a defensive line) were nevertheless, considering the small number of troops available, arduous undertakings. Fort Corcoran, with its auxiliary works, Forts Bennett and Haggerty, and the block-houses and infantry parapets around the head of the Aqueduct, Forts Ranyon, Jackson,a nd Albany (covering our debouches from the Long Bridge), and Fort Ellsworth, on Shooter's Hill, Alexandria, were mostly works of large dimensions. During the seven weeks which elapsed between the crossing of the Potomac and the advance of General McDowell's army the engineer officers under my command were so exclusively occupied with these works (all of which were nearly completed ant the latter date), as to make impracticable the more general reconnaissances and studies necessary for locating a line of defensive works around the city and preparing plans and estimates of the same..

The works just mentioned ont he south of the Potomac, necessary for the operations of any army on that shore, were far from constituting a defensive system which would enable an inferior force to hold the long line from Alexandria to Gerogetown or even to secure their heights of Arlington..

On the retreat of our army such was our situation. Upon an inferior and demoralized force, in presence of a victorious and superior enemy, was imposed the duty of holding this line and defending the city of Washington against (as was then deemed probable) above or below..

Undecided before as to the necessity, or at least the policy, of surrounding Washington by a chain of fortifications, the station left no longer room to doubt. With our army too demoralized and too weak in numbers to act effectually in the open field against the invading enemy, nothing but the protection of defensive works could give any degree of security. Indeed, it is probable that we owe our exemption from the real disaster which might have flowed from the defeat of Bull Run-the loss to the enemy of the real fruits of his victory-to the works previously built (already mentioned), and an exaggerated idea on his part of their efficiency as a defensive line..

The situation was such as to admit of no elaborate plans nor previously-prepared estimates. Defensive arrangements were improvised and works commenced as speedily as possible where most needed. A belt of woods was felled through the forest in front of Arlington and half-sunk batteries prepared along the ridge in front of Fort Corcoran and at suitable points near Fort Albany, and a battery of two rifled 42-pounders (Battery Cameron) was established on the heights near the distributing reservoir above Goergetown to sweep the approaches to Fort Corcoran..

Simultaneously a chain of lunettes (Forts De Kalb, Woodbury, Cass, Tillinghast, and Craig), was commenced, connecting Fort Corcoran and the Potomac on the right with Fort Albany on the left, and forming a continuous defensive line in advance of the heights of Arlington. The wooded ridge which lies north of and parallel to the lower course of Four Mile Run offered a position from which the city, the Long Bridge, and the plateau in advance of it could be overlooked and cannonaded. While our external line was so incomplete, it was important to exclude the enemy from its possession. Access to it was made difficult be felling the forest which covered it (about 200 acres), and the large lunette (Fort Scott) was commenced as soon as the site could be fixed (about the middle of August). The subsequent establishment of our defensive line in advance throws this work into the same category with Forts Corcoran, Albany, Runyon, &c., as an interior work, or second line, but it.