On the 14th of April Colonel Michler was detached from the staff of the commanding general of the Army of the Potomac, and directed, in conformity with instructions from Lieutenant-General Grant, to proceed to Petersburg, in order to examine and direct military surveys of the respective intrenched positions held by the two opposing armies during the siege and prepare plans of the same, combined with a detailed and accurate topographical map of the adjacent country; also to assume charge of the surveys of the different battle-fields and lines of operations from the James River to Appomattox Court-House.

By the 30th of June, the termination of the fiscal year for which this report is called, the field-work had been far advanced, but in consequence of the necessity of continuing during favorable weather the survey of the several hundred square miles through which it extends, little or no office duty was accomplished-only sufficient drawing to answer necessary purposes at the time; and consequently the maps at that time were not sufficiently far advanced, and could not exhibit the large amount and the nature of the work accomplished.

Narrative collated from reports of Major Mendell.

PONTOON TRAINS.

The companies of the U. S. Engineer Battalion with the Army of the Potomac were under the immediate command of Major Mendell, of the Corps of Engineers, with Captain Turnbull, Lieutenants Mackenzie, Benyaurd, Howell, Cuyler, and Heap, whose services, with the men under their command, are given in Major Michler's reports.

The pontoon trains for service in the field, and to accompany the several army corps, were under the command of Lieutenant-Colonel Spaulding, of the Fiftieth New York Volunteers. The services rendered by this branch of the Engineer Department were indispensable to the success of the army. Without these transportable bridges the armies could not have moved through a country intersected with numerous rivers, wide and deep, and oftentimes with rapid currents, as well as ebb and flood tides. The material of this branch of our service is modeled from the French wooden trains and the Russian canvas trains. These trains, particularly that with light caval boats, have, during this war, for the first time been proved advantageous and efficient and adapted to our country. They have been very generally used by the armies in the West and South, as well as the armies in the East. The officers having charge of these trains and their construction have devised and adopted many useful modifications in the details. Lieutenant-Colonel Spaulding has added much to these modified improvements. The accompanying tabular statement from his report will exemplify the use and value of this indispensable branch of the engineer service.*

From the above statement it appears that the total number of pontoon bridges built was thirty-eight, and their aggregate length 6,458 feet.

During the whole time covered by this report he believes the pontoon trains have been promptly on time when ordered, the bridges rapidly and skillfully built, and all other engineering operations of the command faithfully performed.

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*See Series I, Vol. XXXVI, Part I, p. 316.

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