The line once established by the location of the larger forts, the process already employed on the Virginia
side was used to fill in the gaps.
Supporting works of usually less strength, were placed within rifle-range along the crest.
The problem of resting the left of the line on the Potomac
, however, was more difficult.
There were two matters of paramount importance, the consideration of which indicated a position for the line quite different from that indicated by the topography.
It must be remembered that the Chain Bridge
crossed the Potomac
about three miles above Georgetown
, and the receiving reservoir which supplied most of Washington
with water was about three and one-half miles from the latter place.
The value of the bridge and reservoir rendered their protection necessary.
But the high ground, upon which naturally the line of forts should be placed, ran toward the Potomac
on a line south of Powder-Mill Run, the stream supplying the reservoir, which approached the river at the point where the bridge crossed.
It was obvious that works placed on these heights would not protect the reservoir, and that the bridge would be in the zone of fire of any force attacking the forts.
Hence the line of works was broken, and three isolated works, afterward united into one, were placed on high ground to the north of the reservoir, and far enough above the bridge to prevent artillery fire from reaching it.
South of Anacostia Branch the problem at first appeared to be capable of solution by placing bridge-heads, or small forts covering the approaches to the bridges, on the south side.
There were two bridges, one at the navy-yard, about two miles up the creek, and Benning's Bridge, some two and onehalf miles above the first.
In addition, it appeared that there should be at least one large Fort overlooking and protecting the navy-yard and the arsenal, which latter was on the point at the confluence of the Anacostia
and the Potomac
, and which contained large quantities of war-supplies of all kinds.
A more critical examination, however, showed the necessity of