CHAP. VII.

MECHANICAL POWERS.

MECHANICS is the science which treats of forces applied to bodies, either directly, or through the agency of some machine.

Any cause which alters, or tends to alter a body's state of rest or motion, is called a Force.

The force with which we are most familiar, is that which attracts every body on the earth's surface towards its centre, and which causes the body, if free, to move towards that point; and if restrained, to press against the obstacle which prevents its motion. This force is called Gravity. The amount of the pressure exerted by different bodies varies with their magnitude. Thus, the pressure exerted by a mass of lead containing 2 cubic feet is twice as great as that of a mass containing only 1 cubic foot. Gravity is a uniform force, or in other words, exerts the same influence on all bodies which contain the same amount of matter. The pressure actually exerted by a heavy body in consequence of its tendency to fall to the earth is called its Gravitation; and that which is exerted by a certain definite bulk is called a Pound. All forces may be measured by the number of pounds weight which they could sustain if applied in a direction opposite to that of gravity, i. e. vertically upwards.

Besides the magnitude, it is necessary to take into consideration the Direction of a force.

By this is meant the straight line in which it tends to move the body, if free, and not counteracted by any other force.

Thus, if a force is produced by a thrust of a rod, its magnitude is measured by the number of pounds weight it would support if it were to act vertically upward; and its direction is that of the rod's length.

Forces applied to a body are said to be in equilibrium, when the body, if originally at rest, still remains so after their application, or, if in motion, continues to move in the same manner as before, or as if they had not been applied at all.

Velocity is the degree of quickness or slowness of a body's motion; and, if uniform, is measured by the number of feet

described in a Unit of time. The units of time usually employed are a Second and a Minute. The latter is the unit generally employed when the motion of machines is considered.

The principal Moving Powers are the physical force of men and animals (or living agents); the expansive force of steam and gunpowder; the action of water, and of wind; and the elastic force of metals when formed as springs.

Always directing motion, it is absolutely necessary that an officer should know how (without overtasking) to apply the "moving powers" to the greatest possible advantage, as well as to form a right estimate of "work."

Great care must be taken to have a clear understanding of the proper meaning of the term "power," as used in matters of science. It is quite distinct from strength, or force, or pressure. It always has reference to a quantity of work performed, or which could be performed. For instance, if it be said that a man can lift off the ground a piece of iron of 300 pounds weight, there is no information as to the power of a man -speaking scientifically conveyed by the knowledge of this fact, which merely indicates his strength. But if it were stated how many such pieces he lifted to a given height, and placed upon a truck in an hour or in a minute, the amount of work which he performed would then be known, and this would be the measure of his power.

If one man loaded a truck with pieces of iron weighing 300 pounds each, and another man had to perform a similar duty with pieces of 150 pounds each, the latter must lift double the number of pieces to do the same amount of work. In this case each of these pieces must have been raised from the ground to the truck in half the time of the others; that is, two lifts for one must have been made. The quickness with which the weights were lifted is called the Velocity with which they have travelled, and this, in the case of the light weights, was double that of the heavy; and hence half the weight, raised with double the velocity, gives the same amount of work done within the same space of time, and the same amount of power is said to have been exerted.

In a similar manner, let it be supposed that a man in a given time raises 10 pieces of iron, each weighing 300 pounds, that is 3000 pounds in all, 1 foot high, and that another man raises one

of these pieces in 10 lifts one above another, each of 1 foot high, that is 10 feet in all, this will be the same number of equal lifts, and the two men will have done an equal amount of work. If the weights raised be multiplied respectively by the distances to which they have been raised, the result will be the power; that is, the same amount of power is exerted to raise 300 pounds 10 feet high, 3000 pounds 1 foot high, or 1 pound 3000 feet high.

Moreover the work done by men and animals varies according to the manner in which they exert their strength; and in consequence of their difference of form a general comparison is impossible. A man can carry a weight equal to his own, and yet his average force when thrusting at the height of the chest is reckoned at but 30 pounds. A horse cannot draw up a hill as much as three men could carry up the same; yet on a level road he would draw as much as seven men could. And either of these agents might, by a single great effort perform a piece of work, and becoming exhausted, be unfit for further labour; whilst another by a series of moderate ones, might in due time effect as great a result. So that physical force being intermittent, it can only be estimated by its effects, and hence it is that time becomes an essential part of the consideration, the question embracing, not merely the amount of action, but also the time in which it is developed.

Weight and time, and height or distance, being then interchangeable with each other, it will easily be understood that it becomes a convenient mode of expressing the power of a man or horse, to say that one man or one horse can raise a given number of pounds one foot high in one minute.

A Unit of work represents the labour required to raise 1 pound weight through the space of 1 foot, and has the same relation to work as a foot (which is the measure of space) has to distance, or a pound to weight. The labour expended in raising a body is found by multiplying the weight of the body in pounds by the space in feet through which it is raised. Supposing a man to raise a weight of 1 pound 1 foot high, he does a unit of work; if he lifted a weight of 160 pounds 30 feet high, he would have performed 4800 units of work, or (according to the above rule) 160 x 30 = 4800.

Again, if a man handed thirty 32-pound shot over the gunwale of a boat, 1 foot high, he would perform 960 units of

work. Say that he handed at the rate of one shot a second, and that another man worked at the same rate with a luff jigger and net filled for him with three shot at a hoist, the second man would only perform the same number of units of work; for whilst the force of the first man was exerted through a space of 1 foot in a certain time, that of the other was exerted through three times the space of 1 foot in the same time. In other words, one would hand through 1 foot, and the other would haul through 3 feet of the tackle fall.

Again, if a man carry a 56-pound shot, at the rate of three miles an hour, and another of 42 pounds at the rate of four miles, the effect would be the same, viz. 168.

In the application of physical force, the individual weight of the agent must be considered as well as his muscular force.

If a man be 150 pounds weight, and he went 40 feet up the rigging, and let himself down in a bowline knot on the end of a single whip, the other end being fast to a weight somewhat lighter than his own, he would raise that weight 40 feet, and perform a number of units of work equal to the number of pounds in that weight multiplied by 40. If the end, however, were made fast to some machinery (such, for instance, as the barrel of a winch, to which some weight was attached) the efficiency of the force employed would be 150 × 40 6000, and this can be caused to raise a weight of 6 pounds through a space of 1000 feet, or a weight of 1000 pounds through a space of 6 feet, or a weight of 6000 pounds through a space of 1 foot. For the mechanical effect produced by a machine is measured by the work done in a given time, or by the product of the force exerted, and the distance gone through in a unit of time in the direction of that force.

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But, in thus making use of a man's individual weight, it must be observed that, except whilst raising his body, his muscular force is at rest; whereas in moving about whilst labouring, (as in working the capstan or “walking away" with a tackle fall or single rope,) he exerts his strength, both in thrusting or hauling, in addition to carrying his body; and the more nearly that is a load to him, the less can he do in addition. So that, when we can have a choice in the mode of application, it behoves us to consider how to place our men at their work. "The very best and most effectual posture in a man is that of rowing, wherein he not only