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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 at: tended by hurricanes, calms, and great rains.
Obs.-la the atmosphere, heated air is constanty fiting, and colder air rutes in to supply its place and this event is the principal cause of winds : lae air that flows from the poles towards the equator, in conscare of the rotation of the earth, has less morinn than the ato mayphere into which it passes, and occa oas an eastesis current; the air passing from the equator tanar is na: pales having more motion, occasions a westerly carreat: and by these changes, the different parts of the arms phere are mixed together; cold is subdued by brati moist air, from the sea, is mixed with dry air fronda land; and the great mass wielastie tiuid sarinauding the globe, preserved in a state fitted for the purposes of ve getable and animal life.
607. 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 aveiare temperature being in Great Britain 6:29. In the southern hemisphere, the periods vary by si months,
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 teet 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
ancw at 1 miles high; and there is always snow at the equator, at three miles up, on the Andes.
608. Terrestrial heat is occasioned, less by the direct insulated rays of the sun, than by their reflections from all surrouuding objects at the earth's surface; and by the heat generated by the action of the rays on the surface of bodies.
lo 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 asceut of vapour, the pressure of gravity, &c., are continually generating the electrical Guid,
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 etl'ects. Thus, when vapour is formed, or condensed, the bodies in contact with the vapour, become electrical. If, for instance, a place 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 electric.iy. Sul. phur, when melted, becomes strongly electrical during the time of congelation , and the case seeins to be analo yous, with respect to non-conducting substances in gene ral, when they change their forms. As electricity ap. pears to result from the general powers or agencies of inatter, it is obvious, that it must be continually exhibited in nature, and that a number of important phano meda must depend upon its operation. When aqueous vapour in condensed, the clouds formed nro usually more or less electrical, and the earth below. than beids
brought into as opposite state, by iaductios, a discharge takes place whee me clouds approach within a certaia distance, coostAlucing lightnings and the nudulativa of the air. produced by the discharge, is the cause of thunder i which is more or less is ease, and of tonger or storier duration, according to the quantity of air acted upod, 20d the distance of the przee where the report is beard fron the point of the discharge. It may not be uuiute. restieg to give a further illustration of this idea :--elec. trical effects take place ia on seosible tiine; it has beers found, that e discharge, through a circuit of four miles, is iustantaneous, but sound moves at the rate of about 12 miles in a uninute,-"W, supposing the lightening to ! puse through a space of some miles, the explosion will i be first heard froin 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, las! of all, will be beard from the remote extremity i and the different degrees of the agitation of the air, and Jikewise the difference of the distance, will necount for the different inteusities of the sound, and its apparent reverberations and changes.--DAVY,
609. 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.
Mail, by the freezing of the drops after they have begun to fall as rain.
Dew, or hage, is the falling of the vapours of the day, when they part with their heat in the cool of the evening.
610. The form of the clouds is found to be re gular and systematic; and, within these few years, they have been classed into different kiuds, worthy of being understood and remenbered.
a. The Cirous, those of the greatest elevation and least density, parallel, aud beginning with a few threads: these are accompanied or fol lowed by steady high winds. 10. The Cumulus, convex or conical massey, 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 dragonlah :
SHAKSPBAREL c. The Stratus, a widely extended horizontal sheet, often touching the earth or water, and properly a cloud of the night, being in the mornfug converted into the cumulus.
d. The Nimbus, or rain-cloud, a horizontal sheet; above which the cirrus spreads, and the eumulus enters its side and forms beneath ; neither of the former appearing to raiu by themselves.
611. Fiery meteors sometimes appear; and shooting starts 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, atfords matter of investigation.
Respecting meteors, falling stars, and north
ern 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 phosphorie matter,
Obs.--The lights seen in ruins, which oficu' terrify the ignorant, are nothing more than hydrogen gas in a state of combustion, The cause of candies burning blue in cellars, arises, in like manner, from a zotie gas Doubi. Jess, 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 combi. nation of various gases, Of the fall of stones froin the clouds, there is now no doubts and it is rationally eos. eluded, 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,
612. The discovery of hydrogen gas, which "15 16 times lighter than atmospheric air, suge gested 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 hitlerto unattended by corres. ponding uility, owing to the difficulty of steering the machine, Moos, Blanchard made more than ofty voyeges in all parts of Earope : Mons. Garneria has made Dearly as many; and Mr. Sadler, 30.--See Jøragraph 51L