PUMP

The common pump is so generally understood, that it hardly requires any description. It is a long wooden tube, whose lower end rests upon the ship's bottom, between the timbers, in an apartment called the well, inclosed for this purpose near the middle of the ship's length.

This pump is managed by means of the brake, and the two boxes, or pistons. Near the middle of the tube, in the chamber of the pump, is fixed the lower-box, which is furnished with a staple, by which it may at any time be hooked and drawn up, in order to examine it. To the upper-box is fixed a long bar of iron, called the spear, whose upper-end is fastened to the end of the brake,, by means of an iron bolt passing through both. At a small distance from this bolt the brake is confined by another bolt between two. cheeks,. or ears, fixed perpendicularly on the top of the pump. Thus the brake acts upon the spear as a lever, whose fulcrum is the bolt between the two cheeks, and discharges the water by means of the valves, or clappers, fixed on the upper and lower boxes.

These sorts of pumps, however, are very rarely used in ships of war, unless of the smallest size The most useful machine of this kind, in large ships, is the chain-pump, which is universally used in the navy. This is no other than a long chain, equipped with a sufficient number of valves, at proper distances, which passes downward through a woodeh tube, and returns upward in the same manner on the other side. It is managed by a roller or winch, whereon feveral men may be employed at once; and thus it discharges, in a limited time, a much greater quantity of water than the common pump, and that with less fatigue and inconvenience to the labourers.

This machine is nevertheless exposed to several disagreeable accidents by the nature of its construction. The chain is of too complicated a fabric, and the sprocket-wheels, employed to. wind it up from the ship's bottom, are deficient in a very material circumstance, viz. some contrivance to prevent the chain from sliding. or jerking back upon the surface of the wheel, which frequently happens when the valves are charged with a considerable weight of water, or when the pump is violently worked. The links are evidently too short, and the immechanical manner, in which they are connected, exposes them to. a great friction in passing round the wheels. Hence they are sometimes apt to break or burst asunder in very dangerous situations, when it is extremely difficult or impracticable to repair the chain.

The consideration of the known inconveniences of the above machine has given rise to the invention of several others which should better answer the purpose. They have been offered to the public one after another with pompous recommendations by their respective projectors, who have never failed to report their effects as considerably superior to that of the chain-pump with which they have been tried. It is however much to be lamented, that in these sort of trials there is not always a scrupulous attention to what may be called mechanical justice. The artist, who wishes to introduce a new piece of mechanism, has generally sufficient address to compare its effects with one of the former machines which is crazy or out of repair. A report of this kind indeed favours strongly of the evidence of a false witness, but this finesse is not always discovered. The persons appointed to superintend the comparative effects of the different pumps, have not always a competent knowledge of hydraulics to detect these artifices, or to remark with precision the defects and advantages of those machines as opposed to each other. Thus the several inventions proposed to supplant the chain-pump have hitherto proved ineffectual, and are now no longer remembered.

Reference Works