On the 18th of December, at my camp by the side of the plank-road, eight miles back of Savannah, I received General Hardee’s letter declining to surrender, when nothing remained but to assault.  The ground was difficult, and, as all former assaults had proved so bloody, I concluded to make one more effort to completely surround Savannah on all aides, so as further to excite Hardee’s fears, and, in case of success, to capture the whole of his army.  We had already completely invested the place on the north, west, and south, but there remained to the enemy, on the east, the use of the old dike or plank-road leading into South Carolina, and I knew that Hardee would have a pontoon-bridge across the river.  On examining my maps, I thought that the division of John P. Hatch, belonging to General Fosters command, might be moved from its then position at Broad River, by water, down to Bluffton, from which it could reach this plank-road, fortify and hold it—­at some risk, of course, because Hardee could avail himself of his central position to fall on this detachment with his whole army.  I did not want to make a mistake like “Ball’s Bluff” at that period of the war; so, taking one or two of my personal staff, I rode back to Grog’s Bridge, leaving with Generals Howard and Slocum orders to make all possible preparations, but not to attack, during my two or three days’ absence; and there I took a boat for Wassaw Sound, whence Admiral Dahlgren conveyed me in his own boat (the Harvest Moon) to Hilton Head, where I represented the matter to General Foster, and he promptly agreed to give his personal attention to it.  During the night of the 20th we started back, the wind blowing strong, Admiral Dahlgren ordered the pilot of the Harvest Moon to run into Tybee, and to work his way through to Wassaw Sound and the Ogeechee River by the Romney Marshes.  We were caught by a low tide and stuck in the mud.  After laboring some time, the admiral ordered out his barge; in it we pulled through this intricate and shallow channel, and toward evening of December 21st we discovered, coming toward us, a tug, called the Red Legs, belonging to the Quarter-master’s Department, with a staff-officer on board, bearing letters from Colonel Dayton to myself and the admiral, reporting that the city of Savannah had been found evacuated on the morning of December 21st, and was then in our possession.  General Hardee had crossed the Savannah River by a pontoon-bridge, carrying off his men and light artillery, blowing up his iron-clads and navy-yard, but leaving for us all the heavy guns, stores, cotton, railway-cars, steamboats, and an immense amount of public and private property.  Admiral Dahlgren concluded to go toward a vessel (the Sonoma) of his blockading fleet, which lay at anchor near Beaulieu, and I transferred to the Red Legs, and hastened up the Ogeechee River to Grog’s Bridge, whence I rode to my camp that same night.  I there learned that, early on the morning of December 21st, the skirmishers had detected the absence of the enemy, and had occupied his lines simultaneously along their whole extent; but the left flank (Slocum), especially Geary’s division of the Twentieth Corps, claimed to have been the first to reach the heart of the city.