The Royal Mail Company’s steamer Atrato, from the West Indies, with the mails and passengers, via Plymouth, yesterday arrived at Southampton. She brings four officers and 100 met),. being detachments of Imperial and Colonial Corps from Jamaica, St. Lucia, and Trinidad, who will be quartered at the Alexandra Palace. On being formed up in the shed alongside the ship, Captain North, from the Coronation Office, addressed the troops in the name of the Duke of Connaught, Welcoming them to England, and hoping that they would have a pleasant visit.