Battery Harris was a concrete gun emplacement that housed two rapid-fire five-inch rifled guns mounted on balanced pillar carriages. Completed in 1900, it was named for Col. David Harris, commander of an American artillery regiment during the British attack on Baltimore in 1814. Originally it had two five-inch gun emplacements, but after the guns were removed in 1917, a Coincidence Range Finder station was constructed in the north side emplacement and linked to the three-inch rapid-fire guns of Batteries Lazear and Claggett. For a coast artillery fort to be effective, gun crews had to know their distance from enemy ships, and soldiers in range finding stations determined those distances. The concrete structure on the left side of Battery Harris housed a Coincidence Range Finder with a single eyepiece at the center and two windows at either end through which images of the target entered; those images were reflected to the center, where a prism turned the light toward the eyepiece, and the operator adjusted them until they merged into one coincident image, then determined the target's distance by triangulating the angles between the two windows and the target. Each gun had an eighteen-foot barrel and weighed twenty-four tons, and gun crews moved the eighty-pound rounds, containing the projectile, propellant powder, and a primer, by hand from storage to the breech. The pillar mounting could be cranked above the parapet when firing and lowered when inactive. The battery's rapid-fire five-inch guns and the three-inch guns of Batteries Lazear and Claggett were intended to engage and destroy fast torpedo boats, mine sweepers, and landing craft that could leave the shipping channel and enter shallow water near the shore.