Concerning the Witham, Joseph Priestley says, in his {121}"Historical Account of the Navigable Rivers, Canals and Railways of Great Britain" (1831), it has been thought that previous to the Norman Conquest the river was a tideway navigation for ships to Lincoln. That it was navigable at a very early period he thinks may be inferred from the fact that the Fossdike Canal, "an ancient 'Roman Work,'" was scoured out by Henry I. in the year 1121 for the purpose of opening a navigable communication between the Trent and the Witham at the city of Lincoln in order that that place, which was then in a very flourishing condition and enjoying an extensive foreign trade, might reap all the advantages of a more ready communication with the interior. Another most important group of rivers, from the point of view of inland navigation, was the series which have their outlet in the Humber. This group includes the Yorkshire Ouse and the Trent, both naturally navigable. The Ouse (York) is formed by the confluence of the Ure and the Swale sixty miles above the Trent Falls, where, after passing through York, Selby, and Goole, it joins the Trent and forms the Humber estuary. Under a charter granted by Edward IV., in the year 1462, the Lord Mayor and Aldermen of York were to "oversee and be conservators" of this river, as well as of the Aire, the Wharfe, the Derwent, the Don, and the Humber, all of which are connected with it. Of the city of York, as he found it in or about the year 1723, Defoe says:-- "No City in England is better furnished with Provision of every Kind, nor any so cheap, in proportion to the goodness of Things; the River being so navigable and so near the Sea, the Merchants here trade directly to what port of the world they will; for Ships of any Burthen come up within thirty Mile of the City, and small Craft from sixty or eighty Ton, and under, come up to the very City." The navigable Trent was for many centuries the chief means of communication between south and north, and Nottingham, as the capital of the Trent district, became a place of great importance. It was along the Trent that the King's messengers passed on their way to York, in preference to braving the dangers of the road through Sherwood Forest. The burgesses of Nottingham were required to take charge of them as soon as they came to the river and conduct them safely to {122}Torksey, whose burgesses, in turn, had to take them to the Humber, and so on up the tidal Ouse to York.