War of the Rebellion: Serial 126 Page 0169 UNION AUTHORITIES.

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Boards of engineers have been detailed to consider what modifications are necessary at each and every work along our sea- coast to adapt them to resist the powerful armaments that European fleets, singly or combined, may be enabled to bring across the Atlantic, over the bars of our ports and harbors, to attack them.

The details of the operations during the year at the several works on the Atlantic, Gulf of Mexico, Lake and Pacific Coasts, derived mostly from the reports of the superintending engineers, are annexed.

PRISON DEPOTS.

The prison depots also called for the labors of engineer officers. Point Lookout, at the mouth of the Potomac, was subject to sudden attack from marauding parties and detached cavalry from armies operating against Baltimore and Washington, which, with the immense body of prisoners, made it necessary to fortify the position against attempts to liberate them by forts commanding both the interior and exterior. Major Stewart, assisted by Lieutenant Cantwell, and afterward Colonel Brewerton, constructed these defensive works.

THE PRISON DEPOT AT JOHNSON'S ISLAND.

Cleveland Harbor, Lake Erie, had to be defended against attempts of the prisoners, and succor by water from expeditions organized in the friendly and neutral territories of Great Britain in Canada. Major Casey and Captain Tardy were assigned to and performed this service, constructing a water battery at the mouth of the harbor against a force approaching by water, and temporary field forts on Johnson's Island.

SURVEYS, MAPS, AND TOPOGRAPHY.

The surveys for the armies in the field, embracing the topography of the country passed over and particular sites occupied, have been referred to in other parts of this report.

The extent of the labors performed by the officers on duty in the Bureau has been the engraving, lithographing, photographing, and issuing 24,591 sheets for officers in the field and various branches of the service requiring this information, leaving still on hand a few copies of each publication for reference and the calls of the War Department.

The survey of the northwestern lakes has been in progress for several years, to obtain for the commerce of the States whose industry is promoted by that extended interior navigation the safety that a perfect and correct knowledge of the shores and bottom alone can attain. It is being conducted under the direction of Major Reynolds, with the same care and accuracy that has been bestowed upon the coast survey and other national geodetic surveys. During the year two maps have been prepared from the field-notes published and issued to the navigators of the lakes. One gives the west end of Lake Superior and the other the northeastern part of Lake Michigan. Three others have been prepared and are now ready for engraving, giving the Portage River and the Bay of L"Anse, on Lake Superior, and a third giving the north end of Green Bay.