e must engage himself within
the broadening triangle which lies between the Clain and the Vienne: these
rivers join their waters just above Chatellerault itself.

The main road from Chatellerault to Poitiers runs on the further side of
the Clain from this triangle, and the Black Prince, by engaging himself in
the wedge between the rivers, would thus have a stream between his column
and the natural marching route of any force which might approach him from
the fortified city which he feared.

Further, he was well provided for part of this march through the triangle
between the rivers by the existence of a straight way formed by the old
Roman road which runs through it, and may still be followed. He could not
pursue this road all the way to Poitiers (which town it ultimately reaches
by a bridge over the Clain), but somewhere half-way between Chatellerault
and Poitiers he would diverge from it towards the east, and so avoid the
latter stronghold and make a straight line for Les Roches. This it would
be the easier for him to do because the soil in that countryside is light
and firm and traversed by very numerous cross-lanes which serve its
equally numerous farms. Only one considerable obstacle interrupts a
passage southward through the triangle between the rivers. It is the
forest of Mouliere. But the Black Prince's march along the Roman road
would skirt this wood to the west, and by the time his approach to
Poitiers compelled him to diverge from the Roman road eastward, the
boundary of the forest also sloped eastward away from it.

His first day's march upon this last lap, as it were, of his escape was a
long one. By the road he took it was no less than fifteen miles, and at
the end of it he gathered his column into Chatellerault, a couple of miles
from the place where the Clain and the Vienne meet, and where the triangle
between the two streams through which he proposed to retreat begins. At
the same hour that the Black Prince was bringing his men into
Chatellerault, John was leading the head of _his_ column into La Haye. He
was just one day's march behind the Plantagenet.

There followed an unsoldierly and uncharacteristic blunder on the part of
the Black Prince which determined all the strange cross-purposes of that
week.

The Black Prince having made Chatellerault, believed that he had shaken
off the pursuit.

In explanation of this error, it must be remembered that the population so
far north as this was universa