at all times of 
the day and at all seasons of the year, but Mr Mallet, froma © 
study of nearly 7000 recorded shocks, thinks that they are rather 
more common at the winter solstice. He says, however, that his 
tables show a decided periodicity in large eartthquakes, the max- 
imum being in the middle of each century, with a second epoch, 
rather less powerful, toward the close of the century. 
These facts show, as might be expected, that earthquakes are 
not due to cosmical causes, the varying positions of the sun and 
moon having, perhaps, not so much influence as they have on the 
weather, and we seem almost driven to choose between explosions 
of gas and splitting of rock for the cause. Perhaps both may be 
vere caus@, but there are some reasons for giving preference to 
the latter, All large earthquakes are followed, and are very often 
preceded by several minor shocks, which gradually get smaller 
and smaller, This is just what we should expect in the rupture 
