METALLIC BURNING MIRRORS.
The classical student will remember that Archimedes
burned the fleet of Marcellus, by means of burning-glasses,
from the heights of the fortifications of his
native city of Syracuse. Unfortunately, any account of
the system of catoptrics, or the science of reflections,
employed by the ancient Syracusan in their construction
is lost to us, and many modern writers have
gone so far as to doubt the fact altogether. The
knowledge of the properties, however, of concave
mirrors which we have just been acquiring, will enable
us to form a pretty good guess as to the means adopted
by Archimedes for the destruction of the enemy's fleet.
The ancients, not having the means of either casting or
grinding such enormous mirrors, must have constructed
them of a large number of small ones, so arranged
that the images of the sun reflected by them would all
fall in the same place, or nearly so. In this case, the
larger the number of mirrors, the greater would be the
burning effect. In order to explain the reflection of
rays incident upon the surface of concave mirrors, we
supposed them to consist of an immense number of
plane mirrors placed in a curve, so that the reflected
rays might all meet in one point; but on examining
into the history of burning mirrors, we find that the
plan has been adopted in reality in a great number of
