During the building of this pretty structure an unsuccessful attempt was made to ground the piers while eleven men baled water from the river. Then a dam was built, and trenches were dug to prevent the water from overflowing the dam. This I gather from the ballad, but the wording is not at all graphic in any technical matter.[106] We are not told why cofferdams[107] were not tried. In the Middle Ages cofferdams were known as brandryths or brandereths; by this name they are mentioned in the Contract Deed for the building of Catterick Bridge over the Swale, A.D. 1421; and they were large enough to obstruct most rivers, for they had to surround enormous piers, and the thickness of their sides was never less than from four to six feet. It is interesting to note, in this connection, that during the construction of Old London Bridge, between 1176 and 1209, the Thames “was turned another way about by a trench,” which, according to Stow, began east near “Rotherhithe, as is supposed, and ended in the west about Patricksey, now termed Battersea.” In those days no embankments controlled the Thames at London; wide shores, littered with the odds and ends of a waterside life, were playgrounds for the ebb and flow of the tidal waters; and the main purpose of the “trench” or canal was to lessen the risk of floods while the huge piers were being founded. Stow’s words give us to understand that _all_ the water in the Thames “was turned another way about”; a very important feat of civil engineering. Perhaps the purpose of the canal was not so thorough; perhaps it drew from the river sufficient water to lower its normal level by several feet and to diminish the force of the tidal current. In any case, however, Stow’s evidence has great interest.