MECHANICS.

INTRODUCTORY.

Why are certain truths termed physical?

Because they explain the greater part of the phenomena of nature, the term physical being derived from the Greek word signifying nature; an appellation distinguishing them from chemical truths, which regard particular substances, and from vital truths, which have relation only to living bodies.-Arnott. Why is an atom so called?

Because of its origin from a Greek word signifying that which cannot be farther divided; or, an exceedingly minute resisting particle.

Why is the term attraction used?

Because the atoms of which the visible universe is built up, whether separate, or already joined into masses, tend towards all other masses, with force proportioned to their proximity: as, when any body presses or falls towards the great mass of the earth, or when the tides on the earth rise towards the moon. Why is the term repulsion used?

Because, under certain known circumstances, as of heat diffused among the particles, their mutual attraction is countervailed or resisted, and they tend to separate with force proportioned to their proximity : as, when heated water bursts into steam, or when gunpowder explodes.

Why is the term inertia used?

Because it denotes that the atoms, in regard to motion, have about them what may be figuratively called a stubbornness, tending always to keep them in their existing state, whatever it may be; in other words, that bodies neither acquire motion, nor lose motion, nor bend their course in motion, but in exact accordance to some force applied.

This, and the three preceding definitions, are derived from the Synopsis of Dr. Arnott's valuable Elements of Physics, Part I. third edit. 1828; the author pertinently observing, that "a person comprehending fully the import of these four words, atom, attraction, repulsion, inertia, may predict or anticipate correctly, very many of the facts and phenomena which the extended experience of a life can display to

him."

Why are not men sensible of the rapid motion of the earth?

Because all things move at the same rate. Whatever common motions objects may have, it does not interfere with the effect of a force producing any new relative motion among them. All the motions seen on earth are really only slight differences among the common motions: as, in a fleet of sailing ships, the apparent changes of place among them are, in truth, only slight alterations of speed or direction in their individual courses.

Why does a spire or obelisk stand more securely on the earth, than a pillar stands on the bottom of a moving wagon?

Because the motion of the earth is uniform, and not that the earth is more at rest than the wagon. Were the present rotation of our globe to be arrested but for a moment, imperial London, with its thousand spires and turrets, would be swept from its valley towards the eastern ocean, just as loose snow is swept away by a gust of wind.-Arnott.

Why does a ball, let drop from the hand, fall with greater velocity the nearer it approaches the earth?

Because, owing to the inertia of matter, any force continuing to act on a mass which is free to obey it, produces in the mass a quickening or accelerated motion; for, as the motion given in the first instant, continues afterwards without any farther force, merely on account of the inertia, it follows that as much more motion is added during the second instant, and as much again during the third, and so on. A falling body, therefore, under the influence of attraction, is, as it were, a reservoir, receiving every instant fresh velocity and momentum (or quantity of motion). The height of a precipice, or the depth of a well, may be judged of with considerable accuracy, by marking the time required for a body to fall through the space. A body falls four times as far in two seconds as in one, although the velocity, at the end of two seconds, is only doubled.-Arnott.

A body falls by gravity precisely 16 1-16 feet in a second, and the velocity increases according to the squares of the time: viz.

In "(quarter of a second) a body falls "(half a second)

1 second

2 ditto

3 ditto

1 foot.

ditto

4

The power of gravity at two miles distance from the earth, is four times less than at one mile; at three miles, nine times less; and so on. It goes on lessening, but is never destroyed.

Meteoric stones, falling from great heights, bury themselves deep in the earth, by the force of their gradually acquired velocity.

Why are we said to know of nothing which is absolutely at rest?

Because the earth is whirling round its axis, and

round the sun; the sun is moving round his axis, and round the centre of gravity of the solar system; and, doubtless, round some more remote centre in the great universe, carrying all his planets and comets about his path.

One of the grand laws of nature is, that all bodies persevere in their present state, whether of motion or rest, unless disturbed by some foreign power. Motion, therefore, once begun, would be continued for ever, were it to meet with no interruption from external causes, such as the power of gravity, the resistance of the medium, &c.

Dr. Arnott adduces several familiar illustrations of motions and forces. Thus, all falling and pressing bodies exhibit attraction in its simplest form. Repulsion is instanced in explosion, steam, the action of springs, &c. Explosion of gunpowder is repulsion among the particles when assuming the form of air. Steam, by the repulsion among its particles, moves the piston of the steam-engine. All elasticity, as seen in springs, collision, &c. belongs chiefly to repulsion. A spring is often, as it were, a reservoir of force, kept ready charged for a purpose; as when a gun-lock is cocked, a watch wound up, &c.

Why does a billiard ball stop when it strikes directly another ball of equal size, and the second ball proceed with the whole velocity which the first had?

Because the action which in parts the new motion is equal to the reaction which destroys the old. Although the transference of motion, in such a case, seems to be instantaneous, the change is really progressive, and is as follows: The approaching ball, at a certain point of time, has just given half of its motion to the other equal ball; and if both were of soft clay, they would then proceed together with half the original velocity; but, as they are elastic, the touching parts at the moment supposed, are compressed like a spring between the balls; and by their expanding,

and exerting force equally both ways, they double the velocity of the foremost ball, and destroy altogether the motion in the other.

Why is the uniformity of motion essential to rational conjecture or anticipation as to future events?

Because, it is by assuming, for instance, that the earth will continue to turn uniformly on its axis, that we speak of to-morrow and of next week, &c. and that we make all arrangements for future emergencies: and were the coming day or season, or year, to arrive sooner or later than such anticipation, it would throw such confusion into all our affairs that the world would soon be desolate.

To calculate futurities, then, (observes Dr. Arnott) or, to speak of past events, is merely to take some great uniform motion as a standard with which to compare all others; and then to say of the remote event, that it coincided, or will coincide, with some described state of the standard motion. The most obvious and best standards are the whirling of the earth about its axis, and its great revolution round the sun. The first is rendered very sensible to man by his alternately seeing and not seeing the sun, and it is called a day; the second is marked by the succession of the seasons, and it is called a year. The earth turns upon its axis about 365 times while it is performing one circuit round the sun, and thus it divides the year into so many smaller parts; and the day is divided into smaller parts, by the progress of the earth's whirling being so distinctly marked, in the constantlyvarying directions of the sun, as viewed from any given spot on the face of the earth. When advancing civilisation made it of importance to man to be able to ascertain with precision the very instant of the earth's revolution, connected with any event, various contrivances were introduced for the purpose. Such have been sun-dials, where the shadow travels pro