it in the evening, than he would do were there no refraction; and at a mean rate, about seven minutes on any day of the year. The refraction varies, however, very much with the state of the atmosphere. In cold dry weather the air is more dense than in warm weather; consequently the refraction is greater in cold weather than in warm; and for the same reason, it is greater in cold countries than in hot ones. A remarkable instance of this is mentioned by Dr. Smith in his Optics, where he states, that some Hollanders who wintered in Nova Zembla, in the year 1596, were surprised to find that, after three months' constant darkness, the sun began to appear seventeen days sooner than the time by computation deduced from the latitude, which was 76 degrees. Now this phenomenon can only be accounted for by the extraordinary refraction of the sun's rays in passing through the cold dense air in that climate. At the same altitudes, the sun, moon, and stars, all undergo the same refraction; for at equal altitudes, the rays which proceed from any of these bodies suffer the same inclination. The horizontal refraction being the gteatest, causes the sun and moon, at rising and setting, to appear of an oval form; for the lower edge of each being seen through denser atmosphere than the upper edge, is more refracted; consequently, the perpendicular diameter must appear shortened, while the horizontal diameter (which is not affected by refraction) remains the same, and in this way the oval appearance is produced. For the same reason, two fixed stars that are nearer the horizon, and right above each other, appear nearer than when they are high above the horizon; and if they are both in the horizon, but at some distance from each other, then they will appear at a less distance than they really are; for the refraction makes each of them higher, and consequently must bring them into parts of their respective vertical circles which are nearer to each other, because all vertical circles converge and meet in the zenith. Hence, in all astronomical calculations allowance must be made for refraction, before the true altitude of any celestial body can be obtained. Tables, containing this allowance for all altitudes, is to be found in every work on practical astronomy. The atmosphere not only occasions celestial bodies to appear higher than they really are, by bending the rays of light as they pass through it, but it also affects terrestrial bodies in the same way. The quantity of this refraction is, however, found to vary considerably with the different states of the atmosphère, and is therefore very uncertain. But at a mean rate it may be taken at one-fourteenth part of the distance expressed in degrees in a great circle: or, according to Professor Playfair, it is about one-seventh part of the correction for the earth's curvature, answering to the distance between the observer and the object.* *For example, suppose the distance between a person and any conspicuous object to be 5 English miles, and he wishes to know how much it is elevated by refraction. The correction for the earth's curvature on this distance is 163 feet, one-seventh part of which is 2 feet 45 inches, which it raised by refraction. Many curious and even whimsical effects of terrestrial refraction are mentioned by various authors; but as our business is chiefly with astronomical refraction, we shall only mention a few of these effects, which have been lately noticed by some of the most intelligent philosophers of the present day. UNUSUAL REFRACTION OF THE ATMOSPHERE. Although the phenomena of unusual refraction have been often observed by atronomers and navigators, yet they do not seem to have attracted particular notice till the year 1797. The unusual elevation of coasts, mountains, and ships, have been long known under the name of looming; and the same phenomena, when accompanied with inverted images, have been distinguished in France by the name of mirage. Mr. Huddart seems to have been the first person who described an inverted image beneath the real objects; he accounts for this, and other phenomena of elevation, by supposing that, in consequence of the evaporation of the water, the refractive power of the air is not greatest at the surface of the sea, but at some distance above it, increasing gradually from the surface of the sea to a line, which he calls the line of maximum density, and thence diminishing gradually till it becomes insensible. He then shows, that, in passing through such a medium, the rays of light would move in curve lines convex upwards, when they passed above the line of maximum density, and convex downwards when they passed below that line. Hence, two pencils from the object will arrive at the eye, which will produce an inverted image of the object. In the year 1798, the Rev. Dr. Vince, of Cambridge, made a series of interesting observations at Ramsgate, on the unusual refraction of the atmosphere. He made his observations with a terrestrial telescope, magnifying between thirty and forty times, when the height of the eye was about twenty-five feet above the surface of the sea. Sometimes the height of the eye was eighty feet; but this produced no variation in the phenomena. On the 1st of August, between four and eight o'clock, P. M. he saw the topmast of a ship as at A, above the horizon xy of the sea: at the same time, he also discovered in the field of view two complete images, B, C, of the ship in the air, vertical to the ship itself, B being inverted, and C erect, having their hulls joined. As the ship receded from the shore, less and less of its mast became visible; and as it descended, the images B and Cascended; but as the ship did not recede below the horizon, Dr. Vince did not observe at what time, and in what order, the images vanished. He then directed his telescope to another ship A, whose hull was just in the horizon x y, and he observed a complete inverted image B, the mainmast of which just touched that of the ship itself. In this case there was no second image as before. While the ship A moved along, B followed its motion, without any change of appearance. Dr. Vince observed a number of other ships, which produced variously formed images; but our limits will not permit us to give a particular description of them, nor of similar appearances which have been observed on various occasions at land.* We shall, however, notice a few of the experiments made by Dr. Wollaston, to illustrate his theory of the cause of unusual refraction. According to this ingenious philoshpher, the varying density of the atmosphere is the principal cause of the singular appearances which we have just mentioned; and from a number of interesting experiments, he found, that the results were perfectly conformable to this hypothesis. He took a square phial, and poured a small quantity of clear syrup into it, and above this an equal quantity of water, which gradually incorporated with the syrup, between the pure water and the pure syrup. The word syrup written on a card, and held behind the bottle, appeared erect through the pure syrup; but when seen through the visible medium of the syrup and water, it appeared inverted with an erect image above. *This subject has been fully treated by Dr. Wollaston, in the Philosophical Transactions for 1810. Dr. Wollaston then put nearly the same quantity of rectified spirit of wine above the water, and he observed a similar appearance; only in this case, the true place of the object was seen uppermost, and the inverted and erect images below. When the variations of density are great, the object may be held close to the phial; but when they become more gradual, the object is only elongated, and in order to be seen inverted, must be held one or two inches behind the phial. By examining an oblique line seen in this way, he found, that the appearances continue many hours even with spirit of wine; with syrup, two or three days; with sulphuric acid, four or five; and still longer with a solution of gum-arabic. Dr. Wollaston next heated a poker red hot, and looked along the side of it at a paper ten or twelve feet distant. A perceptible refraction took place at the distance of three-eighths of an inch from it. A letter, more than three-eighths of an inch distant, appeared erect as usual; at a less distance there was a faint reversed image of it; and still nearer the poker was a second image erect. Although the experimental method of illustrating the phenomena of unusual refraction, as given by Dr. Wollaston, is in every respect an excellent one, yet the employment of different fluids does not représent the case which actually exists in nature. The method employed by Dr. Brewster seems more agreeable to nature. His method consists in holding a heated iron above a mass of water, bounded by parallel plates of glass. As the heat descends through the fluid, a regular variation of density is produced, which gradually increases from the surface to the bottom. If the heated iron be withdrawn, and a cold body substituted in its place, or even the air allowed to act alone, the superficial strata of water will give out their heat so as to have an increase of density from the surface to a certain depth below it. Through the medium thus constituted, all the phenomena of unusual refraction may be seen in the most beautiful manner; the variation of density being produced by heat alone. OF THE CLOUDS. Ye mists and exhalations that now rise MILTON. Clouds are generally supposed to consist of vapour which has been raised from the sea and land by means of heat. These vapours ascend till they reach air of the same specific gravity with themselves, when they combine with each other, become more dense and opaque, and then become visible. The thinner or rarer the clouds are, the higher do they ascend in the air; however, it seldom happens that their height exceeds two miles. The greater number of clouds are suspended at the height of one mile; and when they are highly electrified, their height is not above eight or nine hundred yards. While Don Ulloa was in South America, measuring a degree of the meridian, he was for some time stationed on the summit of Cotapaxi, a mountain about three miles above the level of the sea, where he says the clouds could be seen at a great distance below, and that he could hear the horrid noise of the thunder and tempests, and even see the lightnings issue from the clouds far below him. The wonderful variety observable in the colours of clouds is owing to their particular position with respect to the sun, and the different reflections of his light. The various figures which they so readily assume, is supposed to proceed from their loose and voluble texture, revolving in any form, according to the direction and force of the wind, or to the quantity of electric matter which they contain. Sometime we see a cloud that's dragonish; A vapour, sometime, like a bear or lion, A towered citadel, a pendent rock, A forked mountain, a blue promontory, With trees upon't that nod unto the world, And mock our eyes with air. That which is now a horse, even with a thought, As water is in water. SHAKSPEARE. About the tropics the clouds roll themselves into enormous masses, as white as snow, turning their borders into the forms of hills, piling themselves upon each other, and exhibiting the shapes of mountains, caverns, and rocks. There," says St. Pierre, "may be perceived amid endless ridges, a multitude of valleys, whose openings are distinguished by purple and vermillion." These celestial valleys exhibit, in their various colours, matchless tints of white, melting into shades of different colours. Here and there may be observed torrents of light issuing from the dark sides of the mountains, and pouring their streams, like ingots of gold and silver, over rocks of coral. These appearances are not more to be admired for their beauty than for their endless combinations, for they vary every instant. What, a moment before, was luminous, becomes coloured; what was coloured, mingles into shade; forming singular and most beautiful representations of islands and hamlets, arched bridges stretched over wide rivers, immense ruins, huge rocks, and gigantic mountains. Among the Highlands of Scotland the clouds also display the finest outlines, and assume the most beautiful figures; more especially when viewed from their rugged and lofty summits. These bold and magnificent scenes are finely described by Dr. Beattie in the following lines: |