BLOCK, the name given to a pulley, or system of pulleys, mounted in a frame or shell, but considering them as detached from the ropes which run through them. When speaking of the block with its rope, the seaman uses the phrase of a tackle of single or double blocks ; hence the term block is applied to the pulley or pul lies, with its frame or shell, and its band or strap.
The ship's block consists of its sheaves or pullies, which are circular pieces of wood (usually lignum vita:,) or sometimes brass or cast metal, with a groove, turned on its edge for the reception of the rope ; and in the best blocks, called soaked sheaves, the sheave has a brass bush fitted into the centre, with a hole through it to receive the pin on which the sheave re volves. The pin is made of lignum vitz,*cocus, or a ,West India wood called green heart ; but the best blocks.have iron pins. The pin is supported by pass. ing through the sides of the shell of the block, which is made of elm, ash, or other tough wood, with a hole morticed through it to receive the sheave, and confine it to revolve steadily though freely upon its pin, and at the same time keep the rope from getting off the groove in the edge of the sheave. When the block contains two or more sheaves, as many mor, tices are made. Sometimes the same pin serves all the sheaves; and at other times, the sheaves are placed one above the other, having of course separate pins. The strap is a rope, or, in some cases, a band of iron; encompassing the shell of the block, in a notch or scoring cut round the block to receive it : The strap terminates in an eye of rope, or hook of iron, by means of which one of the blocks of a tackle is at tached to the object upon which it is to act as a me chanical power, while the other block is suspended from some fixed support. The former is called the running block, and the latter the standing block.
The blocks in use among shipping are so nume rous, having different names according to the pur poses to which they are applied,. and the manner in which their straps and tackles are fitted up, that a mere enumeration of all their names would take con siderable room. They may, in general, be divided into single, double, triple, and four-fold blocks, ac cording to the number of sheaves they contain. The shells of very large blocks are made of separate pieces of wood, as the cheeks of the shell, its partitions, &c. These are called made blocks. The shell is formed of several pieces of elm plank, suited to the thickness of the cheeks, sheave holes, and partitions, and is strongly bolted toget:ier by three bolts at each end, driven through and riveted with a washer at the points.
Blocks are again divided into thick and thin blocks; the former being intended to receive large ropes, and the latter smaller ones. The following may serve as a general idea of the mode of making any of the common kinds of blocks in the way, before the introduc tion of machinery. The shells being sawn to their length, breadth, and thickness, the corners or angles are sawn off. The workman then gauges or marks out the size of the sheave hole in the middle, one six teenth larger than the thickness of the sheave, and ' once the thickness longer than the diameter, for a single sheaved block. In blocks of two sheaves, the partition is kept in the middle, and is one sixth less than the sheave hole; each sheave hole is gauged on the two opposite sides, and in manner for blocks with a greater number of sheaves. The blocks are then jambed up edgewise with wedges' in a slave or frame, and the sheave holes are made in this manner; the length and breadth are first gauged out, and holes are bored half way through the block, along the part gauged out, with an auger of the size of the sheave hole ; then the sheave hole is gauged, and bored on the opposite side iivube same manner, so as to meet the opposite holes. Blocks from ten inches
and upwards have one hole bored at each end, and cut through with a chisel, and the wood is sawed out with a rib saw. All blocks have the sheave holes cleared through by chisels, and by burrs at the corners. Blocks that are to have iron straps should have the strap fitted on before the wood is cut out of the middle. The hole for the pin is bored through the middle of the block, one-tenth less than the diameter of the pin. The outsides and edges of the shell are next rounded off by the stock shave, and neatly finished by the spoke shave. In the royal navy, blocks are left thick upon the edges of the cheeks ; but in the merchant ships, the edges are somewhat thinned off to a small square, and some what rounded off. The scores, which are the grooves to receive the strap, are gauged out along the out sides of the cheeks, and tapered in depth, from no thing at the pin to half the thickness of the strap at the ends of the block for a single score, and the same on each side of the pin for double scores, which are made when the block is to have double straps. The scores are gauged down across the breast of the block to half the size of the strap, in order to allow for the serving. After the score is cut, the sheaves are fit ted ; they are one-tenth thicker than the diameter of the rope intended for running on them, and five times that thickness in diameter. The hole for the pin should be bored through the centre of them by a bit fixed in the mandrel of a turning lath, or with a stock arid bit, and opened out with an auger one sixteenth larger than the pin, that it may easily turn. They are then put in a lath, and turned smooth, and the outer circumference hollowed one-third of its thickness, that the rope may embrace it closely. The diameter of the pin is the thickness of the sheave, and is turned in a lathe, except its head, which is left octagonal to prevent its turning in the block, and the pin is driven through the holes in the block and sheaves. After the sheaves are fitted, the inside of the sheave hole, at one end of the block, is gabged hollow to admit the rope, and correspond with the sheaves ; and a small neat chamfer is taken off the' edges. The following articles will explain some of the different kinds of blocks used in shipping Snatch block, is a single sheave, with a notch cut through one of its cheeks, to admit the rope or fall to be lifted in a,.1 out of the block without putting its end through first. (See a figure of this in Plate LVII. Fig. 1.) The strap does not in this surround the block, but is put through a hole bored through the divided The figure is represented with two tails, which may be made up for a hook, a thimble, or eye, according to the situation where it is to be used, which is generally for the main or fore sheet blocks of square rigged vessels. It is a convenient block for heaving any rope in the navy. The snatch blocks are iron bound, terminating at the notched end of the block, with a swivel hook or an eye bolt, large enough to receive several turns of lashing, which fastens the block to its fixed support. That part of the strap over the notch in the side lifts up with a hinge, and is confined down, when the rope is in the block, by a small pin put across through the end of the pin of the sheave, which projects up from the block sufficiently to pass through an eye made in the hinge part of the strap. The strap on the other part of the block is let into the block, and confined by the pin and some nails. These blocks are used for heavy purchases, where a warp or hawser is brought to the capstan, See Plate LVII. Fig. 2.