tion wholly ceases. The dark lines in the cut show the true, and the dotted the apparent positions.

80

In the cut, the depth of the atmosphere, as compared with the globe, is greatly exaggerated. Even allowing it to be 50 miles deep, it is only 1 th of the semi-diameter of the globe, which is equal to only about th of an inch upon a common 13-inch globe. But it was necesary to exaggerate, in order to illustrate the principle.

2

647. The amount of displacement of objects in the horizon, by atmospherical refraction, is about 33', or a little more than the greatest apparent diameter of either the Sun or Moon. It follows, therefore, that when we see the lower edge of either apparently resting on the horizon, its whole disc is in reality below it; and would be entirely concealed by the convexity of the Earth, were it not for refraction.

648. Another effect of refraction is, that the Sun seems to rise about three minutes earlier, and to set about three minutes later, on account of atmospherical refraction, than it otherwise would ; thus adding about six minutes, on an average, to the length of each day.

The atmosphere is said to be so dense about the North Pole as to bring the Sun above the horizon some days before he should appear, according to calculation. In 1596, some Dutch navigators, who wintered at Nova Zembla, in latitude 76°, found that the Sun began to be visible 17 days before it should have appeared by calculation; and Kepler computes that the atmospheric refraction must have amounted to 5o, or 10 times as much as with us.

649. The twilight of morning and evening is produced partly by refraction, but mainly by reflection. In the morning, when the Sun arrives within 18° of the horizon, his rays pass over our heads into the higher region of the atmosphere, and are thence reflected down to the Earth. The day is then said to be dawn, and the light gradually increases till sunrise. In the evening, this process is reversed, and the twilight lingers till the Sun is 18° below the horizon. There is thus more than an hour of twilight both morning and evening.

In the arctic regions, the Sun is never more than 18° below the horizon; so that the twilight continues during the whole night.

650. In making astronomical observations, for the purposes of navigation, &c., allowance has to be made for refraction, according to the altitude of the object, and the state of the atmosphere. For this purpose tables are constructed, showing the amount of refraction or every degree of altitude, from the horizon to the zenith.

647. Amount of displacement of celestial objects by refraction? What follows? 648. Influence of refraction on length of days? How about the North Pole ? 649. Cause

of troilight?

650. What allowance for refraction? Tables?

CHAPTER XVII.

AURORA BOREALIS AND PARALLAX.

651. THE sublime and beautiful phenomena presented by the Aurora Borealis, or northern lights, as they are called, have been in all ages a source of admiration and wonder alike to the peasant and the philosopher. In the regions of the north (and indeed in many other places) they are regarded by the ignorant with superstitious dread, as harbingers of evil; while all agree in placing them among the unexplained wonders of nature.

These lights, or meteoric coruscations, are more brilliant in the arctic regions, appearing mostly in the winter season and in frosty weather. They commonly appear at twilight near the horizon, and sometimes continue in that state for, several hours without any sensible motion; after which they send forth streams of stronger light, shooting with great velocity up to the zenith, emulating, not unfrequently, the lightning in vividness, and the rainbow in coloring; and again, silently rising in a compact majestic arch of steady white light, apparently durable and immovable, and yet so evanescent, that while the beholder looks upon it, it is gone.

At other times they cover the whole hemisphere with their flickering and fantastic coruscations. On these occasions their motions are amazingly quick, and they astonish the spectator with rapid changes of form. They break out in places where none were seen before, skimming briskly along the heavens; then they are suddenly extinguished, leaving behind an uniform dusky track, which, again, is brilliantly illuminated in the same manner, and as suddenly left a dull blank. Some nights they assume the appearance of vast columns; exhibiting on one side tints of the deepest yellow, and on the other, melting away until they become undistinguishable from the surrounding sky. They have generally a strong tremulous motion from end to end, which continues till the whole vanishes.

652. Maupertius relates, that in Lapland, "the sky was sometimes tinged with so deep a red that the constellation Orion looked as though it were dipped in blood, and that the people fancied they saw armies engaged, fiery chariots, and a thousand prodigies." Gmelin relates, that, "in Siberia, on the confines of the icy sea, the spectral forms appear like rushing armies; and that the hissing, crackling noises of those aerial fireworks so terrify the dogs and the hunters, that they fall prostrate on the ground, and will not move while the raging host is passing."

Kerguelen describes "the night between Iceland and the Ferro Islands, as brilliant as the day”—the heavens being on fire with flames of red and white light, changing to columns and arches, and at length confounded in a brilliant chaos of cones, pyramids, radii, sheaves, arrows, and globes of fire.

653. But the evidence of Captain Parry is of more value

651. What said of the Aurora Borealis? How regarded by the ignorant? Where most brilliant? In what weather? Describe? 652. Observations of Maupertius, Gmelin, and Kerguelen? 658. Observations of Capt. Parry?

than that of the earlier travelers, as he examined the phenomena under the most favorable circumstances, during a period of twenty-seven consecutive months, and because his observations are uninfluenced by imagination. He speaks of the shifting figures, the spires and pyramids, the majestic arches, and the sparkling bands and stars which appeared within the arctic circle, as surpassing his powers of description. They are, indeed, sufficient to enlist the superstitious feelings of any people not fortified by religion and philosophy.

654. The colors of the polar lights are of various tints. The rays or beams are steel grey, yellowish grey, pea green, celandine green, gold yellow, violet blue, purple, sometimes rose red, crimson red, blood red, greenish red, orange red, and lake red. The arches are sometimes nearly black, passing into violet blue, grey, gold yellow, or white bounded by an edge of yellow. The luster of these lights varies in kind as well as intensity. Sometimes it is pearly, sometimes imperfectly vitreous, sometimes metallic. Its degree of intensity varies from a very faint radiance to a light nearly equaling that of the Moon.

655 Many theories have been proposed to account for this wonderful phenomenon, but there seems to be none which is entirely satisfactory. One of the first conjectures on record attributes it to inflammable vapors ascending from the Earth into the polar atmosphere, and there ignited by electricity. Dr. Halley objects to this hypothesis, that the cause is inadequate to produce the effect. He was of opinion that the poles of the Earth were in some way connected with the aurora; that the Earth was hollow, having within it a magnetic sphere, and that the magnetic effluvia, in passing from the north to the south, might become visible in the northern hemisphere.

656. That the aurora borealis is, to some extent, a magnetical phenomenon, is thought, even by others, to be pretty clearly established by the following considerations :

(1.) It has been observed, that when the aurora appears near the northern horizon in the form of an arch, the middle of it is not in the direction of the true north, but in that of the magnetic needle at the place of observation; and that when the arch rises towards the zenith, it constantly crosses the heavens at right angles, not to the true magnetic meridian.

654. What said of the colors, &c., of these polar lights? 655. Is there a satisfactory explanation of these phenomena ? What conjecture? Dr. Halley's objection? His own singular opinion? 656. What evidences that the Aurora Borealis is of magnetic origin?

(2.) When the beams of the aurora shoot up so as to pass the zenith, which is sometimes the case, the point of their convergence is in the direction of the prolongation of the dipping needle at the place of observation.

(3.) It has also been observed, that during the appearance of an active and brilliant aurora, the magnetic needle often becomes restless, varies sometimes several degrees, and does not resume its former position until after several hours.

From these facts, it has been generally inferred that the aurora is in some way connected with the magnetism of the Earth; and that the simultaneous appearance of the meteor, and the disturbance of the needle, are either related as cause and effect, or as the common result of some more general and unknown cause.

657. Dr. Young, in his lectures, is very certain that the phe nomenon in question is intimately connected with electro-magnetism, and ascribes the light of the aurora to the illuminated agency of electricity upon the magnetical substance.

It may be remarked, in support of the electro-magnetic theory, that in magnetism, the agency of electricity is now clearly established, and it can hardly be doubted that the phenomena both of electricity and magnetism are produced by one and the same cause; inasmuch as magnetism may be induced by electricity, and the electric spark has been drawn from the magnet.

658. Sir John Herschel also attributes the appearance of the aurora to the agency of electricity. This wonderful agency, says he, which we see in intense activity in lightning, and in a feebler and more diffused form traversing the upper regions of the atmosphere in the northern lights, is present, probably, in immense abundance in every form of matter which surrounds us, but becomes sensible, only when disturbed by excitements of peculiar kinds.

PARALLAX OF THE HEAVENLY BODIES.

659. Parallax is the difference between the altitude of any celestial object seen from the Earth's surface, and the altitude of the same object seen at the same time from the Earth's center; or it is the angle under which the semi-diameter of the Earth would appear, as seen from the object.

The true place of a celestial body is that point of the heavens in which it would be seen by an eye placed at the center of the Earth. The apparent place is that point of the heavens where the body is seen from the surface of the Earth. The parallax

657. Dr. Young's opinion? What remark in support of his views? 658. Sir John Herschel's opinion? 659. Parallax? True place of a celestial body? Apparent? When parallax greatest? Least? Called what, and why? What objects the greatest parallax?

of a heavenly body is greatest when in the horizon, and is thence called the horizontal parallax. Parallax decreases as the body ascends towards the zenith, at which place it is nothing.

The adjoining cut will afford a sufficient illustration. When the observer, standing upon the Earth at A, views the object at B, it appears to be at C, when, at the same time, if viewed from the center of the Earth, it would appear to be at D. The parallax is the angle BCD or AB E, which is the difference between the altitude of the object B, when seen from the Earth's surface, and when seen from her center. It is also the angle under which the semi-diameter of the Earth, A E, is seen from the object B.

As the object advances from the horizon to the zenith, the parallax is seen gradually to diminish, till at F it has no parallax, or its apparent and true place are the same.

PARALLAX OF THE PLANETS.

F

This diagram will also show why objects nearest the Earth have the greatest parallax, and those most distant the least; why the Moon, the nearest of all the heavenly bodies, has the greatest parallax; while the fixed stars, from their immense distance, have no appreciable horizontal parallax-the semi-diameter of the Earth, at such a distance, being no more than a point.

B

660 As the effect of parallax on a heavenly body is to depress it below its true place, it must necessarily affect its right ascension and declination, its latitude and longitude. On this account, the parallax of the Sun and Moon must be added to their apparent altitude, in order to obtain their true altitude.

The true altitude of the Sun and Moon, except when in the zenith, is always affected, more or less, both by parallax and refraction, but always in a contrary manner. Hence the mariner, in finding the latitude at sea, always adds the parallax, and subtracts the refraction, to and from the Sun's observed altitude, in order to obtain the true altitude, and thence the latitude.

661. The principles of parallax are of great importance to astronomy, as they enable us to determine the distances of the heavenly bodies from the Earth, the magnitudes of the planets and the dimensions of their orbits.

The Sun's horizontal parallax being accurately known, the Earth's distance from the Sun becomes known; and the Earth's distance from the Sun being known, that of all the planets may be known also, because we know the exact periods of their sidereal revolutions, and, according to the third law of Kepler, the squares of the times of their revolutions are proportional to the cubes of their mean distances. Hence, the first great desideratum in astronomy, where measure and magnitude are concerned, is the determination of the true parallax.

At a council of astronomers assembled in London some years since, from the most

660. Effect of parallax? How obtain true altitude? How differ from refraction? How then obtain true altitude?

661. Use of parallax? How employed? Note?