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the great Pacific Ocean, that the inhabitants have no idea of a change of wind, or of a storm.

Obs.--It is remarked, that the cause of all winds begins at the part towards which they flow or follow.

612. The variation of the parallel over which the sun passes vertical, from 23) north to 23} south, necessarily affects the winds at each pole; the heights of mountains, and local causes of heat in particular situations, also generate constant changes in the wind, in some northern and middle latitudes.

In other situations, where mountains aid the local influence of the sun, regular winds are produced which flow one half the year in one direction and one half in another.

These are called monsoons ; they prevail in most parts of India, and their changes are attended by hurricanes, calms, and great rains.

Obs. In the atmosphere, heated air is constantly rising, and colder air rushes in to supply its place; and this event is the principal cause of winds; the air that flows from the poles towards the equator, in consequence of the rotation of the earth, has less motion than the atmosphere into which it passes, and occasions an easterly current; the air passing from the equator towards the poles having more motion, occasions a westerly current; and by these changes, the different parts of the atmosphere are mixed together ; cold is subdued by heat; moist air, from the


is mixed with dry air from the land; and the great mass of elastic fluid surrounding the globe, preserved in a state fitted for the purposes of vegetable and animal life.

613. In the northern hemisphere, January, is every where, the coldest month; and its average temperature in Great Britain is 40°; and July and August are the hottest months; the average temperature being in Great Britain 62'. In the southern hemisphere, the periods vary by six months.

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The average temperature of the tropics is 80; and of the equator 84.

The temperature diminishes also according to the height above the sea ; 800 feet in Great Britain, making a difference of three degrees; and three miles on the Andes, a difference of 54 degrees.

In Great Britain, there would be perpetual snow at 1} miles high; and there is always snow at the equator, at three miles up, on the Andes.

614. Terrestrial heat is occasioned, less by the 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 fluide

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 vapor, 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 analogous, with respect to nonconducting 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 duraration, 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 takes 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 12 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.

615. Rain, snow, and hail, are formed in the clouds,' I by any sudden change in the atmosphere.

Snow, by the cloud becoming frozen before its particles have collapsed into water.

Hail, by 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.

616. 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 Circus, 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 dragorish;
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 wbich is now a borse, even with a thought,
The rack dislimns, and makes it indistinct,

As water is in water. 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 circus spreads, and the cumulus enters its side and forms beneath ; neither of the former appearing to rain by themselves.

617. 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'-thewisp, 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 pheno


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mena ; 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, 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.

618. 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 was less than the weight 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. vions. Blanchard made more than fifty voyages in all parts of Europe : Mons. Garnerin has made nearly as many; and Mr. Sadler, 30.-See paragraph 521.

XXVIII. Acoustics and Music. 619. 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.

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