Discoveries and Inventions of the Nineteenth Century

only a very short piece of the screw is used—say the length of one revolution—and that this is placed in the dead-wood. Such was the construction of the earlier screw-propellers, but now a still shorter portion of the screw is used; for instead of a complete turn of the thread, less than one-sixth is now the common construction. Such a strip or segment of the screw-thread forms a blade, and two, three, four, or more blades are attached radially to one common axis. The blades spring when there are two from opposite points in the axis, and in other cases from points on the same circle. The blades of the propeller are cut and carved into every variety of shape according to the ideas of the designer, but the fundamental principle is the same in all the forms. It need hardly be said that the particles of the water are by no means fixed like those of the wood in which a screw advances. But as the water is not put in motion by the screw without offering some resistance by reason of its inertia, this resistance reacting on the screw operates in the same manner, but not to the same extent, as the wood in the other case. When we know the pitch of the screw, we can calculate what distance the screw would be moved forward in a given number of revolutions if it were working through a solid. This distance is usually greater than the actual distance the ship is propelled, but in some cases the vessel is urged through the water with a greater velocity than if the screw were working in a solid nut. The shaft which carries the screw extends from the stem to the centre of the ship where the engines are placed, and it passes outward through a bearing lined with wood, of which lignum vitæ is found to be the best kind, the lubricant for this bearing being not oil but water. The screw would not have met with the success it has attained but for this simple contrivance; for it was found that with brass bearings a violent thumping action was soon produced by the rapid rotation of the screw. The wearing action between the wood and the iron is very slight, whereas brass bearings in this position quickly wear and their adjustments become impaired. The screw-shaft is very massive and is made in several lengths, which are supported in appropriate bearings; there is also a special arrangement for receiving the thrust of the shaft, for it is by this thrust received from the screw that the vessel is propelled, and the strain must be distributed to some strong part of the ship’s frame. There is usually also an arrangement by which the screw-shaft can, when required, be disconnected from the engine, in order to allow the screw to turn freely by the action of the water when the vessel is under sail alone. Fig. 60.—Screw Propeller. A screw-propeller has one important advantage over paddle-wheels in the following particular: whereas the paddle-wheels act with the best effect when the wheel is immersed in the water to the depth of the lowest 133float, the efficiency of the screw when properly placed is not practically altered by the depth of immersion. As the coals with which a steamer starts for a long voyage are consumed, the immersion is decreased—hence the paddle-wheels of such a steamer can never be immersed to the proper extent throughout the voyage; they will be acting at a disadvantage during the greater part of the voyage. Again, even when the immersion of the vessel is such as to give the best advantage to the paddle-wheels, that advantage is lost whenever a side-wind inclines the ship to one side, or whenever by the action of the waves the immersion of the paddles is changed by excess or defect. From all such causes of inefficiency arising from the position of the vessel the screw-propeller is free. The reader will now understand why paddle-wheel steamers are at the present day constructed for inland waters only. A great impulse was given to steam navigation, by the substitution of iron for wood in the construction of ships. The weight of an iron ship is only two-thirds that of a wooden ship of the same size. It must be remembered that, though iron is many times heavier than wood, bulk for bulk, the required strength is obtained by a much less quantity of the former. A young reader might, perhaps, think that a wooden ship must float better than an iron one; but the law of floating bodies is, that the part of the floating body which is below the level of the water, takes up the space of exactly so much water as would have the same weight as the floating body, or in fewer words, a floating body displaces its own weight of water. Thus we see that an iron ship, being lighter than a wooden one, must have more buoyancy. The use of iron in ship-building was strenuously advocated by the late Sir W. Fairbairn, and his practical knowledge of the material gave great authority to his opinion. He pointed out that the strains to which ships are exposed are of such a nature, that vessels should be made on much the same principles as the built-up iron beams or girders of railway bridges. How successfully these principles have been applied will be noticed in the case of the Great Eastern. This ship, by far the largest vessel ever built, was designed by Mr. Brunel, and was intended to carry mails and passengers to India by the long sea route. The expectations of the promoters were disappointed in regard to the speed of the vessel, which did not exceed 15 miles an hour; and no sooner had she gone to sea than she met with a series of accidents, which appear, for a time, to have destroyed public confidence in the vessel as a sea-going passenger ship. Some damage and much consternation were produced on board by the explosion of a steam jacket a few days after the launch. Then the huge ship encountered a strong gale in Holyhead Harbour, and afterwards was disabled by a hurricane in the Atlantic, in which her rudder and paddles were so damaged, that she rolled about for several days at the mercy of the waves. At New York she ran upon a rock, and the outer iron plates were stripped off the bottom of the ship for a length of 80 ft. She was repaired and came home safely; but the companies which owned her found themselves in financial difficulties, and the big ship, which had cost half a million sterling, was sold for only £25,000, or only about one-third of her value as old materials. The misfortunes of the Great Eastern, and its failure as a commercial speculation in the hands of its first proprietors, have been quoted as an illustration of the ill luck, if it might be so called, which seems to have attended several of the great works designed by the Brunels—for the Thames Tunnel was, commercially, a failure; the Great Western Railway, with its magnificent embankments, cuttings, and tunnels, has reverted to 134the narrow gauge, and therefore the extra expense of the large scale has been financially thrown away; the Box Tunnel, a more timid engineer would have avoided; and then there is the Great Eastern. It is, however, equally remarkable that all these have been glorious and successful achievements as engineering works, and the scientific merit of their designers remains unimpaired by the merely accidental circumstance of their not bringing large dividends to their shareholders. Nor is their value to the world diminished by this circumstance, for the Brunels showed mankind the way to accomplish designs which, perhaps, less gifted engineers would never have had the boldness to propose. The Box Tunnel led the way to other longer and longer tunnels, culminating in that of Mont Cenis; but for the Thames Tunnel—once ranked as the eighth wonder of the world—we should probably not have heard of the English Channel Tunnel—a scheme which appears less audacious now than the other did then; if no Great Eastern had existed, we should not now have had an Atlantic Telegraph. Possibly this huge ship is but the precursor of others still larger, and it is undoubtedly true that since its construction the ideas of naval architects have been greatly enlarged, and the tendency is towards increased size and speed in our steam-ships, whether for peace or war. Fig. 61.—Section of Great Eastern Amidships. Fig. 62.—The Great Eastern in course of Construction. The accidents which had happened to the ship had not, however, materially damaged either the hull or the machinery; and the Great Eastern was refitted, and afterwards employed in a service for which she had not been designed, but which no other vessel could have attempted. This was the work of carrying and laying the whole length of the Atlantic Telegraph Cable of 1865, of which 2,600 miles were shipped on board in enormous tanks, that with the contents weighed upwards of 5,000 tons. The ship has since been constantly engaged in similar operations.[1] The Great Eastern is six times the size of our largest line-of-battle ships, and about seven times as large as the splendid steamers of the Cunard line, which run between Liverpool and New York. She has three times the steam power of the largest of these Atlantic steamers, and could carry twenty times as 135many passengers, with coal for forty days’ consumption instead of fifteen. Her length is 692 ft.; width, 83 ft.; depth, 60 ft.; tonnage, 24,000 tons; draught of water when unloaded, 20 ft.; when loaded, 30 ft.; and a promenade round her decks would be a walk of more than a quarter of a mile. The vessel is built on the cellular plan to 3 ft. above the water-line; that is, there is an inner and an outer hull, each of iron plates ¾ in. thick, placed 2 ft. 10 in. apart, with ribs every 6 ft., and united by transverse plates, so that in place of the ribs of wooden ships, the hull is, as it were, built up of curved cellular beams of wrought iron. The ship is divided longitudinally by two vertical partitions or bulkheads of wrought iron, ½ in. thick. These are 350 ft. long and 60 ft. high, and are crossed at intervals by transverse bulkheads, in such a manner that the ship is divided into nineteen compartments, of which twelve are completely water-tight, and the rest nearly so. The diagram (Fig. 61) represents a transverse section, and shows the cellular construction below the water-line. The strength and safety of the vessel are thus amply provided for. The latter quality was proved in the accident to the ship at New York; and the former was shown at the launch, for when the vessel stuck, and for two months could not be moved, it was found that, although one-quarter of the ship’s length was unsupported, it exhibited no deflection, or rather the amount of deflection was imperceptible. Fig. 62 is from a photograph taken during the building of the ship, and Fig. 63 shows the hull when completed and nearly ready for launching, while the vignette at the head of the chapter exhibits the big ship at anchor when completely equipped. The paddle-wheels are 56 ft. in diameter, and are 136turned by four steam engines, each having a cylinder 6 ft. 2 in. in diameter, and 14 ft. in length. The vessel is also provided with a four-bladed screw-propeller of 24 ft. diameter, driven by another engine having four cylinders, six boilers, and seventy-two furnaces. The total actual power of the engines is more than that of 8,000 horses, and the vessel could carry coals enough to take her round the world—a capability which was the object of her enormous size. The vessel as originally constructed contained accommodation for 800 first-class passengers, 2,000 second class, and 1,200 third class—that is, for 4,000 passengers in all. The principal saloon