Bure to the neighbourhood of Worsted.

In this connection the Thames is of an especial interest, for it had,
in proportion to its length, the greatest section of navigable
non-tidal water of any of the shorter rivers in Europe. Until the
digging of the Thames and Severn Canal at the end of last century it
was possible, and even common, for boats to reach Cricklade, or at any
rate the mouth of the Churn. And even now, in spite of the pumping
that is necessary at Thames head and the consequent diminution of the
volume of water in the upper reaches, the Thames, were water carriage
to come again into general use, would be a busy commercial stream as
high up as Lechlade.

This exceptional sector of non-tidal navigable water cutting right
across England from east to west, and that in what used to be the most
productive and is still the most fertile portion of the island, is the
chief factor in the historic importance of the Thames.

From Cricklade to the navigable waters of the Severn Valley is but a
long day's walk; and one may say that even in the earliest times there
was thus provided a great highway right across what then was by far
the most thickly populated and the most important part of the island.

A third section in all such rivers (and, from what we have said above,
a short and insignificant one in the case of the Thames) may be called
the _head-waters_ of the river: where the stream is so shallow or so
uncertain as to be no longer navigable. In the case of the Thames
these head-waters cover no more than ten to fifteen miles of country.
With the exception of rivers that run through mountain districts this
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 be