Of these rocks, called Loggan or Laggan stones, there are several among the picturesque barriers of the British coast.

Dr. S. Hibbert has very recently described a natural rocking-stone of granite, near the village of Loubeyrat, in the province of Auvergne, France. This stone appears to have been an object of religious worship, for, on the top of it were two figures, a cross, and a pedestal. Under the figures the word pardon is traceable, and other letters which probably alluded to the number of days of pardon which the cross gave to the venerator. The natural phenomenon of the rocking-stone probably became an object of superstitious veneration to its neighbourhood, and the figures and cross were the adoring tributes of the natives. Dr. Hibbert, however, thinks that the particular use to which rockingstones were applied will ever remain in obscurity: "as they are products of every country where loose detached rocks of a particular structure have been submitted to the operation of atmospheric agents, it is to be expected that the fables assigned to their origin would be regulated by the peculiar mythology of the people among whom they have become the object of notice and wonder."

Why have all shot manufactories lofty towers, as seen on the southern bank of the Thames?

Because, in the manufacturing of shot, the liquid metal is allowed to fall like rain from a great elevation, as through these towers, and the cohesive principle gives rotundity to grains of shot. In its descent, the drops become truly globular, and before they reach the end of their fall they are hardened by cooling, so that they retain their shape.

Why does a porter lean forward when carrying a load?

Because his position must be regulated by the centre of gravity of his body and the load taken together. If he bore the load on his back, the line of direc

tion would pass beyond his heels, and he would fall backwards. To bring the centre of gravity over his feet he accordingly leans forward.-If a nurse carry a child in her arms, she leans back for a like reason.

Why does a young quadruped walk much sooner than a child?

Because a body is tottering in proportion to its great altitude and narrow base. Now, the child has this latter, and learns to walk but slowly, because of the difficulty, perhaps in ten or twelve months, while the young of quadrupeds, having a broad supporting base, are able to stand, and even to move about almost immediately: but it is the noble prerogative of man to be able to support his towering figure with great firmness, on a very narrow base, and under constant change of attitude.-Arnott.

Why are the "safety coaches" built with the wheels far apart, and the luggage-receptacles beneath the body? Because they may have a broader base, and thus be less liable to overturn.

Why do builders use the plummet, or plumb line? Because, when applied to a body, it is a visible indication of the line of its centre of gravity.

Why do certain structures remain secure, although they have lost their perpendicularity?

Because the line of their centre of gravity remains within the base. The famous tower of Pisa was built intentionally inclining, to frighten and surprise; it is 130 feet high, and overhangs its base 16 feet. At Bologna are two celebrated leaning towers, one of which, the Asinelli, is 350 feet high, and 33 feet out of the perpendicular. The other, the Garisenda, is about 130 feet in height, and inclines 8 feet from the perpendicular. Montfaucon, the celebrated antiquary, attributes the leaning of these towers to the sinking of the earth. He says, it appears, upon examination, that when the Garisenda tower bowed, a great part of

it went to ruin, because the ground that the inclined side stood on was not so firm as the other, which may be said of all other towers that lean so; for "besides these two here mentioned, the tower for the bells of St. Mary Zobenica, at Venice, leans considerably to one side. So also at Ravenna, I took notice of another stooping tower, occasioned by the ground on that side giving way a little. In the way from Ferrara to Venice, where the soil is marshy, we see a structure of great antiquity leaning to one side. When the whole structure of the Garisenda stooped, much of it fell, as appears by the top."

The Monument, near London Bridge, inclines so much that timid people sometimes doubt its stability, and some years since its fall was a point of discussion. Salisbury and other of our cathedral spires or towers have lost something of their perpendicularity; Chesterfield, in Derbyshire, is proverbial for its zig-zag or wry spire.

The Monument is of the Doric order, and rises from the pavement to the height of 202 feet, containing within its shaft a spiral stair of black marble of 345 steps; the plinth is 21 feet square. It was begun in 1671, but was not completed till 1677; stone being scarce, and the restoration of London and its cathedral swallowing up the produce of the quarries. Mr. Elmes, in his Life of Sir Christopher Wren, the architect, tells us that the Monument was "at first used by the members of the Royal Society for astronomical experiments, but was abandoned on account of its vibrations being too great for the nicety required in their observations. This occasioned a report that it was unsafe; but its scientific construction may bid defiance to the attacks of all but earthquakes for centuries." The more recent fear of its instability was therefore only a revival of this alarm; which probably obtained some credence among weak persons, from its being erroneously attributed to Fellows of the Royal Society:

Why is it physically advantageous to turn out the toes?

Because the supporting base of a man consists of the feet and the space between them; and turning out the toes, without taking much from the length of the base, adds a good deal to the breadth.-Arnott.

Why do very fat people usually throw back their heaa and shoulders?

Because, by so doing, they keep the centre of gravity of the body over the base.

SIMPLE MACHINES.

Why have the "simple machines," as the lever, wheel and axle, plane, wedge, screw, and pulley, been long called the "mechanic powers?"

Because they were first used to raise great weights, or overcome great resistances. Hence the common error in supposing that they generate force, or have a sort of innate power for saving labour; whereas, neither simple machines nor mechanic powers save labour, in a strict sense of the phrase.

Why, then, are these machines advantageous?

Because they allow a small force to take its time to produce any requisite magnitude of effect. Thus, one man's effort, or any small power, which is always at command, by working proportionally longer, will answer the purpose of the sudden effort of many men, even of hundreds or thousands, whom it might be most inconvenient and expensive, or even impossible to bring together.

Why are there so many vain schemes for perpetual motions, and new mechanical engines of power?

Because the projectors do not understand the great truth, that no form or combination of machinery ever did or ever can increase, in the slighest degree, the quantity of power applied. Hence the futility of supposing that a lever, or great pendulum, or spring, or

heavy fly-wheel, &c. can ever exert more force than has passed into it from some source of motion.-Arnott.

THE LEVER.

Why is a beam or rod of any kind, resting at one part on a prop or axis, which becomes its centre of motion, a lever?

Because such a contrivance was first employed to lift (levare, Latin) weights.

The lever, in mechanics, compensates power by space, and what is lost in power is gained in time. If the lever be 17 feet long, and the pivot or fulcrum be a foot from one end, an ounce placed on the other end will balance a pound placed on the near end. If, instead of an ounce, we place upon the long end the short end of a second beam or lever, supported by a fulcrum one foot from it, and then place the long end of this second lever upon the short end of a third lever, whose fulcrum is one foot from it; and if we put upon the end of this third lever's long arm an ounce weight, that ounce will move upwards a pound on the second lever's long arm; and this moving upwards, will cause the short end to force downwards 16 pounds at the long end of the first lever, which will make the short end of the first lever move upwards, although 256 pounds be laid upon it. The same effect continuing, a pound on the long end of the third lever, will move up a ton and three-quarters at the short end of the first lever, so that the touch of a child's finger, will move as much as two horses can draw.-Notes in Science.

Why did Archimedes reasonably enough say, "Give me a lever long enough, and a prop strong enough, and with my own weight I will move the world?"

Because there is no limit to the difference of intensity in forces, which may be placed in opposition to each other by the lever, except the length and strength of the material of which the levers must be formed. But he would have required to move with the velocity