is an old couplet referring to the appearance of the moon, and is supposed to foreshadow the weather by the size of the halo, which is caused, as we know, by the existence of vesicular vapour in the atmosphere.
Mock Suns, or parhelia, and mock-moons, or paraselenæ, are continually observed in cold climates, where the tiny ice particles are so abundant in the air. These phenomena were recognized by the ancients, and halos round the sun can be observed by means of darkened glasses. We annex an illustration of a mock sun and moon seen on the continent of Europe. Readers of Mr. Whymper’s “Scrambles in the Alps” will remember the gorgeous, and to the guides mysterious, fog-bow or sun-bow seen as the survivors of the first and most fatal ascent of the Matterhorn in 1865 were tremblingly pursuing their descent over the upper rocks of that mountain.
The Mirage, or Fata Morgana, is a very curious but sufficiently common phenomena, and in the Asiatic and African plains it is frequently observed. When the weather is calm and the ground hot, the Egyptian landscape appears like a lake, and the houses look like islands in the midst of a widely-spreading expanse of water. This causes the mirage, which is the result of evaporation, while the different temperatures of the air strata cause an unequal reflection and refraction of light, which give rise to the mirage. Travellers are frequently deceived, but the camels will not quicken their usual pace until they scent water.
The Fata Morgana and the inverted images of ships seen at sea are not uncommon on European coasts. Between Sicily and Italy this effect is seen in the Sea of Reggio with fine effect. Palaces, towers, fertile plains, with cattle grazing on them, are seen, with many other terrestrial objects, upon the sea—the palaces of the Fairy Morgana. The inverted images of ships are frequently perceived as shown in the illustration (fig. 729), and many most extraordinary but perfectly authentic tales have been related concerning the reflection and refraction of persons and objects in the sky and on land, when no human beings nor any of the actual objects were within the range of vision.
It will be well to explain this phenomenon, and the diagrams will materially assist us in so doing, for the appearances are certainly startling when realized for the first time. The Spectre of the Brocken we see mimics our movements, and we can understand it. But when apparently solid buildings appear where no buildings have been erected,—when we see—as has been perceived—soldiers riding across a mountain by a path, or ledge, perfectly inaccessible to human beings even on foot, we hesitate, and think there is something uncanny in the sight. Let us now endeavour to explain the mirage.
Suppose that in the annexed diagram the space enclosed between the letters A, B, C, D, be a glass vessel full of water. The ship is below the horizon, the eye being situated at E—the glass vessel of course representing the atmosphere charged with moisture. The eye at E will perceive the top of the mast of the ship, S, and we may imagine a line drawn from E to S. Then put a (short focus) convex lens at a just above this (imaginary) line, and a concave one, b, just over it. Through the former an inverted ship will be seen, and an erect one through the latter at S′ and S″ respectively. We now have the effect in the air just as reproduced in nature by the difference in temperature in air strata, which cause it to act like a concave lens when the density of the water diminishes towards the centre, and like a convex lens when it is increased.
Fig. 730.—Explanation of Mirage.
This can be proved by heating the air (by hot irons) above the glass vessel filled with oil, and the effects will be just the same as through the lenses. Dr. Wollaston obtained the mirage by using a clear syrup,—about one-third of the vessel full,—and filling it with water. The gradual mingling of these fluids will produce the phenomenon. The illustration in the margin (fig. 731) shows us the rays proceeding from the ship’s hull, and refracted into the line reaching the eye, above the line proceeding from the mast, so the ship appears hull uppermost; the rays cross at x. But if they did not cross before they reach the eye, the image would appear as at s´ p´ in an erect position.
