LAUNCH OF A STEAMER.—The first of a line of merchant steamships intended for the Australian trade, and built and owned by Messrs. Wigram and Sons, was launched yesterday from Blackwall yard in the presence of several thousand spectators. The ship ran off the stocks in admirable style at flood tide, Miss Wyndham, daughter of Col. Wyndham, of Petersfield, christening her ‟the London,” On breasting the water she was immediately towed up to Greenwich, where she will take her engines from Penn’s yard. The London is of 1,750 tons register, and she will sail for Melbourne in October next, under the command of Captain Martin, an experienced Australian navigator.