Blocks, in the Navy, and Marine Architecture, a species of pulley very extensively used for moving heavy weights, by means of ropes or chains passing over the pulleys; also occasionally in architectural and other works. A block consists of one or more pulleys, called sheaves, which are generally formed of lignum vitae, or some hard wood inserted between cheek-pieces forming what is called the shell of the block, and turning upon a pin passing through the shell and the centres of the sheaves. Blocks are of various forms, each having a particular name; the following cut represents a common single block; a is the shell, b the sheave, c the pin. Blocks are suspended by straps, either of rope or iron; the latter are called iron-strapped blocks, and have frequently a swivel-hook. A combination of two blocks, one of which is attached to the load to be raised, is called a tackle, and the power is to be estimated by the space through which the fall (which is that part of the rope to which the power is applied) passes, compared with the space through which the load is raised, deducting for friction, which is great, owing to the rigidity of the ropes, and the small diameter of the sheaves; these, for nautical purposes, are necessarily limited by considerations as to weight and space.
The friction is also considerably increased, in certain circumstances, under which blocks are applied. When there is more than one sheave in the same block, the fall comes last over the outside sheave; and that sheave, if the exertion of power be in a line nearly parallel to the direction in which the load is drawn, always endeavours to get into a line with the point of suspension; for the great friction to be overcome preventing the equal transmission of the power throughout the combination, and the outside sheave having to sustain not only the pressure of its own share of the load, but also the additional strain sufficient to overcome the friction of the other sheaves, and the vis inertias of the entire load; it must, therefore be considerably depressed, and in consequence of this oblique direction of the block, the lateral friction of the sheaves becomes so great, as in some cases nearly to equal the power. Figs. 1 and 2 represent blocks so constructed as to allow the fall to pass over the middle sheave, by which means it will be immediately beneath the point of suspension. Fig. 1 is the invention of the celebrated Smeaton, who employed blocks of this description in erecting the Eddystone Lighthouse.
The upper block a contains 6 sheaves ranged in two tiers, and the lower block b contains also 6 sheaves, also ranged in two tiers; the lower tier of sheaves in a, and the upper tier of sheaves in b, being more than two diameters of the rope smaller than the other sheaves, the mode of reeving the rope is as follows. Beginning in the middle, the rope is reeved over the large sheaves as far as it will go; thence going to the first of the smaller sheaves, they are reeved throughout; thence again to the outer one of the remaining large sheaves, and ending upon the middle sheave of the upper block. The principal objection to this method is, that it requires a combination of at least twelve sheaves, and is not therefore applicable to general purposes. The construction shewn in Fig. 2, which is the invention of Mr. Jones, of High Holborn, can be applied to any number of sheaves from 4 upwards. The cut represents a pair of blocks of 2 sheaves each. To the upper block a is attached another block b, the sheave of which stands at right angles to the former, and is called the cross-sheave; the lower block c contains two sheaves abreast, (shewn diverging,) in order that the cross sheave may not be of a very small diameter.
The method of reeving is to begin upon the middle upper sheave, and when arrived at the outer sheave, to pass to the cross sheave, which carries the rope over to the outer sheave, on the opposite side, and then proceed again in the order of the sheaves.
The annexed figure represents an improved cat-block, invented by Mr. Bothway, and rewarded by the Society of Arts. The advantages which this block possesses over those in common use, are thus stated by Mr. Bothway. " In all large class ships in the royal navy, the unwieldy nature of the usual cat-block requires that two men should be sent out on the anchor, a most perilous service in rough weather; whereas mine only requires one man at any time, because he has not to sustain the whole weight of the block, as in the former case, but only that of the hook. And in vessels smaller than line of battle ships, in blowing weather, when the ship pitches heavily, the anchor may be hooked without a man going on it, by his standing on the head, and guiding the hook of the block to the anchor, by a staff and hook, similar to a boat-hook. This facility i is gained by the mobility of the swivel in its socket, so that the man has not the weight of the block to turn, in order to insert the hook in the ring of the anchor.
Should the anchor, when hooked in the dark, or otherwise, cause a turn in the fall, the hook being on a swivel joint, the turn will come out before the strain comes on the block; and when the anchor is foul it can also be hooked with great facility.
In my thirty-two years' service l have seen the wooden cat-blocks swell so much in cold climates, that the sheaves have become immovable; mine, being of metal, are liable to no such inconvenience." Another great advantage may be derivable from Bothway's cat-block being applicable to other uses; whereas the old ones are not. For instance, by merely having a spare socket or two fitted with hooks of various sizes, it may take a strap for gear-blocks, or it may be converted into a lashing-block without the hook and socket, but with the socket bolt. In the figure, which represents a perspective view of the block, it will be seen that the hook, instead of being formed in one with the strap, turns with a swivel head, in a socket which hangs from a pin passing through the lower end of the shell. Although entirely formed of metal, they are lighter than wooden ones with their iron bindings, and capable of the same service.
There is another species of blocks, which are termed "Dead eyes," and are used for tightening or setting up, as it is called, the standing rigging of ships. It consists merely of a circular block of wood, with a groove on its circumference, round which the lower end of the shroud, or an iron strap, is fastened; three holes passing through the face, (ranged in a triangle,) to receive the laniard or smaller rope, which forms a species of tackle for tightening the shrouds.
There are no sheaves in the dead eye, but the edges of the holes are rounded off to prevent cutting the laniard, but this very imperfectly answers the purpose; as from the roughness of the grain of the wood, which is usually elm, and from the stiffness of the rope, the laniard renders with difficulty, and from the great strain to which it is subjected, it is frequently broken. A very simple and effectual improvement has been made in this respect by Mr. Carey, Surveyor of Shipping, at Bristol, by inserting a half sheave of lignum vitae into each of the holes, which causes the laniard to render with greater facility, and the shroud to be set up in half the usual time.
Fig. 1 shews the dead eye; Fig. 2 a section of the same; and Fig. 3 one of the half-sheaves. It will be seen from the manner of inserting the half sheaves, as shown in Fig. 2, that they cannot fall out, for the more pressure there is on them, the faster they will be.
The annexed figure represents a block of a peculiar description, intended for forming a kind of rope-road to a stranded ship. When a vessel thus circumstanced has had a rope thrown over by Capt. Manby's apparatus, or any other means, considerable difficulty has been found in reeving an ordinary pulley for the conveyance of the crew to the shore. In the figure, it will be seen that the pulley divides at the hook, or shackle, into two equal parts, so that it may be instantaneously passed on to a stretched rope, and, by means of a cord from the ship, persons may pass securely and quickly backwards and forwards. The little bar which traverses the opening is fixed at one end by a joint and fits into a mortise, as shown; the use of it being to confine the rope in its place, when any vehicle, or other apparatus, is slung or suspended to it.