In order that this order of battle, this long thin line of guns, may not be injured or broken at some point weaker than the rest, there is at the same time felt the necessity of putting in it only ships which, if not of equal force, have at least equally strong sides. Logically it follows, at the same moment in which the line ahead became definitively the order for battle, there was established the distinction between the ships 'of the line', alone destined for a place therein, and the lighter ships meant for other uses.