The earthwork at the back of retaining walls is frequently so much saturated with water that it has very little more consisteney than that fluid ,itself has ; and it is therefore necessary, in order to resist its tendency to slip, to calculate the strength of the wall upon the auppo sition that the wall is intended to resist the pressure of a semi-fluid mass denser than water. In ninny places near London, and in sea ports upon alluvial deposits, the best rule seems to be to make the thickness of the walls equal on the average to half their height. This rule will hold good with most deliquescent clays, such as the gault, Oxford clay, the clays of the carboniferous series, ; all of these are in fact as much exposed to slip as the London clays, from which the previous illustrations have been principally derived.
SLIP, The slips used for ship-building are inclined surfaces (the upper parts of which are kept constantly above the water line, and the lower parts are carried to the requisite depth below the water) for the purposed of building and launching the hull, either into tidal or constantly deep water. They may be placed either in an inclined direction to the line of the shore or normally to it, according to the width of the piece of water Into which the ship is to be launched, to the set of the currents, or to the exposure of the situation ; and there must always be a clear space of deep water beyond the lower end of the slip, equal to at least twice the length of the vessels to be built on it. In most government ship-building yards several slips are grouped together, in which case they must be separated by level platforms of sufficient width to allow the carpenters to prepare the timber required for the framing, or by platforms of 60 to 100 feet in width, inasmuch as from 30 to 50 feet are required on each side of a ship for this pur pose. Wherever it is possible so to place a series of slips, they should be so arranged as to ensure an equal degree of light and heat to the sides of the ships built upon them; for, on account of the length of time a ship is usually upon the stocks, the materials, which might happen to be exposed Loa considerable excess of either of those actions, would be likely to have marked differences in their specific gravities at the period of launching ; or, in other words, the weights of the two sides of the ship might be very different. The depths of water it has been found advisable to secure over the extremity of the slips at the moment of launching a vessel are considered to be as follows :— But in all cases it is assumed that the hulls are launched " light." The depths, moreover, are calculated by measuring from the under side of the keels; and as the latter are usually placed in a groove of about 3 feet wide by 16 or 18 inches deep, in the centre of the slip, it is only upon the line of this groove that the above-mentioned depths are indispensable.
The sides of a building-slip, respectively on the right and left of the ;move for the keel, must in all cases be precisely similar; and, wherever possible, they should be made rectilineal in their transverse section. The longitudinal section should also be rectilineal; audit would appear ,hat the angle of inclination should vary with the weight of the vessel Ilion the stocks : thus, for three-deckers, an inclination of is neces ;any, whereas, for frigates, an inclination of h, or even of would iuffice. Very small craft—such as sloops, yachts, &c.—are most con renieutly launched, however, from slips having very steep inclinations.
.t is usual to make the lower immersed ends of the slips for the ionstruction of large vessels about 150 or 160 feet long beyond the aid of the portiou devoted to the stocks and hull, the Litter portion icing at the present day made about 300 feet long; so that a building lip for a large man-of-war is now usually from 450 to 500 feet long at maximum. The width of the sides of the slips, measured trans •ersely to the line of the keels, should never be less than from ?ne-third to one-half of the maximum breadth of the vessels to be ruilt upon them, and in practice it seems to range between 22 and 28 sot. The platforms on which the timber is worked by the sides of hese planes are raised above their level ; but in so doing it is essential o observe—I, that any rain-water falling upon them must be carried way from the slip ; 2, that no interference must be allowed with the ree circulation of air round the hull; 3, that the shores, either for wilding or for launching, should have a sufficient width of base ; and , that the vessel, in leaving the ways, should have sufficient space to e able to sway freely within moderate limits.
The majority of the building-slips in English dockyards are at the resent day covered, at least hi the length occupied by the vessels ordi arily built on them. Of course the dimensions, in length, breadth, nd height, of such structures must bo regulated by the dimensions of vessels themselves; and as naval architecture appears at the present ay to be in course of change in all these respects, it may be dangerous pretend oven to state any general laws on the subject. It may Alice, then, to mention that, for the old class of 120-gun ships, the ids over the slips were usually made about 300 feet long by 110 feet lear span between the points of support, and the ridge was usually apt at about 120 feet above the depression 'for the keel ; the framing f the roof must be kept about 6 feet, at least, above the loftiest ortiou of the vessel ; and the opening at the end, through which she as to be launched, must have at least the same amount of clear apace all directions; the side supports of the roof must be placed at such 'stances apart as to allow the framing which may be put together on me side platforms to be set on end before being carried to their definite >shims, and in some of the best slips they are placed at intervals of tun 30 to 40 feet. In fact, the conditions to be attained by the roofs .ected over building-slips are, that the work and the workmen should i at all times protected from the inclemencies of the weather, whilst 'ere must be an efficient ventilation and a good distribution of light, gether with facilities for the transport and hoisting of the materials. he use of iron for ship-building will no doubt greatly modify the !tails to be adopted in all these matters ; but the principles of fectually protecting the whole structure, and of admitting the midis tnrbed execution of the various manipulations, must remain unchange able. It would seem, indeed, that iron ships required to be protected from the effects of the atmosphere during their construction even more carefully than wooden ones ; and "the fleet of the future" may tax the skill of our constructors to cover their hulls whilst on the slips even more than the old liners have done. Amongst the most important and the most successful covered building-slips, those erected at Woolwich and Deptford may be specially cited.