(l) (6)2/8 = 4 1/2ton as the maximum strain on the-.running end of the " fall." Then P = 4 1/2 tons, and 6 P = R = 27 tons, as the theoretical weight. For each sheave in use we must deduct 1/10 of 27, or 2 7/10 for friction. Then as we have 6 sheaves in use, 6 x 2 7/10 = 16 2/10, as the total amount to be deducted from the theoretical gain, leaving as the actual power 27 - 16 2/10 or 10 8/10 tons.

The foil owing table gives the observed ratio of useful to theoretical work done in different tackles with white rope fall, viz.; -

Theoretical power of tackle.

Percentage of useful work.

Value of P.

Single

90 per cent.

1.lw

2:1

81.0 „

0.62w

3:1

75.0 „

0.45w

4:1

68.0

0.37w

5:1

61.5

0.32w

6:1

59.0

0.28w

If anything, the table gives a higher efficiency than would be obtained in ordinary use. It is highly necessary to keep blocks in good order, to see that all the parts are sound, that the sheaves are quite free on the pin and properly lubricated. Time spent in attending to these important details is well spent, as the friction on badly lubricated sheaves reduces their efficiency very much. Sometimes in using blocks it is found that they twist, and cause the rope to ride against itself; new rope especially. To prevent this twisting, a bar is sometimes placed through a part of the blocks, or at right angles to the "returns " close to the block. One of the best plans is to lash a handspike across the block; a light line may be lashed to each end and act as guys. This twisting of the tackles during use is a constant source of annoyance and waste of power often at a time when the power is wanted.

It has been found by experiment that if the blocks are allowed to twist one complete turn, the power required to overcome friction will be increased 40 per cent.

New ropes are bad to deal with at first, owing to their tendency to twist, or, perhaps we should say, untwist, or unlay. It is difficult to uncoil a coil of rope without getting it full of " kinks." When practicable, the coil of rope should be placed on a roller, and the end walked away with. Ropes intended for reeving "tackles" should be well stretched first. In order to do this, lay the rope out to its full length; connect one end to the barrel of a winch, and the other end to a holdfast, with a swivel-eye to admit of the rope unlaying itself. As the winch is worked and the rope made taut, it will begin to unlay; continue the strain up to the " working strength" of the rope, according to the following table of " working strengths," when it may be left taut for an hour, and then reeved.

Table Of Hawser Laid Cordage

Circumference.

Safe working load in tons.

in.

New.

Worn.

4 1/2

2 6/7

2 1/2

4

2 2/7

2

3 1/2

1 5/7

1 1/2

3

1 2/7

1 1/8

2 1/2

6/7

3/4

2

4/7

1/2

1 1/2

3/7

1/4

1

1/7

1/5

Ropes used with a thimble eye are safer than when ropes are slung over hooks, or fastened by knots. The hook of a block will soon destroy a rope under great pressure. It is true economy to use eyes where possible and convenient. In addition to the " knots " given under " Tying and Splicing," p. 368, the following will be found useful in connection with blocks. C shows a " selvagee " formed of returns of spun yarn in a circle bound together. They are used principally for attaching the hook of a tackle, the "selvagee" being passed round the object and hooked into the tackle. To make a "selvagee," place two pins at a distance from each other, equal to the intended length of the selvagee;" wind returns of spun yarn round the pickets until the " selvagee " is thick enough; then bind them together by half-hitches, with the running end at a distance of 1 1/2 in. apart.

D shows the application to a rope. Lay the middle part of the " selvagee " over the rope, then bring both bights under and around the rope in opposite directions, until the bights are close, then place the hook in both bights.

E shows a double bend, useful for bending a small rope on to a larger one; it will be noticed that the smaller rope is passed twice round the larger one. In F the bend is drawn taut.

G shows a block hook, with a few turns of spun yarn taken round in order to prevent its clearing itself when hooked to anything, and is termed " mousing " a hook.

H is a fishermen's bend. Two complete turns are taken round the ring, or other object, through the two turns next to the ring, over its own standing part, thus forming one half-hitch; the second hall-hitch is taken round the standing part alone.

I is a rolling bend. To make it, take a half-hitch with the running end round the standing part, lash them together just beyond the hitch, and size the end to the standing part; each part is exactly alike. It is sometimes called a "hawser bend."