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the attention of Providence to all his creatures. But if I understand you, the inhabitants of the poles have in their winter a fortnight's light and darkness by turns.

Tutor. This would be the case for the whole six months that the sun is below the horizon, if there were no refraction ;* and no substitute for the light of the moon. But by the atmosphere's refracting the sun's rays, he becomes visible a fortnight sooner, and continues a fortnight longer in sight than he would otherwise do, were there no such property belonging to the atmosphere. And in those parts of the winter, when it would be absolutely dark in the absence of the moon, the brillianey of the Aurora Borealis is probably so great, as to afford a very comfortable degree of light. Mr. Hearne, in his travels near the polar circle, has this remark in his journal: "December 24. The days were so short, that the sun only took a circuit of a few points of the compass above the horizon, and did not at its greatest altitude rise half way up the trees. The brilliancy of the Aurora Borealis, however, and of the stars, even without the assistance of the moon, made amends for this deficiency, for it was frequently so light all night, that I could see to read a small print."

The subject of refraction will be very particularly explained when we come to Optics.

These advantages are poetically described by our Thomson:

By dancing meteors then, that ceaseless shake
A waving blaze refracted o'er the heavens,
And vivid moons, and stars that keener play
With double lustre from the glossy waste:
Ev'n in the depth of Polar Night, they find
A wond'rous day: enough to light the chase,
Or guide their daring steps to Finland-fairs.
WINTER, 1. 859.

CONVERSATION XXXIX.

Of Mercury.

Tutor. Having fully described the earth and the moon, the former a primary planet, and the latter its attendant satellite, or secondary planet, we shall next consider the other planets in their order, with which, however, we are less interested.

MERCURY, you recollect, is the planet nearest the sun; and Venus is the second in order. These are called inferior planets.

Charles. Why are they thus denominated? Tutor. Because they both revolve in orbits which are included within that of the earth; thus (Plate v. Fig. 2.) Mercury makes his annual journey round the sun in the orbit a ; Ve

nite space, and accordingly must sometimes involve in it the other planets, and eclipse them, which is contrary to fact. Therefore, since the earth is neither larger than, nor equal to the sun, it must be the lesser body. We will now proceed to the eclipses of the sun.

Charles. How are these occasioned?

Tutor. An eclipse of the sun happens when the moon м, passing between the sun s and the earth T (Plate VII. Fig. 15.) intercepts the sun's light.

James. The sun then can be eclipsed only at the new moon.

Tutor. Certainly; for it is only when the moon is in conjunction that it can pass directly between the.sun and earth.

Charles. It is only when the moon at her conjunction, is near one of its nodes, that there can be an eclipse of the sun?

Tutor. An eclipse of the sun depends upon this eircumstance: for unless the moon is in, or near one of its nodes, she cannot appear in the same plane with the sun, or seem to pass over his disk. In every other part of the orbit she will appear above or below the sun. If the moon be in one of the nodes, she will, in most cases, cover the whole disk of the sun, and produce a total eclipse; if she be any where within about 16 degrees of a node, a partial eclipse will be produced.

The sun's diameter is supposed to be divided into 12 equal parts, called digits, and in every

partial eclipse, as many of these parts of the sun's diameter as the moon covers, so many digits are said to be eclipsed.

James. I have heard of annular eclipses, what are they, sir?

Tutor. When a ring of light appears round the edge of the moon during an eclipse of the sun, it is said to be annular, from the Latin word annulus, a ring: these kind of eclipses are occasioned by the moon being at her greatest distance from the earth at the time of an eclipse, because in that situation the vertex or tip of the cone of the moon's shadow does not reach the surface of the earth.

Charles. How long can an eclipse of the sun last?

Tutor. A total eclipse of the sun is a very curious and uncommon spectacle and total darkness cannot last more than three or four minutes. Of one that was observed in Portugal, 150 years ago, it is said that the darkness was greater than that of the night:-that stars of the first magnitude made their appearance ;and that the birds were so terrified that they fell to the ground.

James. Was this visible only at Portugal?

Tutor. It must have been seen at other places, though we have no account of it. The moon, however, being a body much smaller than the earth, and having also a conical shadow, can with that shadow only cover a small part of the

VOL. I.-S

earth; whereas an eclipse of the moon may be seen by all those that are on the hemisphere which is turned towards it. (See Plate VII. Fig. 15 and 12.)

You will also observe, that an eclipse of the sun may be total to the inhabitants near the middle of the earth's disk, and annular to those in places near the edges of the disk, for in the former case the moon's shadow will reach the earth, and in the latter, on account of the earth's sphericity, it will not.

Charles. Have not eclipses been esteemed as omens presaging some direful calamity?

Tutor. Till the causes of these appearances were discovered, they were generally beheld with terror by the inhabitants of the world, which is beautifully alluded to by Milton in the first book of Paradise Lost, line 594:

-As when the sun, new risen,

Looks through the horizontal misty air
Shorn of his beams, or from behind the moon,
In dim eclipse, disastrous twilight sheds
On half the nations, and with fear of change
Perplexes monarchs.

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