jected to extreme tension while the inner ones are actually compressed. The result is that the outer layers give way and are followed by the others in succession toward the inside. This is the explanation also of the weakness in a sharp nip of any kind, whether due to a splice, a hitch, a bad lead or a bend around a pin or post.

Observe that whereas in hoisting, the maximum tension comes on the hauling part, in lowering, it comes on the standing part.

If the system is at rest, friction will tend to keep it at rest and will reduce the power needed at either end to maintain equilibrium. Thus if we have a mass of 800 lbs. hanging from the lower block of a two-fold purchase, we should require 200 lbs. on the hauling part to maintain equilibrium if there were no friction; but since in practice there is and must be friction, we shall be able to prevent motion by much less than 200 lbs. As an extreme case, we may imagine the friction so great that nothing is needed on the hauling part to prevent the lower block from moving-the weight of 800 lbs. not being sufficient to "overhaul" the tackle. Similarly a mass of 200 lbs. on the hauling part may be held at rest by much less than 800 lbs. on the lower block, the friction acting as before to prevent motion.

If we attempt, as a matter of convenience, to assign an approximate numerical value to the loss by friction, we shall find it convenient to represent this loss as a percentage added to the load; and experience shows that it is a safe general rule to increase the load by 10 per cent for each sheave over which the fall leads and then to consider that this increased load is being lifted by a frictionless purchase. Accordingly, to find the power required on the hauling part, or (what is the same thing) the maximum tension on the rope, we add the percentage for friction as above, and divide by the number of parts at the movable block. (See § IV.)

No attempt is made, in the above "Rule-of-Thumb," to take account of variations in speed due to variations in power applied, or of the fact that friction increases with the speed. The rule is for average working conditions, and includes a sufficient margin of safety to cover all practical cases.

It will be clear from the preceding that much power is wasted where the hauling part of a purchase is taken through an extra (fixed) sheave merely to give a fair lead. A familiar case of this is that in which a fall leading from aloft is taken through a block on deck to be manned or led to a winch. Such leads are

often unavoidable; and even where not so, the gain in convenience may more than offset the loss of power; but it must not be forgotten that this loss of power is considerable. The same reasoning applies to an unnecessary sheave in the fixed block of a purchase, which has of course the same effect as an unnecessary sheave anywhere else.

If one tackle is attached to the hauling part of another, the power of the combination is the product of the powers of the tackles composing it.

It is important to observe that in tackles as in all other mechanical appliances, "what is gained in power is lost in speed." This is clearly brought out by a comparison of Figs. 1 and 2 of Plate 29. In Fig. 1, a single whip, if the hauling part moves through one foot, the weight at the other end moves through the same distance. Both space and power are equal at the two ends of the rope. In Fig. 2, a runner, the power is doubled at the block, but the block moves through only half the distance moved by the hauling part. And so at every sheave of a movable block. If the power is multiplied six times, as by a three-fold block, the hauling part must move through six feet to move the block one foot. This may be a very important point where space is limited. Tackles are of value not only as multiplying power but as applying the power more smoothly and uniformly. So, too, in easing away, they prevent the surging which is almost unavoidable with a single part, and at the same time make it possible to lower more slowly and with much more exactness; since, as has been explained, the motion at the block of a purchase is only a fractional part of what is given on the fall. The greater the number of parts to the tackle, the greater the gain in this respect as well as in the power.

For heavy work with a three- or four-fold purchase, the fall should be rove with the hauling part leading from the middle sheaves of the blocks instead of from the outer ones. This involves a turn in the parts, but reduces the chance of slewing the blocks in their straps. For light purchases, it is a common practice to make the standing part fast to a becket worked into the strap. This will not answer for heavy work. If the standing part is to be made fast to the block, it should be taken around and secured between the block and the hook (Fig. 1, Plate 32). It is a still better plan to make it fast to the weight to be lifted,

TACKLES IN USE.

close alongside the block, instead of to the block itself. The hauling part of a tackle should be kept as nearly as possible parallel to the other parts. A divergence from this line means a loss of power.

As the power of a tackle depends upon the number of parts at the movable block, we have the rule that whenever circumstances permit, the block having the greater number of partsor in other words, the block from which the hauling part leads -should be attached to the weight to be moved. This rule is often disregarded because it is usually impracticable to take the hauling part from the lower block directly to the winch; and the most natural way of giving it a lead from the derrick-head is to place there the block of the tackle from which this part leads. A better plan is to let the hauling part come from the lower block, and to take it up through an independent leading block at the derrick-head; thus preserving the full power of the tackle. In many cases it is necessary to use still another leader at the heel of the derrick.

§ III.

Tackles may be designated either according to the number of sheaves in their blocks; as, single, double, three-fold, etc.; or according to the purpose for which they are used; as, Yardtackles, Stay-tackles, Fore-and-aft-tackles, etc. Still other designations, not so easily accounted for, are luff-tackles, guntackles, Spanish-burtons, etc.

Tackles are almost invariably rove of manila rope.

Plate 29 shows diagrammatically various forms of tackles with the theoretical gain in power due to each and the approximate actual gain when friction is taken into account. Plates 30, 31 and 32 illustrate certain common applications of tackles on shipboard.

A Single-whip (Fig. 1, Plate 29). A single block, fixed. A Runner (Fig. 2, Plate 29). A single block, movable. A Whip and Runner would be a whip hooking to the hauling part of a runner.

A Gun-tackle (Figs. 3 and 4, Plate 29). Two single-blocks. A Luff (Figs. 5 and 6, Plate 29). A single and double-block. Sometimes called a watch-tackle."