Two or three thousand men had collected in the neighborhood of Jacksonville, Florida, which had been selected as the principal rendezvous of the expedition. These men awaited the arrival of a steamer from New-York, which had been chartered by parties there. The Government, however, had already received intelligence of their plans, and instructions were at once sent to the United States Marshal at New-York, to prevent the departure of the steamer. This officer, accompanied by a police force, sailed down the bay in search of the suspected craft. In the mean time it was found that the steamer Cleopatra, a large boat, formerly employed on the Sound as a passenger boat, was the vessel indicated. She was then lying at one of the piers on the North River, and was immediately seized and placed under the supervision of the United States authorities. She was alleged to be bound to Galveston, Texas. A large quantity of coal was found on board, and a great number of water casks, and but few arms or ammunition of any kind. A file of marines from the Navy Yard was placed on board, and all communication with the shore forbidden. No final disposition has yet been made of the vessel, though orders were received to deliver her cargo to any person who may establish his ownership to the articles found on board. At the same time, notice was received by the Marshal that a number of Germans and others had assembled at South Amboy for the purpose of embarking on some secret expedition, and one of the Deputy Marshals was sent there for the purpose of procuring information. Disguising himself as a German emigrant, he obtained sufficient evidence to warrant the arrest of the following six persons: William T. Rogers, Jr., John L. O'Sullivan, Capt. Lewis, of the steamboat Creole, a member of the former expedition; Major Louis Schlesinger, one of the Hungarian refugees; Pedro Sanchez Yznaga, a Cuban refugee; and Dr. Daniel H. Burtnett. Each of the parties was held to bail in the sum of $3,000, to appear for examination. The movement must have been of considerable magnitude, but there was evidently a want of concert among its members, which may have led to its abandonment. From what could be ascertained, it was not the intention of the leaders to organize the expedition in this country, but to sail to some point beyond the limits of the United States, and there concentrate their forces for the invasion.