with asbestos saturated with sulphuric acid. Berzelius gives the following as the best composition for the matches: 30 parts powdered chlorate of potassa, 10 of powdered sulphur, 8 of sugar, 5 of gum arabic, and a little cinnabar. The sugar, gum, and salt, are first rubbed together into a paste with water; the sulphur is then added, and the whole being thoroughly beaten together, small brimstone matches are dipped in, so as to retain a thin coat of the mixture upon their sulphuretted points. A similar compound is employed in percussion gun-locks: gunpowder, made into a paste with water and chlorate of potash, is dropped in small copper caps adapted to the tubular touch-hole of the gun, when a blow inflames the powder, and communicates to that in the barrel. Fulminating mercury is, however, now substituted for this composition, which is found to rust the touch-hole.

Note to "STEAM," page 113.

In the whole range of English literature, perhaps there is nothing more curious than the following prophecy respecting Steam, in Dr. Darwin's Botanic Garden, published in 1789, but written, it is well known, at least twenty years before the date of its publication:

Soon shall thy arm, unconquer'd steam, afar
Drag the slow barge, or drive the rapid car;
Or on wide waving wings expanded bear
The flying chariot through the fields of air.
Fair crews triumphant leaning from above

Shall wave their fluttering 'kerchiefs as they move;
Or warrior bands alarm the gaping crowd,
And armies shrink beneath the shadowy cloud :
So mighty Hercules o'er many a clime

Waved his huge mace in virtue's cause sublime;
Unmeasured strength with early art combined,
Awed, served, protected, and amazed mankind.

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KNOWLEDGE FOR THE PEOPLE:

OR THE

PLAIN WHY AND BECAUSE.

· PART VII.—MECHANICS.

LONDON:

NOTH

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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 P

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 waggon ?

Because the motion of the earth is uniform, and not that the earth is more at rest than the waggon. 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.