The level of the docks at Manchester, which is 60 feet 6 inches above the ordinary level of the tidal portion of the canal, will be reached by four sets of locks. The locks will be of a size sufficient to admit the largest merchant steamers afloat. Each set comprises (a) a large lock, 550 feet by 60 feet; (b) a smaller lock 300 feet by 40 feet for ordinary vessels; and (c) one lock 100 feet by 20 feet, for small coasters and barges. All will be capable of being worked together.
Each set of locks will be worked by hydraulic power, enabling vessels to be passed in 15 minutes. It has been ascertained by careful gaugings that the rivers Irwell and Mersey (which will be diverted into the upper reaches of the canal) will supply more than sufficient water for the locks, even in the driest season.
There will be tidal gates at the entrance to the canal, which will be worked as locks at low water, so that large vessels can enter and leave at almost any state of the tide, instead of only during a period of 40 minutes of each tide as at Liverpool. Small vessels will be able to enter and leave at any time.
It is claimed that vessels will be able to navigate the canal with safety at a speed of five miles an hour, and it is estimated that the journey from the entrance at Eastham to Manchester will be accomplished in eight hours, which is less time than is now taken to cart goods from ship to rail in Liverpool, and to carry them thence by rail to Manchester.
One of the most interesting operations to be carried out in connection with the canal works, will be the removal and rebuilding of the aqueduct which Brindley constructed for the Bridgwater Navigation in 1765. The aqueduct and the neighbouring viaduct ([shown in the old print at p. 344]) pass over the Mersey and Irwell Navigation at such a height as to allow the passage through the archways of small vessels. To accommodate the larger vessels that will pass up the Ship Canal, the archways of the aqueduct and viaduct would have to be more than double the height. This was the engineering difficulty which the Ship Canal promoters had first of all to encounter and by many it was regarded as insuperable. The suggestion was made that the Ship Canal should end at a point below the aqueduct. Mr. E. Leader Williams, the engineer of the company has, however, proposed to construct a short diversion of the Bridgwater Canal immediately over the line at which it would cross the Ship Canal. The length of the Bridgwater over the Ship Canal will then be formed in the manner of a long movable iron caisson or trough, somewhat deeper in the centre than at the two ends, supported by and turning (when required) upon a circle of live rollers. This caisson is to be filled with water to a depth equal to that of the canal itself, and is to be fitted at either end with watertight gates, which are also to be fixed at either end of the approaches from the canal.
The Bridgwater Canal.—
(a) Across the Irwell (b) Barton Bridge.
Upon the completion of this work, the central portion of Brindley’s aqueduct will be removed, the ends being allowed to remain. The manner of working the new aqueduct will be as follows:—The operator in charge of the machinery will, on descrying an approaching steamship, cause the four watertight gates at the ends of the caisson and of the approaches to be closed, and will then, by means of hydraulic machinery, cause the caisson to revolve for a quarter of a circle upon the live roller which will support it, thus leaving a perfectly clear passage for the vessel. Through this passage, up- or down-going vessels will be able readily to steam, and when clear of the aqueduct the process will be reversed—that is to say, the attendant will cause the caisson to turn back into its original position, and will have his watertight gates opened once more, when the line of the Bridgwater traffic will be clear again, after a very brief interval, and without any loss of the water in the canal.
At the ends of the existing line of the canal (after the removal of Brindley’s old aqueduct) it is proposed to construct hydraulic lifts as already stated, by means of which it will be competent to lower barges with full cargoes (the barges remaining afloat throughout the whole operation) from the Bridgwater to the Ship Canal, or, vice versâ, to raise them from the Ship Canal to the Bridgwater, thus making Barton a point of interchange of traffic between the high and the low level navigation.