The Progress of the Charleston Siege.

The siege of Charleston threatens to be as protracted an undertaking as the siege of Vicksburg, and for the same reason, that Charleston has been made as strong as good natural features of defense aided by most scientific artificial means can make it. Though its defense will be stubborn, its fall is a certainty, if the means adequate to that end, which we possess, are properly used. The naval force is strong enough, but a larger land force would seem to be necessary not only to overpower the garrison of the fort in an assault, but to keep the enemy from recovering lost ground by strong sorties, which may be continually resorted to both from the interior garrison and the outside force upon the neighboring islands. According to report, one of these attacks has recovered the ground previously gained by our troops on Folly Island. The siege so far closely resembles the operations at Vicksburg. The first assault was a surprise which gave a footing on Morris Island and led to the capture of some of the batteries. The subsequent assaults have been made in the face of an expecting foe and have proven to be failures.

The attack on Fort Wagner on the 18th seems to have been particularly unfortunate, exposing us to a loss of upwards of fifteen hundred in killed and wounded.

These assaults, therefore, prove to be only weakening, as they wore at Vicksburg. Regular siege approaches will probably have to be relied upon till the assaulting party can get near enough to the intrenchments to escape their most destructive fire and be precipitated in such strength upon the work as will make its capture sure. The "sword of Gideon" is a good instrument in resolute hands, but the trenching spade and pick are valuable and necessary precursors of its use. If they are available upon the sandy beach of Morris Island, they will probably be relied upon hereafter. If they are not, the ingenuity of Gen. Gilmore may be tried in constructing artificial movable breastworks for his siege guns, as was done at Vicksburg, where they were made out of cotton bales, pushed forward upon wheeled trucks, just as a besieged town was approached before gunpowder was invented, by wooden towers, upon wheeled platforms, from the turrets and loopholes of which, the archers assailed the garrison on the walls. It is the greatest ingenuity covering the greatest fertility of resources and the most perfect mechanical appliances in war, next to the greatest celerity of movement which wins success. Gen. Gilmore seems to have this ingenuity — being one of the best civil and military engineers in the country — and he has been supplied with the most approved patterns of rifled cannon and small arms.

There is one thing, however, which detracts from the estimation we should have of his just appreciation of the means which science supplies for his aid. We see by the accounts that he has constructed a tower, within cannon range of Port Wagner, from which to observe the enemy's batteries. This might be a useful construction, if the use of balloons had not always supplied the most perfect floating observatory that any general could need. It is just as available upon the sea-shore as in the interior, where Gen. McClellan, whose scientific education led him to appreciate its advantages, used them for observing the enemy's force, as at Fair Oaks. Gen. Hooker, we see it reported, since broke up the balloon corps, and, lacking this means of discovering what the enemy in front of him at Chancellorsville was about, suffered his right wing to be suprised by Jackson, who marched his whole division in flank around Hooker's front, and drove him back in disorder. A balloon would enable Gen. Gilmore to observe safely every battery, and even to communicate by telegraphic means his knowledge below to an assaulting party, which would save many hundreds of lives sacrificed in finding out the strength of the enemy's works and the position of his guns.