er, 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 BC, (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 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 shadow. An instance of this kind, observed at Paris in 1750, is mentioned by 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 me. dium 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. 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 18° 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 180, before it will be above the hori zon. 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 ap 6 pear 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 be low 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? CHAPTER XV. Latitude and Longitude. SEC. I. LAtitude. LATITUDE, as before stated, is the distance north or south from the equator. It is reckoned on the meridian in degrees; which, like those of all other circles are subdivided into minutes, and again into sexagesimal parts. (The centre of the meridian like that of the equator, and other great circles of the globe, is considered at the centre of the earth. The great circles of the globe, extended into the visible heavens, are considered (as celestial circles, always lying in the same plane with those on the earth.) (The position of the heavenly bodies, therefore, in re gard to these circles, may be used in determining the latitude and longitude of places.) The latitude of a place may be determined by finding the distance of its zenith from the celestial equator, If therefore the zenith distance of a heavenly body and its declination be known, the latitude of the place of observation may be ascertained.) The declination of a heavenly body, as before defined, is its distance, north or south from the celestial equator. The zenith distance of a heavenly body may be obtained by observing its meridian altitude, or by two altitudes.(Four corrections are required in finding the altitude of the sun or moon; semi-diameter, depression of the horizon, parallax and refraction. [For tables to find these, see the author's larger work, and other works on astronomy.] The semi-diameter and parallax of a planet can be but a few seconds. They are imperceptible in a star. Suppose, that on the 4th of July 1831, the sun's declination was found to be 22° 55′ 39′′ north, when it passed the meridian of New York; and at that time the sun's true zenith distance was found to be 17° 46' 21" south; what is the latitude of that city? Declination Answer 22° 55′ 39′′ 17° 46′ 21′′ 40° 42′ 0 If Arcturus, the noble star mentioned in the book of Job, be in 20° 20′ north declination, as placed on the British celestial globe, and be observed to pass the meridian of Boston 22° 3' south of the zenith; what is the latitude of the city? With a little attention the student may easily determine, whether he ought to add or subtract in making |