because such a state of things would invite rather than discourage attack, and secondly, because the enemy would have vastly more to gain by success and vastly less to lose by failure than we would.  This can be accomplished much easier than is generally supposed, either by earthen parapets of sufficient thickness or by iron turrets or casements.  It is evident that the weight of metal used in these structures may be vastly greater than could be carried on shipboard.  Great weight of metal is no objection on land, but, aside from its cost, is a positive advantage.  This is evident when we consider the enormous quantity of energy stored in the larger projectiles moving at high velocities.  For example, we often hear of the sixteen inch rifle whose projectile weighs about one ton, and this enormous mass projected at a velocity of 2,000 feet per second would have a kinetic energy of 60,000 foot tons, or it would strike a blow equal to that of ten locomotives of 50 tons each running at 60 miles an hour and striking a solid wall.  Any structure designed to resist such ponderous blows must, therefore, 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.