METEORITES.

Y The "shooting stars" common on fine winter nights, or during the prevalence of dry easterly winds in summer time, have very much the appearance of real stars; darting in straight lines obliquely downward, but sometimes horizontally, and leaving no train. Those of warm summer evenings are, some of them, very brilliant, but have curvilinear motions. A third kind, of a bluish white colour, (generally seen in the intervals of showery weather, and often preceding high winds,) after being extinguished in their course, leave it marked, for some seconds, by trains which widen and become dissipated. Various hypotheses have been offered to explain these, some referring them to combustion in consequence of chemical action, and some to electricity.

But meteoric stones, or aerolites, have frequently been found to have descended "from the bosoms of large luminous meteors," in various regions of the globe; and from strict chemical research on stones having so fallen, at various times, in England, Italy, Germany, the East Indies, and other places, it has been ascertained that they are all compounded of the same chemical principles, and nearly in the same proportions! "At whatever period, or in whatever part of the world, they may have fallen, these stones have appeared, as far as they have been examined, to consist of the same substances, and to have nothing similar to them, not only among the minerals in the neighbourhood of the places where they were found, but among all that have hitherto been discovered in our earth."-Dr. Ure, Chem. Dict. p. 618. (The pupil may see some large and very interesting fragments from some of these bodies, at the end of the saloon appropriated to geological specimens, in the British Museum.)

Z

Minute bodies may be revolving,* like the new planets,

It has even been given as the opinion of some philosophers, that these falling meteoric bodies may be the minuter fragments of that very planet, once revolving as a whole, but now burst into parts, the largest of the parts of which are circling as the Asteroids; and that the smaller fragments are thus occasionally precipitated upon Mars, or Jupiter, or the Earth, according as, in wandering through space, those fragments may come within the spheres of the several attractions of these planets.

(see L, p. 270,) which are so small that their masses may be inappreciable, and there may be many still smaller. Nor is this an unwarrantable presumption; many such do come within the sphere of the earth's attraction, are ignited by the velocity with which they pass through the atmosphere, and are precipitated with great violence on the earth. The fall of meteoric stones is much more frequent than is generally believed; hardly a year passes without some instances occurring; and if it be considered that only a small part of the earth is inhabited, it may be presumed that numbers fall into the ocean, or on the uninhabited land, unseen by man. They are sometimes of great magnitude; the volume of several has exceeded that of the planet Ceres. One which passed within 25 miles of us, was estimated. to weigh about 600,000 tons, and to move with a velocity of about 20 miles per second- —a fragment of it alone reached the earth.”—Mrs. Somerville.

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zz Others have conjectured that these meteoric bodies have their origin in the volcanoes, which are thought to have been perceived, on the surface of the moon. It is calculated that, if the eruptive force of such a volcano were equal to projecting a body from its crater at the rate of nearly 11,000 feet per second, such a body would, necessarily, be carried beyond the attraction of the moon's mass; and instead of falling back again upon her, would come within the earth's control; and although perhaps retain ing, by inertia, the orbitual direction of the moon, (3, p. 212,) and therefore continuing for some time to circle about us as a satellite, might ultimately be disturbed by the attraction of the sun, fall down into our atmosphere, and thus arrive in a fused or heated state, at the earth's surface.

PROBLEMS, ETC.

SECTION III.

RELATING TO THE TRADE WINDS, THE MOON'S NODES, ECLIPSES, THE TIDES, THE PRECESSION OF THE EQUINOXES, ETC.

THE WINDS-TRADE WINDS, ETC.

B The heat of the sun, in producing partial expansion and buoyancy in the atmosphere, is the originating cause of the currents which we call winds.

The velocity and, consequently, the momentum or force of the winds, vary between limits considerably extended.

A gentle pleasant wind varies from

A brisk gale

A storm

A violent tempest

A hurricane (destructive of buildings and gigantic trees)

**The force of wind within the limits given above, as estimated by the pressure it exerts against a square foot of surface placed perpendicularly to its course, varies from an ounce to nearly 50 pounds.

If our globe were to cease rotating, and one spot only were exposed to the vertical sun, the air would be constantly rising from that spot like the smoke from the chimney of a furnace, and the atmosphere from all parts around would be, as constantly, in motion towards that spot, to supply its deficiency.

As the earth is turning round, and there is thus a belt of such heated parts, and a belt of atmosphere ascending from them;-only with less activity than in the case just supposed, because the sun's heat is not, as in that case, concentrated;-towards this heated belt, currents set in from the hemispheres north and south of it, to pour an incessant supply of cooler and heavier air.

D But, as the calm atmosphere of any latitude has the actual motion only of that latitude, no part of any one of these supplying currents has, for some time, so much westward motion as the lower latitudes of the rotating surface over which it has to pass; it therefore lags on the surface, and appears to the inhabitants of the latitude it may be crossing, to have lost all its fellow tendency from the west, as they outstrip it and are thus carried against it towards the east.

The very unequal distribution of land on the surface of our globethe various nature of the land surface itself, as more or less adapted to the reception and communication of heat, (R and S, p. 173,)—and other local circumstances, interfere much, at all seasons, with the regularity and direction of winds, even in the Torrid Zone. In our own country, and those similarly circumstanced in the Temperate Zones, the winds are proverbially inconstant; although from the same cause of differing velocity in the latitudes north and south of us, from which they come, our N. wind is a N.E. one, and our S. wind a S.W. one; and these winds blow in England 300 days in the year.

F In the expanse of the vast Pacific and the Atlantic, at considerable distances from land, the winds arising from the influence of the vertical sun prevail almost uninterruptedly as the well-known N.E. and S. E. Trade-Winds.

VELOCITIES OF DIFFERENT LATITUDES.

291

PROBLEM A.

TERRESTRIAL GLOBE.

To find the result of the transference of a portion of the Atmosphere into a higher or lower Latitude.

Admonitory questions.

a What is the meaning of Inertia? D, p. 207. What is the meaning of Velocity? F, p. 207.

c If the Earth were to leave off rotating for an instant, would the sea and the atmosphere immediately stop their eastward course? 6 and 7 of page 212.

d Of places in lat. 60 and lat. 41, which have the greater actual velocity (although affected with the same angular motion); and in what ratio do their velocities differ? M, on p. 226.

e What effect has a heated body when applied to the lower portions of a fluid? V, and 4 of V, , p. 174.

f Is the transparent atmosphere affected with currents by the sunbeams as they pass through it to the earth's surface? Y, p. 175.

g With what motion, as well as vertical motion, is the atmosphere affected whilst ascending from the Earth, at the Equator (or in any latitude,) in consequence of heat? 3, 4, and 5, of p. 212.

RULE. (When the difference of velocity is required.) Find (by Rule to Prob. XVIII., p. 101,) the velocities per minute of the two places proposed :—the difference between these two velocities will be the excess or deficiency of eastward velocity in the atmosphere, supposed to be suddenly transferred. If the difference be an excess, the wind will be westerly; if it be a deficiency, the wind will be easterly.

1. If the inhabitants "under the line" are becalmed, at what rate per minute is their atmosphere moving by the earth's rotation on its axis?

2. If the atmosphere at Quito, or the north of Celebes,