Correspondence, memoranda, diaries, memoirs, reports, notebooks, orders, memoirs, sketches, and other papers primarily concerning Comstock's Civil War service as chief engineer of the Army of the Potomac.

Topics include the siege of Vicksburg, Miss.; the capture of Fort Fisher, N.C.; the fords on the Rapidan and Rappahannock rivers, Va.; the defenses at Aquia Creek, Va. and Harpers Ferry, W. Va.; Ulysses S. Grant's resignation as U.S. secretary of war under Andrew Johnson; James K. Polk's 1847 annual message to Congress concerning the Mexican War; Comstock's service as president of the U.S. Mississippi River Commission (1884) including improvement of the Mississippi River; the ancestry of Jesse Root Grant; and William Petit Trowbridge.

Correspondents include Henry Martyn Adams, B.S. Alexander, Daniel Butterfield, James Buchanan Eads, Ulysses S. Grant, H.W. Halleck, A.A. Humphreys, George Gordon Meade, David D. Porter, John McAllister Schofield, Philip Henry Sheridan, William T. Sherman, Daniel Edgar Sickles, E.D. Townsend, Seth Williams, and Horatio Gouverneur Wright.