Again, when the air space is twenty times greater than the space occupied by the powder the force of the powder is but 1,066 pounds to the square inch; when ten times greater it is increased to 2,525 pounds per square inch, and when the powder is equal to half that of air space the force is increased to 32,200 pounds to the square inch. When the powder fills and occupies the whole space without air space about it, as in an iron sphere (shell) so filled, the force is increased to 113,000 pounds to the square inch. In the case under consideration, the powder occupies the space equal to the hold of a vessel, and that of the air is infinite. Unless, then, we can conduct this expansive force of the powder in the vessels, immense as it may be, by a tube, or other means, to the object to be destroyed, or by means of an immense projectile in contact with the powder, we can expect no result calculated to destroy the garrison, and much less any of the solid ramparts and massive walls distant a quarter of a mile from the center of the power proposed to be used, a power that decreases rapidly with the distance from the object to be overcome. In all cases to which reference will now be made, the projectile force of the powder acting upon solid matter in immediate contact with the powder, thus making these solids the projectiles, they (the solids) have been the cause of injury, and in no case has the air been the projectile put in motion by the explosion that destroyed or injured buildings remote therefrom.

The reflux of the atmosphere filling the vacuum suddenly created by exploding the powder has in every case been the cause of forcing open and outwards doors, &c.; but the motion of translation or propulsion from the explosion, through the medium of the elastic vapor of the atmosphere, is not perceptible to any injurious degree. I consider that the explosion of a vessel load of gunpowder at the nearest point it can approach Fort Caswell or Fort Fisher, can produce no useful result toward the reduction of those works, and that no such vessel as the one proposed to be so loaded can be navigated and placed at the nearest point to these forts, provided the fort is garrisoned and its guns are served with hollow projectiles and hot shot.

FORT FISHER.

Fort Fisher is another work proposed to be destroyed, with its garrison, in the same manner as Fort Caswell. This work is situated on a sand-hill above the light-house at the northern entrance into Cape Fear River. It is unlike Fort Caswell in being formed exclusively of earthen or sand ramparts and ditch, without masonry, retaining its shape from the slopes given to the parapets, scarp, and counterscarp, with the aid of gabions and other temporary expedients. Our knowledge of it is imperfect, but enough is known to enable us to form as correct a judgment of the effects of the proposed explosion in the open air in front of it, as though we possessed as much detail as in the preceding case of Fort Caswell.

The distance of Fort Fisher from the water is not known with certainty. The light-house situated in this fort, or immediately adjacent thereto, was at the date of our last survey (1856) 440 yards from the water line on a N.65 E. course, and the nearest point to which a vessel drawing twelve feet water could float was 950 yards on the same course, very nearly double the distance of that at fort Caswell, and hence exposed to much less liability to injury from the proposed explosion. All my remarks in relation to the destruction of Fort Cas-

41 R R-VOL XLII, PT III