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.