The sliding caisson derives its nomenclature from the methods adopted in the removal of the caisson from the dock entrance by sliding or hauling it into a recess in the dock walls prepared for its reception, and in such a recess the caisson is stowed away, and forms no barrier to the ordinary procedure of docking or undocking vessels.

The methods by which such sliding and hauling operations are performed will be further described, together with the precautions to be observed.

The floating caisson, on the contrary, is so designed in the mutual adjustment and balancing of its weight and displacement as, by a slight modification of the former, it can either be raised out of, or sunk into, the grooves which are prepared for its reception in the sill and dock walls of the dock entrance, its final removal from the entrance being accomplished by warping or towing the caisson away to some convenient berth prepared for it.

It is in the necessity of providing such a berth that an objection is found in some cases to the employment of a floating caisson as compared with a sliding caisson. In the former case the space taken up by berthing the caisson in a basin or wet dock, or in a tidal stream, or alongside a wharf wall or jetty, may be of importance, although provision is sometimes made in the construction of the dock walls for a recess to receive the caisson, so as to be protected from the risk of grazing or colliding with passing vessels.

From this point of view the sliding caisson possesses the advantage, as above described, of being completely stowed out of the way, in its own recess, by means of the hauling apparatus provided for that purpose.

On the other hand, it is found that the cost of the sliding caisson is approximately twice that of the floating caisson, even if the cost of building the special recess is left out of the calculation, while the amount of machinery is greater, and the risks of a breakdown correspondingly increased, although it must not be overlooked that the floating caisson is exposed to certain risks of its own in the process of removal from, or reimplacement in its groove, which are not shared in quite the same degree by the slider.

Both types of caisson possess, or should possess, the capability, when properly designed, of removal from their position for the purpose of dry-docking for repairs and examination and painting or scraping.

For this purpose the sliding as well as the floating caisson must possess the requisite degree of stability as a floating body for all the usual operations of warping, towing, and docking; and practical experience has indicated what amount of stability such structures should possess when designed in the manner shown by the examples which follow, and which will be further discussed.

Whatever functions as a bridge which a caisson may be called upon to fulfil, either as means of transport for passenger traffic only, or as a roadway for mixed railway and road traffic of a heavy description, it must, as a dam at least, fulfil one important condition, viz. that of Watertightness, and of keeping water out of the dry dock at the highest tides with the least possible amount of leakage, and consequent demands upon the drainage pumps.

The problem of forming a watertight joint around three out of four of the sides of a superficial area amounting to as much as 5000 square feet, and having a lineal dimension of nearly 200 feet run, exposed to an hydraulic pressure which in some cases amounts to as much as 53 feet 6 inches of head, capable of being easily unsealed, and as easily sealed up again, is an interesting one of great practical importance, and were it approached from abstract mathematical grounds, or from the point of view of theoretical hydraulics, would afford scope for sufficient speculation.

Fortunately, the practical experience of many years has shown that the contact produced by the hydraulic pressure against the caisson, between two surfaces, the one of planed greenheart or other suitable hard timber, and the other of patent axed granite or other suitable hard masonry, is sufficient to produce a joint practically watertight over its entire superficies, and capable of being made and unmade with ease and celerity, while the amount of Watertightness possessed by a well-designed and properly fitted caisson is greater than that possessed by the majority of the best constructed gates.

Upon the practical experience thus gained on this most important detail is based the design of the keel and stems of sliding and floating caissons, being the important preliminary in the design of the entire structure, these keels and stems having to supply the required reactions of the supports as against the hydraulic pressure of the full head of water acting against the entire exposed surface of the caisson at highest tides on one side, the other side being dry.

In Fig. 394 is given a detail of the arrangement of timber keels to a floating caisson, showing the contact of the timber with the masonry of the sill and the extent of the watertight seal. In this case the groove is 2 feet 3 inches in width, and a clearance of 4 inches is allowed, by which amount the caisson "fleets" over, when the direction of water pressure is reversed. In Fig. 395 is given the detail of the timber stems in the groove cut in the masonry of the sides of the dock entrance, this groove being a continuation of the groove cut in the sill, and of the same width, but 2 feet deep in place of 1 foot 3 inches, the 10-inch space shown in Fig. 395 being provided to enable the caisson to swing out of the groove, in combination with the additional clearance gained by the batter of the sides of the entrance, when the caisson has floated up a few feet, the 10-inch space being an important factor in the operation.