Sunday, 19 June 2011
Engraving of the week
'Weir with movable bridge' from 'Life on the Upper Thames' 1875. Author –H R Robinson.
Strangely Robinson does not use the term ‘Flash Lock’ in his description of this primitive contrivance for altering the levels of rivers in order to primarily facilitate navigation by boats and barges, although he does describe the operation as ‘flashing a weir’.
I will let Robinson describe the whole procedure …. ‘ The most primitive way of overcoming the difficulty (of shallows obstructing navigation) has been to erect a movable dam all across the river, below the shallow; the boards of the dam,being of course, high enough to keep back sufficient water to enable a boat to float over the shallow. By this means the boat descending the stream meets with no impediment till it reaches the dam or “weir”, (pronounced “wire” by the riverside people. The boards composing the dam are then removed and the boat proceeds with great rapidity, owing to the increased volume of water by which it is carried along. The temporary depth thus produced by the body of water descending enables the boat to descend over many shallows below the weir.
The different parts of the weir are first the cill or fixed beam , laid securely across the bottom of the stream; then, directly over this, but considerately above the surface of the water, is placed a second but movable beam. Against and in front of these parallel beams a set of loose boards is placed upright and close together like a door. These loose boards are called paddles, and the long handles with which they are furnished rest against the upper beam, the pressure of the stream serving to hold them in their places. Between the paddles are placed upright supports termed “rimers;” and when a second set of paddles is employed above the first to obtain a greater depth of water,this set is called the “overfall”.
As the largest barge is far from occupying the fall width of the stream, it is practically found that only a portion of the bridge is required to be movable. In the illustration the man who is putting down the paddles is standing on the movable part called the “swing bridge”. It revolves on a pivot close to the edge of the water and the weight is balanced by the increased thickness of the beam at the landward end on which is often placed a great stone or other heavy substance. The upper beam and hand rail across that part, are of course, removed before the bridge is swung round and it is for this purpose that the two handles which may be noticed are added.........'
The weir in its most primitive form was just a dam and there is evidence that the Vikings dammed the river in order to ease navigation. Later they were used by millers and fisheries and then adapted to the movable type discussed by Robinson to aid navigation.
Many rivers had flash locks and on the Thames ,some lasted into the 20th century –particularly above Oxford where the last at Eaton Hastings survived until 1938.The last surviving capstan used for hauling boats up river against the flow was found at Hurley recently and restored in 1999. Pictures of this and other interesting stuff can be found under locks at http://www.the-river-thames.co.uk