38 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



mouth of the Thames, ahout twenty miles below the Nore, forming 

 the northern side of the Swin, or King's Channel, which, on account 

 of its depth, is much frequented by large ships, as also by colliers 

 and other vessels from the North Sea, and where a floating light i.s 

 now maintained. This spot is a shifting sand, and is dry at low- 

 water spring-tides. The plan is to erect a tlxcd lighthouse of tim- 

 ber framing, with a lanteni, and residence for the attendants. For 

 this purpose, in August last, operations were connnenced to fiirm the 

 base of an octagon, 40 feet diameter, with Mitchell's mooring screws, 

 one of which was fixed at each angle, and another in the centre ; 

 each of these are 4 feet 6 inches diameter, attached to a shaft of 

 wrought iron about 25 feet long, and 5 inches diameter, and. conse- 

 quently, presenting an immense liorizontal resisting surface. For 

 the purpose, a stage for fixing the screws, a raft of timber, 30 feet 

 square, was floated over the spot, with a capstan in the centre, which 

 was made to fit on the top of the iron shaft, and firmly keyed to it; 

 a povi er of about .30 men was employed for driving the screws ; their 

 united labours were continued until the whole force of the ,30 men 

 could scarcely turn the capstan : the shafts were left standing about 

 5 feet above the surface of the sands. The fixing of llie nine screws, 

 including the setting out the foundation and adjusting the raft, which 

 liad to be replaced eveiy tide, did not occupy more than nine or 

 ten days. 



This is the portion of tlie «ork liitherto eflected. and its continua- 

 tion will be proceeded in when the proper season comes in the ensu- 

 ing spring. Upon this foundation the superstructure of timber is to 

 be constructed, consisting of a principal post, strongly braced and 

 secured, with angle-posts made to converge until tliey form a diame- 

 ter of about 16 feet at the top, giving the superstructure the appear- 

 ance of the frustrum of an octangular pyramid, the feet of the angu- 

 lar posts and braces are well secured and keyed down to the tops of 

 the iron shafts, and the whole is connected at top and bottom with 

 strong horizontal ties of wood and iron. The entire height of the 

 superstructure will be 30 feet above the top of the iron shafts; up to 

 a point about 12 feet above high-water mark spring tides the work 

 Avill be open ; the part above will be enclosed as a residence for the 

 attendants ; in the centre and above this will be erected a room or 

 lantern of about 10 feet diameter, from which the lights are to be 

 exhibited. 



The interval that has elapsed since the screws were fixed has fully 

 proved the security of them, which, although driven into sand, seem 

 as if fixed into clay, and in this state they have remained since the 

 summer. The whole process confers the greatest credit both on the 

 engineers and Mr. Mitchell, the patentee of the screws who super- 

 intended the work, (assisted by his son) and we feel happy to hear 

 that his ingenious invention daily obtains a greater extension. 



The importance of this experiment certainly called for a trial, and 

 it was with due liberality that the Trinity Board sanctioned the ex- 

 pense. To them it involves the question of a better security of the 

 light, and a less expense in its maintenance, both objects justifying 

 the experiment, and counter-balancing the expense of prime cost in 

 such construction. The insecurity of floating lights has been too 

 manifestly productive of disastrous consequences not to call for a 

 remedy, and it will be for(unate if by this means it be obtained. 

 Within the last mouth the Nore light was blown from her mooring; 

 and the breaking away of the North-west Light of the Mersey is 

 supposed to have led to the lamentable shi)iwreeks at liivcrpool. 



AVc can perceive only one objection which can be started, and that 

 is rather to be determined by experience than conjecture, that is how 

 far the edifice is liable to be washed away by slonns as one of the 

 lOdystone buildings was ; but this, in our opinion, will be mainly 

 l)rovided against by the unity of constniction and the breadth of base 

 well secured I o the shafts by the screws. 



The progress of this work will naturally be watched with interest, 

 for it is one which in its influence is not limited to this individual 

 case. It is of much more importance than chain pieis, as it will 

 enable us to obtain a foundation in positions where (hey cannot at 

 present be used. It must be remembered that the screw can be em- 

 ployed where the pile is of no avail, and that it possesses a much 

 stronger hold, and has greatei' durability. 



We shall thus, therefore, be able to construct piers and break- 

 waters in localities inaccessible, and be enabled to render important 

 senice to the interests or commerce. We think, too, that the screw 

 itself would be of great utility in securing the end chains of suspen- 

 sion-bridges, as its powers of resistance can bcextcndtd toanyneces- 

 sary degree by an increase in size. The greater employment of the 

 screws, which would arise from their successful application, will 

 have a further beneficial effect in enabling the jialcntce to supply 

 them at a diminished expense, which, under their present limited 

 sale, is necessarily high. 



The carpenters' work of the superstructure is about to be contracted 

 for, which is intended to be erected and put together at the Wharf 

 at Blackw^all to save time of fitting, &c., at the spot. 



In order for the better understanding of our description of the 

 mooring screws at page 22 in our last volume, we have obtained of 

 Mr. Weale the use of the wood engravings illustrating his valuable 

 publication, the " Public Works of Great Britain." "They will ex- 

 hibit, in a nmch clearer light, tlie construction of the moorings. The 

 screws described above for the foundation of the lighthouse, difl'er in 

 some respects from these engravings ; instead of being furnished 

 with a chain and shackle, they have a wrought iron shaft connected 

 to the screw as before described. 



Fig. 4. 



Figure 1 shows the screw mooring as prepared for use. o is a 

 spiral or screw flange of about one tuni and a half, having a hollow 

 cylindrical centre, ns shownin^^»)T2, and of cast-iron in one piece; 

 Z> is a wruught-iron spindle, which passes through the cylindrical 

 socket of the screw flange, somewlmt tapering in form, as shown in 

 fiqurc 3, and wlien driven up tight, is fixed thereto by a forelock, 

 w-liich passes through botl ; it is formed with a square head, c, to 

 receive the key for Bcrcwing it into the ground ; rf is a collar of 



wrought iron, the front of which is shoWn in figure 4, fitted so as to 

 turn freely on the upper part of the shaft of the spindle below the 

 collar. Figure 2 shows the upper surface of the spiral flange; 

 fgurc 3, the spindle ; and .figure 4, the collar and shackle. The 

 shackle is fixed to the spindle by means of the loose collar, in order 

 to prevent the dragging round, and the consequent fouling of the 

 chain, whilst the spindle is 



> being turned in or out of the ground. 



