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The Galaxy.

The Galaxy, or Milky Way, is a luminous zone in the heavens. The beautiful cloudy whiteness, by which it is distinguished, is found by modern astronomers to proceed from the collected rays of innumerable stars not discernible by the naked eye. "That the milky way," says Dr. Herschel, " is a most extensive stratum of stars of various sizes, admits no longer of the least doubt."

A group of stars is a collection of them of any figure, closely compressed together, like the trees in a crowded forest.

Clusters of stars are regarded by Dr. Herschel among the most magnificent objects in the heavens. They differ from groups in their beautiful and seemingly artificial arrangement. (Plate vii. Fig. 6.)

Nebula are light spots in the heavens, sometimes denominated cloudy stars. (Plate vii. Fig. 7.) Some of them are found to be clusters of telescopic stars. The most noted nebula was discovered by Huygens, in 1656. It is between the two stars in the sword of Orion. In one part of it, (Plate viii. Fig. 1,) a bright spot upon a dark ground seems to be an opening into a brighter and more distant region. Nebula were discovered by Dr. Halley and others. "But to Dr. Herschel," says Enfield, "are we indebted for catalogues of 2000 nebulæ and clusters of stars, which he himself has discovered." Dr. Brewster says, "2500."

What an astonishing view of the works of creation is opened upon us by the night! With wonder and delight, we greet the return of day. The beauty, and even the sublimity, of this world are lighted up to us by the splendor of the morning. But how surpassed are these by the infinite grandeur presented to our view by the nocturnal heavens! To the night we are indebted

for the most exalted conceptions we can form of the immensity and sublimity of Jehovah's works. We cannot contemplate them without the most profound awe! We behold, not a solitary world, but a system of worlds, kept in perpetual harmony by the Sun; not one sun and one system only, but millions of suns and of systems, ranged in endless perspective, all revolving in harmonious order! How inconceivably great, and wise, and good, must be the AUTHOR AND GOVERNOR OF SUCH

A UNIVERSE!

What

Why are the stars called fixed stars? What reason have we to consider their distance immense ? What does Mr. Ferguson think may be the distance of the nearest star? How long would a cannon ball be in flying to it? Why must we conclude the stars shine by their own light? What are the fixed stars now considered? How may they be distinguished from planets? What number of stars is visible to the naked eye in either hemisphere? What are most of those bright spots which are taken for stars in a clear evening? What number of stars does the British catalogue contain? By improved telescopes, how is the number found to be? What number of stars did Dr. Herschel see pass his telescope in a quarter of an hour? How do some stars, seemingly single to the naked eye, appear viewed through a good telescope? What is a binary sidereal system? Can you mention some stars which are double, treble, or quadruple? Do stars seen at any time ever disappear? Can you give the particulars of some such stars? was remarkable in the new star discovered by Kepler in 1604 ? What may stars of this kind be? What was the opinion of Maupertuis? What renders doubtful the term fixed, as applied to the stars? Who observed an advancement of the solar system in absolute space? Who gave the first explanation of this? Who pointed out the part in the heavens to which the solar system is advancing? How have the stars been arranged into classes or orders? Was the arrangement of the stars into magnitudes before or after the invention of telescopes? What other arrangement of the stars has been handed down to us from antiquity? What did the ancients conceive companies of stars as having? How did they apply names to constellations? Can much similarity be perceived between the figure presented by the stars in a constellation and the animal or other object by which it is represented? How many constellations are reckoned ancient? many constellations are there? How many in the zodiac? how many north, and how many south? How many constellations does Prior enumerate in his Lectures, in the zodiac, north and south? What is the galaxy? From what does the whiteness of the galaxy proceed? What is a group of stars? What are clus

How

ters of stars? What are nebula? When did Huygens discover the most noted nebula? Where is it? To whom are we indebted for extensive catalogues of nebula and clusters of stars? Which opens on us the most sublime view of the Creator's works, the day or the night? Can we form any idea of the immensity of Jehovah's works?

CHAPTER XIII.

Refraction.

REFRACTION of light is the incurvation of a ray from its rectilinear course. In passing obliquely from one medium into another of different density, a ray is turned towards the perpendicular, drawn to the surface of the denser medium; but it is turned from the perpendicular drawn to the surface of a rarer medium, in passing from one more dense.

Refraction elevates a body, or makes it appear above its true place. An object always appears in the last direction in which a ray of light from it passes to the eye of the observer; though before it may have been refracted or reflected many times. A straight rod partly immersed in water appears crooked, when viewed obliquely to the surface of the water. The rays of light coming from the part immersed are refracted, or turned out of their course, in coming into the air, a rarer medium. The eye, therefore, viewing that part in the direction of the ray after refraction, sees it above its true place. Put a piece of money into a bowl, and retire till the money is just hidden by the edge of the bowl; let an attendant pour water into the bowl, the piece will rise into view. To the eye at A, (Plate vi.

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Fig. 9,) the money at a is hidden by the edge of the bowl, but a ray of light from the piece immersed in water is refracted at the surface, and passes in the direction b A. Hence it becomes visible, appearing

at b.

A ray of light passing from one medium into another, in a line perpendicular to the intervening surface, suffers no refraction. It is true, that, when a person looks into a pail, or other vessel of water, the bottom seems elevated some distance above the ground or bench on which the vessel stands. But it must be remembered, that no single point of the surface is, in such a view, perpendicular to both eyes. And when the observer uses but one eye, from a single point only does a ray of light pass perpendicularly to that eye. In such case, the imagination gives an elevated appearance to the whole. In wading a river, the water by you appears about its true depth; but at a little distance forward, it appears more shallow than it will be found on trial, the bottom seeming elevated by refraction. No doubt this has often been the cause of drowning.

The light of heavenly bodies is refracted, in coming to us, by the atmosphere of the Earth. This refraction is greatest at the horizon. Decreasing upwards, it becomes nothing at the zenith. When a medium is equally dense in every part, the refraction is at the surface. But it is not so in a varying medium. The atmosphere of the Earth decreases in density from the surface upward to its utmost height. A ray of light, therefore, must be more and more refracted, and pass in a curvilinear course through the atmosphere.

Refraction brings a heavenly body into view, when it is below the horizon. Let A B C, (Plate v. Fig. 10,) be a part of the Earth's atmosphere; B F, the sensible horizon to an observer at B; D, a place of the Sun below the horizon. Should a ray of light, passing in the direction D F, strike the atmosphere at F, it would be refracted by the increased density of the air all the way

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from F to the surface of the Earth at B, and would sent the Sun in the line of its last direction B E. The Sun would, therefore, appear in the horizon, while it is below that circle.

The density of the air and the refracting power are increased by cold. The higher the latitude, therefore, in general, the greater the refraction. We are told by Mr. Ferguson, that, in 1596, some Hollanders wintered in Nova Zembla, and that the Sun arose to them seventeen days sooner than, by calculation, it would have been above the horizon. Though refraction increases the length of the day over the night more in high latitudes than in those that are near the equator, yet, from the principle, it must be perceived, that the day is increased in all latitudes. The excess of the day over the night by refraction, exclusive of twilight, in latitude 43° has been computed at about six minutes, three in the morning and three in the evening.

The refraction of the atmosphere is sometimes the cause of a curious phenomenon, the Sun and Moon both visible, when the Moon is eclipsed by the Earth's shad

ow.

An instance of this kind, observed at Paris in 1750, is mentioned by Mr. Phillips in his Astronomy.

The disk of the Sun or Moon, when in or near the horizon, appears elliptical. The lower limb being more elevated by refraction than the upper, not only by the atmosphere itself, but often by floating vapor, the outline of the disk must be changed from a circle to an elliptical form.

What is refraction of light? In passing obliquely from one medium into another of different density, how is a ray turned? Does refraction make a body appear above or below its true place? In what direction does an object always appear? Why does a rod immersed partly in water appear crooked? What is the cause that a piece of money at the bottom of a bowl will appear to rise, on water being poured into the bowl? Is a ray of light affected by refraction in passing perpendicularly from one medium into another? What refracts a ray of light in coming from a heavenly body to us? Why does a ray of light pass in a curvilinear course through the atmosphere of the Earth? Why can a heavenly body

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