re,
have enormous weight, or it will be overturned or driven bodily from
its foundations. If the armor itself is not thick enough to give the
required weight as well as resistance to penetration, the additional
stability must be supplied by re-enforcing it with heavy masses of
metal or masonry. It is evident, therefore, that _quality_ of metal is
less important than _quantity_, and that so long as it is sufficiently
tough to resist fracture, a soft, cheap metal, like wrought iron or
low steel, is better adapted for permanent works than any of the fancy
kinds of armor that have been tested for naval purposes. As an
illustration of this, we may compare compound or steel-faced armor
with wrought iron as follows: The best of the former offers only about
one-third greater resistance to penetration than the latter, or 12
inches of compound armor may equal 16 inches of wrought iron, but the
cost per ton is nearly double; so that by using wrought iron we may
have double the thickness, or 24 inches, which would give more than
double the resistance to penetration, in addition to giving double the
stability against overturning or being driven bodily out of place. But
our guns may be reasonably well protected by earthen parapets without
any expensive armor by so mounting them that when fired they will
recoil downward or to one side, so as to come below the parapet for
loading. This method of mounting is called the disappearing principle,
and has been suggested by many engineers, some of whose designs date
back more than one hundred years. We may also mount our guns in deep
pits, where they will be covered from the enemy's guns, and fire them
at high elevation, so that the shell will fall from a great height and
penetrate the decks of the enemy's ships. This is known as mortar
firing, but the modern ordnance used for this purpose is more of a
howitzer than a mortar, being simply short rifled pieces arranged for
breech loading. All our batteries should, of course, be as far from
the city or other object to be protected as possible, to prevent the
enemy from firing over and beyond the batteries into the city.

But, with all these precautions, the enemy might put on all steam and
run by us either at night or in a dense fog, and we must have some
means of holding him under the fire of our guns until his ships can be
disabled or driven away. This object is sought to be accomplished by
the use of torpedoes anchored in the channels and und