direct insulated rays of the sun, than by their reflections from all surrounding objects at the earth's surface; and by the heat generated by the action of the rays on the surface of bodies.

In some other respects, the earth has been compared to a vast electrical machine; and the action of the sun's rays, the winds, water, the ascent of vapour, the pressure of gravity, &c., are continually generating the electrical fluid.

The air being a non-conductor, the clouds become variously electrified; and, from various causes, discharge their electricity either between each other, or to the earth; producing shafts of lightning, accompanied by explosions and the echoes of explosions, called thunder.

Obs. Certain changes in the forms of substances, are always connected with electrical effects. Thus, when vapour is formed or condensed, the bodies in contact with the vapour, become electrical. If for instance, a plate of metal, strongly heated, be placed upon an electrometer, and a drop of water be poured upon the plate, at the moment the water rises in vapour, the gold leaves of the electrometer diverge with negative electricity. Sulphur, when melted, becomes strongly electrical during the time of congelation; and the case seems to be analagous, with respect to non-conducting substances in general, when they change their forms. As electricity appears to result from the general powers or agencies of matter, it is obvious, that it must be continually exhibited in nature, and that a number of important phenomena must depend upon its operation. When aqueous vapour is condensed, the clouds formed are usually more or less electrical; and the earth below them being brought into an opposite state by induction, a discharge takes place when the clouds approach within a certain distance, constituting lightning; and the undulation of the air, produced by the discharge, is the cause of thunder; which is more or less

intense, and of longer or shorter duration, according to the quantity of air acted upon, and the distance of the place where the report is heard from the point of the discharge. It may not be uninteresting to give a further illustration of this idea:-electrical effects take place in no sensible time; it has been found, that a discharge, through a circuit of four miles, is instantaneous;" but sound moves at the rate of about twelve miles in a minute.-Now, supposing the lightning to pass through a space of some miles, the explosion will be first heard from the point of the air agitated, nearest to the spectator; it will gradually come from the more distant parts of the course of the electricity; and, last of all, will be heard from the remote extremity; and the different degrees of the agitation of the air, and likewise the difference of the distance, will account for the different intensities of the sound, and its apparent reverberations and changes.-DAVY.

599. Rain, snow, and hail, are formed in the clouds, by any sudden change in the atmosphere. Snow, by the cloud becoming frozen before its particles have collapsed into water.

Hail, by the freezing of the drops after they have begun to fall as rain.

Dew, or haze, is the falling of the vapours of the day, when they part with their heat in the cool of the evening.

600. The form of the clouds is found to be regular and systematic; and, within these few years, they have been classed into different kinds, worthy of being understood and remembered.

a. The Cirrus, those of the greatest elevation and least density, parallel, and beginning with a few threads: these are accompanied or followed by steady high winds

b. The Cumulus, convex or conical masses, of dense structure, formed in the lower atmosphere

and the cloud of the day, but increasing about sun-set: these threaten thunder.

Sometime, we see a cloud that's dragonish ;
A vapour, sometime, like a bear or lion;
A tower'd citadel; a pendent rock;
A forked mountain; a blue promontory,
With trees upon 't that nod unto the world,
And mock our eyes with air.-

That which is now a horse, even with a thought,
The rack dislimns, and makes it indistinct,

As water is in water.

SHAKSPEARE.

c. The Stratus, a widely extended horizontal sheet, often touching the earth or water, and properly a cloud of the night, being in the morning converted into the cumulus.

d. The Nimbus, or rain-cloud, a horizontal sheet; above which the cirrus spreads, and the cumulus enters its side and forms beneath; neither of the former appearing to rain by themselves. 601. Fiery meteors sometimes appear: and shooting stars are very frequent. Stones, also, have been often known to fall to the earth.

Northern lights, or aurora borealis, are frequently very interesting; and the ignis fatuus, or will-o-the-wisp, affords matter of investigation. Respecting meteors, falling stars, and northern lights, nothing certain is known of their origin or

cause.

Shooting stars are supposed to be electrical phenomena; and the ignis fatuus is ascribed to hydrogen gas set on fire by phosphoric matter.

Obs. The lights seen in ruins, which often terrify the ignorant, are nothing more than hydrogen gas in a state of combustion. The cause of candles burning blue in cellars, arises, in like manner, from azotic gas. Doubtless, also, the noises and explosions which take place on

opening rooms long closed, or in which fruit has been suffered to decay, arise from the combustion and combination of various gases. Of the fall of stones from the clouds, there is now no doubt; and it is rationally concluded, that they arise from the explosion of meteors, and the co-mixture of gases; but in what way these are generated, must long remain a question.

602. The discovery of hydrogen gas, which is 15 times lighter than atmospheric air, suggested the plan of filling a silken balloon with it: and of its ascent in air, with an aeronaut appended to it, provided the whole did not exceed the weight of an equal bulk of atmospheric air.

Accordingly, balloons have been filled with hydrogen gas, created by mixing iron-filings, water, and sulphuric acid; which have carried through the atmosphere two, three, and four persons at a time.

Obs.-This is one of the most splendid discoveries of modern philosophy; but hitherto unattended by corresponding utility, owing to the difficulty of steering the machine. Mons. Blanchard made more than fifty voyages in all parts of Europe: Mons. Garnerin has made nearly as many; and Mr. Sadler 30.—See paragraph 504:

XXVII. Acoustics and Music.

603. Sound is an effect of vibration, and is produced by diverging waves of the air. This is evident, from the vibration of stringed instruments; and from the effect on water in musical glasses.

Sound, like heat, appears to depend on the reflection of the surrounding bodies, and also on the density of the air.

Aeronauts can scarcely hear each other speak, when high in the atmosphere; and the discharge of a pistol from an air-balloon, produces scarcely any report, for want of reflecting bodies.

Obs. That bodies move, or tremble, when they produce sound, is evident in drums, bells, and other instruments, whose vibrations are distinctly perceptible; and it is equally clear, that a similar vibration is excited in the air; because bells, glasses, basins, and musical strings, will sound, merely by the action propagated from other sounding bodies, and will not sound in a vacuum.

604. The vibrations that produce sound, have been aptly compared to the circles produced by throwing a stone into the water; but judging by their effect on the water in a musical glass, the undulations are more pungent and decided.

A bell rung under water, gives the same tone as in air; and water is known in other respects to be a conductor of sound. Wood and the earth appear, also, to be conductors of sound.

605. Sounds, or their undulations, are found to travel at the rate of 1142 feet in a second, or about thirteen miles in a minute.

Hence, as any corresponding light is comparatively instantaneous in its progress, the distance of the report of thunder, or of a piece of cannon, may be exactly calculated.

Sounds also are reflected like light; and hence we have echoes, which are like plain mirrors, and whispering galleries, and repeating caves, like so many concave mirrors.

Obs.—Every building standing alone, is an echo, when addressed at a proper distance; but, if there are trees or other objects to the right or left, the various echoes destroy each other.