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ters of stars? What are nebulæ ? When did Huygens discover the most noted nebula ? Where is it? To whom are we indebted for extensive catalogues of nebulæ 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.

when a person

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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 6.

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,

looks into a pail, or other vessel of water, the bottom seenis 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 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; BF, the sensible

liorizon 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

from F to the surface of the Earth at B, and would present 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

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.

OW.

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

be seen by us when it is below the horizon? What effect has cold on the refracting power of the air? What curious circumstance is related by Mr. Ferguson? What excess of the day over the night, is it computed, refraction gives in latitude 43°? Of what curious phenomenon is the refraction of the atmosphere sometimes the cause ? Why does the disk of the Sun or Moon, when in or near the horizon, appear elliptical ?

CHAPTER XIV.

Twilight.

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Twilight, or crepusculum, is the light of the morning before sun-rising, and of the evening after sun-setting. It is the result of refraction.

The atmosphere of the Earth extends about 45 miles in height; or, at that distance from the Earth's surface, it is sufficiently dense to refract the rays of the Sun. Hence, when the Sun is about 180 below the horizon, the morning twilight begins, and the evening twilight ends.

The evening twilight is said, however, to be longer than that of the morning. The elevation of the atmosphere by the heat of the day and the vapor exhaled by rarefaction, may, by affecting the refracting power of the air, prolong the evening twilight.

The continuance of twilight must increase with the distance from the equator, and be very long in high latitudes. At the poles, the Sun is never more than about 23° 28' below the horizon. If there be polar inhabitants, therefore, they must be blessed with a long twilight. To them it must be more than 50 days after the Sun sets, before it will be 180 below the horizon, and, on its return, the same time, after it approaches within 18°, before it will be above the horizon.

The immense benefit of the atmosphere must be contemplated with admiration. Not only, by the chemical operations of air, does it cause our blood to flow, and diffuse warmth through our bodies ; but, by its reflecting and refracting powers, it gives beauty to the day. It gives also an easy and pleasing transition from night to day, and from day to night, and enlarges the borders of the day even into the regions of night. Astronomers generally concur with Dr. Keill, “ that it is entirely owing to the atmosphere, that the heavens appear bright in the day time. For without it, only that part of the heavens would be luminous in which the Sun is placed ; and, if we could live without air, and should turn our backs to the Sun, the whole heavens would appear as dark as in the night. In this case, also, we should have no twilight, but a sudden transition from the brightest sunshine to dark night, immediately upon the setting of the Sun, which would be extremely inconvenient, if not fatal to the eyes of mortals.

What is twilight? How high is the atmosphere? How far below the horizon is the Sun when the morning twilight begins, and the evening twilight ends? Which have the longest twilight, the people near the equator, or those in high latitudes? How long must the twilight be at the poles ? Can you name some particular benefits derived to us from the atmosphere ? According to Dr. Keill, what would be the consequence of our being without an atmosphere?

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