The usefulness of a river to its country depends on several things. In the first place, it must be able to carry goods—to act as a convenient highway along which the traffic can descend through the valley towards the busy places near the mouth. That is to say, it must be navigable to barges and small boats throughout a considerable portion of its length. In the second place, there must be the goods to carry. That is to say, the river must pass through a countryside which can produce in great quantity things which are needed. In the third place, the chief port of the river must lie in such a position that it is within comparatively easy distance of good foreign markets. Now let us see how these three conditions apply to the River Thames. Firstly, with regard to the goods themselves. If we take our map of England, and lay a pencil across it from Bristol to the Wash, we shall be marking off what has been through the greater part of English history the boundary of the wealthy portion of Britain, for only in modern times, since the development of the iron and coal fields, and the discovery that the damp climate of the north was exactly suited to the manufacture of textiles, has the great industrial North of England come into being. England in the Middle Ages, and on till a century or more ago, was an agricultural country; its wealth lay very largely in what it grew and what it reared; and the south provided the most suitable countryside for this sort of production. The consequence was that the Thames flowed right down through the centre of wealthy England. All round it were the chalk-ranges on which throve the great herds of long-fleeced sheep that provided the wonderful wool for which England was famous, and which was in many respects the main source of her prosperity. In between the hills were the cornfields and the orchards. And dotted all down the course at convenient points were thriving towns, each of which could, as it were, drain off the produce of the area behind it, and so act as a collecting and forwarding station for the traffic of the main stream. The River, too, was quite capable of dealing with the great output, for it was navigable for barges and small boats as far as Lechlade, a matter of 150 miles from the mouth, and its tributaries were in most cases capable of bearing traffic for quite a few miles into the right and left interior. Moreover, its current at ordinary times was neither too swift nor too sluggish. So that, with the wealth produced by the land and the means of transport provided by the River, the only things needed to make the Thames one of Europe’s foremost rivers were the markets.