36 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



PFebruaby, 



tended to meet any case, and may be said to be limited only by 

 the power available for forcing them into the ground. Either the 

 screw-pile, or the screw-mooring can be employed in every de- 

 scription of ground, hard rock alone excepted : for its helical form 

 enables it to force its way among stones, and even to thrust aside 

 medium-sized boulders. In ports, harbours, estuaries, and road- 

 steads, rock is, however, seldom met with, except in detached 

 masses, the ground being usually an accumulation of alluvial de- 

 posit, which is well adapted for the reception of such foundations, 

 and is also that in which they are generally most required. The 

 ground screw has been already extensively used for several pur- 

 poses, and its applicability to many others will be evident from a 

 succinct account of its present employment. 



Fig. 2. 



PORT OF L0ND0^4 



buoy shackled at the other end (fie;. 3). This sinker, which is a 

 block of stone or iron, is either laid upon the surface of the 

 ground, or Is placed in an excavation prepared for its reception. 



Fig. 3. 



UMEHOUSE 



The fixed, or permanent moorings most commonly used are of 

 two kinds ; the span-chain mooring, and the sinker or mooring- 

 block. The former of these (fig. 2) consists of a strong chain of 

 considerable length, stretched along the ground (across the river), 

 and retained by heavy anchors, or mooring-blocks, at either end, 

 and to the middle of the ground-chain the buoy-chain is shackled. 

 Many of these moorings were formerly laid down in the Thames, 

 at great expense, and they still continue to be the only moorings 

 in use at the royal dockyards. The disadvantages of these moor- 

 ings are their expense and the obstruction they present to naviga- 

 tion ; for should a vessel cast anchor in the neighbourhood, it is 

 almost certain to get hooked into the cliain, from which it can only 

 be freed by the assistance of a chain-lighter, at considerable e.x- 

 pense and loss of time. In fact, mooring-chains across the an- 

 chorage were adopted to insure the anchors of vessels liringing up, 

 when the harbour was crowded, and in such cases the harbour flat 

 was obliged to attend to clear the anchor at slack water. Messrs. 

 Hemmans, Parkes, and others, state the expense of each govern- 

 ment mooring of this kind to be about 2,500/. 



The other kind, which is more generally employed, consists of a 

 heavy sinker, to which a strong chain is attached, extending to a 



i-ig. 4. 



The advantages of this system are its simplicity and cheapness of 

 construction, with the power of holding nearly equally well in 

 every direction; but, on the other hand, when the sinker is sub- 

 jected to a heavy strain, it is generally found deficient in holding- 

 power, because it chiefly depends upon the specific gravity of the 

 mass, and because the ground, which must be greatly broken up 

 for its reception, offers only a feeble resistance to its subsequent 

 extraction. Another disadvantage is, that being generally confined 

 to shallow bays and harbours, vessels are often as seriously injured 

 by grounding upon' them, as when they come in contact with 

 anchors, which it is contended should never be permitted to be 

 used in harbours or rivers. Moorings are sometimes formed of 

 timber frames, loaded with stone and buried in the bed of the 

 river, as in the harbour of Sunderland, where they have been ex- 

 tensively applied by Mr. Meik. 



These evident defects in the ordinary systems of mooring, in- 

 duced the author, whilst 

 seeking for a simple, 

 eft'ective, and at the 

 same time inexpensive 

 mode of holding the 

 buoy-chain down, to 

 adopt a modification of 

 the screw-pile (fig. 4) ; 

 because it offered great 

 facilities for entering 

 tlie ground, and when 

 arrived at the lequired 

 depth, it evidently af- 

 forded greater holding 

 jiower than any other 

 Ibrm. 



K\ery description of 

 earth is more or less 

 adhesive, and the great- 

 er its tenacity the 

 larger must be the por- 

 tion disturbed before 

 the mooring can be dis- 

 jilaced by any direct 

 force. The mass of 

 ground thus affected, in 

 the case of the screw- 

 mooring, is in the form of the frustrum of a cone, inverted; that 

 is, with its base at the surface, the breadth of the base being in 

 proportion to the tenacity of the ground; this is pressed on by a 

 cylinder of water equal to its diameter, the axis of which is its 

 depth, and the water again bears the weight of a column of air of 

 the diameter of the cylinder. The comparative masses of ground 

 which must be removed in dragging up the two kinds of moorings 

 are shown by the dotted lines in figs. 3 and 5. 



It is evident, therefore, that if a cast-iron screw, of a given 

 area, be forced into the earth to a certain depth, it must afford a 



