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 vitte, 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 par ticular 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 arc 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 in creased, 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 via inertire of the entire load; it must, therefore be consi derably 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 Light house. 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 fol lows. 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 re maining large sheaves, and ending upon the middle sheave of the upper block. Tha 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 di verging,) in order that the cross sheave may not be of a very small diameter. The method of reev ing 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.