HAEBOUES 463 Booms. apacity
- bar-
>m-* r ierc e t imber estruc- _ isects. 3 8 feet ; but the formula for distances so short as these would give waves about a foot higher, and in heavy storms perhaps waves not much less than 5 feet iray exist, but on such occasions vessels could not safely use the quay. [ n order to tranquillize harbours of small reductive power, logs of timber called booms, having their ends secured by projecting into grooves cut in the masonry on each side, are placed across the entrance of the inner basin or dock. From 15 to 25 logs are usually dropped into those grooves, or as many more as will insure close contact of the lowest log with a sill-piece placed in the bottom of the harbour. They are also warped down or fixed with an iron hasp at the coping course, without which precaution the swell is found to enter the harbour from underneath. By this contrivance, which forms a temporary wall, the waves are checked, and completely prevented from spread ing into the interior basin. Twelve-inch booms of pine timber about 33 feet long have been broken at the small fishing village of Mousehole, near Penzance, and elm logs have in consequence been substituted. At Hynish in Argyllshire, booms of the same timber and scantling and 20 feet long have been very fre quently broken. The marine dynamometer at this place gave 1^ ton per square foot. The capacity of commercial harbours for trade varies so muc h ^th the exposure and size of vessels, that it is diffi- CU ^ to approximate to the truth. At Eamsgate, for instance, there were found to be about 6 vessels to an acre in. the outer harbour, while there were about 14 in the inner and better protected basin, where too there was perhaps a greater pro portion of small vessels. In the Scotch fishing harbours the number of boats used to be reckoned at from 85 to 115 per acre; but of late their size has been much increased, and probably not more than from 80 to 90 could be accom modated. The Cornish boats at New Lynn, according to the Channel pilot, vary from 60 to 80 per acre. The employment of timber in harbour works is of great antiquity. It seems first to have been used in forming- boxes which were filled with stones, and at a more recent period in the formation of open frameworks through which the current could pass freely. Vitruvius mentions moles, consisting of timber filled with stones and cement, as having been used by the harbour-builders in his days. 1 The earliest drawing of timber piers that we have met with is that of the ancient port of Dunkirk in 1699, a cress section of which from Belidor s Architecture Hydraulique is given in Plate VIII., fig. 4. This kind of structure is still not uncommon on the coasts of the English Channel. In sheltered bays where a deep-water landing-place is all j g require^ and where the bottom is sandy or soft, tim ^ er ma 7 be employed with great advantage. Even in somewhat exposed situations it can also be used for tidal harbours ; but the fatal evil in places where there is no admixture of fresh water is its rapid destruction by marine insects. In the Atlantic Ocean the Teredo navalis, and at many places in the German Ocean the Limnoria tere- brans, which was first discovered by the late Mr Kobert Stevenson at the Bell Piock Lighthouse in 1810, are very destructive of most kinds of timber. Mr Stevenson found that inemel was destroyed by the Limnoria at the rate of about 1 inch inwards per annum. He found that green- heart, beefwood, African oak, and bullet tree were scarcely 1 It was probably to some such work that Horace refers in Ode 1, Book iii. : " Contracta pisces sequora sentiunt, Jactis in alt um molibus; hue frequens Cfementa dimittit redemptor." A harbour contract dated 1394, in all probability the earliest which has been preserved, appears in the Registrum Nigrum de Aberbrothoc a collection of ancient documents printed by the Bannatyne Club, and edited by the late Professor Cosmo Innes attacked, while teak stood remarkably well and locust tolerably well, though suffering at last. Greenheart timber, though not absolutely impenetrable, as appears from Mr Stevenson s experiments, is the great specific in seas where the worms are destructive. But subsequent experience has proved that on the coasts of Scotland it is, in some cases, readily attacked, and at these places creosoting is of comparatively little use, as the worm manifests no repugnance to timber which has been even very recently impregnated. In addition to the experiments on timber, twenty-five different kinds and combinations of iron were tried at the Bell Rock, including specimens of galvanized irons. All the ungalvanized irons were found to oxidize with much the same readiness. The galvanized specimens resisted oxidation for three or four years, after which the chemical action went on as quickly as in the others. Although the association of zinc with iron protects, so long as it lasts, the metal with which it is in contact, it must be remembered that this immunity is obtained at the expense of the zinc, the tendency of which to oxidization is propor tionally exalted so soon as any part of the iron is exposed. The important experiments of Mr Mallet on specimens sunk in the sea showed that the amount of corrosion decreased with the thickness of the casting, and that from Yjj-th to yg-th inch in depth, in castings 1 inch thick, and about y^yth inch of wrought iron will be destroyed in a century in clear salt water. But there is reason to believe from a cannon ball which was picked up on the shores of the island of Inchkeith that the decay was in that case |th inch to the century. Cast-iron gratings periodically im mersed by the tide at the Bell Rock lost at the rate of 1 inch in a century. Even when the castings were externally sound their strength was reduced one-half in about 50 years. In judging of the qualities of different quarries for harbour purposes the importance of a high specific gravity ought not to be overlooked, as appears from the table given on page 464. The superiority of granite to greenstone is proved by the following experiments on the times required to abrade y^th inch of each kind of stone, made with the same weights and grinding agents, and with equal cubes of each material : 30 minutes were required for Queensferry (Carlin Nose) greenstone; 40 minutes for greenstone from Barnton near Edinburgh ; 60 minutes for Peterhead granite. Mr Murray of Sunderland also established, many years ago, the superior power of resistance to attrition possessed by granite over greenstone. Portland cement rubble and concrete, a most valuable material, now very commonly used in Britain as well as on the Continent, may be said to have to a large extent revolutionized harbour building; for it admits of being- employed in many different ways, and can, if due care be taken as to the quality of the cement and purity of the sand and gravel, be used with perfect confidence. The following specification has been followed extensively at different places : The Portland cement is to be obtained from the best manufac turers. It is to be ground extremely fine, and is to weigh not less than 115 R> per bushel ; and each cargo will, on its arrival at the works, be tested in the following manner. The cement is to be made into small blocks 1 inch square and 8 inches long. After being made, these blocks are to be immersed in water for 7 days and then tested by being placed on two supports 6 inches apart, when they must stand the transverse strain produced by a weight of 70 lb placed in the centre. Slabs or cakes are also to be made and placed in water, and after immersion for 24 hours they are not to show any signs of cracking or any softness on the surface. The cement is to be tested before any of it is used, and if it be found unsatisfactory it must be removed from the works. When a cargo is landed it must be tested from four different bags, and it must also be tested weekly as the works proceed, and the results forwarded to the engineers. The cement must be brought direct Dura- bility iron> Matei f cor struct Port-
land
