18. Coursed Masonry.-Another way, much used in harbor work, is to build up the work from the bottom (which must be first roughly leveled) with large stones, carefully lowered into their places; and this is a very successful method where the stones are of sufficient size and weight to enable the work to withstand the run of the sea. The divingbell affords a ready means of verifying the position of each stone as it is lowered.

19. Béton.-On the continent, foundations under water are frequently executed with blocks of béton or hydraulic concrete, which has the property of setting under water. The site of the work is first inclosed with a row of sheet piling, which protects the béton from disturbance until it has set. This system is of very ancient date, being described by Vitruvius, and was practised by the Romans, who have left us many examples of it on the coast of Italy. The French engineers have used béton in the works at Algiers, in large blocks of 324 cubic feet, which were floated out and allowed to drop into their places from slings. This method, which proved perfectly successful, was adopted in consequence of the smaller blocks first used being displaced and destroyed by the force of the sea.

20. Caissons. A caisson is a chest of timber, which is floated over the site of the work, and, being kept in its place by guide piles, is loaded with stone until it rests firmly on the ground. The masonry is then built on the bottom of the caisson, and when the work reaches the level of the water the sides of the caisson are removed.

This method of building has been much used on the continent of Europe.

21. An improvement on the above method consists in dredging out the ground to a considerable depth, and putting in a thick layer of béton on which to rest the bottom of the caisson.

22. There is a third method of applying caissons which is

practised on the continent of Europe, and which is free from the objections which commonly attend the use of caissons. A firm foundation is first formed by driving piles a few feet apart over the whole site of the foundation. The tops of the piles are then sawn off under water just enough above the ground to allow of their being all cut to the same level The caisson is then floated over the piles, and, when in its proper position, is sunk upon them, being kept in its place by a few piles left standing above the others, the water being kept out of the caisson by a kind of well, constructed round each of these internal guide piles, which are built up into the masonry. This method of building in caissons on pile foundations is shown in figs. 3 and 4. The piers of the

Fig. 3.

Pont du Val Benoît at Liége, built A. D. 1842, which carries the railway across the Meuse, have been built on pile foundations, in the manner here described.

23. Solid Foundations laid in Cofferdams.-There are many circumstances under which it becomes necessary to

lay the bottom dry before commencing operations. This is done by inclosing the site of the foundation with a watertight wall of timber, from within which the water can be pumped out by steam power or otherwise. Sometimes, in shallow water, it is sufficient to drive a single row of piles only, the outside being protected with clay, as shown in fig 5; but in deep water two or even four rows of piles will be Fig, 5.

required, the space between them being filled in with wellrammed puddle, so as to form a solid water-tight mass. (See fig 6.) The great difficulties in the construction of a

cofferdam are 1st, to keep it water-tight; and, 2nd, to support the sides against the pressure of the water outside, which in tidal waters is sometimes so great as to render it necessary to allow a dam to fill to prevent its being crushed,

24. In order to save timber, and to avoid the difficulty of keeping out the bottom springs, it has been proposed by a French engineer, after driving the outer row, to dredge out the area thus inclosed, and fill it up to a certain height with béton. The cofferdam is then to be completed by driving an inner row of piles resting on the béton, and puddling between the two rows in the usual manner; and the masonry is carried up on the béton foundation thus prepared. This construction is shown in fig. 7.

25. Concrete is a valuable material when applied in a proper manner, viz., in underground works where it is confined on all sides, and is, consequently, subjected to little cross strain; but it is not fit to be used above ground as a substitute for masonry, and will not bear exposure to water.

26. Concrete is made of gravel, sand, and ground lime, mixed together with water; the slaking of the lime taking place whilst in contact with the sand and gravel. It is difficult to give any definite proportions for the several ingredients, but the principle to be followed in proportioning the several quantities of sand and stones should be to form as much as possible a solid mass, for which purpose it is desirable that the stones should be of various sizes, and angular rather than rounded. The common material is unscreened gravel, containing a considerable portion of sand and large and small pebbles, but small irregular fragments of broken

stone, granite chippings, and the like, are of great service, as they interlace each other and bind the mass together. The proportion of lime to sand should be such as is best suited to form a cement to connect the stones. This must depend in a great measure on the quality of the lime used; the pure limes requiring a great proportion of sand, whilst the stone limes, and those containing alumina, silica, and metallic oxides, require a much smaller proportion.

27. The lime and gravel should be thoroughly incorporated by being repeatedly turned over with shovels, sufficient water being added to ensure the thorough slaking of the lime without drowning it. Concrete should not be thrown into water, because ordinary stone lime will not set under such circumstances; and it should be carefully protected from any wash or run of water, which would have the effect of washing out the lime, and leaving the concrete in the state of loose gravel. Concrete made in the way just described swells slightly before setting, from the expansion due to the slaking of the lime, and does not return to its original bulk. This property makes it valuable for underpinning foundations and similar purposes.

28. Béton.-Béton may be considered as hydraulic concrete; that is, concrete made with hydraulic lime; and is chiefly used in submarine works, as a substitute for masonry, in situations where the bottom cannot be laid dry. It differs from ordinary concrete inasmuch as the lime must be slaked before mixing with the other ingredients, and it is usual to make the lime and sand into mortar before adding the stones. Concrete also is used hot, whilst béton is allowed to stand before being used, in order to ensure the perfect slaking of every particle of lime. Belidor directs that the mortar shall first be made, with puzzolana, sand, and quicklime. When the mortar is thoroughly mixed, the stones are to be thrown in (not larger than a hen's egg), and also iron dross well pounded; the whole is then to be thoroughly incorpor