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.