either by magnitude or commanding positions of works, or both, has great strength, if broken, the enemy has yet another line to carry before he can reach the bridges or the heights opposite Washington. If he attempts the left flank of the Arlington lines, by the Columbia turnpike, he takes a line of attack through comparatively low ground, swept to a greater or less degree by cross-fires of front-fires from Forts Ward, Blenker, Barnard, Richardson, Craig, Tillinghast, and Albany. The route from Ball's Cross-Roads, approaching the center and right flank of the Arlington lines, is, from the configuration of the ground, not thus closely swept and commanded. It forms the most practicable approach; it leads most directly to the point to be gained. All the ground in front, to the distance of a mile, is, however, in fact, swept in flank by the 100-pounders and other rifled guns of Fort Richardson, and of Batteries Cameroon and Parrott, at an extreme range of 2 miles, and from the 100-pounder of Fort Ward and the two 100-pounders of Battery Kemble at an extreme range of 3 1/4 miles, while it is under the direct fire, to a distance of at least 1,000 yards of the works (closely contiguous to each other), of the line.

The Commission are of opinion that this part of the line needs further strengthening,and recommend the following:

1st. A work at the red house, which shall strengthen the extreme flank of the line of the Potomac, and enfilade the long and deep ravine on the right and front of Fort De Kalb.

2d. A work on the spur behind Forts Cass and Tillinghast, which shall see into the gorges of these works, give an important fire upon the high ground in front of the line, and flank that line from Fort Woodbury to Fort De Kalb. This work will give great additional strength to Fort Corcoran, enabling it to be held, even should the two works in its front fall, and thus will enable us to maintain a tete-de-pont at the aqueduct, which cannot be held after Fort Corcoran falls.

3d. The construction of batteries for field guns along the intervals of the works,or in the lines of rifle-pits, wherever favorable locations offer themselves.

4th. The construction of sufficient bomb-proofs, to shelter the garrisons of the works named, Fort Corcoran included.

5th. The strengthening of the tete-de-pont at the aqueduct.

The Commission also recommend the construction of two works in advance of the line, at points which have been examined and indicated--- one opposite the interval between Forts Craig and Tillinghast, the other opposite the interval between Forts Woodbury and Cass, and 700 or 800 yards in front, these works to have stockaded gorges.

Fort Ethan Allen.- This is a large work, bastioned on its exposed fronts, and pretty well adapted to its important position. The Commission recommend that a retrenchment be made to cross the gorge of the west bastion; that the fire on this capital be increased by placing 20-pounder rifled guns in the adjacent flanks, and two of the same class of guns on the pan-coupe of the salient; that additional bomb-proofs be built, so as to furnish sleeping accommodations for one-half the garrison; that the parapets of the northwest front be thickened to 14 feet; that additional platforms be provided for field guns,and that traverses be constructed on the northwest and south fronts.

Fort Marcy.-Bomb-proofs are in course of construction,as well as additional platforms for guns. The Commission make no further recommendations.

The two works just named form no part of the Defenses of Washington, strictly speaking, but are of the utmost importance as a tete-de-pont