As previously stated, the breakwater was founded upon a mound of quarried and deposited limestone rubble, in a depth of water varying from 45 to 65 feet below low water, and brought up to a level of 36 feet below low water, this being the depth at which it was considered the foundation courses of the superstructure could be laid with safety.

That portion of the surface of this mound upon which the caisson was intended to rest was carefully levelled by divers and brought to a fair surface by the deposit of fine material or small stuff, care being taken that no large stones projected above the finished level.

The caisson, having attained the draught before mentioned, was towed into position over the prepared site above described, and, the valves to the end chambers being opened, was sunk through the space, about 4 feet, intervening between the prepared surface of the mound and the under surface of the caisson at low water.

The possibilities of bad weather and heavy seas rendered this, perhaps, the most critical juncture in the entire scheme, and the succeeding operations about to be described were planned with the view of obtaining in the shortest possible time such a preponderance of dead weight over displacement as should reduce the risk of any shifting of the caisson out of its place by heavy seas to a minimum.

The programme described in the following table was therefore planned and carried out as closely as circumstances permitted, and with entire success, the concreting being carried on with the utmost possible despatch, while mooring chains were used as an additional safeguard against any shift of the caisson.

Summary Of Conditions After Sinking

An examination of the last column of the foregoing table, which gives the excess in tons of dead weight over buoyancy, will show the general increase in stability as the processes of pumping out and concreting went on, and the final result of the operations above described was the gradual building up of a monolithic structure, which, when first launched, had a total weight of about 281 tons, but in its completed state weighed nearly 9000 tons, and formed the first stage in the construction of a breakwater, of which the caisson itself formed a portion of the length, and was possessed of at least as much stability as any other section of the breakwater when finished.

Upon this caisson, completed in the manner described, were erected the Titan cranes,1 by whose means the whole of the sloping blockwork was set. One Titan was first erected, as shown in Fig. 393, and, forming its own road as it advanced, was gradually worked off the caisson to make room for a second Titan of similar dimensions and power, working in the opposite direction to the first. By these two machines the whole of the remainder of the breakwater, including the heads, was constructed, and no further temporary staging of any kind was required, notwithstanding that the work was entirely cut off from the shore, with no communication, temporary or otherwise, except that of the floating craft transporting the concrete blocks from a blockmaking yard on the mainland.

The concrete blocks were laid in their sloping position by means of Fidler's patent block-tilting apparatus, designed by the author for this purpose.

It is not of course to be claimed that the system here adopted would be universally applicable, or would offer a sufficient guarantee against all sea risks in all situations. Varied exposures, and the local risks to be met, will obviously rule the decision as to the best method to be adopted; but any further consideration of this branch of the subject is outside the range of these notes.

Caissons for closing dock entrances - The entrance to a graving or dry dock may be closed and rendered watertight by one or the other of two methods, viz. by gates or by some form of movable dam or caisson.

It is not within the scope of these notes to institute a complete comparison between these two methods of construction, or to enter into all the arguments for or against either type on the grounds of relative economy, efficiency, or ready handling.

1 For a description of these machines, the reader is referred to the article on "Titan Cranes " in the Supplement to the Encyclopaedia Britannica, by Mr. Walter Pitt, M.I.C.E.

Fig. 393 (Scale 1 inch = 24 feet).

But for present purposes it may be pointed out that where the conditions of the site and the requirements of the dockyard or the commercial port are such as to demand a line of railway across the dock entrance capable of carrying with safety the heaviest rolling loads which have to be handled, such as boilers, heavy articles of machinery, guns, or the like, then the caisson possesses advantages over its rival, inasmuch as such a line of railway cannot conveniently be carried over a pair of dock gates of the ordinary design, but would demand a swing bridge of very considerable dimensions and cost, in addition to the gates themselves.

A bridge of this description would, moreover, frequently prove an obstruction to wharfing operations, and occupy valuable space.

The caisson, under such conditions, on the other hand, contains within itself the capacities of both gates and bridge combined, as the entire structure can easily be made of sufficient strength as a roadway to transport across the dock entrance the heaviest loads which can ever in practice be brought upon it, while at the same time fulfilling the functions of a movable dam and completely sealing the dock entrance against the maximum water pressures which can occur at the highest tidal levels.

Caissons for dock entrances may be divided broadly into two principal classes, viz. sliding and floating caissons, or, as the latter class is sometimes denominated, ship caissons, due probably to some fancied resemblance to ship forms of construction, especially when built on lines of curvature and outline approximating to those of ordinary vessels, although such outlines are sometimes adopted more from motives of appearance, or possibly of precedent, rather than from any practical advantages to be gained.