fied in breadth, and will appear as if seen through a telescope.

The mirage at Hastings was probably due to this

cause.

That such a lateral displacement is possible, is evident from the remarkable mirage beheld by Messrs. Soret and Jurine, on the Lake of Geneva, in Sept. 1818, and which is shown in figure 30.

Fig. 30.

P

M

R

LATERAL MIRAGE.

456. The curve A B C represents the east bank of the lake. A boat, with all her sails set, was at P, advancing towards Geneva, and was seen, by the aid of a telescope, in the direction of G P, from Jurine's house, at the distance of six miles. As the boat successively occupied the positions M N R, a lateral image was clearly seen at the corresponding points M' N' R', approaching with the boat, but appearing to recede to the left of G P, while the boat receded to the right. When the sun shone full upon the sails, the image was visible to the naked eye.

Describe the lateral mirage seen at Geneva (figure 30.), and explain its

eause.

The direction of the sun's rays at the time of observation, is shown by the arrow F D.

457. This curious phenomenon is supposed to have been caused in the following manner. The air at M N R, had been in the shade all the morning, while that at M' N' R' had been warmed by the sun; the two portions therefore were of different densities, and the surface which separated the warm air from the cold was probably vertical. The rays of light proceeding from the boat might, in this case, fall upon the vertical surface of the warm stratum as upon a mirror, (Art. 461,) and being thence reflected to the eye of the observer, an image of the boat would appear behind this surface. Thus, if LIO was a part of this surface, the ray MI from the boat at M might be reflected from it in the direction I G, and an image would then be seen by an observer at G, in the direction GI M'.

458. IMAGES BELOW THE OBJECT. Illusions, like those which were seen by the French army, arise from a condition of the atmosphere exactly the reverse of that which occasions the images to appear above the object.

Upon the arid plains of Asia and Africa, when the sand is intensely heated, the temperature of the air decreases and its density increases from the surface upwards to a certain height, where it is nearly uniform. A ray of light, therefore, proceeding from an elevated object towards the ground, must necessarily pass through strata of decreasing density, and will consequently, upon principles already explained, after a number of refractions be reflected upwards, causing images to be seen below their objects.

459. For the sake of illustration, suppose (fig. 31.) that 1-2, 2-3, 3-4, 4-5, &c., are atmospheric strata decreasing in density from the height, 6, to the surface of the ground at 1. Let D represent a palm tree, which is seen in its true position by the eye at P, through an atmosphere of uniform density. The rays of light e a,

Under what circumstances are images seen below the object?
What is then the state of the atmosphere, in respect to temperature?

c b, which proceed from the top and bottom of the tree, in passing successively from denser into rarer media, will be constantly bent upwards, until at last they suffer total reflection at y and x; and then, crossing each other, are again refracted through the upper strata converging to the eye at P. An inverted image D2 will therefore be seen below the real object in the direction of the last refracted rays P b2 c2 and P a2 e2.

460. The observer is led to imagine these images in the midst of a lake from the circumstance, that the ascending currents of warm air, mixing with the colder strata, impart a tremulous motion to the images seen through them; and thus they appear to be agitated, as if floating upon a slightly ruffled surface. A difference of three or four degrees in temperature is sufficient to occasion appearances of this kind.

461. IMAGES PRODUCED BY REFLECTION. It is probable that the mirage is sometimes produced by reflection only, as from a plane mirror, and the instance witnessed

Illustrate from figure 31.

by Capt. Mundy when travelling in India, and thus related in his Journal, may have proceeded from this cause. "A deep, precipitous valley below us, at the bottom of which I had seen one or two miserable villages in the morning, bore in the evening a complete resemblance to a beautiful lake; the vapor which played the part of water ascending nearly half way up the sides of the vale, and on its bright surface trees and rocks being distinctly reflected. I had not been long contemplating this phenomenon, before a sudden storm came on, and dropped a curtain of clouds over the scene."

The fata morgana is attributed to the reflection of the rays of light from the surface of the sea and vapor. 462. The reflecting surface of a stratum of air may possibly at times possess a concave form, so as to present a magnified image of the object. (C. 732.) In this way, the gigantic images seen by Dr. Tschudi were probably produced; the reflecting surface of the stratum of air being nearly vertical.

463. Another instance of this kind occurred, during the last war with England, when Commodore Hardy was lying off Boston. A figure of a sailor of a colossal size, was seen by his whole ship's crew reflected in the heavens, during a peculiar state of the atmosphere.

464. To the same cause must be attributed the extraordinary phenomenon, which occurred in the parish of Migne in France, on the 17th of December, 1826. It was Sunday, and 3000 persons were engaged in the exercises of the Jubilee. As a part of the ceremony, a large red cross, twenty-five feet high, was planted beside the church, in the open air. Towards the close of the day, while one of the clergy was addressing the multitude, and reminding them of the miraculous cross beheld in the sky by Constantine and his army, a cross was seen at that moment in the heavens, directly before the porch of the church, and at the height of two hun

Explain in what manner images are produced by reflection.
Are they ever magnified?

How is this accounted for?

Give instances.

dred feet above the ground. Its length was nearly one hundred and forty feet, its breadth from three to four, and it shone with a bright silvery hue, tinged with red.

The assembled multitude were struck with awe, many regarding it as a miracle, and such was the extraordinary sensation produced throughout the country, that a committee was appointed to investigate this phenome

non.

From the circumstances detailed in their report it is evident, that the cross in the sky was the magnified image of the cross before the church, and reflected from the concave surface of some atmospheric mirror.

The

image possessed exactly the shape and proportions of the wooden cross, it was tinged with the same color, and the state of the air at the time was favorable to the formation of such images.

465. SPECTRE OF THE BROCKEN. The gigantic spectre which is supposed to haunt the Hartz mountains in Hanover, and is seen at sunrise from the Brocken, the loftiest peak of the range, is produced in a different It is in fact nothing more than the shadow of the observer, cast upon the thin vapors then floating in the sky. The cut below represents the spectre, as seen by Mr. Haue and another person, on the 23d of May, 1797.

manner.

Standing on the summit of the Brocken at sunrise,

How is the spectre of the Brocken explained?