MECHANICS.
There is no subject of such importance as Mechanics, as its principles are founded upon the properties of matter and the laws of motion; and in knowing something of these, the tyro will lay the foundation of all substantial knowledge.
The properties of matter are the following: Solidity (or Impenetrability), Divisibility, Mobility, Elasticity, Brittleness, Malleability, Ductility, and Tenacity.
The laws of motion are as follows:—
1. Every body continues in a state of rest or of uniform rectilineal motion, unless affected by some extraneous force.
2. The change of motion is always proportionate to the impelling force.
3. Action and reaction are always equal and contrary.
EXPERIMENT OF THE LAW OF MOTION.
In shooting at “taw,” if the marble be struck “plump,” as it is called, it moves forward exactly in the same line of direction; but if struck sideways, it will move in an oblique direction, and its course will be in a line situated between the direction of its former motion and that of the force impressed. This is called the resolution of forces.
BALANCING.
The center of gravity in a body is that part about which all the other parts equally balance each other. In balancing a stick upon the finger, or upon the chin, it is necessary only to keep the chin or finger exactly under the point which is called the center of gravity.
THE PRANCING HORSE.
Cut out the figure of a horse, and having fixed a curved iron wire to the under part of its body, place a small ball of lead upon it. Place the hind legs of the horse on the table, and it will rock to and fro. If the ball be removed, the horse would immediately tumble, because unsupported, the center of gravity being in the front of the prop; but upon the ball being replaced, the center of gravity immediately changes as position, and is brought under the prop, and the horse is again in equilibrio.
TO CONSTRUCT A FIGURE, WHICH BEING PLACED UPON A CURVED SURFACE, AND INCLINED IN ANY POSITION, SHALL, WHEN LEFT TO ITSELF, RETURN TO ITS FORMER POSITION.
The feet of the figure rest on a curved pivot, which is sustained by two loaded balls below; for the weight of these balls being much greater than that of the figure, their effect is to bring the center of gravity of the whole beneath the point on which it rests; consequently the equilibrium will resist any slight force to disturb it.
TO MAKE A CARRIAGE RUN IN AN INVERTED POSITION WITHOUT FALLING.
It is pretty well known to most boys, that if a tumbler of water be placed within a broad wooden hoop, the whole may be whirled round without falling, owing to the centrifugal force. On the same principle, if a small carriage be placed on an iron band or rail, it will ascend the curve, become inverted, and descend again, without falling.
TO CAUSE A CYLINDER TO ROLL BY ITS OWN WEIGHT UP-HILL.
Procure a coffee-canister, and loading it with a piece of lead, which may be fixed in with solder, the position of the center of gravity is thus altered. If a cylinder so constructed be placed on an inclined plane, and the loaded part above, it will roll up-hill without assistance.
THE BALANCED STICK.
Procure a piece of wood, about nine inches in length, and about half an inch in thickness, and thrust into its upper end the blades of two pen-knives, on either side one. Place the other end upon the tip of the fore-finger, and it will keep its place without falling.
THE CHINESE MANDARIN.
Construct out of the pith of the elder a little mandarin; then provide a base for it to sit in, like a kettle drum. Into this put some heavy substance, such as half a leaden bullet; fasten the figure to this, and in whatever position it may be placed, it will, when left to itself, immediately return to its upright position.
TO MAKE A SHILLING TURN ON ITS EDGE ON THE POINT OF A NEEDLE.
Take a bottle, with a cork in its neck, and place in it, in a perpendicular position, a middle-sized needle. Fix a shilling into another cork, by cutting a nick in it; and stick into the same cork two small table-forks, opposite each other, with the handles inclining outwards and downwards. If the rim of the shilling be now poised on the point of the needle, it may easily be made to spin round without falling, as the center of gravity is below the center of suspension.
THE DANCING PEA.
If you stick through a pea, or small ball of pith, two pins at right angles and defend the points with pieces of sealing-wax, it may be kept in equilibrio at a short distance from the end of a straight tube, by means of a current of breath from the mouth, which imparts a rotary motion to the pea.
OBLIQUITY OF MOTION.
Cut a piece of pasteboard into a circular shape, and describe on it a spiral line; cut this out with a pen-knife, and then suspend it on a large skewer or pin. If the whole be now placed on a warm stove, or over the flame of a candle or lamp, it will revolve with considerable velocity. The card, after being cut into the spiral, may be made to represent a snake or dragon, and when in motion will produce a very pleasing effect.