We can only guess, but I would suggest that three factors combined to establish such a state of things: First, the little boats used for inland transport and the vehicles dependent upon roads and therefore upon bridges would seek a place of trans-shipment at some point upon the river where it had not yet become too broad or too rough, and so long as this place was accessible from the sea, the higher up river it was the better for them. Next must be counted the security of a perfectly land-locked harbour for the smaller craft, which formed so much the largest part of maritime transport. A perfectly land-locked natural harbour upon the coast was a very rare thing. Let the stretch of water be only of the size of Southampton Water at its mouth, or of the Solent, and they would be wrecked in a high wind, but rivers everywhere afforded a secure protection when once one had entered their narrow channel. Thirdly, we must consider the advantage which such sites presented against the attacks of pirates, and the better opportunities for defence which a considerable inland town possessed, with its resources in the surrounding fields and population over the smaller seaports. In considering the first of these points we must remember that there was always a tendency for the central depôt or main commercial town to arise somewhat inland and to be served by subsidiary ports, if there were no direct access to it by water--and even if there were. Canterbury is an example of this, so is Winchester, so is Amiens, and so is Arras, and so is Caen, and so is Rennes, and a host of others. In balancing the various advantages offered by various sites for the establishment of a market, a preponderating advantage must always be a position lying in the midst of several centres of production, and, since man is a land animal, these sites would normally lie inland. When they were served by a river so much the better; when they were served by a great and secure river, they could not fail to grow as London has grown. As part of this ease of access must be reckoned, the peculiar character of the Thames, much more open to the wind than the Seine, not blocked by any island, affording once within the estuary a constant depth amply sufficient for all vessels until quite recent times. But all this would not have given London its place had not a city established at the lowest crossing of the Thames exercised in a peculiar degree that “draining power” of which I have spoken. We have seen how the system of British roads necessarily converged upon London, and if we consider one or two other features in English topography, we shall see why London provides a common depôt for nearly the whole of English exports in a fashion which no other city could show for any other equally large area of production.