THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



352 



mation of the Caledonian Canal, and it was also used for deepening the 

 channels through the shoals in Loch Dochfour and Loch Ness. The length 

 of the vessel is 80 feet by 23 feet in width ; the bucket frame is -12 feet long, 

 with ''5 buckets, worked bv a condensing steam engine of 6 h. p. The di- 

 mensions of all the principal parts of the machinery are given minutely, with 

 accounts of several experiments for extending the use of the dredger. On 

 one occasion, as it was found that the buckets had much difficulty in pene- 

 tratioK the hard mountain clay, every alternate bucket was removed and a 

 pair of steel cutters substituted for each, in the expectation that the clay 

 would be loosened and the succeeding bucket would take it up more easily. 

 They did not, however, act satisfactorily, and a risk of fracture was incurred 

 which induced the abandonment of the plan. On another occasion, in form- 

 ing a portion of the canal between Loch Ness, and the lochs at Fort Augustus, 

 where the height of the ground above the water averaged from 20 to 30 feet, 

 and the excavation was required to be about 16 feet beneath it ; that part of 

 the cutting above the water level was commenced by manual labour, while 

 the dredging machine did the excavation under water ; it was soon found, 

 however, that the engine having completed its share of the task, continued 

 to undermine the upper portion, which, being of a loose nature, fell into the 

 water, and was raised by the buckets so rapidly that the manual labour could 

 not compete with the machine, and it was then used to complete the under- 

 taking which it did in eight months, having in that time excavated about 

 170 000 cubic yards of material. When working in favourable situations the 

 qnantitv generally raised equals 90 tons per hour: 1' of the buckets are 

 discharged per minute, with an expenditure of coal of about 15 cwt. per day 

 The communication was illustrated by two detaUed drawings of the boat and 

 its machinery. 



[October, 



" Description of the Maplin Sand Lighthouse at the mouth of the River 

 Thames." By John Baldry Redman, Grad. Inst. C. E. 



The paper commences with an enumeration of the various channels and 

 sandbanks at the mouth of the Thames, with the floating lights, beacons, and 

 buoys marking the entrances of the Channel, and gives the objections to 

 floating lights, and the reasons for selecting the Maplin Sand as the position 

 for a fixed lighthouse. „ ,,_ ■ . ii,„ 



In the year 1837 a survey was made by Mr. Walker, the engineer to the 

 Trinity House, and by boring it was ascertained that the first six feet of the 

 sand was close and compact, but below that for 20 feet the bonng rod went 

 more easily as it descended, and it was found that it became mingled with 

 arRillaceous earth as the depth increased. 



It was then decided to use for the foundations Mr. Mitchell s screw moor- 

 ings and in 1838 the patentee, under Mr. Walker's directions, commenced 

 fixing nine cast-iron screws of 4 feet diameter so as to form an octagon with 

 one screw in the centre ; attached to each of these screws was a cast iron 

 pile 5 inches in diameter and 26 feet long, which was inserted into the sand 

 21 feet below low-water mark. On account of the constant shifting of the 

 sand from around the piles it was determined to place a raft or grating of 

 timber around and between them ; the surface of the raft was covered with 

 faggots of brushwood well fastened to the timbers, and upon them was de- 

 posited 120 tons of rough Kentish ragstone, by which the raft was secured in 

 its situation, and after a time no farther changes occurred in the level of the 

 surface of the sand. In the summer of 1840 the superstructure was com- 

 menced ; it consisted of nine hollow iron columns or pipes curved at the top 

 to a radius of 21 feet towards the centre ; they were secured upon the piles, 

 and two series of continuous circular horizontal ties bound them together, 

 while they were connected with the centre column by diagonal braces— all of 

 wrought iron. Upon these columns is built a wooden dwelUng for the light- 

 keepers, in the upper part of which is placed a French dioptric light of the 

 second order, its centre being 45 feet above the mean level of the sea, and at 

 that elevation can be seen from a ship's deck at a distance of nine or ten 

 miles ; a bell is fixed on the gallery which is sounded by machinery at inter- 

 vals during dark and foggy nights. :, ,, j 

 The communication gives all the details of the dimensions and the mode 

 of fixing the cast iron screws and piles made by Messrs. Rennie's— the iron 

 work by Messrs. Gordon's of Deptford— the wood work by Messrs. Gates and 

 Home of Poplar, and the lanthorn by Messrs. Wilkins— and the whole is 

 illustrated by a series of drawings which fully describe this useful construe 

 lion, which has hitherto withstood the most violent attacks of the sea to 

 which it is exposed. . 



Jtemarks.—lu answer to questions from the President, Mr. Wdkins stated 

 that he had been in the Eddystone and the Maplin Sand lighthouses during 

 severe gales of wind ; that as might be conceived from the nature of the con- 

 struction, the latter building was more affected than the former by the strik- 

 ing of heavy seas; the motion appeared to be more like torsion than simple 

 vibration, which he attributed to the waves striking the ladder and its pro- 

 jecting stage, and thus tending to twist the upper part. S^U the motion 

 was not such as would cause injury to the budding. 



The President pointed out two diagonal braces extending downward from 

 ihe end of the ladder stage to the piles on either side, which had been intro- 

 duced in order to counteract the twisting described by Mr. Wilkins. In con- 

 structions of this nature it was of importance to oppose as little resistance as 

 possilile to the seas, especially in the upper part of the budding, a system of 

 bracing had therefore been adopted, which consisted principally of two series 

 of continuous circular horizontal ties between the piles at the several heights 



of 6 feet and 15 feet above low-water mark of spring tides. From the ex- 

 ternal ring of piles two sets of diagonal stays extended to the centre pillar, 

 forming strong triangular trusses in the direction of each pile, and two sets 

 of horizontal stays stretched between the piles and the centre pillar at the 

 levels of the circular bands. The amount of direct vibration was very small, 

 and he did not conceive that the twisting motion which had been described 

 was snflicient to warrant the introduction of diagonal braces, which would 

 materially augment the surface upon which the waves would act. 



Mr. Vi'gnoles directed the attention of the meeting to the system of di- 

 agonal bracing between the piles which had been adopted at the Port Fleet- 

 wood lighthouse ; he apprehended that as the principal force of the waves 

 would be exerted against that part of the structure which was above the 

 high-water level, the diagonal braces extending between the upper part of 

 the piles and the level of low-water were preferable to the horizontal con- 

 tinuous bands of the Maplin Sand Ughthouse, although assisted by the system 

 of radiating central truss braces which it possessed : he conceived that both 

 buildings weie strong enough for the purposes for which they were con- 

 structed, but he preferred the mode of bracing adopted in the Port Fleetwood 

 house, the vibration of which he knew to be very small, although situated m 

 an exposed position where the rise of the tide is 30 feet. _ 



Mr. Donkin observed that there could not exist a doubt of the introduction 

 of diagonal braces rendering the building stronger ; how far they were neces- 

 «arv, or might be prejudicial in offering additional resistance to the passage 

 of the waves, should be well examined before adopting them. He considered 

 the position of the suspended ladder decidedly objectionable, as any torsion 

 caused by the waves striking it must tend to dislocate the fibre of the ma- 

 terial of the piles and to fracture them. 



Mr. Farcy believed the construction of the Maplin Sand house to be better 

 adapted than the Fleetwood liouse for resisting the direct action of waves, 

 but the diagonal bracing of the latter enabled it to withstand torsion better 

 than the hoop bracing of the former. He inquired why the lower part ot 

 the light-keeper's house was made conical, as he apprehended that it would 

 receive a heavier blow from a wave than if it had been flat. 



The President replied that the main body of the waves seldom or never 

 rose so high as the bottom of the house, and that the conical form allowed 

 the air and sprav to rise up and be guided off without affecting the b>"l<i"ig 

 as it would do if the bottom was flat. With regard to the torsion, that had 

 only been felt at first, when the ladder extended too low down and received 

 a constant succession of blows from every wave, which naturally communi- 

 cated a vibration to the whole structure ; the ladder was now shortened, and 

 nothing of the kind was felt : the waves scarcely, even in the roughest wea- 

 ther, struck the suspension stage or the boat. He preferred the continuous 

 horizontal bracing, which bound all the piles firmly together like the staves 

 of a barrel ; and from observations he had made, he believed the amount of 

 vibration to be greater in the Port Fleetwood lighthouse than in that at the 



"inTnswer'to a question from Mr. G. H. Palmer, the President said that at 

 present there was not any indication of a change in the condition of the cast 

 iron from its contact with the salt water. 



Professor Brande was unable to give any additional evidence on the oD- 

 served facts connected with the change suffered by cast iron exposed to the 

 action of salt water, or in mines and in various other positions— from experi- 

 ments which he had made, he was led to beUeve that many of the appear- 

 ances observed in the changes of cast iron arose rather from a peculiar me- 

 chanical combination of the molecules, than from a difference in the chemical 

 constitution of the metal ; no difference could be detected by analysis in the 

 metal which had undergone change and that which had not. It should be 

 remarked that the contact of two metals was not essential to cause galvamc 

 action : a film of oxide upon the surface of the body of metal formed a very 

 active galvanic pile : hence arose the necessity for preventing oxidation by 

 proper paints or varnish before using pieces of cast iron in exposed situ- 



"''Mr-'Parey observed that in the early engines constructed by Woolf in 

 CornwaU, in which the packing segments were of gun-metal and the body ot 

 the piston was of cast iron, wherever the two metals were in contact the iron 

 was turned to plumbago; this had been particularly observed where high- 

 pressure steam was used : it might be a question whether the temperature of 

 the steam, and the quantity of mineral water carried over with the steam by 

 the large amount of priming of the engines in that day, had not materially 

 contributed to produce the effect. 



Mr P Taylor believed that the temperature of the steam had not any 

 connexion with the subject; in the metallic packing of steam pistons of low- 

 pressure marine engines, which he had constantly under repair at Marseilles, 

 wherever the wedge pieces were of gun metal, the backs of the cast iron seg- 

 ments were converted into plumbago, while those surfaces of cast iron which 

 were ground together and worked against each other remained unchanged ; 

 the same might be said of the rubbing surfaces of cast iron against gun-metal ; 

 it appeared, therefore, that the formation of an oxide was necessary to com- 

 mence the change. He repudiated the use of cast iron in situations where 

 these changes were to be apprehended ; he would employ ^"""S'^* '™''',^' 

 although that did become oxidized, it retained its relative strength to the 

 last, whereas cast iron, when changed into plumbago, retained its bulk out 

 lost nearly all power of cohesion. „,rnn«pd to 



Mr. John Taylor said that in Cornwall the cast >™n P'""P:''"'„'''P?Zgh 

 the action of niine water were very speedily destroyed; and even aHhough 

 li inch thick, they could be cut to pieces with a knife when first taken out 



