The detailed design of such engines is somewhat outside the scope of these notes, and need not be further alluded to, except to state that a liberal margin of power is always judicious, in order to deal with contingencies, such as the increase of resistance on the sliding ways due to deposited mud, or temporary obstruction, or the jamming of the caisson stems and keels against their faces by the tidal outflow or inflow of water through the dock entrance.

With respect to the latter contingency, it must be remarked that ample culvert area should always be provided in the masonry construction of the dock as an essential feature in the safe and proper working of the caisson. A closed dock in direct communication with tidal waters will always demand special attention to these details, especially if the tidal range be considerable, as if insufficient culvert area is provided, the water can neither flow into the dock on a rising tide, nor flow out on a falling tide with sufficient velocity to maintain the water level on both sides of the caisson.

The resulting small head of water on one side or the other will be sufficient, if it exceeds certain limits, to prevent the opening of the caisson altogether, or, in a lesser degree, to increase the power required for, and add a certain amount of risk to, the operation.

In practice it is found that while a caisson may be workable with one to two inches difference of water level, a difference of as much as four inches might render it unsafe to attempt to open the caisson. Hence the necessity of a sufficiency of area in the culverts, and, it may be added, of a correct hydraulic coefficient in the calculations for flow and capacity. To meet any doubt in these respects, sluices are sometimes arranged in the body of the caissons themselves to supplement the culverts in the dock sides. Difficulties of the kind here described are, however, very frequently reduced to a minimum by limiting the operation of opening and closing the dock entrance within the period of slack water at the top of the tide, or, where the depth of sill admits, at low water. Sluices have also been provided in caissons for the purpose of the removal of mud by sluicing, but such methods are not usually now considered the most effective methods of removal of deposit, which is more generally dealt with by dredging operations of the usual character.

It is desirable that all portions of the hauling machinery should be above high water as far as is practicable, for observation and access for repairs.

This requirement implies that the point of application of the hauling force to the caisson body is at a high level above the points of resistance at the sliding ways, and the pull on the chains has a correspondingly large tilting moment, due to its leverage. This tendency to tilt or "kick" must be met either by appropriate ballasting of the caisson, or by a reduction in the frictional resistance of the sliding ways, or both combined.

This requirement leads us to some further consideration of the details of the sliding ways or roller paths upon which caissons of this type travel, and it will be found that a considerable diversity of practice exists.

On the one hand, numerous caissons have been successfully handled for years, which simply slide upon the prepared surfaces of granite ways or cast-iron plates, the keels of the caisson bearing upon the ways being simply sledge runners, while, on the other hand, it has been considered necessary to mount the caisson upon a continuous series of fixed rollers under water, and in one recent modern example so many as forty-four rollers have been thus employed.

In other cases the caisson has been mounted upon wheels, travelling upon rails of a suitable section.

The sledge-runner principle is of an exceedingly simple type, not liable to give much trouble, but involving a considerable amount of friction to be overcome, while the wear on the steel plates has been found to be considerable in the course of years, especially at the "toe" or forward end, where the pressure is increased by reason of the tilting moment of the hauling chains.

On the other hand, a multiplicity of rollers with their axles and bearings under water, although reducing frictional resistance, is open to the objection that difficulty may be caused if the bearings should rust up after prolonged immersion.

In certain recent examples, including some of the largest sliding caissons in existence, a combination of the two types of the sledge runner and the roller has been designed by the author and introduced with success. This arrangement is shown in Kg. 399, which is a transverse section of a sliding caisson represented by example No. 10 in the table on p. 427.

The caisson is furnished with a pair of keels, consisting of a flat bar, 9" X 1", countersunk riveted to steel angles on the skin of the caisson, as shown in Fig. 397. These keels slide upon prepared granite ways, as shown, laid at an inclination of one in forty. But, in addition to the keels, two cast.steel rollers are provided on the centre line of the caisson, one near each end, running upon a cast.steel path laid on the centre line of the caisson ways for the entire length of its travel, both across the entrance and in the caisson camber. The relation between the level of the prepared granite ways and the upper surface of the cast.steel pathway is very carefully adjusted and maintained, so that the three plane surfaces are strictly parallel at the same inclination, and at a certain fixed distance apart vertically (see Fig. 399).

The rollers are operated upon by a pair of hydraulic rams, placed one near each end of the caisson. These rams are worked by pumps and manual labour at that end of the caisson farthest from the hauling engines, while valves are attached to the pumps so as to enable the pressure to be placed upon the rollers alternately, or, if necessary, on both at once. Thus, if the caisson is being hauled into the camber, the cylinder nearest the hauling engine (the forward cylinder for the time being) is subjected to hydraulic pressure, and the caisson keels are lifted by a small amount (about ½ inch to ¾ inch) off the granite ways, the whole weight being sustained by the roller and central pathway.

Fig. 399. Scale 1 inch = 10 feet.