It has not been possible to obtain a complete and correct list of all the regiments engaged in that battle. Some regiments, after the battle of South Mountain, were detached from their brigades, and sent on special service; others were kept in the rear, to guard the trains; others were sent on flank movements. But much care has been taken in the description of that battle to give the exact position of the divisions engaged, and also the brigades, so that it will be comparatively easy to ascertain the general position of most of the regiments. ORGANIZATION OF THE ARMY OF THE POTOMAC, APRIL, 1862. CAVALRY RESERVE. BRIG.-GEN. P. ST. G. COOK. _Emory's Brigade._ 5th U. S. Cavalry. 6th " " 6th Penn. " _Blake's Brigade._ 1st U. S. Cavalry. 8th Penn. " Barker's Squadron, Ill. Cavalry. ARTILLERY RESERVE. COL. HENRY J. HUNT. Graham's Battery "K" & "G" 1st U. S. 6 Napoleon guns. Randall's " "E" 1st " 6 " " Carlisle's Battery "E" 2d U. S. 6 20-pds. Parrott guns. Robertson's " 2d " 6 3-in. ordnance " Benson's " "M" 2d " 6 " " " Tidball's " "A" 2d " 6 " " " Edwards's " "L" & "M" 3d " 6 10-pds. Parrott " Gibson's " "C" & "G" 3d " 6 3-in. ordnance " Livingston's " "F" & "K" 3d " 4 10-pds. Parrott " Howe's " "G" 4th " 6 Napoleon " De Russy's " "K" 4th " 6 " " Weed's " "I" 5th " 6 3-in. ordnance " Smead's " "K" 5th " 4 Napoleon " Ames's " "A" 5th " 6 { 4 10-pds. Parr.}" { 2 Napoleon }" Diederick's " "A" N. Y. Art. Batt'n 6 20-pds. Parrott " Voegelie's " "B" " " " 4 " " " Knieriem's " "C" " " " 4 " " " Grimm's " "D" " " " 6 32-pds. Howitzers. ---- 100 guns. VOLUNTEER ENGINEER TROOPS. GEN. WOODBURY. 15th New York Volunteers. 50th " " " REGULAR ENGINEER TROOPS. CAPT. DUANE. Companies "A," "B," and "C," U. S. Engineers. ARTILLERY TROOPS WITH SIEGE TRAIN. 1st Connecticut Heavy Artillery. _Col. Tyler._ SECOND CORPS. GEN. SUMNER. _Cavalry._ 8th Illinois Cavalry. _Col. Farnsworth._ One Squadron 6th New York Cavalry. RICHARDSON'S DIVISION. _Artillery._ Clark's Battery "A" & "C" 4th U. S. 6 Napoleon guns. Frank's " "G" 1st N. Y. 6 10-pds. Parrott guns. Pettit's " "B" 1st " 6 " " " Hogan's " "A" 2d " 6 " " " _Infantry._ _Howard's Brigade._ 5th N. H. Vols. 81st Penn. " 61st N. Y. " 64th " "