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must be made for the effects of refraction. Dr. Bradley's tables of refraction were formed by observing the zenith distances of the sun at his greatest declinations, and the zenith distances of the pole-star above and below the pole; the sum of these four quantities is equal to 180°, diminished by the sum of the four refractions; whence the sum of the four refractions was obtained; and from the law of the variation of refraction determined by theory, he assigned the quantity due to each altitude. The mean horizontal refraction is about 35'6", and at the height of forty-five degrees it is 58'•36. The effect of refraction upon the same star above and below the pole was noticed by Alhazen, a Saracen astronomer of Spain, in the ninth century; but its existence was known to Ptolemy in the second, though he was ignorant of its quantity.
The refraction of a terrestrial object is estimated differently from that of a celestial body; it is measured by the angle contained between the tangent to the curvilineal path of the
where it meets the eye, and the straight line joining the eye and the object. Near the earth's surface the path of the ray may be supposed to be circular; and the angle of this path between tangents at the two extremities of this arc is called the horizontal angle. The quantity of terrestrial refraction is ob
T. Masiped by a heavy shower after e msity and hot weather. Single E L une mees of cojects at sea, arising from scan ane i zererature, which are not so 2012 Srmet so the water on account of ses sa te sit, occur more rarely, and
surz a than similar appearances ani. 3:35, Captain Scoresby, whose obsamas ja se pisamena of the polar seas are * De Sagnised his father's ship by its incei mate in se air, although the vessel itself s trinnHe afterwards found that
Tas na miles beyond the horizon, and ir mis ISEL. Two images are sometimes
sea the air over a ship, one direct ME 1 CRT igrerted, with their topmasts or the meeting, according as the inverted mare tire or below the direct image. Dr. TILINI has proved that these appearances are PECHE I i rraction of the rays through media rem destes, by the very simple experimet inn along a red hot poker at a distant De Tv Saras are seen, one direct and DI Tera in consequence of the change manis set in the density of the adjacent sr. Tided the same effect by a saline or SLITIR Riyaza with water and spirit of wine trasme pe
tained by measuring contemporaneously the elevation of the top of a mountain above a point in the plain at its base, and the depression of that point below the top of the mountain. The distance between these two stations is the chord of the horizontal angle; and it is easy to prove that double the refraction is equal to the horizontal angle, dimis nished by the difference between the apparent elevation and the apparent depression. Whence it appears that, in the mean state of the atmosphere, the refraction is about the fourteenth part of the horizontal angle.
Some very singular appearances occur from the accidental expansion or condensation of the strata of the atmosphere contiguous to the surface of the earth, by which distant objects, instead of being elevated, are depressed; and sometimes, being at once both elevated and depressed, they appear double, one of the images being direct, and the other inverted. In consequence of the upper edges of the sun and moon being less refracted than the lower, they often appear to be oval when near the horizon. The looming also, or elevation of coasts, mountains and ships, when viewed across the sea, arises from unusual refraction. A friend of the author's, on the plains of Hindostan, saw the whole upper chain of the Himalaya mountains start into view, from a sudden change in the den
sity of the air, occasioned by a heavy shower after a very long course of dry and hot weather. Single and double images of objects at sea, arising from sudden changes of temperature, which are not so soon communicated to the water on account of its density as to the air, occur more rarely, and are of shorter duration than similar appearances on land. In 1818, Captain Scoresby, whose observations on the phenomena of the polar seas are so valuable, recognised his father's ship by its inverted image in the air, although the vessel itself was below the horizon. He afterwards found that she was seventeen miles beyond the horizon, and thirty miles distant. Two images are sometimes seen suspended in the air over a ship, one direct and the other inverted, with their topmasts or their hulls meeting, according as the inverted image is above or below the direct image. Dr. Wollaston has proved that these appearances are owing to the refraction of the rays through media of different densities, by the very simple experiment of looking along a red hot poker at a distant object. Two images are seen, one direct and another inverted, in consequence of the change induced by the heat in the density of the adjacent air. He produced the same effect by a saline or saccharine solution with water and spirit of wine floating upon it.