The transition from rivers to canals, and from larger canals to smaller ones, is expedited by the possibility afforded of, on the arrival at the locks, dividing the vessel in a space of a few minutes; of passing with the semi-vessel, singly, the various smaller locks or the shallow canal, after which the two sections may be re-combined and navigated again as one vessel.  The process of “folding up” the two vessels will of course take longer than that of separation.

On rivers, the channels of which are interrupted by sand banks and rapids, the same operation may be carried out, thus avoiding the expense and delay necessitated by, perhaps, repeated “lightering,” i.e., reduction of the cargo.

Thus, the through traffic on large rivers like the Danube, with its repeated obstacles to navigation, such as the “iron gate,” and several sand-banks known and dreaded by bargemen, would be materially facilitated, any necessity for unloading part of the cargo being obviated; moreover, such a duplex vessel composed of two semi-vessels affords the advantage of utilizing to a fuller degree the power of traction, and one large vessel will be more convenient for traffic than two smaller ones.

Further, the mode of construction of the semi-vessels—­both ends of which are of a similar pattern—­allows of their being navigated up and down a water channel without the necessity of turning them round; provision having also been made for the fixing of the rudder at either end, which would therefore merely require exchanging.  This is of some advantage in narrow river beds and canals, and applies equally to the duplex vessel as to the single semi-vessels.

[Illustration:  FIG. 1.]

[Illustration:  FIG. 2.]

[Illustration:  FIG. 3.]

[Illustration:  FIG. 4.]

[Illustration:  FIG. 5.]

Each semi-barge on its part is also constructed of two equal halves—­which are, however, inseparable—­and as there is no distinct stem or stern, any one of these semi-vessels will fit any other semi-vessels of the same dimensions, and can be attached to the same by means of the coupling apparatus, and the two “folded up” into one duplex vessel.  This process does not present any material difficulties.  The two single boats on being coupled together can be made to lean over toward each other, by filling their lateral water compartments, to such an extent that the further closing up can be easily effected by means of specially constructed windlasses.  In the case of petroleum vessels the “folding up” operation is facilitated by the circumstance that the petroleum may be made to serve the purposes of water ballast.

As regards the size and tonnage of the new vessels, this will of course depend on the local condition of the rivers and canals to be navigated.  Thus a vessel destined for traffic on canals with locks of varying dimensions will have to be adapted to the dimensions of the smallest existing lock.