By the whole weight is, of course, understood only the surplus of weight over the buoyancy of the air-chamber and the immersed materials, an amount which will vary with the state of the tide and the weight of water in the upper tanks.

By these means the sliding friction of the keels is converted into the rolling friction of the roller, and the result is a large diminution of pull in the hauling chains, a corresponding reduction in the amount of the tilting moment, and a saving of wear and tear upon engines and gearing.

As the after roller has no weight upon it, or only as much as may be deemed desirable without lifting the caisson, the keels at the after end remain in contact with the granite ways, and the caisson being supported on three points is stable, roughly resembling the condition of a wheelbarrow when being pulled along with its legs upon the ground wheel foremost, the friction of the keels at the after end being, however, reduced by that proportion of the weight which is taken by the after roller.

On the return journey these conditions are reversed, end for end, the pressure being released from the roller nearest the hauling engine (the after roller for the time being) is transferred to the now forward roller, and the same results ensue, while the alteration in the conditions is obtained simply by the turning of a valve, together with any additional pumping up of the rams required by leakage, etc.

At the close of either journey both cylinders are relieved from pressure, and the caisson then settles down upon its keels and upon the granite ways, the rollers and central path being relieved of their duty. On this principle there are only two rollers to be kept in order, and the examination and repair of them is provided for by the arrangement of the hydraulic cylinders. By an adjustment of the valves above mentioned, the pressure can be transferred to the under side of the rams, and the rollers lifted off their pathway to the level of the lowermost deck of the caisson, where access can be had to them by divers, the stroke of the rams being made sufficiently long for this purpose.

Arrangements are also made by which the rollers can, if required, be detached, hoisted to the uppermost deck, and landed, without docking the caisson.

In Figs. 400 and 401, sections through various portions of the caisson are given, and Fig. 402 is an end elevation. Fig. 401 shows the general system of internal cross-bracing, forming a series of vertical girders, square to the planes of the solid plate girders which form the watertight upper and lower decks of the air chamber.

The central cast-steel roller path shown in Fig. 399 is further shown in detail in Figs. 403 and 404, Fig. 403 being a cross-section of the casting, showing its attachment to the bed stones, the holding.down bolts being run with lead. Fig. 404 shows a plan and side elevation of one length of the roller path.

Fig. 400. Scale 1 inch = 10 feet.

Fig. 401. Scale 1 inch = 10 feet.

These castings are in lengths of 9 feet 7 inches, a dimension which experience in the foundry proved to be satisfactory for this section and material.

The lengths of roller path are machined on the upper and bearing surfaces, while the ends are machined by a special tool to the birdsmouth joint shown in the figure, this form of joint being adopted in order to mitigate the shock of the concentrated rolling load in passing over the joint from one length of path to the next.

Fig. 402. Scale 1 inch = 10 feet.

The quality of cast steel from which these castings are made is well shown by the results of numerous tests in the table of tensile and bending tests on cast steel bars, p. 69. The test bars were cast with the lengths of roller path, and cut off after annealing for testing purposes.

The bolt-holes at joints are bored, and bolts turned, with the object of obtaining as rigid a connection between the lengths as possible.

The width of tread of the cast-steel roller travelling over the path is 6 inches on the flat, and the upper surface of the path is 9 inches on the flat, so that the fleeting of the caisson through the allowed space of 3 inches is provided for, the edges of the roller being rounded.

Fig. 403. Scale 1½ inch = 1 foot.

The details of the roller are given in Figs. 405, 406, 407, 408, Fig. 405 being a side elevation, 406 a part front elevation and part section, 407 a plan, and 408 a sectional plan of part of the wheel casing.

The roller is 2 feet 6 inches diameter, with a total width of 6½ inches, turned true on its circumference and bored for steel spindle. The pressure of the hydraulic ram is brought upon the roller spindle by means of the casting shown, which partially encases the wheel, the upper surface, being shaped to a semi-spherical seating, receiving the lower end of the hydraulic ram similarly spherical shaped, as shown in Fig. 406.

Fig. 404. Scale ½ inch = 1 foot.

The entire apparatus is provided with gun-metal guides, attached to the riveted steel framework of the caisson, for the purpose of lifting the roller as above described. India-rubber scrapers are attached to the casting fore and aft of the roller to remove mud or other obstacles from the pathway.