The lower pair of gates are beveled off to an acute which termi nates at the outside of the upper edge • and to correspond with this, the lower edges of the middle gates are beveled off in a contrary way, to lap over the others, as exhibited in the drawing. From this arrangement, it will be perceived that the middle gates will be partly in the water before it runs over the lower gates ; and hence, the second gates will be shut as soon as the water begins to run over the first. The same arrangement is made with respect to the middle and upper gates, except that the upper edges of the latter are beveled to an angle on the inside, to fit a contrary bevel on the top of the gateway. It may be here observed that none of the flood-gates are made to open so far back as to become parallel with the side of the tunnel, conse quently they are always in a situation to be acted upon by the water; and, that the whole of the gates, as well as the framework in each arch, meet in the middle of the arch, as flood-gates on canals do, at such an angle as to afford the greatest resistance to the pressure of the fluid. To prevent the out ward lateral pressure of the sides of the framing against the brickwork, and the injury it might thereby sustain, the opposite sides of the framing in each arch are connected by tie-beams, similar to those used in roofing.
It will not be necessary to make the whole area of the arch to open, as a comparatively small opening will be sufficient for conducting the operation at the miners. The opening gates are therefore represented as occupying only a small portion in the middle of the strong framing which fills up the arch.
It will of course be of thegreatest importance that the threshold, or celia, against which the bottoms of the first gates shut, and for that purpose the por tion of the road-way which passes through the gates, is so balanced and sup ported, that a very small portion of water accumulated under it will disengage its supports, and project part of the road-way or platform outside of the gates. The threshold may be further secured, if necessary, by a covering of canvas, so attached to the gates as to be rolled off by them in the act of shutting.
The joinings between the frame and brick-work, as well as the joinings round the gates, are made air and water-tight, by triangular packings of leather or other soft material, which are drawn into the crevices by a series of screw-bolts through to the outside of the gates, where the workmen can, at their leisure, screw the packing up after all the gates are shut.
The method of moving the gates forward, and of securing them in their places, is shown in Fig. 2, where t represents a vertical section of a set of flood gates, supported in its place by three pair of strong beams, represented at s, fastened together at r; the other ends of these beams are attached to the flood gates, three on each aide, at a small distance from the edge. The piece r rests upon a friction-roller or small wheel, and against a powerful screw-jack o, which is supported by the abutment p, fixed into the bottom of the tunnel, and kept in its place by the vertical beam q.
When the gates are to be moved forward, the triangular packing round the edges of the frame must be released, and moved back, by unscrewing the bolts, which keep it in its place, and then the gates are forced forward on the smooth supports, on which they rest, by the screw o • and when they have been moved to their assigned place, the screw is returned into its box, and the abutments are brotight up, and the whole apparatus again properly secured.
The box represented to the left of the eastern arch is sufficiently capacious to hold two or three men ; it is provided with two doors, one of which opens into the box, and the other into that part of the tunnel which would be full of water when the flood-gates are all closed. The use of this box is for a man, harnessed in James's diving apparatus, to enter the part filled with water, for the purpose of exploring and examining the works, and bringing out any thing of importance, requiring to be removed from the water. The man, having pro vided in his diving apparatus a sufficient supply of air for the time he intends to remain in the water, enters the box, and closes the door ; he then, by means of a stop-cock, admits the water into the box, when the door between the box and the interior can be easily opened to admit him. In coming out, he has only to re-enter the box, shut the communication between the box and the inte rior, and then, by a stop-cock, let the water contained in the.box issue into the open part of the tunnel.
This may be repeated as often as occasion may with very little escape of water, and with perfect safety to the diver; for the ingress of the water being entirely prevented by the flood-gates, it will be perfectly quiescent within them, and no danger is to be apprehended from a change or derange ment of any part of the works taking place during the miner's inspection. The small space between the shield and flood-gates would soon be filled, when all would become stationary ; and consequently the principal cause of damage to the works, the rushing of a large quantity of water with great violence, would be removed. If it should be objected, that the time occupied in filling the space between the gates and the shield would be too short to allow the workmen 03 escape ; it may be answered, that very little time would be required for all the workmen to get outside, where they would be perfectly safe, and might leisurely view the progress of the water in filling the space, and closing the gates : besides, an irruption of the water, under such circumstances, would be of so little cdttse quence, that there would be no occasion for detaining the men in attempting to atop the torrent, till their lives are in danger. The water within being perfectly still, the bed of the river might be made good, the water pumped out, and the work might be going on again, in the course of a day or two after an irruption, if checked by such means as are herein described.
At the period we are writing, (February 183.1,) the work at the Thames tunnel is at a stand. A brick wall has been completed at the further extremity of the excavation, which is made water-tight, and the interior of the tunnel is as perfect as though no accidents had happened.