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SWEET nurslings of the vernal skies,
Relics ye are of Eden's bowers,
Fallen all beside-the world of life,
Ye dwell beside our paths and homes,
Our paths of sin, our homes of sorrow,
They cannot brook our shame to meet-
Ye fearless in your nests abide-
For ye could draw th' admiring gaze
Alas! of thousand bosoms kind
OF MACHINES IN GENERAL, AND THEIR USES.
THERE are many kinds of force which men have employed for the purpose of producing motion. Thus, a common pump is worked by the hand; carts and carriages are drawn by horses; the motion of a clock is due to gravity acting on its weights; the downward flow of water is made to turn water-wheels; a windmill and a ship are driven by the wind; and, not to mention others of less importance, the elastic force of steam is the great moving power of our time. But it will often happen that none of these moving powers, or prime movers, as they are called, can be directly applied at the point where it is wanted; or it may act in a different direction from that in which we wish motion to take place; or, from various other causes, it may be difficult so to regulate its application, as to secure the exact accomplishment of the desired effect. Suppose that a heavy stone, for example, has to be raised twenty feet. It is impossible for any considerable number of men to get hold of
it, and, even if they could, it would be equally impossible for them to raise it farther than their arms can reach. But, by means of a contrivance called a crane, they will be able to accomplish their purpose without the least difficulty. This is only one out of innumerable cases in which some instrument or contrivance is necessary in order to modify a moving power, so as to adapt it to the performance of a particular kind of work. Now, every such instrument or contrivance is called a machine. Some simple machines, used by artizans or mechanics, are also called tools; and some others, of larger size and more complex structure, are often spoken of as engines; but the word "machine" is, properly speaking, applicable to them all.
It is not the purpose of a machine to produce motion, nor has it any power to do so. It cannot even move itself; and it will accordingly remain at rest, unless some moving power act upon it. But it transmits the force, communicated to it by a moving power, from one point to another, alters its direction or intensity, and, in general, turns it to the best account in the performance of any given work.
Hence we have always at least two things to consider in examining a machine, namely, the work to be done, implying a certain resistance to be overcome, and the force or power, by whose action on the machine that work is performed. The former is technically called the weight. Sometimes, indeed, weight is the actual resistance which has to be overcome, as, for example, in raising a stone or other heavy substance; but even when this is not the case, we still speak of the power and the weight. This cannot be said to be incorrect, for every kind of force or resistance may be conveniently represented by a weight exactly sufficient to produce the same effect.
Machinery is employed for an immense variety of purposes. By it many results are easily attained, which neither the hand nor any other moving power could directly accomplish. We owe to it much of our progress in civilization. Few of the comforts, the luxuries, or the ornaments
of life could become ours without its invaluable aid. It is the indispensable handmaid of all the useful arts-of agriculture, mining, manufactures, commerce, navigation. Even in the most common transactions of everyday life, machines are ever in our hands. The merchant uses a balance to weigh, and a knife to cut, the goods which a cart, or a steamengine, has brought him. The balance, the knife, the cart, the steam-engine, are all machines. When we replenish the grate, we use a shovel or a pair of tongs; these are machines too. So is the poker with which we afterwards stir up the fire. The clock or watch which tells us the hour is a machine of great complexity; and scarcely less intricate, perhaps more so, is the printing-press with which this book was printed. Not to multiply examples unnecessarily, the motions of our own bodies are to a great extent mechanical, and certain parts of their structure, especially in the limbs, afford some of the best illustrations of the principles of machinery.
It has been said that no machine can of itself produce motion or create force, yet it may enable a comparatively feeble force, such as the muscular strength of a man, to overcome a resistance much greater than itself. How is this? Does the machine really add to the energy of the power? It does not, for the increase of power is gained at the expense of time. If the weight of a ton, for example, is to be raised one foot by a force equivalent to a hundred weight, the force must act through a space of twenty feet. So it is in every other case; the greater the weight is, when compared with the power, the smaller must be the space through which it will be moved by the motion of the power through any given space. In short, a feeble power will raise a heavy weight, but it will do it slowly in proportion to its feebleness.
There are certain simple elements of which all machines, however complex, are composed. They are called the Mechanical Powers. Of these there are usually reckoned six-the lever, the wheel and axle, the pulley, the inclined plane, the wedge, and the screw.
A LEVER is a solid rod or beam turning on a fixed axis,
which is called the fulcrum or prop. It may be either straight or bent, but the straight lever is that which most frequently occurs, and whose effects are most easily understood. The delightful game of see-saw affords an example with which every boy is familiar. The fulcrum is the point on which the plank rests and round which it turns; the weight of either boy may be reckoned the power; that of the other is the weight. Here the fulcrum is between the power and the weight, and the power may be either less or greater than the weight, or they may be exactly equal. This is the most common kind of lever, and is usually called a lever of the first kind. P and W are the power and weight, acting at their respective ends of the lever of which F is the fulcrum; their distances from F are called the
arms of the lever. The longer the arm PF is, when compared with the arm WF, the smaller will be the force P required to move or balance a given weight W. Thus if P is a force equal to 1 lb., and acts at a distance of one foot from F, it will balance a weight of 12 lbs. suspended at the distance of one inch on the other side of F. Hence power is gained in this kind of lever by moving the fulcrum towards the end at which the weight or resistance is to be overcome. But it is clear that it must be gained at the expense of time, for the weight will move through a shorter distance, and therefore more slowly than the power, in proportion as the arm which supports it is shorter than the other. When a quarrier employs a crow-bar to move a heavy stone, his hand is the power, and he places a small stone below the crow-bar as a fulcrum. The poker rests in the same way, on one of the bars of the grate, which is its fulcrum; the coals in the grate are the weight. A pumphandle is another familiar example of the same kind of