structure. It is known in history as the Wall of
Severus; and so solid, substantial, and permanent was the work, that
the traces of it have not entirely disappeared to the present day.

The wall extended across the island, from the mouth of the Tyne, on
the German Ocean, to the Solway Frith--nearly seventy miles. It was
twelve feet high, and eight feet wide. It was faced with substantial
masonry on both sides, the intermediate space being likewise filled
in with stone. When it crossed bays or morasses, piles were driven
to serve as a foundation. Of course, such a wall as this, by itself,
would be no defense. It was to be garrisoned by soldiers, being
intended, in fact, only as a means to enable a smaller number of
troops than would otherwise be necessary to guard the line. For these
soldiers there were built great fortresses at intervals along the
wall, wherever a situation was found favorable for such structures.
These were called _stations_. The stations were occupied by garrisons
of troops, and small towns of artificers and laborers soon sprung up
around them. Between the stations, at smaller intervals, were other
smaller fortresses called castles, intended as places of defense, and
rallying points in case of an attack, but not for garrisons of any
considerable number of men. Then, between the castles, at smaller
intervals still, were turrets, used as watch-towers and posts for
sentinels. Thus the whole line of the wall was every where defended
by armed men. The whole number thus employed in the defense of this
extraordinary rampart was said to be ten thousand. There was a broad,
deep, and continuous ditch on the northern side of the wall, to
make the impediment still greater for the enemy, and a spacious and
well-constructed military road on the southern side, on which troops,
stores, wagons, and baggage of every kind could be readily transported
along the line, from one end to the other.

[Illustration: WALL OF SEVERUS]

The wall was a good defense as long as Roman soldiers remained to
guard it. But in process of time--about two centuries after Severus's
day--the Roman empire itself began to decline, even in the very seat
and center of its power; and then, to preserve their own capital from
destruction, the government were obliged to call their distant armies
home. The wall was left to the Britons; but they could not defend it.
The Picts and Scots, finding out the change, renewed their assaults.
They battered