Moon is in those signs which rise or set with the smallest angles, she rises or sets with the least difference of time; but when she is in those signs which rise or set with the greatest angles, she rises or sets with the greatest difference of time.

Let the globe, for example, be rectified to the latitude of New York, 40° 42' 40", with Cancer on the meridian, and Libra rising in the east. In this position, the ecliptic has a high elevation, making an angle with the horizon of 7240.

But let the globe be turned half round on its axis, till Capricorn comes to the meridian, and Aries rises in the east, then the ecliptic will have a low elevation above the horizon, making an angle with it of only 2510. This angle is 47° less than the former angle, and is equal to the distance between the tropics.

In northern latitudes, the smallest angle made by the ecliptic and horizon, is when Aries rises; at which time Libra sets; the greatest is, when Libra rises and Aries sets. The ecliptic rises fastest about Aries, and slowest about Libra. Though Pisces and Aries make an angle of only 2510 with the horizon when they rise, to those who live in the latitude of New York, yet the same signs, when they set, make an angle of 72°. The daily difference of the Moon's rising, when in these signs, is, in New England, about 22 minutes; but when she is in the opposite signs, Virgo and Libra, the daily difference of her rising is al most four times as great, being about one hour and a quarter.

As the Moon can never be full but when she is opposite to the Sun, and the Sun is never in Virgo or Libra except in our autumnal months, September and October, it is evident that the Moon is never full in the opposite signs, Pisces and Aries, except in those two months. We can therefore have only two full Moons in a year, which rise, for a week together, very near the time of sun-set.-The former of these is called the Harvest Moon, and the latter, the Hunter's Moon.

Although there can be but two full Moons in the year that rise with so little variation of time, yet the phenomenon of the Moon's rising for a week together so nearly at the same time, occurs every month, in some part of her course or the other.

In Winter, the signs Pisces and Aries rise about noon; hence the rising of the Moon is not then regarded nor perceived.

In Spring, these signs rise with the Sun, because he is then in them; and as the Moon changes while passing through the same sign with the Sun, it must then be the change, and hence invisible.

What results from this in regard to the Moon? How may this be illustrated on the globe In northern latitudes, what signs rise and set with the least angles? What with the greatest? What parts of the ecliptic rise fastest, and which slowest? Give an example. What is the daily difference of the Moon's rising and setting, in these sigus, in the latitude of New York? How many full Moons in a year, which rise with so little dif ference of time? Why are not these phenomena observed in the same signs, in IVin ter, Spring, and Summer?

In Summer, they rise about midnight, when the Moon is in her third quar ter. On account of her rising so late, and giving but little light, her risin passes unobserved.

To the inhabitants at the equator, the north and south poles appear in the horizon; and therefore the ecliptic makes the same angle southward with the horizon when Aries rises, as it does northward when Libra rises; consequently the Moon rises and sets not only with angles nearly equal, but at equal intervals of time, all the year round: Hence, there is no harvest Moon at the equator. The farther any place is from the equator, if it be not beyond the polar circles, the angle which the ecliptic makes with the horizon gradually diminishes when Pisces and Aries rise.

Although in northern latitudes, the autumnal full Moons are in Pisces and Aries; yet in southern latitudes it is just the reverse, because the seasons are so:-for Virgo and Libra rise at as small angles with the horizon in southern latitudes, as Pisces and Aries do in the northern ; and therefore the harvest Moons are just as regular on one side of the equator as on the other.

At the polar circles, the full Moon neither rises in summer, nor sets in winter. For the winter full Moon being as high in the ecliptic as the summer Sun, she must continue, while passing through the northern signs, above the horizon; and the summer full Moon being as low in the ecliptic as the winter Sun, can no more rise, when passing through the southern signs, than he does.

THE HORIZONTAL MOON.--The great apparent magnitude of the Moon, and indeed of the Sun, at rising and setting, is a phenomenon which has greatly embarrassed almost all who have endeavoured to account for it. According to the ordinary laws of vision, they should appear to be least when nearest the horizon, being then farthest from the eye; and yet the reverse of this is found to be true. The apparent diameter of the Moon, when viewed in the horizon by the naked eye, is two or three times larger than when at the altitude of thirty or forty degrees; and yet when measured by an instrument her diameter is not increased at all.

Both the Sun and the Moon subtend a greater angle when on the meridi. an, than they do in the horizon, because they are then actually nearer the place of the spectator, by the whole semi-diameter of the Earth."

Explain why there is no Harvest Moon at the equator. The farther any place is from the equator, how is the angle between the ecliptic and the horizon, when Pisces and Aries rise? Do the Harvest Moons happen as regularly, and in the same months, on the south side of the equator, as on the north? Why does not the full Moon rise in summer, nor set in winter, to the inhabitants of the polar circles? According to the ordinary laws of vision, how ought the magnitudes of the Sun and Moon to appear, when they are nearest the horizon? What is the fact? How much larger does the Moon appear to the naked eye, when in the horizon, than when at the altitude of thirty or forty degrees? Where, in reality, do the Sun and Moon subtend the largest angle? Why is it so?

This apparent increase of magnitude in the horizontal Moon, is chiefly an optical illusion, produced by the concavity of the heavens appearing to the eye to be a less portion of a spherical surface than a hemisphere. The eye is accustomed to estimate the distance between any two objects in the heavens by the quantity of sky that appears to lie between them; as upon the Earth we estimate it by the quantity of ground that lies between them. Now when the Sun or Moon is just emerging above the eastern horizon, or sinking beneath the western, the distance of the intervening landscape over which they are seen, contributes, together with the refraction of the atmosphere, to exaggerate our estimate of their real magnitudes.

CHAPTER XXV.

REFRACTION-TWILIGHT.

The rays of light in passing out of one medium into another of a different density, deviate from a straight course; and if the density of the latter medium continually increase, the rays of light in passing through it, will deviate more and more from a right line towards a curve, in passing to the eye of an observer. From this cause all the heavenly bodies, except when in the zenith, appear higher than they really This bending of the rays of light, giving to the heavenly bodies an apparent elevation above their true places, is called Refraction.

are.

It is in consequence of the refracting power of the atmosphere that all heavenly bodies are seen for a short time before they rise in the horizon, and also after they have sunk below it. At some periods of the year the Sun appears 5 minutes longer, morning and evening, and about 34 minutes longer every day, at a mean rate, than he would do were there no refraction. The average amount of refraction for an object half way between the horizon and the zenith, or at an apparent altitude of 45°, is but one sixtieth of a degree, a quantity hardly sensible to the naked eye; but at the visible horizon it amounts to 33' of a degree, which is rather

How is the apparent increase of magnitude in the horizontal Moon, accounted for? How are the rays of light affected in passing out of one medium into another, of a differ ent density? How, if the density of the latter medium continually increase? What as tronomical phenomenon results from this cause? What is this bending of the rays of right out of their course called? What effect does refraction have upon the apparent rising and setting of the heavenly bodies? How much longer do we see the Sun, morning and evening, than we should, if there were no refraction? What is the average amount of refraction for an object half way between the horizon and the zenith? What is i the horizon?

anore than the greatest apparent diameter of either the Sun or the Moon.

Hence it follows, that when we see the lower eds of the Sun or Moon just apparently resting on the horiz 4, their whole disc is in reality below it, and would be entely out of sight and concealed by the convexity of the Earth, but for the bending, which the rays of light have undergon in their passage through the air to the observer's eye.

The following general notions of its amount, ar 1 law of variations, should be borne in mind:

1. In the zenith there is no refraction; a celestial object, situated directly over head, is seen in its true position, as if there were no atmosphere.

2. In descending from the zenith to the horizon, the refraction continually increases; objects near the horizon appearing more elevated by it than those of a higher altitude.

3. The rate of its increase is nearly in proportion to the apparent angular distance of the object from the zenith. But this rule, which is not far from the truth, at moderate zenith distances, ceases to give correct results in the vicinity of the horizon, where the law becomes much more compli cated in its expression.

The effects of refraction must be familiar to every person who has seen a walking stick partially plunged into a river, or other collection of water. While the stick is held upright, it appears straight, as usual, because there is no refraction in this position; but if it be ever so little inclined, the re fraction takes place, and the stick appears bent; if the inclination be increased, the refraction is also increased.

Another easy and familiar illustration of the effect of refraction may be thus obtained-Put any small object, as a piece of money, into an empty basin, as near the centre as possible, and retire to such a distance as just to lose sight of the object. Let an assistant then pour water in the basin, and the object will soon appear. Retire again till it is no longer seen; let more water be added, and it will again appear. The experiment may be re. peated till the basin is full. The edge of the basin may be supposed to represent the horizon; the water, the atmosphere; and the piece of money, the Sun, or other object which is thus made to appear by the power of refraction, when otherwise it would be invisible.

It follows from this, that one obvious effect of refraction must be to shorten the duration of night and darkness, by prolonging the apparent stay of the Sun and Moon above the horizon. But even after they appear to have set, the influence of the atmosphere still continues to send us a portion of their light; not, indeed, by direct transmission, but by reflection:-for as long as the Sun continues to illuminate

What interesting facts result from this truth? What is the first general law of atmospheric refraction? What is the second general law? What is the third? Mention a familiar instance of refraction often seen in water. Mention some familiar cxperiment, to illustrate refraction, and show its application to astronomy? How does this principle affect the duration of nocturnal darkness? By what principle is it that the atmosphere sends us a portion of the solar light, for a considerable time before the S rises, and after it has set}

any portion of the atmosphere which is above the horizon, the light from this portion is reflected to the Earth, and it is this that causes twilight.

In the morning, when the Sun arrives at 18° below the horizon, his rays pass over our heads into the higher region of the atmosphere, and are thence reflected, or as it were, bent down to the Earth. The day is then said to dawn, and the light gradually increases until the Sun appears above the horizon this is called Morning Twilight, or Aurora, which the heathens personified as a goddess. They assigned to her the office of opening the Gates of the East, to introduce the chariot of Apollo or Phœbus.

In the evening, after sunset, the rays of the Sun continue to illuminate the atmosphere, till he sinks 18° below the horizon, and a similar effect, called the Evening Twilight, is produced, only in an inverse progression, for the twilight now gradually becomes fainter till it is lost in dark night.

The quantity of reflection and the duration of twilight are much influenced by the changes which are perpetually taking place with respect to the heat and cold, the dryness or moisture, &c. of the atmosphere. The height of the atmosphere, also, has an influence in determining the duration of twilight: Thus in winter, when the air is condensed with cold, and the atmosphere upon that account lower, the twilight will be shorter; and in summer, when the limits of the atmosphere are extended by the rarefaction and dilation of the air of which it consists, the duration of the twilight will be longer. And for the same reason, the morning twilight, (the air being at that time condensed and contracted by the cold of the preceding night,) will be shorter than the evening twilight, when the air is more dilated and expanded.

It is entirely owing to the reflecting power of the atmosphere that the heavens appear bright in the day time. For without such a power, only that part of the heavens would be luminous in which the Sun is placed; and, if we should turn our backs to the Sun, the whole heavens would appear as dark as in the night, and the stars, even at noon day, would be seen as clear as in the nocturnal sky.

In regions of the Earth situated towards the poles, the Sun, during their summer months, is never more than 18° below the horizon; consequently their twilight continues

What is Twilight? How is it occasioned? How is the Evening Twilight produced? By what are the quantity of reflection, and the duration of twilight, considerably influenced? Why is twilight shorter in winter? Why longer in summer? Why is the morning twilight shorter than the evening twilight? To what is it entirely owing, that the heavens appear bright in the day time? How would the heavens appear, if it were not for this power? What are the duration and advantages of twilight in high latitudes?