1839.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



121 



The last stone of the arch was laid Jan. 31, 1838, by the chairmaD 

 of the trustees, J. H. Pelly, Esq., F.R.S., &c. &c., when a bronze 

 medal of Queen Victoria was deposited in the bed of the stone, inscribed 

 upon the edge with the occasion, date, name, &c. &c. 



The bridge was publicly opened nn Thursday, Feb. I4th, 1839, by 

 the Sheriff of Essex, William Cotton, Esq., F.R.S., &c., and a retinue 

 of carriages driving from the Essex side, meeting on the centre of the 

 bridge, the Sheriff of Middlesex, Alderman Thomas Wood, accom- 

 panied by the chairman of the trustees, and followed by a long retinue 

 of carria-es, containing the trustees, the engineers, &c. &c. 



The form of the bridge, as shown in fig. 1, is a very flat segment, 

 the rise nut being more than three feet, and consists of an oblique arch 

 of an elliptical form, the wing walls extending at each end of the 

 bridge terminated with granite pede-tals surmounted by lamp irons. 

 The following are the principal dimensions of the bridge : — 



ft. in. 

 Span of arch, measured on the face - - - - 66 



Span of arch, measured square with the abutment line - 64 

 Rise of arch - - - - - - - -139 



Thickness of abutments - - - - - -15 



Length of bridge at wings - - - - - 146 



Width of bridge in clear of parapet - - - - 40 



Width of carriage way - - - - - -30 



Width of each footpath 5 



The arch stones are 4 feet thick at the springing, and 2 feet 6 inches 

 in the crown. 



The contract for the new structure, with the temporary accommoda- 

 tion for the public during the erection of the new bridge, is s;ated to 

 be about 11,000/. The stone used for the external face is blue Aber- 

 deen granite, backed with the masonry of the old structure. The 

 foundations are laid upon a bed of strong gravel several feet below the 

 bed of the river, and a protection of sheet piling is driven in front of 

 the masonry several feet into the solid ground. 



It was expected that in excavating the bed of the river, for the 

 fnundations of the new bridge, some antiquities would have been dis- 

 covered, but in fact few articles of any interest were found, and those 

 of trifling va ue. The mdst interesting were some brass tokens, two of 

 which are more particularly connected with the subject before us ; a 

 few silver coins of little value, some ancierit iron keys, with the re- 

 mains of an iron spear h ad, nearly comprise the catalogue of all that 

 was found. 



We must not omit to notice thit in the demolition of the old bridge 

 it was found that the masonry of the arches was not originally covered 

 by gravel, &c., to form a roadway, as it is now usual, but that tie 

 carriages and iiorses went directly upon the stone-work of the arches, 

 and that ruts of the wheels had been worn in places to a depth of nine 

 inches, and holes were worn through, evidently made by the tread of 

 the horses. 



ISOLATED HARBOURS OF REFUGE— EXAMPLES OF 

 NATURAL FORMATION, 



BY HYDE CLARKE, ESQ., C. E. 



As I have strongly advocated, and I believe introduced, tlie prin- 

 ciple of insulation as applied to harbours, it is not unnatural that I 

 should take some interest in that of isolation, which is certainly one 

 of the most important principles wliich has lately been elucidated. 

 Its author, Mr. Tait, may congratulate himself on the acknowledge- 

 ment of its merits which it has so generally received, and I sliall be 

 happy if the few remarks I now contribute should afford any addi- 

 tional reason for its support. 



It is certainly augural of the advancement of harbour engineering, 

 that instead of plans being merely confined to local circumstances, 

 so much attention is now being devoted to the illustration of gene- 

 ral principles ; which, however, can only be drawn from nature. 

 The importance of philosophical instruction to the engineer is 

 powerfully inculcated when we see the manner in which objects, 

 apparently so remote, are brought to bear upon our immediate pur- 

 suits. It is true, indeed, that this is a principle of philosophy, but 

 it is one, the importance of which does not seem always to be recog- 

 nised, although it should be remembered that even the abstract 

 sciences are derived from the observation of natural phenomena, and 

 that the laws of mechanics are equally developed in the motions of 

 the heavenly bodies, as in the construction of tlie animal and vege- 

 table kingdoms. The face of nature is, in fact, the great book of 

 truth, written by an unerring hand ; and it is upon the precepts there 

 taught that equally in science, literature, and art, all excellence 

 depends. 



Mr. Tait has, indeed, informed me that it was from the Isle of 

 Wight that he partially conceived the idea of his own plan, and 



upon that basis Mr. Rooke has also founded his modifications. To 

 shew, however, that this is not an individual example, but part of a 

 general rule, I now present same instances of natural isolated 

 harbours, whicli, while they support the principle advocated by my 

 two philosophical friends, may throw some inductive light upon the 

 probable result of future operations. 



Fig. 1. 



K*^6 



PORTO DO ILHEO. 



The figure B here inserted is the representation ofan isolated har- 

 bour in active operation. It is called Porto do Ilheo, and is situated 

 in the island of St. Michael's, one of the Azores, opposite the iowit of 

 Villa Franca, and not above six miles from that of Poute Delgada, 

 one of the most important shipping places for the fruit and wine 

 trade. From the nature of the basin it may be readily referred to a 

 volcanic formation, and this is supported by its general contigurd- 

 tion, as much as by its situation with deep soundings around it. It 

 has all the appearance of an extinct crater ; consisting of a wall 

 of rock rising in most places 30 to 50 feet high, with a hollow centre, 

 and an opening on one .side. As nearly as I can recollect, it is about 

 a quarter or half a mile across, and at low water lias, in the centre, 

 from eight to ten feet water, with a slieeting of mud covering the 

 bottom. The mouth is to the nortli-east, of course opposite to the 

 prevailing winds, and it is the only place in the islands which is 

 sheltered against the westerly winds, but it is unprotected in the 

 soutli-east when a heavy swell runs in its neighbourhood. The 

 island is often used for careening, and it is the general place of 

 refuge in westerly winds for the vessels from Ponte Delgada, w hich 

 run here until the fury of the storm is abated, and lie without a wave 

 to ruffle them, except that sometimes there is a dash of spray 

 through a breali in the west wall. Its importance was impressed 

 upon me at the time I \\'as engaged with the Atlantic Steam Naviga- 

 tion Company, the first which was brought forward in 1836, when 

 my plan was to have used the Azores as a central station for com- 

 munication to North and Soutli America and the West Indies. The 

 use of this island as a depot was strongly enforced upon me by a 

 merchant captain well acquainted with it, and I proposed to have 

 had it deepened and an entrance made, accessible at all times of 

 tide. 



All such cases, it must be remarked, are of volcanic formation, 

 and the entrance is almost uniformly found opposed to the prevailing 

 winds. No satisfactory account of the cause of this has, however, 

 been given, and the reasons assigned by Lyell are far from clear or 

 convincing. In the recess called Rowley Shoal, off the north-west 

 coast of Australia, however, as described by Captain King, in 

 Lyell's Geology,* where the east and west monsoons prevail alter- 

 nately, the open side of one crescent-shaped recess, the Imperieuse, 

 was turned to the east, and of another, the Mermaid, to the 

 west. 



In the Yellow Sea there is a remarkable horse- shoe volcanic 

 harbour, mentioned in Macartney's Voyage to China, and the 

 Chagos Isles in the Indian Ocean, described by Horsburgh, whicli 

 have their openings to the north-west, are most important to naviga- 

 tors. These harbours are well known for their security, and ships 

 can enter and depart with ease. 



The Coral Islands, in Polynesia, are nearly all of this horse-shoe 

 formation, so that of thirty-two examined by Captain Beechey (t), 

 twenty-nine liad lagoons in the centre, tlie land being merely a nar- 

 row ring. Some of these lagoons were as deep as tliirty-eight 

 fathoms, and the largest was thirty miles in diameter. 



The follo\^'ing cuts represent a view of Whitsunday Island and a 

 section. 



• Vol. 11 , p. 233. 



C'cclwy's Voyage*, I'art 1. 



