ontemporaneously on a vast superficial area. In that case, if
the upheaving force increase gradually in energy, it will at length be
restrained by only the slightest degree of superiority in the antagonist
or repressive power, and then the equilibrium may be suddenly destroyed
by any cause, such as an ascending draught of air, which is capable of
depressing the barometer. In this manner we may account for the
remarkable coincidence so frequently observed between the state of the
weather and subterranean commotions, although it must be admitted that
earthquakes and volcanic eruptions react in their turn upon the
atmosphere, so that disturbances of the latter are generally the
consequences rather than the forerunners of volcanic disturbances.[785]

From an elaborate catalogue of the earthquakes experienced in Europe and
Syria during the last fifteen centuries, M. Alexis Perrey has deduced
the conclusion that the number which happen in the winter season
preponderates over those which occur in any one of the other seasons of
the year, there being, however, some exceptions to this rule, as in the
Pyrenees. Curious and valuable as are these data, M. d'Archiac justly
remarks, in commenting upon them, that they are not as yet sufficiently
extensive or accordant in different regions, to entitle us to deduce any
general conclusions from them respecting the laws of subterranean
movements throughout the globe.[786]

_Permanent elevation and subsidence._--It is easy to conceive that the
shattered rocks may assume an arched form during a convulsion, so that
the country above may remain permanently upheaved. In other cases gas
may drive before it masses of liquid lava, which may thus be injected
into newly opened fissures. The gas having then obtained more room, by
the forcing up of the incumbent rocks, may remain at rest; while the
lava congealing in the rents may afford a solid foundation for the newly
raised district.

Experiments have recently been made in America, by Colonel Totten, to
ascertain the ratio according to which some of the stones commonly used
in architecture expand with given increments of heat.[787] It was found
impossible, in a country where the annual variation of temperature was
more than 90 degrees F., to make a coping of stones, five feet in
length, in which the joints should fit so tightly as not to admit water
between the stone and the cement; the annual contraction and expansion
of the stones causing, at