section of a river's course is nearly always small in proportion to
the rest; but the Thames, just as it has the longest proportion of
navigable water, has also by far the shortest proportion of useless
head-water of all the shorter European rivers.

There is a further discussion as to what is the true source of the
Thames, and which streams may properly be regarded as its head-waters:
the Churn, especially since the digging of the canal, having a larger
flow than the stream from Thames head; but whichever is chosen, the
non-navigable portion starts at the same point, and is the third of
the divisions into which the valley ranges itself when it is
considered in its length, as a highway from the west to the east of
England. The two limits, then, are at London Bridge and at Cricklade,
or rather at some point between Lechlade and Cricklade, and nearer to
the latter.

But a river has a second topographical and historic function. It
cannot only be considered longitudinally as a highway, it can also be
considered in relation to transverse forces and regarded as an
obstacle, a defence, and a boundary.

This function has, of course, been of the highest importance in the
history of all great rivers, not perhaps so much so in the case of the
Thames as in the case of swifter or deeper streams, but, still, more
than has been the case with so considerable and so rapid a river as
the Po in Lombardy or the uncertain but dangerous Loire in its passage
through the centre of France. For the Thames Valley was that which
divided the vague Mercian land from which we get our weights, our
measures, and the worst of our national accent, and cut it off from
that belt of the south country which was the head and the heart of