Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/852

834 moving on the same axis, to which one of them is firmly fixed, but on which the other is made to slide ; so that these two wheels may be placed at any given distance from each other, and blocks of any size admitted between their two rims or peripheries. For this purpose, both rims are divided into ten equal parts, for the reception of ten blocks, which are firmly fixed between the two wheels. When the double wheel with its ten attached blocks is put in motion, the outer surface of the blocks, or those which are farthest from the centre, strike against the edge of a chisel or gouge fixed in a movable frame, which, being made to slide in a curved direction in the line of the axis, cuts those outward faces of the blocks to their proper curvature. A contrivance is attached to the cutting tool which allows of the curvature being altered in any re quired way. One side being shaped, the ten blocks are then, by a single operation, each turned one fourth part round, and another side is exposed to the cutting instru ment moving in the same direction as before. A. third side is then turned outwards, and after that the fourth side, when the whole ten blocks are completely shaped. The velocity with which the wheels revolve, and the great weight with which their peripheries are loaded, would make it dangerous to the workmen or bystanders, if, by the violence of the centrifugal force, any of the blocks should happen to be thrown off from the rim of the wheelsj to prevent the possibility of such an accident, an iron cage or guard is placed between the workman and the machine. The last operation is performed by the scoring-machine, which cuts a groove to receive the binding or strapping of the block. The binding may be of iron or rope, and is very frequently of wire rope.

The Sheaves.—The machinery employed for making this part of the block consists of a circular saw, by which the log is cut into plates of the thickness required for the sheaves, according to their several diameters. These plates are next carried to a crown saw, which bores the central hole, and at the same time reduces them to a perfect circle of the assigned diameter. The sheave, thus shaped, is next brought to the coaTcing-machine, a piece of mechanism not inferior in ingenuity to the shaping machine for the shells. A small cutter, in traversing round the central hole of the sheave, forms a groove for the insertion of the cook or lush, the shape of which is that of three semicircles, not concentric with each other, nor with the sheave, but each having a centre equally distant from that of the sheave. The manner in which the cutter traverses from the first to the second, and from this to the third semicircle, after finishing each of them, is exceedingly ingenious. So very exact and accurate is this groove cut for the reception of the metal coak, and so uniform in their shape and size are the latter cast, the casting being made not in sand but in iron moulds, that they are invariably found to fit each other so nicely that the tap of a hammer is sufficient to fix the coak in its place. The coaks are cast with small grooves or channels in the inside of their tubes, which serve to retain the oil or grease for the pins. The sheave, with its coak thus fitted in, is now taken to the drilling-machine, which is kept in constant motion. In casting the coaks a mark is left in the centre of each of the three semicircles. This mark is applied by a boy to the point of the moving drill, which speedily goes through the two coaks and the intermediate wood of the sheave. Fdvets are put in these holes and clenched by hand. The next operation is performed by the facing-machine, which has two cutters, so arranged as to finish the side and groove the edge simultaneously ; then the hole for the pin is enlarged to its exact size by the broaching-machine. The pins, which form a very important part of the block, are now made at Portsmouth, not of iron but of steel, carefully tempered by special appliances. They are turned by a Klf-acting lathe, and are then reduced to the exact required diameter, and polished in the pin-polishing machine. They are also, in this machine, subjected to a proof strain pro portional to their sectional area, and thus the strength of the pin is guaranteed. The blocks are invariably made of English elm, the grain of the wood running lengthways of the block ; but in Germany recently, blocks have been made with the grain of the wood running across the block, the reason being that they are less likely to be split by the pressure on the pin of the sheave. The sheaves are made of lignum vita?. Three machines of each description for each operation, up to and including the facing-machine, are required. The smallest sized machines will make blocks of from 4 inches to 7 inches in length, the second size from 8 inches to 11 inches, and the largest from 12 inches to 17 inches. Two sizes of the Iroaching-machinc, and one pin-polishing machine, are sufficient. Blocks larger than 17 inches are made by hand, 26 inches being the largest used in the Royal ISTavy. As will be seen from the foregoing account, all machine- made blocks are cut out of a solid piece of wood; whereas hand-made blocks, larger than about 8 inches, are usually made in pieces, filled in at the ends and riveted together. It is questionable whether a block so made is not stronger than one cut out of the solid, as in the latter case the short-grained wood at the ends of the mortises is very liable to give way. In hand-made blocks the brass coak or tail of the sheave is not made of the peculiar shape described for machine-made blocks, but is usually of a circular shape. The machinery for Portsmouth Dockyard, on Brunei s plans, was made by Maudslay, whose firm now the very eminent firm of Maudsiay Sons and Field has since supplied block-making machinery to the Spanish, Turkish, and Russian Governments, and also to Chatham Dockyard ; the last mentioned, however, has never been used, as the machinery at Portsmouth is capable of supplying all the dockyards, the demand for blocks being much less for the steamships and ironclads than it was formerly for the old sailing ships. The first cost of this machinery is so great that no private firm has yet ventured to set it up, and the whole of the blocks used in merchant ships are made by hand-labour, assisted by a lathe and two or three other simple mechanical contrivances.  BLOCKADE. It appears to have been the ancient practice of belligerents at the outset of a war to forbid by proclamation all trade on the part of neutrals with the enemy, and to treat as enemies all those who contravened the proclamation ; and neutrals acquiesced tacitly in this practice until the commencement of the 17th century. In the course of that century the ancient practice came into question, as imposing on the commerce of neutrals an in convenience not justified by any adequate necessity on the part of belligerents, and it has since fallen into desuetude. Belligerents, however, have still maintained, without any question on the part of neutrals, the practice of intercepting supplies going over sea to an enemy under certain condi tions, namely, when a belligerent has invested an enemy s port, with the intention of reducing the enemy to surrender from the failure of supplies, and for that object a stoppage of all supplies to such port has become a necessary opera tion of the war. Any attempt, under such circumstances, on the part of a neutral merchant to introduce supplies into the invested port is a direct interference with the operations of the war, and is inconsistent with neutrality, and it accordingly subjects the offending party to be treated 