om the destruction of the rocks on which the glacier is
formed. The effect of a torrent in deepening its bed will explain the
mechanical agency of fluid-water, though this effect is infinitely
increased, and sometimes almost entirely dependent, upon the solid
matters which are carried down by it. An angular fragment of stone in
the course of ages moved in the cavity of a rock makes a deep round
excavation, and is worn itself into a spherical form. A torrent of rain
flowing down the side of a building carries with it the silicious dust,
or sand, or matter which the wind has deposited there, and acts upon a
scale infinitely more minute, but according to the same law. The
buildings of ancient Rome have not only been liable to the constant
operation of the rain-courses, or minute torrents produced by rains, but
even the Tiber, swollen with floods of the Sabine mountains and the
Apennines, has often entered into the city, and a winter seldom passes
away in which the area of the Pantheon has not been filled with water,
and the reflection of the cupola seen in a smooth lake below. The
monuments of Egypt are perhaps the most ancient and permanent of those
belonging to the earth, and in that country rain is almost unknown. And
all the causes of degradation connected with the agency of water act more
in the temperate climates than in the hot ones, and most of all in those
countries where the inequalities of temperature are greatest. The
mechanical effects of air are principally in the action of winds in
assisting the operation of gravitation, and in abrading by dust, sand,
stones, and atmospheric water. These effects, unless it be in the case
of a building blown down by a tempest, are imperceptible in days, or even
years; yet a gentle current of air carrying the silicious sand of the
desert, or the dust of a road for ages against the face of a structure,
must ultimately tend to injure it, for with infinite or unlimited
duration, an extremely small cause will produce a very great effect. The
mechanical agency of electricity is very limited; the effects of
lightning have, however, been witnessed, even in some of the great
monuments of antiquity, the Colosaeum at Rome, for instance; and only
last year, in a violent thunderstorm, some of the marble, I have been
informed, was struck from the top of one of the arches in this building,
and a perpendicular rent made, of some feet in diameter. But the
chemical effects of electrici