Rack blocks, are a range of small single blocks, made from one solid, by the same proportions as single blocks, with ends in form of a dove's tail for the lashing by which they are fastened athwart the bowsprit, to lead in the running ropes : they are sel dom used.
Shoe blocks, are two single blocks, cut in a solid piece, transversely to each other ; they serve for legs and falls of the but are seldom used.
Shoulder block, is a large single block, left nearly square at the upper end of the block, and cut sloping in the direction of the sheave. Shoulder blocks are Used on the lower yard arms, to lead in the topsail sheets ; and on topsail yards, to lead in the top gallant sheets, and by means of the shoulder are kept upright, and prevent the sheets from jambiag between the block and the yard ; they are also used at the outer end of the boornkins, to lead in the fore tackle. Sec Plate LVIII. Fig. 2.
Sister blocks, arc similar to two single blocks, and are formed out of a solid piece, about 20 inches long, one above the other. Between the blocks is a scoring for a middle seizing : A round head is turned at each end, and hollowed underneath to contain the end seizings ; along the sides, through which the pins are driven, is a groove or scoring, large enough to re ceive part of the topmast shrouds, in which it is seized. These blocks receive the lifts and reef tackle pendents of the topsail yards.
Spring block, a new kind of block, invented by Francis Hopkinson, Esq. of Philadelphia, and de signed to assist a vessel in sailing, by increasing the acting spring of her rigging. It is proposed to apply it to all such parts of the rigging as will admit of it with safety and convenience, and where its operation will be most advantageous, but particularly to the sheet ropes, and, if practicable, to the dead eyes, in lieu of what are called the chains. A, (Plate LVI I I. Fig. 3. ) is a block made in the usual manner, having a ring or eye B at one end ; c is a spiral spring linked at one end to the hook DE, and at the other to the ring F, which is to be annexed to an eye-bolt at the timber- head, or by some other means, to the place where it is to be applied. The spring c must be of well tem pered steel, and proportioned in strength to the ser vice it is to perform. When used, two of these blocks are employed, one attached to the timber heads and the other to the sail. Within the cavity
or pipe, formed by the spiral spring, there must be a chain of suitable strength, called a check chain, (represented in the centre of the spring,) connected by the links to the DE and ring F. When the spring is not in action, this chain is slack ; but' when the spiral spring is extended by the force of the wind, as far as it can be without danger of in jury, the .check chain must then begin to bear to prevent its further extension, and if strong enough, will be an effectual security against failure. The in ventor of this machine apprehends, on good grounds, that a vessel thus furnished will be less liable to heel ; and that she will receive the impulses of the wind to better advantage, and sail with a more lively and equable motion, than if rigged in the common way. We have never heard of its being tried; but fear the weight of a spring sufficiently strong to have any effect on a large sail, would be very unmanageable, particularly aloft, from its weight, and would also he very expensive. There is no doubt if this, or a similar contrivance, could be applied to a sail, it would have a good effect : for instance, when a ship is sailing with a certain rate, if a sudden blast of wind comes, but does not continue, it will not advance the vessel at all; be-., cause it does not continue long enough to communi cate an increased momentum to so large a mass as a ship, though, at the same time, it may make her heel or Ditch violently. If her rigging is fitted with these springs, she would receive the impulse of the same blast in a regular and progressive manner, which would tend to increase her velocity instead of causing her to heel. We think it would be very improper and dangerous to apply any thing of this kind to the shrouds. We recollect meeting with a similar inten tion, to be effected by a cylinder, fitted with a pis. ‘•• ton, which, when drawn out, would cause a vacuum, and act as a strong spring. By this means a sufficient elastic force might easily be obtained ; but it would be difficult, nay, impracticable, to preserve such a cylinder in an acting state at sea, unless it Were situa ted beneath the deck, and defended from the weather, &c., and then it could not be applied to the upper sails.