Harper's Ferry, like a large fortress, is a srtong position if held by troops enough. The number required to hold it is rather out of proportion to its strategic importance. As we must guard the great connecting link between the Ohio and Potomac, the Baltimore and Ohio Railroad, it is necessary to have a secure debouche into the Shenandoah Valley. Hence, we must hold Harper's Ferry. So, should the theater of war again be Northern Virginia, there might be urgent reasons for holding Harper's Ferry, even though it required 8,000 or 10,000 men. On the other hand, the point might lose its importance, and it might become desirable to abandon it. I have, therefore, left anxious to restrict the fortifications and the number of guns of position as much as possible, that, if evacuated, everything valuable might be easily removed.

I am, very respectfully, your most obedient,

J. G. BARNARD,

Brigadier-General.

P. S. - A sketch* of the place and works will be sent you as soon as a copy can be made. I examined Loudoun Heights very carefully, riding over every point where there seemed any possibility of an enemy's planting artillery, and came to the conclusion that, under the artillery fire of Maryland Heights and Fort Duncan, it would be impracticable for an enemy to plant artillery upon them.

[Inclosure.]

Memorandum for Colonel Raynolds, concerning the fortifications of Harper's Ferry.

The position of Bolivar Heights is too extensive to be held by us, and it is enfiladed by the guns of Fort Duncan, and, taken obliquely by the guns of the 30-pounder battery, an enemy cannot occupy it with his artillery.

The line actually fortified on the Harper's Ferry heights is, I think, excellent. It can be held by one regiment. It should be put in gold condition, and a platform and embrasure prepared on each flank on the bastioned trace, to run in a field gun, if required. On the right flank, rifle-pits should be extended, so as to see down the steep slope to the river, and a place for a field gun should be prepared either on the bank or at the foot of the slope, for a gun to sweep the road and the railroad. On the left flank, likewise, there should be an emplacement for two field guns to sweep the Winchester road and railroad.

With these precautions, and the norther side of the Potomac held in a proper manner, it will be almost impossible to attack this line.

Northern shore of the Potomac. - The key points to the defense of Harper's Ferry are Maryland Heights and Fort Duncan, and on the former particularly depends the defense. The most practicable line of attack is from the north. The enemy will first attack the summit of the mountain, either from Solomon's Gap, or by ascending the lateral slopes of the mountain north of us, and then advance, deploying and endeavoring to turn the flanks of our position. I cannot regard the present arrangements as altogether adequate. all the artillery fire is confined to the crest of the mountain. There is no flanking fire to sweep the point of our lines of rifle-pits, around and over which the enemy would try to force his way. The citadel does well enough as a keep, but should not have been the sole support of the rifle-pits.

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*To appear in Atlas.

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