The French system as applied in their light gun-boats has the same wooden outer skin, with the ordinary iron frame. The outer planking is double, gaining great longitudinal strength by breaking seams. There is no diagonal bracing. This system of outer planking is the one used in the English Navy. In the French transports of the Annamite class a system of alternate framing is followed, with wooden ceiling and double outside planking. In these vessels additional longitudinal strength is gained by the use of heavy iron box-stringers in place of the ordinary water-ways. _Iron Construction._ In tracing the development of iron construction it is necessary to pass from the consideration of unarmored vessels to the armored types, as the pure iron construction is limited almost exclusively to these vessels. The advantages of iron over wood may be summed up as being, 1st, lightness combined with strength; 2d, durability when properly treated; 3d, ease and cheapness of construction and repair; 4th, safety when properly constructed and subdivided. Its disadvantages are: (1) easy penetration of the bottom by rocks or by other pointed substances; (2) fouling of the bottom and consequent loss of speed; (3) the immense holes made, not only by taking out solid pieces, but, what is worse, the long rents or tears made by a penetrating shot through the thin side-plates and frames. Fast cruisers cannot be built of iron alone on account of the fouling, and the smaller the ship the greater the harm from this cause. War-vessels of any kind are excluded from this construction on account of the vulnerability of the sides, combined with the impossibility to stop a shot-hole which is starred with long rents. In the heavy iron-clad, however, the third disadvantage is done away with by the application of armor. The second is partially overcome by the surplus engine-power, and the first is neutralized by the double bottom, wing passages and compartments which the large roomy hull allows to be introduced.