It is not difficult to imagine how mankind first conceived the idea of making use of floating structures to enable him to traverse stretches of water. The trunk of a tree floating down a river may have given him his first notions. He would not be long in discovering that the tree could support more than its own weight without sinking. From the single trunk to a raft, formed of several stems lashed together, the step would not be a long one. Similarly, once it was noticed that a trunk, or log, could carry more than its own weight and float, the idea would naturally soon occur to any one to diminish the inherent weight of the log by hollowing it out and thus increase its carrying capacity; the subsequent improvements of shaping the underwater portion so as to make the elementary boat handy, and to diminish its resistance in the water, and of fitting up the interior so as to give facilities for navigating the vessel and for accommodating in it human beings and goods, would all come by degrees with experience. Even to the present day beautiful specimens exist of such boats, or canoes, admirably formed out of hollowed tree-trunks. They are made by many uncivilized peoples, such as the islanders of the Pacific and some of the tribes of Central Africa. Probably the earliest type of _built-up_ boat was made by stretching skins on a frame. To this class belonged the coracle of the Ancient Britons, which is even now in common use on the Atlantic seaboard of Ireland. The transition from a raft to a flat-bottomed boat was a very obvious improvement, and such vessels were probably the immediate forerunners of ships. It is usual to refer to Noah's ark as the oldest ship of which there is any authentic record. Since, however, Egypt has been systematically explored, pictures of vessels have been discovered immensely older than the ark--that is to say, if the date usually assigned to the latter (2840 B.C.) can be accepted as approximately correct; and, as we shall see hereafter (p. 25), there are vessels _now in existence in Egypt which were built_ about this very period. The ark was a vessel of such enormous size that the mere fact that it was constructed argues a very advanced knowledge and experience on the part of the contemporaries of Noah. Its dimensions were, according to the biblical version, reckoning the cubit at eighteen inches; length, 450 feet; breadth, 75 feet; and depth, 45 feet. If very full in form its "registered tonnage" would have been nearly 15,000. According to the earlier Babylonian version, the depth was equal to the breadth, but, unfortunately, the figures of the measurements are not legible.