Washington, May 31. Slaves wero flocking to tho Fort. It was discovered that thirty of tho el aves belonged to ono man in Richmond. He obtained permission to visit the fort to confer with Gen. ISutler on the subject of getting his live property back. 11e asked the General if he might bo allowed to convey them back. The General said that they carne there of their ovvn acoord, and that they could go back with hun it they desired to. They were asked if they wished to return with their mastcr. They quickly decfded that they preferred to to remain with the soldiers in tho fort. The elaiinant then said that if the General would allow him to tako his .slaves to Riuhmond, he would manumit them. Gen. Butler said that he could not make any such arrangement. He, the cliiimant, could go to Richmond with or without the slaves, and they could go or stay, as they pleased. The claimant, fiuding himself in a bad fix, manumitted the tlnrty slaves on the spot, and left them in the fort free men, and left himself for Richmond. A gentleman, arrived to-day from Fortress Monroc, reporls that when he left yesterday four hundred and fifty slaves had arrived at the fort, and they etated that a general uprising of the slave population was expected. JÊ3ÈT" A Washington letter writcr Says: " Gen. Jumes, proprietor of tho celebrated rifled cannon, proposed, several weeke aincc, to take one or more of his hundred pound shot and sliell I mcnts down the Potomac and silence all the batteries that could be disoovered in ; the Potomac or Cheaspeak Bay. The j osition was refused. The proposal was made ngain to-day, and the Secatary of : of the Xavy aoceptud it. The General has leffc for New York to execute his pledges."