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waterways, and for deep-draught ships well within the thousand yards' range. In other words, the fort would have to stand its battering, if attacked under such circumstances, at uncomfortably close quarters. Strength and invulnerability ought, therefore, to be its primary qualifications.
In respect to strength, its very massiveness and weight of oval wall, must require, if even the materials were simply piled together, a very considerable force to throw them down ; but materials cost money, and rolled iron in large masses a good deal, too, both for manufacture and carriage. Any proper system of iron-fort building should clearly, then, be based on the best mechanical plans of constructing with the use of the smallest amount of material employed in its cheapest forms. The structural cohesion of the whole fabric, and its power, in all its details, of clinging together, should also be leading features in all designs of this class, having in view the shattering and destructive effects of any heavy cannonade by modern artillery.
It is impossible, unfortunately, to take the new iron fort at Plymouth as an example of remarkable engineering, or of first-rate constructive or scientific skill; but it is, nevertheless, a topic of grave interest both in a national point of view and in respect to the immediate protection it is capable of giving to one of the most important naval arsenals of this country.
In the memorable contest carried on at Shoeburyness in 1868 an exciting interest was created between the rival targets constructed upon the plans of Colonel Inglis, of the Fortification Branch of the Royal Engineers, and that designed by Mr. John Hughes, with the hollow stringer backing; the work of the latter, victorious in respect to endurance and resistance, has already been described in this magazine (vol. vii. p. 345).
The design for the Plymouth iron fort, which at that time was embodied in one of the competing targets, has since been carried into execution upon the same main principles which at that date proved so deficient of strength and cohesion in the representative casemate. Various parts and details have been modified, as much as they could be, to bring in those improvements which the experience of the artillery attacks and the suggestions of criticism had dictated. We have, then, in the final production a formidable structure, although not a scientific example of the best order ; but a fort, nevertheless, of considerable strength, and, for the duties it might at the present time be called upon to perform, sufficient probably in its essential qualifications. One of the main reasons constantly repeated for adhering to the defective principles of the fort was an asserted economy in the use of the narrow bars. If, however,
we put the cost of the Plymouth fort, as it now stands, with the estimate of a comparative structure upon Mr. Hughes's plan, we shall find that no saving has been effected by employing bars instead of hollow stringers; whilst the advantage of the latter would be very great in respect to defensive resistance.
The accompanying woodcut will show the general structure of the walls of the Plymouth fort as they now stand. The front armour (a) is of rolled plates 5 inches thick; behind this is a layer of shingle concrete 6 inches thick; next a layer of
SECTION OF IRON WALL, SHOWING SYSTEM OF STRUCTURE. a. Face armour ; • Concrete; b. Vertical bars; c. Horizontal bars ; Iron concrete;
d. Wood plank, with iron struts at intervals ; x x x. Bolts, backing ; p. Perpendicular; G. Glacis; m m. Masonry foundation.
vertical iron bars (1) 167 inches wide and 5 inches thick ; then a similar layer of bars (c) laid horizontally ; behind this a plastering of iron turnings and asphalte 1 inch thick. In the rear the wall is supported by vertical iron bars 12 inches deep and 5 inches thick, the intervals (about 10 inches) being packed with wood planks set edgewise. A glance at the composition shows that there is not the slightest mechanical cohesion in it, but that the whole mass is simply kept together by the bolts. If these give way, the disruption of the wall seems inevitable. Moreover, the bulging effect of projectiles striking the front will open out the incoherent backing, if it do not actually draw the bars and timbering out of their footings. The concrete behind the face-plating will be quickly disintegrated by any heavy battering, and then the bolts being loosened in their hold, the armour will clatter about in certainly a very noisy manner, if with no further damaging results; but how the multitude of bars will keep together in position under such circumstances must be left to the constructors to explain, or probably for Providence hereafter to determine.
In the Gibraltar shield, after the first failures of the bolts, others upon the pattern devised by Major Palliser were introduced. These had plus screw-threads, and were tapered away in the middle of the shank, so that when the strain was brought upon the bolt, the weakest part yielded and stretched. The objection to these bolts was, that they could not be used in ships, as there would be a cavity all round them. They stood, however, well enough in the land shield. In the Hughes shield there was another class of bolts of a much better character, founded by Mr. Parsons upon a similar basis. These were hollow cylinders with plus screw-threads, the cavities at the ends being closed by steel plugs. The cylinders or shanks, having thus the least substance stretched uniformly throughout, and being made of good ductile iron, the stretching limit was very considerable, and they were thus capable of withstanding admirably the strains brought upon them by the shot. They were cheap, too, as well as very strong. Subsequently, Lieutenant English, R.E., produced and patented a compound bolt with a carefully turned ball and socket-head on both ends, and a spiral spring-washer for buffing the rear nuts. The plan may be ingenious, but cui bono? If cheaper and less complicated bolts do the work required of them, why employ the costly which can do no more? None have ever stood severer tests than Mr. Parsons', or served their purpose better.
What the effect of a concentrated broadside of the heaviest guns might be on the new Plymouth fort would be very hard to estimate; and happily the heaviest guns at this moment are alone possessed by England. It will be remembered, however, that at the Shoeburyness trials a salvo of three guns broke an orifice in the 15-inch structure through which a General entered the casemate. No salvo was ever fired against the 20-inch portion there, and the doubt therefore remains whether a lamination of 15 inches of iron plus 6 inches of concrete will keep out a concentrated broadside from such ships as the Hercules or the Devastation. If so, the credit will be due to the improvements made in the mechanical distribution of the materials by the force of critical comments.
SOUTH AFRICA AND ITS DIAMONDS.
By T. RUPERT JONES, F.G.S., PROFESSOR OF GEOLOGY AT THE STAFF AND ROYAL MILITARY COLLEGES,
“ DARKERTON is being surveyed, and before long it will have
I its Market-square, Church-square, High-street, West-end, back slums, and river villas. ... The Church Committee are calling for tenders for bricks. ... The Standard Bank premises will shortly be completed; and the British Commissioners' Offices will not be very much longer on the road.” Store is added to store ; and the jeweller's store “is as safe as if it were in the best watched site in London-nay, it is safer. .. At Pniel great attention is being given to sanitary measures. ... Its natural situation entitles it to be called the very gate of the Diamond Fields. . . . Pniel and Parkerton are likely to live and flourish.” “ Hebron is to have its races on January 2," and Pniel also on the same day; whilst the necessaries and comforts of life, together with music-halls, liquors, quackmedicines, and other accompaniments of civilisation, are announced as ready to hand at the above-named settlements in the Klip-drift Diamond-fields, by the “ Diamond News and Vaal Advertiser," No. 10, December 17, 1870, out of which paper and its “ Supplement” (published at Pniel) the foregoing extracts have been taken. The newspaper and the nascent towns are, of course, due to the concourse of diamondseekers, and of tradesmen supplying their wants, right and left of the Pniel Mission Station, near the “Great Bend” of the Vaal River, 480 miles north-east of Cape Town, upwards of 300 miles due north of Port Elizabeth (Algoa Bay), and about 290 miles west by north of D’Urban (Port Natal).
There are numerous spots at which local proprietors, squatters, and regular diggers have met with diamonds along the valley of the Lower Vaal, as at Hebron, Klip-drift (or ford), Pniel Mission-station and its neighbouring grounds, Bultfontein (or Du Toit's Pan), Zitzikamma, Vogelstruis Pan, Sitlacomie's Village, Sifonell's Village, Gong-gong, Cawood's Hope, Nicholson's Pan, Kalk farm (Litkhatlong, near the junction of the Hart with the Vaal at the “Great Bend”), and elsewhere, near the junction of the Modder and Vaal. Also below the junction of the Ky Gariep (Vaal) with the Nu Gariep, to form the Gariep (Orange), diamonds have been found on several farms along the latter river, about twenty miles north-west of Hopetown. The finding-places, however, of the precious gem are not confined to the immediate neighbourhood of the Orange River and its great northern branch. The valleys of two, at least, of the tributaries of the latter (Vaal), namely the Modder and the Vet, have yielded specimens in their upper branches, near Fauresmith (eighty miles south of Pniel) and near Winburg (seventy miles from the Vaal) respectively. High up the Vaal, also, at Bloemhof, twelve miles south-west of Potscherfstroom, diamonds are said to occur ; and even above Bloemhof some diamonds have been found (I am informed by Mr. C. L. Griesbach) on the Maquassi Spruit, a stream running into the Vaal from the north, and some 250 miles away on the northeast, from the Hopetown District. Throughout this region, however, no place has hitherto proved to be so rich in diamonds as the neighbourhood of Pniel above-mentioned. Hebron, an old mission-station, about ten miles up the river, on its north bank, and within a great loop of the stream, has been a productive digging-ground. The Klip-drift (or ford), by which the waggon-track from the south crosses into Betchuana Land, gives its name to the rich diggings five miles lower down the river than Pneil, mostly on the north side of the river, but including some grounds belonging to the missionary station, enclosed in a fine curve of the river, and traversed by the road to the south on its leaving the “drift.” Four or five miles further down are the Gong-gong diggings, on the left bank of the river as it winds along on its north-westward course before receiving its northern tributary, the Hart, and making its “Great Bend” to the south-west ; and the new diggings called Cawood's Hope are just opposite. All the diamond-diggings about Pneil, and to the north-east and north-west, are called the “ Diamond-fields of Pneil,” or 66 of the Lower Vaal,” or the “ Klip-drift Diamond-fields ;” whilst the diggings near the ford indicate the “Klip-drift Diamond-field ” in particular.
This rich diamond-bearing district, traversed by the winding Vaal, and suddenly occupied by an energetic digging and trading community from all parts of the world, belongs partly to the Pniel Missionary Establishment, within the limits of the Orange River Free States; but the north side of the valley* is claimed
* Referred to as “Adamanta” in the “Grahamstown Journal” of January 30.