ut 



SLIP. 



SLIP. 



ii JO 



such depth as to ensure their not exerting any detniure force 

 the letter sources of danger are far more complicated, and they require 

 to be dealt with very carefully, on account of the numerous con 

 dderatiom attached to the change of state the materials often aseuin 

 IK !' .re they get into motion. 



t'mform'ly permeable materials, such as broken stones, gravel. 

 Jungle, are not exposed to slip after they hare onoe taken their op;] 

 I>o8e, because the gravity of the separate stones is sufficient fa 

 keep them in their places whilst the water falling upon their expose* 

 surfaces is percolating through them. Very fine sands, however, are 

 able to be rendered semi-fluid under some conditions, and they the 

 hare no angle of repose, and are able to spread in every direction. 

 Some ;clays, such as the Oxford clay, the gault, and the London clny 

 which seem to have originally been the alluvial mud* of marin 

 estuaries, subsequently dried and consolidated, are susceptibl 

 absorbing again quantities of water ablo to bring them into their origina 

 state ; and it seems, from the experience of our railway and cona 

 engineers, that many years must pass before cuttings executed in those 

 formations attain permanent conditions of stability. Instances hav 

 been known in which slips have taken place in the Oxford clay forma 

 tion when the slopes have been even 10 to 1 (10 base to 1 in vertica 

 height) ; and the New Cross cutting of the Brighton Railway, in) th 

 London clay, slipped in the winter 1860-61, although it had forma 

 part of the cutting for the old Croydon canal executed nearly sixt; 

 years since. Slips, it ,may be added, occur in undisturbed uatura 

 deposits exposed to the action of the sea, or of running water, and th< 

 same class of accidents, above described as occurring in the cuttings 

 and embankments executed by the hand of man, occasionally Imppei 

 in consequence of the action of the ordinary laws of nature. \Vhei 

 they do happen in coal mines, or in analogous positions, owing to the 

 interference with the lateral support of the strata worked through, the 

 slips are technically known under the name of " creeps," and the) 

 constitute by for the most dangerous, because the most irresistible 

 of the accidents to which coal mining is exposed. 



As slips are principally attributable to the effects of water upon thi 

 materials concerned, it must be evident that the only method <>: 

 preventing or remedying them must be to establish a perfect system 

 of drainage in order to carry off superficial waters without allowing 

 them to soak into the ground so as to dilute the soluble materials, 01 

 to set in motion the more minute particles of the intercallatec 

 permeable strata. The mode usually adopted near London in dealing 

 with the earthworks to be formed around, or for the support of, water- 

 works, where of course slips would be of very serious importance, ia to 

 carry up the embankment in layers, alternately of the clay in its 

 natural state, and of the clay after having been burnt in a heap; 

 vertical dykes or trenches filled in with burnt clay are carried from the 

 top to the bottom of the bank ; and the top is dressed off with the 

 longitudinal and transverse inclinations required to throw the rain- 

 water into a series of surface drains. Theoretically this system is the 

 one to be aimed at in all such works, but it is too often the case that 

 the cost of the precautions it involves induces constructors to neglect 

 some of them ; and again it may happen that the materials of the banks 

 are exceptionally impermeable, and thus justify a more economical mode 

 of treatment. Nevertheless, it must be considered that, especially 

 upon ground having a transverse fall, on embankment formed of the 

 London clay is always exposed to slip, unless it should have been 

 executed in some such manner as the one above mentioned. Broken 

 stone or broken chalk may be used instead of the burnt clay ; but the 

 condition to be aimed at in the application of those materials ia that 

 they should be perfectly permeable, and that the water they may 

 remove should have a free outlet. In deep cuttings, the experience of 

 the New Cross section of the Brighton line proves that the London 

 clay, as it exhibits itself there, will not stand with slopes of less than 

 4 to 1, even when a good system of superficial drains has been executed. 

 In no case will the stiff blue clays stand with slopes of less than 2 to 1, 

 without giving rise to numerous slips ; and those accidents can hardly 

 be prevented, unless the inclination of the slopes is made in the ratio 

 of 3 to 1. An essential precaution to be taken before establishing an 

 embankment of clay is to clear its seating from the vegetable mould 

 which might be originally there, and to provide efficient lateral drains, 

 in order to prevent any land waters from finding their way under the 

 embankment. In order to prevent lateral displacement, or slipping on 

 the bed, on sloping ground, it is frequently the case that toothings are 

 cut in the natural surface, for the 'purpose of increasing the friction, 

 and thus of opposing the tendency to slip. 



The earthwork at the back of retaining walls is frequently so much 

 saturated with water that it has very little more consistency 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 suppo- 

 sition that the wall is intended to resist the pressure of a semi-fluid 

 mass denser than water. In many 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, &c. ; all of these are 

 in fact as much exposed to slip as the London clays, from which the 

 previous illustration* have been principally derived. 

 SLIP, Shif-BuiUiin'j. 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 purposes 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 -h-re or normally to it, according 

 to the width of the piece of water into which the ship is to be launehed, 

 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 ,.f the 

 slip, equal to at least twice the length of the vessels to be Imilt on it. 

 In moot government ship-building yards several slips are p\ 

 together, in which case they must be separated by ! m.- ..i 



sufficient width to allow the carpenters to prepare the timber re 

 for the framing, or by platform* of 60 to 100 feet in width. In 

 as from 80 to fill feet are required on each side of a ship for thin pur- 

 pose. Wherever it is possible so to place a series of slips, tin -y 

 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 leu 

 time a ship is usually upon the stocks, the materials, which mi ;ht 

 hapi>en to be exposed to a considerable excess of either of those action*, 

 would be likely to have marked differences in their speeitie uiavities at 

 the period of launching; or, in other words, the weights of 1 1 > 

 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 : 



For a first rate three-decker . 



For a two-decker 



For a frigate 



For a corvette or small craft 



18ft. Oin. 



It ft. Sin. 



10 ft. 6 in. 



8ft. Sin. 



But in all coses it ia 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 

 8 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 arc 

 indispensable. 



The sides of a building-slip, respectively on the right and left of the 

 groove for the keel, must in all cases be precisely similar; and, wit 

 possible, they should be made rectilineal in their transverse H 

 The longitudinal section should also be rectilineal ; and it would apjH-ar 

 ;hat the angle of inclination should vary with the weight of the 

 upon the stocks : thus, for three-deckers, an inclination of j*j i- \ 

 sory, whereas, for frigates, an inclination of fe, or even of , \v uM 

 suffice. Very small craft such as sloops, yachts, &c. are most con- 

 veniently launched, however, from slips having very steep inclinations. 

 It is usual to make the lower immersed ends of the slip* fur the 

 construction of large vessels about 150 or 160 feet long beyond tin- 

 end of the portion devoted to the stocks and hull, the Litter | 

 >eing at the present day made about 300 feet long ; so that a building- 

 slip 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 keek, should never be less than from 

 jiie-third to one-half of the maximum breadth of the vessels to be 

 milt upon them, and in practice it seems to range between 22 and 23 

 eet. 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 

 x> observe 1 , 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 

 luilding 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 limit.". 



The majority of the building-slips in English dockyards are at the 

 resent day covered, at least in the length occupied by the vessels onli- 

 arily built on them. Of course the dimensions, in length, breadth, 

 and height, of such structures must be regulated by the dimensions of 

 lie vessels themselves; and as naval architecture appears at thr p 

 ay to be in course of change in all these respects, it may be dam 

 o pretend even to state any general laws on the subject. It may 

 uffice, then, to mention that, for the old class of 120-gun ships, the 

 oofs 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 

 ept at about 120 feet above the depression 'for the keel ; the frai 

 t the roof must be kept about 6 feet, at least, above the loftiest 

 ortioii 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 space 

 u all directions ; the side supports of the roof must be placed at such 

 istances apart as to allow the framing which may be put together on 

 ic side platforms to be set on end before being carried to their delinitu 

 ositious, and in some of the best slips they ore placed at intervals of 

 roui 30 to 40 feet. In fact, the conditions to be attained by the roofs 

 ner building-slips are, that the work and the workmen -hould 

 HI at all times protected from the inclemencies of the weather, whilst . 



tore must be an efficient ventilation and a good distribution of light, 

 igcther with facilities for the transport and hoisting of the materials, 

 he use of iron for ship-building will no doubt greatly modify the 

 etails to be adopted in all these matters; but the prim: 

 fectually protecting the whole structure, and of admitting the undis- 





