foot, and the body leaning forward, balance themselves by raising the other in such a manner as to preserve the centre of gravity over that part touching the ground.

On ascending stairs, the body is bent forward, that the centre of gravity may advance with the feet.

When we walk, we change the base from one foot to another; this gives a swaying motion to the body. Thus people cannot walk well together when linked by their arms without keeping in step, that both bodies may have the same motion.

When carrying a burden on the back, we lean forward; if on the chest, we lean backward; if on either arm or shoulder, we lean to the opposite side.

In that useful domestic utensil, the pail, the centre of gravity is near the centre, and the handle, which is the centre of suspension, being fixed vertically over it, the centre of gravity must ascend; this, therefore, keeps the contents safely within the vessel. In some measures, such as those used for coals, which are heavy, and have to be frequently emptied, the handle is placed lower; thus little power is required to upset it, and empty the contents out quickly.

In some instances the centre of gravity is not in the object itself, as may be illustrated in a ring, where it is in the middle of the space measured from every part of the solid circle.

If we take a circular piece of wood, and near the edge drill a hole, and place a piece of wire through it, the wood will steadily hang from this axis; by making the hole exactly in the centre, the point of support will then be the centre of gravity, and the wood may be placed at rest in any position; but if the hole be made below this centrical spot, then the wood is most unstable, and the least motion will cause it to reverse its position, as the

centre of gravity will endeavour to get below the point of suspension. Thus, when the point of suspension is far above the centre of gravity, the balance of the body is more likely to remain undisturbed. It will be seen, then, that in constructing fine balances it is necessary to make the point of suspension just below the centre of gravity; the balance is then delicate and easily moved, which gives value to the instrument.

In many mechanical contrivances, as well as in the truths of science, the position of the centre of gravity is an important fact to ascertain, and it is known simply as any other question in figures. If a rod be five feet long and equally made, the centre of gravity will be exactly in the middle of the length; but if a weight of one pound be fixed on one end, and a weight of four pounds on the other end, then the centre of gravity will be at one foot distance from the four-pound weight, the other weight and four feet of the rod being required to counterbalance the opposite; thus the length of rod at each side from the point of suspension is in exact opposite proportion to the weights-that is, as one to four. This is also called the centre of inertia,5 and is the centre of centrifugal force as well; for were a whirling body not to have its axis made in that part, one portion of the hole in the wheel would wear out much quicker than the remainder.

Were a small ball attached to a larger one by a chain, fired out of a cannon, the two balls would be seen to fly round and round each other, and their centre of gravity not being in either ball, but, according to their proportion, nearer to the larger one, the rotatory motion would be described round this point.

The sun and the earth are bound to each other by attraction, and have a centre of gravity. In so speaking,

we are not now describing atoms of matter, but masses; suppose, then, the earth to be 1, then the sun is 354,936; and the centre of gravity of the sun will be 270 miles from its centre, which is the 300 part of its diameter.

The earth and the moon, by the attraction of the sun, revolve around it, and are as one mass of matter to that great body. The earth is a large ball, and the moon a small one, and they are held together by attraction as if by a bar of iron; thus they form a joint system, having a common centre of gravity. The moon is in bulk but the 49th part of the size of the earth, while from its density it is not more than the 70th of its mass; thus the mutual centre of gravity, which will mark the line of the motion of the two bodies, is a little below the earth's surface.

Quadrupeds, from the broad base they have to support their weight, are able to walk sooner than man. A horse, when moving, first lifts up a hind-leg, leans its body forward, and then lifts up the fore-leg on the same side as the hind-leg it first moved; thus the centre of gravity is advanced. The other hind-leg is next moved, then the fore one on the same side, progressing in this way forward. In trotting he lifts and puts down two feet at once, those diagonally opposite. In galloping he lifts and sets his feet down one by one, though the two fore and hind feet are set down nearly at the same time.

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"Mechanics and Mechanism," by R. S. BURN.

1. BALLAST, heavy matter placed in the hold of a ship to keep it steady when it has no cargo.

2. SCALE-BEAM, a graduated, ladder-like measure. (Lat. scala a ladder.)

3. ITINERANTS, persons who have no fixed abode, but travel from place to place. (Lat. itineris, of a journey.)

4. PRECARIOUS, uncertain, because depending on the will of others, and obtained by prayer or entreaty. (Lat. precor, to pray.)

5. INERTIA, that property of matter, by virtue of which it cannot change its state, whether of rest or motion. (Lat. iners.)

6. CENTRIFUGAL, a tendency to fly off from the centre. (Lat. centrum, centre, and fugio, to flee from.)

7. DIAGONALLY, passing through the corners, or between two angles not adjacent.

SIMPLE MACHINES.

A SIMPLE machine is an instrument, by which weights can be raised, the resistance of heavy bodies overcome, and motion communicated to masses of matter. It is by the application of simple machines, or mechanical powers, that man accomplishes many useful undertakings that, without such contrivances, would be beyond his natural strength.

Complex 1 machines may be traced to be merely peculiar arrangements of simple mechanical powers.

The natural forces or powers at the command of man for producing motion are few, being principally the strength of men and horses, running water, steam, fire, and wind.

It is the ability to regulate, accumulate, and divide the speed of power, and to connect, oppose, and counterbalance different velocities,2 that gives the great value of mechanical power to man. Machines do not beget or increase force, they only apply that which has been communicated to them in an advantageous, easy

manner.

The power applied must be greater than the resistance, otherwise there would be no motion.

The velocity of a body is measured by the space passed over in a given time.

If we notice the arms of a windmill in rapid motion, the outer parts can hardly be seen; while the parts nearer to the centre of motion can be easily distinguished. Now both parts take the same time to perform their journey round, but from the greater space passed over by the ends in the same time, the velocity is proportionally increased.

See, again, those youthful aspirants at our fairs for a ride on a roundabout, who get between the poles to push it along, giving this labour to purchase the luxury. Those near to the riders have to run with all their might, while those near to the axis move at almost a walking pace.

Time is exchanged for power; or, as it is sometimes expressed, "what we gain in power we lose in time.” This is termed the law of virtual velocities, or the golden rule of mechanics. Thus, if a person could raise fifty pounds to a certain height in one minute, and by the help of machinery he raises 500 lbs. to the same height, it will be found that the time occupied in lifting up the 500 lbs. would be ten minutes: thus the tenfold increased power has to have a tenfold increased time, or the work of ten minutes could have been accomplished in ten different efforts in the same time.

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The primary, mechanical powers are the lever, the pulley, and the inclined plane. The wheel and axle are derived from the lever, the wedge and screw from the inclined plane.

"Mechanics and Mechanism," by R. S. BURN.

1. COMPLEX, complicated, composed of many parts.

2. VELOCITY, Swiftuess, rate of motion. (Lat. velox, swift.) 3. PRIMARY, first, chief. (Lat. primus, first, superlative of prior, for.ner.)