of mud deposited on the upper edges of the cover strips and on the rivetheads, the mud being highly charged with acids derived from the decaying river deposit and the salt-water and water from the moors conveyed by the lower Weser and the Geste. The corrosive influence of the deposit is proved by the fact that the decay in question is specially noticeable on the convex side of the curved floodgates, the outer skin of which is permanently immersed in the very muddy water of the outer harbour, whereas on their concave side they are often washed by the water in the harbour which is not so turbid.

"III. The dock gates of the new harbour at Bremerhaven were erected in 1852, and removed as worn out in 1900. The thinning down of the plate was especially noticeable where projecting edges formed ledges upon which mud could settle. Those parts of the gates which had been in contact with oak timber were also in worse condition. At Bremerhaven the water is fairly full of salt and heavily laden with mud.

"IV. The inner gates of the great lock at Harburg, erected in 1880 and removed in 1901 for alteration, were found in very good condition with the exception of a strip about 2 feet wide near the low water-line, where the outer skin was very rough and showed rust spots penetrating inch into the metal. The river-water is completely free from salt and almost free from mud at Harburg, but the water in the harbour is, as yet, strongly polluted by the surface and house drainage of the town, and several chemical factories, besides, discharge their waste water into it full of impurities, the oxidation of all which takes place on the surface of the water; consequently, the plating of the gates is principally damaged near the water-line."

The following statement of results, obtained by the author in some experiments, covering a period of twelve months, serves to illustrate the difficulty of deducing reliable coefficients of corrosion from any but the most extensive investigation. The data obtained are not without intrinsic interest, but in order to be of any practical value, such observations would have to be extended over a considerable number of years. It is a noteworthy feature that the galvanised specimens apparently suffered more than the ungalvanised, and that, during the first three months, the latter, instead of losing, actually gained, weight. This is due partly to the conditions of immersion, and partly to the fact that weight is, after all, no very reliable criterion of the amount of corrosion actually taking place, since some forms of oxidation involve no loss in this respect.

The first six specimens were suspended in a disused clough-shaft, to which the tidal water of the River Mersey had free access, the specimens being placed at mean tide level, so that they were in and out of water for about equal periods. The water was somewhat impregnated with sewage discharged from a neighbouring outfall sewer, and the ungalvanised specimens became coated with a hard deposit, apparently of a calcareous nature, which was removed as far as possible before each weighing by washing in

PRESERVATION OF IRON AND STEEL.

145

clear water and using a stiff scrubbing brush. The gain in weight of certain of the specimens represents the amount of deposit which could not be removed in this way. No further measures were taken to remove the deposit, because it was deemed desirable to maintain the normal conditions of corrosion.

The last three specimens were kept continuously immersed in the water of an inner dock, which was free from contamination.

Precautions were taken to prevent any contact between the various pieces, and all were well washed prior to each weighing.*

Wrought-iron bar, plain,

galvanised,
turned,

Steel bar, plain,

23.89

Cast-iron plate,

77.5

Wrought-iron plate,

Sq. ins. Grains. Grains. Grains. Grains. Grains. Grains. Grains. .37 12 19,473 19,499 19,526 19,510 19,457 16 43 36 87 18,272 18,228 18,170 | 18,197 18,069 203 54 96 31,126 31,175 31,173 31,167 31,083 43 54 38 31,437 31,351 31,282 31,278 31,243 194 58 35 31,172 31,187 31,185 31,143 30,988 184 6,662 | 6,672 6,685 6,671 6,633 29 16,979 16,976 16,965 16,943 16,972 7 75 25 12,903 12,883 12,838 12,814 12,854 75 25 13,519 13,503 13,483 13,453 13,406 | 113

5.50

.78

3.56

3.15

1.21

*09

Mild steel plate,

It may be useful, as well as interesting, to insert here an analysis of the water of the River Mersey, made by Mr. Charles C. Moore, F.I.C., in September, 1897. The sample was taken about the time of high water, and its specific gravity was found to be 1.02254. The water contained the following salts in solution :—

Preservation of Iron and Steel. The two principal measures adopted for preventing corrosion are painting and galvanising.

Painting is an operation which should be repeated, at least, once in three years under normal conditions, and oftener in exposed situations.

* In regard to this last operation, the author desires to acknowledge the kind assistance he received from Messrs. H. Pooley & Son, Ltd.

As a general rule, lead paints* are employed, but it has been suggested that preference should be given to oxide of iron paints, to avoid any tendency to galvanic action between two metallic substances. Care should be taken to remove all rust and scale before applying the paint.

Cast iron on leaving the mould has, or should have, a hard bluish skin, which should be kept intact by an immediate coat of (hydro-carbon) oil or paint. Wrought iron is also sometimes specified to be dipped in oil while hot, but the method is not a very successful preservative, and ironworkers dislike it on account of its messiness.

Dock gates and other marine structures of iron and steel should be thoroughly scraped, cleaned, and painted at frequent intervals-in some cases annually. The materials usually employed for the purpose include— red lead and oil paint, mineral tar, vegetable tar, black varnish, and siderosthen. The surfaces of ironwork in close contact should be painted before being put together. The interior walls of ballast boxes, and other generally inaccessible surfaces, are frequently floated with a thick wash of Portland cement.

Galvanising consists in immersing the iron in a bath of molten zinc, whereby a skin of that metal is formed upon the surface. The process is successful so long as the zinc covering remains intact. When it cracks, or becomes defective in any way, rapid corrosion ensues in the presence of the least damp. The writer's experience of galvanised iron, employed as a material for dock sheds, is that sea air, highly charged with salt and moisture, works havoc with it. Several such sheds, after being a few years in existence, have had to be completely coated with black varnish to preserve them from imminent destruction.

The Angus-Smith treatment, largely adopted for cast-iron pipes, consists in dipping them, at a temperature of 700° F., into a mixture of coal tar, pitch, linseed oil, and resin, at a temperature of 300° F. The process is an admirable method of preservation, and enjoys a considerable reputation.

TIMBER.

The varieties of timber principally in demand for the purposes of dock engineering may be enumerated as follows:

Piles.-Greenheart, Jarrah, Karri, Mora, Pitchpine, Oak, Elm, Beech.

A very common constituent of modern paints is sulphate of barium, of which there are two forms, viz. :-(1) the finely-ground mineral barytes, and (2) blanc fixe, or precipitated sulphate of barium. While both these substances have the same chemical composition, there is a wide difference in their physical conditions, which results in the ground mineral being worthless as an ingredient of paint, whereas the precipitate is just as valuable, owing to its covering power and unalterability. Examination of a sample of each paint under the microscope will easily show the difference between the fragments of crystals in the first case and the amorphous condition of the other.

Gates and Cloughs.-Greenheart, Jarrah, Mora, Oak, Pitchpine, Pine, and Fir.

Deckings (for wharfs and bridges).—Greenheart, Oak, Teak, Elm.
Fenders.-Elm.

Temporary Dams.-Pitchpine.

Timbering for Excavations.—Pitchpine, Spruce deals, Greenheart sheeting piles.

Graving Dock Blocks.-Oak, Birch, Elm, Pitchpine.

As indicative of their comparative values in maritime situations, the following classification of timbers for shipbuilding purposes, by a committee of Lloyd's, will be useful:

Teak, British oak, mora, greenheart, ironbark, sal.
Bay mahogany, cedar.

European Continental oak, chestnut, blue gum, stringy bark.
North American white oak and chestnut.

Larch, hackmatack, pitchpine, English ash.

Cowrie, American rock elm.

Red pine, grey elm, black birch, spruce fir, English beech.
North American hemlock, pine.

Greenheart is a product of British Guiana and the north coast of South America. It is a wood of extreme hardness and durability, with a colour ranging from green to black. Its resistance to crushing is enormous, but it is very brittle and splits under the least provocation. Particularly is this the case during the months of April and May. Great care is therefore required in working it, and when a log is about to be sawn in two, it is often advisable to bind it on each side of the proposed cut with chains and wedges. The wood has a very fine grain and exhibits no distinct annual rings. It is very heavy, ranging from 62 to 75 lbs. per cubic foot, so that it does not float in water. It contains an essential oil which is very poisonous, and which apparently confers upon it some immunity from the attacks of sea-worms. The evidence on the last point, however, is not conclusive. Greenheart is obtainable in logs from 12 to 24 inches square and up to 70 feet in length.

Mora is a light red wood with similar uses to greenheart and is a native of the same district. It is very tough and close-grained, difficult to saw and split, and extremely durable. It can be obtained in logs 18 to 24 inches square and as much as 100 feet in length.

Purpleheart, another neighbouring tree, is also noted for its qualities of durability and strength. It is hard and close-grained, and its colour is

indicated by its name.

ing great shocks.

greenheart.

Owing to its great toughness it is capable of resistLogs can be obtained from 18 to 30 inches square.

Bullet tree is a dark red wood said to be an excellent substitute for It saws easily, takes a smooth finish, and is thoroughly tough and durable. The size of the logs runs up to 3 feet in quarter girth and 50 feet in length.

Kakaralli, though a less known tree, is described as even surpassing. greenheart in its qualities for marine situations, such as durability and resistance to the attacks of worms. It is close-grained, tough and difficult to saw, but easy to plane. It has one drawback in that it can only be obtained in small logs, 10 to 14 inches square, and rarely exceeding 40 feet in length.

Jarrah is an Australian timber, resembling mahogany in colour, also recommended as a valuable substitute for greenheart. It is hard and closegrained, very liable to warp and split and full of clefts, filled with resinous matter. The fibres contain an acid having a pungent odour, said to be very efficacious against sea-worms and insects. Its extreme durability compared with other timbers is incontestible, and it is on record that it has survived the attacks of marine borers long after other woods have succumbed. On the other hand, there are some authenticated instances of its destruction by the white ant and the teredo.

Karri, another Australian native, is hard, heavy, straight-grained, and tough. It is stronger than jarrah but less durable in damp situations, though when entirely immersed it is said to last well. No decisive evidence is forthcoming as to its capacity to resist worms.

Red Gum is another tree possessing the same characteristics as jarrah, with strength and toughness in a higher degree, while its durability is rather less.

Ironbark is one of the hardiest and strongest woods in existence, but it is not so durable in marine situations as the preceding varieties, being admittedly readily attacked by the teredo. In spite of this fact it is much used for piles in harbour works in New South Wales. The wood has a close, straight grain, is very tough and heavy, and is white or yellowish in colour.

Blue Gum, though an undoubtedly useful timber, is only suitable for dry and open situations, and it is depreciated by a tendency to warp and shrink under exposure to the sun. It is straw-coloured.

Stringy bark is a hard, heavy, straight-grained wood, occasionally employed for the superstructure of engineering works. This concludes the Australian series.

Keyaki is a very important timber in Japan, being strong, durable, and easily worked. It is durable in situations alternately wet and dry, and is much used for piles.

Deodar, supposed to be a variety of the Cedar of Lebanon, is a wood of great stiffness, strength, hardness, and durability, well adapted for engineering purposes in India.