of a cannon-ball for millions of ages, to alter the position of the earth by the small part of an inch. This feat of Archimedes is, in mathematical truth, performed by every man who leaps from the ground, for he kicks the world away when he rises, and attracts it again when he falls back.-Arnott.

Why is a finger caught near the hinge of a shutting door so much injured?

Because the centre of action of the door moves through a space comparatively great, and acts with a great lever-advantage on a resistance placed near the fulcrum of the lever where there is little motion. Children pinching their fingers in this way, or in the hinge of the fire-tongs, where there is a similar action, wonder why the bite is so keen.

Why have pincers or forceps such extraordinary power?

Because they are double levers, of which the hinge is the common prop or fulcrum. Dr. Arnott thus illustrates the advantages of this machine:-In drawing a nail with steel nippers, we have a good example of the advantages of using a tool; 1. The nail is seized by teeth of steel, instead of by the soft fingers; 2. Instead of the griping force of the extreme fingers only, there is the force of the whole hand conveyed through the handles of the nippers; 3. The force is rendered, perhaps, six times more effective by the lever length of the handles; and, 4. By making the nippers, in drawing the nail, rest on one shoulder as a fulcrum, it acquires all the advantages of the lever or claw-hammer for the same purpose.

Why do lofty sails often cause open boats to upset?

Because the mast and sails set upon it are as a long lever, having the sails as the power, turning upon the centre of buoyancy of the vessel as the fulcrum, and lifting the balance or centre of gravity as the resist

ance.

PART VII.

D

1

Why may a boy, who cannot exert a direct force of 50lbs., by means of a claw-hammer, extract a nail, to which half a ton might be suspended?

Because his hand, perhaps, moves through eight inches, to make the nail rise one quarter of an inch. The claw-hammer also proves, that it is of no consequence whether the lever be straight or crooked, provided it produces the required difference of velocity between power and resistance. The part of the hammer resting on the plank, is the fulcrum, or prop.

Why does a combination of levers produce such extraordinary power?

Because if a lever, which makes one balance four, be applied to work a second lever which does the same, one pound at the long arm of the first, will balance sixteen pounds at the short arm of the second lever, and would balance sixty-four at the short arm of a third such, &c.-Arnott.

WHEEL AND AXLE.

Why is motion transmitted through a train of wheelwork by the formation of teeth upon the circumference of the wheels?

Because the indentures of each wheel fall between the corresponding ones of that in which it works, and ensure the action so long as the strain is not so great as to fracture the tooth.

Why is a heavy wheel sometimes used as a concentrator of force, or a mechanic power?

Because, by means of a winch, or a weight, or otherwise, motion or momentum is gradually accumulated in the wheel, and is then made to expend itself in producing some sudden and proportionally great effect.

The coining-presses of the Royal Mint are thus impelled by a fly-wheel, and generally complete a coin by one blow; and they strike, upon an average, 60 blows in one minute; the blank piece, previously pre

pared and annealed, being placed between the dies by part of the same mechanism. The number of pieces which may be struck by a single die of good steel, properly hardened and tempered, not unfrequently amounts, at the Mint, to between 3 and 400,000: There are eight presses frequently at work for ten hours each day, and each press produces 3,600 pieces per hour; but, making allowances for occasional stoppages, we may reckon the daily produce of each press at 30,000 pieces; the eight presses, therefore, will furnish a diurnal average of 240,000 pieces.

Why is it an error to account the fly-wheel a positive power?

Because, in common cases, it merely equalizes the effect of an irregular force. Thus, in using a winch to turn a mill, a man does not act with equal force all round the circle; but a heavy wheel, fixed on the axis, resists acceleration and receives momentum, while his action is above par, and returns it again while his action is below par, thus equalizing the movement. Again, in circular motion produced by a crank, when by the pressure of the foot on a treadle, we turn a lathe, or grindstone, or spinning-wheel, the force is only applied during a small part of the revolution, or in the form of interrupted pushes, yet the motion goes on steadily, because the turning grindstone, or wheel, or lathe, merely becomes a fly and reservoir, equalizing the effect of the force.

Why is the common winch in principle a wheel?

Because the hand of the worker describes a circle, and there is no difference in the result, whether an entire wheel be turning with the hand, or only a single spoke of the wheel.

Why is a man on a treadmill compelled to keep perpetually moving?

Because, being placed at the circumference of the wheel, his weight turns it, and he must move forward

as fast as the wheel descends, so as to maintain his position continually at the extremity of the horizontal diameter of the wheel.

The invention of the treadmill is, by some persons, said to have been derived from a squirrel in a cylindrical wire cage.

WHEEL CARRIAGES.

Why have wheel carriages been advantageously substituted for sledges?

Because the rubbing or friction, instead of being between an iron shoe and the stones and irregularities of the road, is between the axle and its bush, which have surfaces smoothed and fitted to each other, and well lubricated.

Why does the wheel aid the progress of a carriage?

Because, while the carriage moves forwards, perhaps 15 feet, by one revolution of its wheel, the rubing part, viz. the axle, only passes over a few inches of the internal surface of its smooth greased bush. Again, the wheel surmounts any abrupt obstacle on the road, by the axle describing a gently rising slope or curve; and by rising as on an inclined plane, and giving to the drawing animal the relief which such a plane would bring.—Arnott.

Why are wheels usually made of a dished form, that is, inclining outwards?

Because they thus acquire astonishing strength, indeed that of the arch, as contrasted with the flat or upright wheel; the dished form is farther useful in this, that when the carriage is on an inclined road, and more of the weight consequently falls upon the wheel of the lower side, the inferior spokes of that wheel become nearly perpendicular, and therefore support the increased weight more safely. The disadvantage of these wheels, however, is, that an inclining wheel naturally describing a curved path, the

horses, in drawing straight forward, have to overcome this deviating tendency in all the wheels.-Arnott.

Why are axles made of steel, and the parts on which they bear of brass?

Because friction is universally diminished by letting the substances which are to rub each other be of different kinds. The swiftness of a skaiter, it may be observed, depends much on the dissimilarity between ice and steel.

Why are the fore-wheels of carriages smaller than the hind-wheels?

Because they facilitate the turning of the carriage. The advantage of the wheel is proportioned to the magnitude; the smaller wheel having to rise a steeper curve. It is not true, however, according to the popular prejudice, that the large hind-wheels of coaches and waggons help to push on the little wheels before them.-Arnott.

From these causes, continues the same ingenious writer, "the difference in performing the same journey of a mile by a sledge and a wheel carriage, is, that while the former rubs over every roughness in the road, and is jolted by every irregularity, the rubbing part of the latter, the axle, glides very slowly over about thirty yards of a smoothed oiled surface, in a gently waving line. It is ascertained that the resistance is thus reduced to 1-100th of what it is for a sledge."

Why do springs not only render carriages easy vehicles on rough roads, but much lessen the pull to the horses?

Because, where there is no spring, the whole load must rise with every rising of the road, and must sink with every depression, and the depression costs as much as the rising, because the wheel must be drawn up again from the bottom of it; but in a spring carriage, moving rapidly along, only the parts below the