If the line of Lake Champlain is, as we have endeavored to show, the most important line in the north; its security by fortifications is a matter of the greatest interest. The works recommended by the Board, consist of a single fort, costing $600,000, at Rouse's Point, on the extreme frontier, and unfortified depots at Plattsburg and Albany. But is this sufficient to accomplish the object? If the hostile army should pass the extreme frontier barrier, what is to retard his advance,--what defensive works are to protect the debouche of the Northern canal, or even to save the great central depot? We know of no foreign engineer who has recommended less than _three_ lines of fortifications for the security of a land frontier; and Napoleon, the Archduke Charles, and General Jomini, agree in recommending at least this number of lines. There may be circumstances that render it unnecessary to resort to a three-fold defence throughout the whole extent of our northern frontier; but upon our main line of communication with Canada,--a line of maximum importance both to us and to the enemy, we know of no reason for violating the positive rules of the art,--rules which have been established for ages; and sanctioned by the best engineers and greatest generals of modern times. Ticonderoga has more than once stayed the waves of northern invasion; and we know of no change in the art of war, or in the condition of the country, that renders less important than formerly the advantages of an intermediate point of support between Albany and the Canadian lines. Indeed it would seem that the connection of the Hudson with the lake by the northern canal had even increased the value of such a point.