The navigation of these streams was important to the Assyrians in two ways. In the first place it was a military necessity that they should be able, _readily and without delay_, to effect the passage of both of them, and also of their tributaries, which were frequently too deep to be forded. Now from very early times it was probably found tolerably easy to pass an army over a great river by swimming, more especially with the aid of inflated skins, which would be soon employed for the purpose. But the _materiel_ of the army--the provisions, the chariots, and the siege machines--was not so readily transported, and indeed could only be conveyed across deep rivers by means of bridges, rafts, or boats. On the great streams of the Tigris and Euphrates, with their enormous spring floods, no bridge, in the ordinary sense of the word, is possible. Bridges of boats are still the only ones that exist on either river below the point at which they issue from the gorges of the mountains. And these would be comparatively late inventions, long subsequent to the employment of single ferry boats. Probably the earliest contrivance for transporting the chariots, the stores, and the engines across a river was a raft, composed hastily of the trees and bushes growing in the neighborhood of the stream, and rendered capable of sustaining a considerable weight by the attachment to it of a number of inflated skins. A representation of such a raft, taken from a slab of Sennacherib, has been already given. Rafts of this kind are still largely employed in the navigation of the Mesopotamian streams, and, being extremely simple in their construction, may reasonably be supposed to have been employed by the Assyrians from the very foundation of their empire.