tween 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
England until the last industrial revolution of our history.

The Thames also has entered to a large, though hardly to a
determining, extent into the military history of the country; to an
extent which is greater in earlier than in later times, because with
every new bridge the military obstacle afforded by the stream
diminished. And finally, the Thames, regarded as an obstacle, was the
cause that London Bridge concentrated upon itself so much of the life
of the nation, and that the town which that bridge served, always the
largest commercial city, became at last the capital of the island.

We have already said that the establishment of the site of London
Bridge was a capital point in the history of the river and the
principal line of division in its course. What were the topographical
conditions which caused the river to be crossed at this point rather
than at another?

It is always of the greatest moment to men to find some crossing for a
great river as low down as may be towards the mouth. For the higher
the bridge the longer the detour between, at the least, _two_
provinces of the country which the river traverses. It is especially
important to find such a crossing as low down as possible when the
river is tidal and when it is flanked upon either side by great
flooded marshes, as was and is the Thames. For under such conditions
it is difficult, especially in primitive times, to cross habitually
from one side to the other in boats.

Now it is a universal rule of early topography, and one which can be
proved upon twenty