Page images
PDF
EPUB

lead dropped upon a man's foot, from a height of a few yards, will not hurt him, but a heavy ball dropped from the same height will bruise his foot. A bullet thrown by the hand against a thin board will make only a slight impression, but a similar bullet discharged from a rifle will pass through the board.

When a body in motion strikes a body at rest, it meets with a resistance equal to the blow struck. The body at rest reacts upon the body in motion with exactly the same force as the latter acts upon the former. Thus, when a ball is thrown against a wall, it rebounds, owing to the reaction of the wall. Birds, in flying, strike the air with their wings, and it is the reaction of the air upon their wings which enables them to rise.

When two or more forces are applied to a body simultaneously, the result is the same as if each of them acted on the body separately. If two forces, for example, act in the same direction, the impetus given to the body will be the sum of their united action. If, on the other hand, they act in opposite directions, the one will counteract, according to the measure of its power, the efforts of the other, and the result will be an impetus equal to the difference between the two forces, and in the direction given by the stronger. Let two boys in a boat pull in the same direction, the speed of the boat will be the sum of what each of them could produce separately; but if the one boy pulls one way, and the other an opposite way, the speed of the boat will only amount to the difference between the two pulls, and will be in the direction given by the stronger of the two. If the pulls are of equal strength, no motion will take place, for forces which are equal and opposite destroy each other.

When two forces, instead of acting in the same line of direction, are applied in directions inclined to each other, the body acted on moves in a line between the two. Thus, if a piece of wood be thrown into a river, when the wind blows right across, it will be carried to the other side, but lower down. If the two forces are inclined to each other at an angle of 90°, the body will move in the diagonal of a square or rectangle; if they are inclined at any other

angle, whether acute or obtuse, it will describe the diagonal of a parallelogram.

N

If a body is acted on by two forces, by one of which it is projected forward in a right line, while by the other it is continually directed towards a fixed point, the result will be a circular motion. For instance, when you whirl a ball which is fastened to your hand by a string, its circular motion is due to two forces-the force which you give it in projecting it forward, and the force of the string which fastens it to your hand; if, during the motion, the string were to break, the ball would fly off in a straight line; because it would then be acted upon only by the force of projection. The force which confines or draws a body to a centre is called the centripetal force; and the force which impels it to fly off is called the centrifugal force. The planets are kept in their orbits by the action of two such forces-the one drawing them towards the sun, the other impelling them to fly off at a tangent; the consequence is-both forces being constant that they revolve in orbits nearly circular. It is only, however, when these two forces balance each other, that the motion is circular. If one of the forces produces a uniform, and the other an accelerated motion, the body will move in another curve than a circle. Thus, when a stone is thrown from the hand in a horizontal direction, it is acted upon by two forces-the force of projection given to it by the hand, and the force of gravity which finally brings it to the ground; but as the motion produced by the projecting force alone is uniform, while that produced by the constantly increasing force of gravity is accelerated, the stone is made to describe a curve, which, but for the resistance of the air, would be a parabola.

Although bodies differ widely in size and in shape as well as in weight, yet there is in each of them one special point where the motive force may be more effectually applied than at any other. This is called the centre of gravity, and it is

the point around which all the parts of a body are so equally balanced that the whole body will move, if it is moved, and remain at rest, if it is supported. A straight line falling perpendicular from the centre of gravity, is called the line of direction; and only when that line falls within its base will a body stand firm. The narrower the base upon which a body rests, the more readily is it upset; but there is difficulty in overturning a body with a broad base. Thus, a cask is easily rolled along, while a square box is moved with difficulty. When a man, in wrestling, is likely to be thrown down, he puts his feet as far asunder as possible. When a rope-dancer finds himself in danger of losing his balance, he shifts the heavy pole which he holds in his hand, so as to change the situation of the centre of gravity. The higher, too, the centre of gravity, the more easily is a body overturned. A coach, empty inside, with passengers and luggage outside, is in more danger than if there were people

[graphic]

inside. A boat is in greater danger of being upset, if the passengers rise up suddenly, than if they keep their seats. In man the centre of gravity is so situated that the line of direction falls between his feet when standing; and it is the same in the case of quadrupeds. A dog cannot long stand on his hind legs, because the centre of gravity is too far forward. Ducks, geese, and swans walk awkwardly for a similar reason. In cats and other animals, which spring upon their prey, the centre of gravity is so situated that they uniformly fall on their feet.

Compiled.

MACHINERY AND ITS USES.

INSTRUMENTS Contrived for the purpose of applying and regulating moving power receive various names according to the degree of their complexity. Thus, an instrument of simple construction, designed to assist the human handsuch as a spade or a saw-is called a tool. An instrument composed of two or more tools which act on each other, is called a machine; while a machine of considerable magnitude, or of more than usual nicety of construction, is termed an engine. Thus we speak of a reaping-machine, and of a steam-engine. In many cases, however, the line of demarcation between tools and machines and between machines and engines, cannot be easily drawn. And it is, therefore, fortunate that we have a term-Machinery-which comprehends them all.

Machinery has no power of motion in itself. Before it can either itself move or give motion to the body to which it is applied, it must have power communicated to it by some external force. It must be itself acted on before it can act on anything else. It is simply a passive instrument interposed between the motive force and the body to be moved; and all that it can do is to receive and deliver the impulse which has been impressed upon it. A spade cannot dig without the hand of the labourer; a mill-wheel cannot turn round without the falling water; a steam-engine cannot propel the paddle-wheel, until the expansive force of steam is brought to act on the piston.

Yet, though machinery cannot create force, it enables us to apply, in the most efficient manner, those natural forces which are at our command; and, when properly constructed and applied, it gives us power to turn those forces to many valuable uses. As a sample of these uses, it may suffice to refer to a few of the ways in which machinery aids muscular power.

In building operations, it is often necessary to raise heavy bodies for instance, blocks of sandstone or of granite-to a considerable height. In all such cases muscular strength

would avail little without the help of machinery. If the size of the block is small compared with its weight, it will not present a surface large enough to allow a sufficient number of men to take hold of it; and even should it admit of being laid hold of by a sufficient number of men, they will not be able to raise it higher than their own arms can reach. But by the intervention of a rope and pulley, any number of men may combine their efforts, and raise the block to almost any height they please.

Machinery affords us similar aid in overcoming friction. and other impediments to motion. No one could drag a heavy block of stone along a rough road; but by placing wheels or rollers under it, the friction is so diminished that the stone is easily moved. In like manner, the inclined plane lessens the opposition of gravity to our efforts in lifting. "A gradually ascending mound of earth," it has been said, "would have enabled our British ancestors to draw up to its place the heaviest block at Stonehenge."

What a help, again, is machinery in operations requiring great exactness and nicety of execution! The most skilful workman is unable, even with the aid of tools, to make a bar of iron so perfectly round that no inequalities shall be found in it; but in a lathe a rod perfectly round can be turned without difficulty. No two hand-made articles are ever, in all respects, exactly alike; but by means of a mould thousands of articles may be formed so perfectly similar, as to be indistinguishable one from another.

Does not machinery, also, enable us to regulate motion-to increase, or diminish, or equalize its velocity at our pleasure? The human hand can move a body, and so can the foot, but not very rapidly, and both hand and foot soon become fatigued by the operation. But by means of a multiplying wheel or a treadle, a rapid movement can be produced and long kept up, with but little fatigue to hand or foot.

[graphic]
« PreviousContinue »