General Van Alen was assigned to the command of Yorktown and vicinity, and having assumed control and made all necessary arrangements for the defense of the posts, the care of property, cleanliness, &c., I withdrew my command.

I am, general, very respectfully, your obedient servant,

F. J. PORTER,

Major-General, Commanding.

Brigadier General S. WILLIAMS,

Asst. Adjt. General, Headquarters Army of the Potomac.

Numbers 20. Reports of Brigadier General John G. Barnard,

U. S. Army, Chief Engineer Army of the Potomac, of operations during the siege.

CAMP NEAR YORKTOWN, VA., May 6, 1862.

SIR: The accompanying drawing (map Numbers 2)* gives with accuracy the outline and armament of the fortifications of Yorktown proper, with the detached works immediately connected therewith (map Numbers 3.). The general outline is almost the same as that of the British works in the Revolution; the trace is somewhat different. The profile is everywhere respectable. The three bastioned forts looking toward our approaches appear to have been earliest built, and have about 15 feet thickness of parapet and 8 to 10 depth of ditch the width varying much, but never being less at tops of counterscarp than 15 feet, and I should think generally much more. The works extending around the town from the western salient of fronts just mentioned appear to have been finished during the past winter and spring. They have formidable profiles, 18 feet thickness of parapet, and generally 10 feet depth of ditch.

The water batteries had generally 18 feet parapet, the guns en barbette. They were (as well as all the works mentioned) carefully constructed, with well-made sod revetments. There were numerous traverses between the guns and ample magazines, how sufficient in bomb-proof qualities I am unable to say. The water batteries were as follows: Numbers 1, five 8-inch columbiads; Numbers 3, three 32s and one 32 navy; Numbers 4, three 32s (1827); Numbers 5, two 32s; Numbers 6, three 9-inch Dahlgrens and one 10-inch Dahlgren; Numbers 7, on beach, three 8-inch columbiads and one 64-pounder, besides a 42-pounder carronade, intended to sweep the shore.

The first two guns of the works on the heights bore upon water as well as the land, and were of heavy caliber. The guns in position on the fronts of attack (the first two of which bore on the water) were as follows, commencing on the left. (See the list herewith, which gives all the guns in position or for which there were emplacements.) The emplacements were all occupied before the evacuation by siege guns, rifled, 4 1/2-inch 24-pounders and 18-pounders.

In Fort Magruder (the first exterior work) there were found one 8-inch columbiad, one 42-pounder, and one 8-inch siege howitzer, the two formed en barbette. The sketch will show the emplacements for guns on field or siege carriages, making, I think, with the foregoing,

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*All maps referred to in this report will appear in Atlas.

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