Besides, such a river, in order that it should be possible for vessels to ascend it from the ocean, must have the surface of its water very near the level of the surface of the ocean.

There can, therefore, be no place to which such waters could be drawn off, unless into a valley below the level of the sea.
All such valleys, whenever they exist in the interior of a country, necessarily get filled with water from brooks and rains, and so become lakes or inland seas.

It is probable, therefore, that it was some other operation which Alfred performed to imprison the hostile vessels in the river, more possible in its own nature than the drawing off of the waters of the Thames from their ancient bed.
Year after year passed on, and, though neither the Saxons nor the Danes gained any very permanent and decisive victories, the invaders were gradually losing ground, being driven from one intrenchment and one stronghold to another, until, at last, their only places of refuge were their ships, and the harbors along the margin of the sea.

Alfred followed on and occupied the country as fast as the enemy was driven away; and when, at last, they began to seek refuge in their ships, he advanced to the shore, and began to form plans for building ships, and manning and equipping a fleet, to pursue his retiring enemies upon their own element.

In this undertaking, he proceeded in the same calm, deliberate, and effectual manner, as in all his preceding measures.