WANTED, A FOUCHE FOR WASHINGTON.--It is high time that a good, sharp detective police officer was set to work to discover the source of the continued leakage of our government's plans. Of our late naval flotilla for Beaufort, we are told that 'The positive destination of our fleet was known even in New Orleans on the 17th ult.,--weeks before it was known in the North! and extra troops were dispatched from points south of Charleston to defend the approaches of that coast.' We are informed that every care was exercised to prevent the destination of the expedition being made public; with how much effect the above quoted paragraph fully demonstrates. In view of this, I repeat that a FOUCHE, a keen detective, is wanted at head-quarters; believing that any man with half the shrewdness of the celebrated 'Duke of Otranto' would pin the traitor in less than twenty-four hours. That such a man can easily be found, any one who has learned what American detectives have done, can readily believe. Active, intelligent, and wide awake, the American who by necessity takes up this life, brings to bear upon his investigations the shrewdness of a savage, the tenacity of an Englishman, and, in a modified degree, the _aplomb_ of a Parisian. No one can read POE'S 'Murder of the Rue Morgue' without recognizing at a glance the latent talent that would have made of the cloudy poet a brilliant policeman, and would have won for him the ducal fortune without the empty title. If we must handle the Southern mutineers in their Rebelutionary war with a velvet glove, let there be an iron hand inside, worked by the high-pressure power of public indignation at their treachery and faithlessness. We should stop this leakage of our plans, cost what it may, and the traitorous Southern correspondent meet the execration of ARNOLD, and the fate of ANDRE. The iron hand should stop the treacherous pen, should choke the wagging tongue. The North demands it. And yet again, since the above was penned, we learn that it has been ascertained by a balloon reconnaissance that a projected flank movement, planned by General McCLELLAN and confided to a very limited number, had been completely anticipated--indicating the basest treachery in a high quarter. Very agreeable this to all interested in the war! And what does it mean? It means that Washington, and not Washington alone, but the entire North, needs purging and purifying from most injurious influences. There are traitors among us everywhere--where two or three are gathered together will be one who sneers at Northern successes, smiles at Southern victory, and is a traitor at heart--ready to be a spy if needed. No wonder that warm friends of the Union sometimes burst out into indignant remonstrance and fierce complaint at such toleration!