Engineers. Captain C. S. Stewart and Second Lieutenant F. U. Farquhar, U. S. Engineers, joined after the army arrived at Fort Monroe.

The necessary bridge equipage for the operations of a large army had been collected, consisting of bateaux with the anchors and flooring material (French model), trestles, and engineer's tools, with the necessary wagons for their transportation.

The small number of officers of his corps available rendered it impracticable to detail engineers permanently at the headquarters of corps and divisions. The companies of regular engineers never had their proper number of officers, and it was necessary, as a rule, to follow the principle of detailing engineer officers temporarily whenever their services were required.

TOPOGRAPHICAL ENGINEERS.

To the corps of topographical engineers was intrusted the collection of topographical information and the preparation of campaign maps. Until a short time previous to the departure of the army for Fort Monroe Lieutenant Colonel John N. Macomb was in charge of this department, and prepared a large amount of valuable material. He was succeeded by Brigadier General A. A. Humphreys, who retained the position throughout the Peninsula campaign. These officers were assisted by Lieuts. H. L. Abbot, O. G. Wagner, N. Bowen, John M. Wilson, and James H. Wilson, Topographical Engineers. This number, being the greatest available, was so small that much of the duty of the department devolved upon parties furnished by Professor Bache, Superintendent of the Coast Survey, and other gentlemen from civil life.

Owing to the entire absence of reliable topographical maps the labors of this corps were difficult and arduous in the extreme. Notwithstanding the energy and ability displayed by General Humphreys, Lieutenant-Colonel Macomb, and their subordinates, who frequently obtained the necessary information under fire, the movements of the army were some-times unavoidably delayed by the difficulty of obtaining knowledge of the country in advance. The result of their labors has been the preparation of an excellent series of maps, which will be invaluable to any army traversing the same ground.

During the campaign it was impossible to draw a distinct line of demarkation between the duties of the two corps of engineers, so that the labors of reconnaissance of roads, of lines of entrenchments, of fields for battle, and of the position of the enemy, as well as the construction of siege and defensive works, were habitually performed by details from either corps, as the convenience of the service demanded.

I desire to express my high appreciation of the skill, gallantry, and devotion displayed by the officers of both corps of engineers, under the most trying circumstances.

During the Maryland campaign I united the two corps under Captain J. C. Duane, U. S. Engineers, and found great advantages from the arrangement.

MEDICAL DEPARTMENT.

For the operations of the medical department I refer to the reports, transmitted herewith, of Surg. Charles S. Tripler and Surg. Jonathan Letterman, who, in turn, performed the duties of medical director of the Army of the Potomac, the former from August 12, 1861, until July 1, 1862, and the latter after that date. The difficulties to be overcome in.