6th. Do not allow ropes to be struck, or trampled on, when the

weight is suspended.

7th. The men should stand as safe as the proper performance of the various duties will permit.

8th. In pulling a rope, the men ought to place themselves in a right line, and haul together. The most advantageous position for pulling is down a slope, or in a descending position.

COMBINATION OF PULLEYS.

A leading block is a fixed pulley, which alters the direction of the power, but does not increase it: Power = Weight. On account of friction the power must exceed the weight a little, in order to raise it.

Vide plate, Mechanics, Fig. 1.

A whip is one moveable pulley, which increases the power without altering the direction.

weight (or 2 to 1).--Vide Fig. 2.

A whip upon whip will afford the same purchase as a tackle having a single and double block, and with much less friction.

A gun tackle consists of two single blocks with fall fixed to the one, then rove through the other, and then through the first. Power = weight (or 2 to 1): or Power = weight (or 3 to 1).

Vide Fig. 3, and 4.

Two double blocks are generally used for very heavy guns.

A luff tackle, or half watch tackle, consists of one double and one single block the fall is fixed to the single, then rove through first sheave of the double, then through sheave of single, and lastly through second sheave of double block. Powerweight (3 to 1): or Powerweight (4 to 1). Vide Fig. 5, and 6.

A runner tackle is the same as a luff tackle, applied to the end of a large rope, called a runner, which is rove through a single block attached to a fixed point, or to a body that is to be moved, or raised; the standing end of the runner being secured to another point.

Power is either 6 to 1, or 7 to 1, or 8 to 1.

A gyn tackle consists of one triple and one double block: the fall is fixed to the double, then rove through first sheave of triple, then through first sheave of double, then through second sheave of triple, then through second sheave of double, and lastly through third sheave of triple block.

Power Vide Fig. 7.

weight (5 to 1); or Power = weight (6 to 1).

If the moveable block of a tackle be strapped with a tail, it is called a tail, or jigger block: and the tackle a tail, or jigger tackle: a block with a hook strapped to it, and attached to a selvage, answers the same purpose.

Two double blocks, with fall fixed to one of them, and then rove through the sheaves of both blocks, will either give Power

(4 to 1): or Power

weight

weight (5 to 1). Fig. 8. Two triple blocks, with fall fixed to one of them, then rove through

sheaves of both blocks, will either give power
Powerweight (7 to 1). Fig. 9.

In the system of pulleys (vide plate, Mechanics) the Power is shown at the hooks of the moveable blocks, which are to be applied to the bodies, or weights, requiring to be moved, or raised. The strain is also shown at the fixed blocks.

The same

In Fig. 3, there are three parts of the rope engaged in supporting the weight-viz., the parts marked 1, 1, 1. Each of them, hence, sustains one-third of it, and the fall of the rope to which the power: tó be attached requires the Power = 1, if weight = 3. principle of calculation is applicable to all systems of pulleys having one fixed block, any number of moveable wheels, and a single rope over all the wheels. Hence, in such a system of pulleys, gravity being applied, there will be an equilibrium, when the weight is as many times the power as there are portions of the rope employed in sustaining the weight. For example, in a system consisting of six moveable sheaves, the same rope going over them all, there will be 12 portions of the rope engaged; and to produce an equilibrium the power must be equivalent to the weight, no allowance being made for friction.

From the foregoing observations, and by referring to the plate, it will be seen that each tackle has two applications, differing in power one from the other; for example, if the double block of a luff tackle is fixed to a weight to be moved, and the single block to a picket, or other fastening, Fig. 6, then, if one man haul on the fall, the power of four men will be applied to the weight (4 to 1), and the power of three men to the picket; but if the double block be fixed to the picket, Fig. 5, and the single block to the weight, then the force of only three men will be applied to the weight (3 to 1), and a power of four men to the picket, or fastening.

When the moveable block of one tackle is fixed to the fall of another tackle, their respective powers are to be multiplied into each other for the power of the combination: thus, if one luff tackle is fixed to the fall of another luff tackle (the double blocks of both tackles being moveable), the power will be 4x4 = 16 (16 to 1): in this, the men haul through 16 feet to move the weight one foot; therefore if the combination be increased until the men haul through 100 feet to move the weight one foot, then the power would be 100 to 1.

The foregoing powers are, however, only true in theory, and are, therefore, called theoretical powers: for owing to the great friction of the pulleys, the stiffness of the ropes, &c., the actual practical powers are far less; so much so, that with a combination giving a power of 48 to 1, a 24-pr. (23 tons weight) suspended, can scarcely overhaul the fall, the friction being so very great.

THE INCLINED PLANE.

The inclined plane forms simply a gradual and sloping instead of a sudden and perpendicular ascent, by which heavy bodies may be

raised to certain heights. The power necessary for raising a weight depends on the difference between the length of the plane and the height to be ascended. If the height be one-third of the length, then one pound will lift three pounds. The force with which a rolling body descends on an inclined plane is to the force of its absolute gravity, as the height of the plane is to its length.

Parbuckling a gun on skids unites the advantage of one moveable pulley with that of the inclined plane.

Rule. As the length of the plane is to its height, so is the weight to the power.

Example.-Required the power necessary to raise 540 lb. up an inclined plane, five feet long, and two feet high.

As 5 2 540: 216 lb. Ans.

:

THE WEDGE.

The

The wedge may be considered as two equally inclined planes joined together at their bases. It has a great advantage over all the other powers, arising from the force of percussion, or blow, with which the back is struck; which is a force incomparably greater than any dead weight, or pressure, such as is employed in other machines. largest masses of timber may by this means be riven, and vessels of war, weighing many thousand tons, are lifted from their supports by the power of a few men, exerted by blows of mallets on wedges inserted for that purpose.

The power of the wedge increases in proportion as its angle is acute. In tools intended for cutting wood the angle is commonly about 30°; for iron from 500 to 600; and for brass from 80° to 90o.

Case 1.-When two bodies are forced from one another, by means of a wedge, in a direction parallel to its back.

Rule..-As the length of the wedge is to half its back, or head, se is the resistance, to the power.

Example. The breadth of the back, or head of the wedge, being 3 inches, and the length of either of its inclined sides 10 inches, required the power necessary to separate two substances, with a force of 150 lb.

As 10 1: 150: 221⁄2 lb. Ans.

Case 2.-When only one of the bodies is moveable.

Rule. As the length of the wedge, is to its back, or head, so is the resistance, to the power.

Example.—The breadth, length, and force, the same as in the last example.

As 10 3 150: 45 lb. Ans.

THE SCREW.

The screw is a spiral thread or groove cut round a everywhere making the same angle with the length of it.

cylinder, and The force of

a power applied to turn a screw round is to the force with which it presses upward, or downward, setting aside the friction, as the distance between two threads is to the circumference where the power is applied; or the advantage gained is as much as the circumference of a circle de scribed by the handle of the winch exceeds the interval, or distance, between the spirals of the screw. Hence the force of any machine. turned by a screw can readily be computed; for instance, in a press driven by a screw, whose threads are each a quarter of an inch asunder, and with a handle, to turn the screw, four feet long; then, if the natural force of a man, by which he can lift, pull, or draw, be 150 lb., and it is required to determine with what force the screw will press when the man turns the handle with his whole force; the diameter of the handle (power) being 4 feet, or 48 inches, its circumference is 48x3 1416, or 1503 nearly; and the distance of the threads being onefourth of an inch, therefore the power is to the pressure as 1 to (150X4)=603, but the power is equal to 150 lb., therefore as 1: 6031: 150: 90480, and consequently the pressure is equal to a weight of 90480 lb. independent of friction.

COMPOUND MACHINES.

Though each of the mechanical powers is capable of overcoming the greatest possible resistance in theory, yet in practice, if used singly for producing very great effects, they would frequently be so unwieldy and unmanageable as to render it impossible to apply them. For this reason it is generally found more advantageous to combine them to. gether, by which means the power is more easily applied, and many other advantages are obtained. In all the mechanical powers, and their combinations, and in all machines, simple as well as compound, what is gained in power is lost in time or velocity; and vice versa, or in other words, the product of the power, and the space through which it moves, is equal to the product of the weight, and the space through which it moves in the same plane. Suppose that a man, by means of a fixed pulley, raises a beam to the top of a house in two minutes, is clear that he will be able to raise six beams in twelve minutes; but by means of a tackle with three lower pulleys, he will raise the six beams at once with the same ease as he before raised one, but then he will be six times as long about it, that is, twelve minutes: thus the work is performed in the same time whether the mechanical power is used, or not. But the convenience gained by the power is very great; for if the six beams are joined in one, they may be raised by the tackle, though it would be impossible to move them by the unassisted strength of one man. No real gain of force is obtained by mechanical contrivances: on the contrary, from friction and other causes, force is always lost; but by machines a more convenient direction can be given to the moving power, and so modify its energy as to obtain effects which it could not otherwise produce.