In a former article, on the surrender of Forts Jackson and St. Philip, which appeared in the May number of THE CONTINENTAL MONTHLY, allusion was made to the efficiency of the mortar flotilla, to which the Coast Survey party, under charge of Assistant F. H. Gerdes, was attached, by special direction of Flag-officer D. G. Farragut. This party rendered hydrographic and also naval service, where such was required, their steamer, the Sachem, being used by the commander of the flotilla like any other vessel under his command. Captain Porter, in his letters to the Superintendent of the Coast Survey, writes, under date of April 29, 1862: "Mr. Gerdes no doubt has written to you and sent you plans. I keep him pretty hard at work. The times require it," &c. _May 16th_.--"I have not spared the Sachem, but treated her like the rest of the vessels, putting her under fire when it was necessary. I look upon the Sachem in the same light as I would upon a topographical party in the army, and if I lose her in such employment, she will have paid well for herself." After the surrender of the Mississippi forts, the mortar fleet met at Ship Island, and the Sachem being directed to join it, arrived there on the 7th of May. Under instructions from the commander, the steamer division of the flotilla stood out for Mobile bar on the 8th, and came to anchor the same evening under the lee of Sand Key, viz.: _Harriet Lane_, Com. J. M. WAINWRIGHT, flagship _Westfield_, Commander W. B. RENSHAW. _Owasco_, Commander JOHN GUEST. _Clifton_, Lieut. Com. CHARLES BALDWIN. _Jackson_, Lieut. Com. S. WOODWORTH. _Sachem_, As't U.S. Coast Survey, F. H. GERDES.