Steamboat Combination.--We learn that the proprietors of steamboats have at length formed an association for the season. All the particulars of the arrangement have not yet transpired, but it is understood to be general, nay universal. Even the Julia Palmer comes into it, and is to be dismantled and laid up. The St. Clair and probably some others are also to be laid up, it is said. Some arrangements are also in progress with the London, running on the Canada side. The fare from Buffalo to Detroit is to be $6 for cabin and $3 for deck passage; from Buffalo to Chicago, &c $12 for cabin and $6 for deck passage. The proportion is not quite just to Detroit. The true ratio according to time and distance is 1 to 3 instead of 1 to 2. We trust that we shall now have regular arrivals, and above all punctual and prompt mails.