UNI OF

OF RAINBOWS, PARHELIA, &c.

101

formed by the rays of the sun falling on the lower parts of the drops of rain; these rays also undergo two refractions; one when they enter the drops, and another when they emerge from them to proceed to the eye but they suffer two, or more, reflections in the interior surface of the drops; hence, the colours of these rays are not so strong and well defined as those in the interior bow, and appear in an inverted order; viz. from the under part they are red, orange, yellow, green, blue, indigo, and violet.

The rays which fall on the drops that produce the interior bow proceed to the eye of the spectator in a direction, that makes an angle of about 42 degrees with the direction in which they entered the drops; and those that form the exterior bow at an angle of about 54 degrees.*

This may be proved by a simple experiment, as follows: let a glass globe, filled with water, be suspended in the sun-shine, and let a person turn his back to the sun, and view the globe at such a distance, that the part of it which is farthest from the sun may appear of a full red colour; then will the rays which come from the globe to the eye make an angle of 42 degrees with the sun's direct rays; and, if the eye remain in the same position, while another person gradually lowers the globe, the orange, yellow, and other colours, will appear in succession, as in the interior rainbow. Again: if the glass globe be elevated, till the side nearest the sun appear red, the rays which come from the globe to the eye will make an angle of about 50 degrees; and if the globe be again gradually raised as before, the rays will successively change from red to orange, yellow; &c. as in the exterior bow.

All rainbows are arcs of equal circles, and, consequently, are all equally large, though we do not always see an equal quantity of them: for the eye of the spectator is the vertex of a cone, and its circular base is the rainbow, of which one half is the greatest portion that can be seen at once.

Although lunar rainbows have been observed, yet they occur but very seldom. A very brilliant and remarkable one was seen in the year 1710, at Glopwell Hall, in Derbyshire, about eight o'clock in the evening. The moon had passed the full about twenty-four hours, and the evening had been rainy; but the clouds were dispersed, and the moon then shone very clear.

This iris lunaris had all the colours of the solar iris exceedingly beautiful and distinct, but faint in comparison with those that are seen when the sun is shining very bright; as must necessarily have been the case, both from the different beams by which it was occasioned, and the disposition of the medium. What most surprised the observer was, the largeness of the arc, which was not much less than that produced by the sun.

Several complete and concentric solar rainbows have sometimes

*The angle which the emerging ray makes with the incident ray in the Interior bow is 42° 2′ for the red, and 40° 17' for the violet; and for the Exterior bow, these angles are 50° 57′ and 54° 7'.-Therefore, the space between the bows is about 9 degrees broad.

been observed in mountainous countries. This extraordinary phenomenon, first seen by Don Ulloa and his companions in the wild heaths of Pambamarca, which he describes as follows," At the side opposite to that where the sun rose on the mountain, at the distance of about sixty yards from the spot where we were standing, the image of each of us was represented as if in a mirror; three concentric rainbows, the last or more exterior colour of one of them touched the first or interior colours of the following one, being centred on the head. On the outside of these, at an inconsiderable distance from them, was seen a fourth arc, purely white. They were all perpendicular to the horizon; and as any one of us moved from one side to the other, he was accompanied by the phenomenon, which preserved the same order and disposition. But what seemed most remarkable was, that, although six or seven persons were standing close together, each of us saw the phenomenon as it regarded himself, but did not perceive it in the others."

A similar phenomenon is described by Mr. Hagarth, F. R.S. as having been seen by him on the evening of the 13th of February, 1780, when ascending a mountain at Rhealt, in Denbighshire.

Another singular phenomenon is sometimes to be seen in the heavens, which very much resembles the sun; and, on that account, it has received the name of Parhelion, or mock sun, An extraordinary appearance of this kind was seen near Marienberg, in Prussia, on the 5th of February, 1674. It was of the same apparent size with the sun, which was several degrees above the horizon at the time, and shone with great lustre. The mock sun appeared under the real one, and seemed to increase in lustre as the true sun descended to the horizon, insomuch, that the reddish colour it first exhibited completely vanished; and it put on the genuine solar light, in proportion as the dise of the real sun approached it. At last the real sun immerged into the counterfeit sun, and remained alone. This phenomenon was considered the more extraordinary, as it appeared perpendicularly under the sun, instead of being to the right or left of it, as parhelia usually are, and of a colour so different from that which mock suns usually exhibit.

One or two appearances of the same kind have been seen in England since that time. On the 28th of August, 1698, about eight o'clock in the morning, there was seen at Sudbury, in Suffolk, the appearance of three suns at the same time, all extremely brilliant, Beneath a dark watery cloud, in the east, the true sun shone with such splendour that the spectators could not look at it; and on each side were the reflections. The circles were not coloured like the rainbow, but white. At the same time, the form of a half moon was visible toward the south, at a considerable distance from the other phenomena, but apparently double the size of the half moon, and of a red colour like that of the rainbow. These phenomena faded away gradually, but continued visible for more than two hours. Two mock suns, an arc of a rainbow, and a halo, were seen at Lyndon, in the county of Rutland, on the 22d of October, 1721, at eleven in the

morning. The parhelia or mock suns were bright and distinct,* They were of a reddish colour towards the sun, but pale or whitish toward the opposite sides, which was also the case with the halo. Still higher in the heavens was an arc of a rainbow, of a curiously inverted form, situated about half way between the halo and the zenith. This arc was as distinct in its colours as the common rainbow, and of the same breadth.

The red colour was on the convex, and the blue on the concave of the arc, which seemed to be about 90 degrees in length; its centre being very near the zenith. On the top of the halo was a kind of inverted bright arc, of considerable extent. This phenomenon was seen on the following day, and again on the 26th of the same month. Several other phenomena are sometimes to be seen in the heavens, but these are too ephemeral to merit any notice here.

OF THE ATMOSPHERE, AND ASTRONOMICAL
REFRACTION.

The earth is surrounded by a thin fluid mass of matter, called the Air or Atmosphere, which revolves with it in its diurnal motion, and goes round the sun with it every year. This fluid is both ponderous and elastic, Its weight is known from the Torricilian experiment, or that of the barometer; and its elasticity is proved by simply inverting a vessel full of air in water.

The atmosphere of the earth's surface being pressed by the weight of all above it, is there pressed the closest together; and therefore the atmosphere is densist of all at the earth's surface; and its density necessarily diminishes the higher up. For each stratum of air is compressed only by the weight of those above it; the upper strata are therefore less compressed, and consequently less dense, than those below them.

The pressure or weight of the atmosphere has been repeatedly determined, by various experiments, to be about fourteen pounds on every square inch of the earth's surface. Hence, the total pressure on the whole surface of the earth is 10,686,000,000 hundreds of millions of pounds avoirdupois.

From a number of experiments made on the density of the atmosphere, at various altitudes, by means of the barometer, it has been ascertained, that if heights, from the earth's surface, be taken in arithmetical progression, the density of the corresponding strata of air decrease in geometrical progression. Thus the density of the atmosphere is reduced one-half for every 3 miles of perpendicular ascent. At seven miles in height, the corresponding density is only ope-fourth; at 10 miles, one-eighth; at 14 miles, one-sixteenth

* A Halo is an extensive luminous ring, which is sometimes seen to surround the sun and moon, and is supposed to be occasioned by the light of these bodies through the intervening clouds. This appearance is most frequent about the

moon.

and so on. Since the density of the air decreases at this rapid rate, it is evident that at a very moderate distance from the surface of the earth, its density would be so much diminished, as to render it incapable of sustaining animal life. From observation and experiment, it is pretty well known that 45 or 50 miles is the utmost height at which the density is capable of refracting a ray of light; and, therefore, this may be considered the altitude corresponding to the least sensible degree of density; for, according to the law of its decrease, just stated, the density at this altitude is above 10,000 times less than at the earth's surface.

One of the most extraordinary and useful properties of the atmosphere, is its reflective power, which causes the heavens to appear luminous when the sun shines; for were it not for this power, the whole of the heavens, and every thing on the earth, would appear black, or completely dark, except what the sun's rays directly impinged upon. The stars would be visible by day as well as by night; and we could see nothing except what was fully exposed to the sun. There could be no twilight, and, consequently, the blackest darkness would immediately succeed the brightest sun-shine when the sun sets; and the transition would be equally sudden from the blackest darkness to the brightest sun-shine when the sun rose. But by means of the atmosphere we enjoy the sun's light, reflected from the aërial particles for some time before he rises, and also for some time after he sets. For when the earth, by its revolution on its axis, has turned any particular place away from the sun, the atmosphere above that place will continue to be illuminated for some time. However, as the sun gets farther below the horizon, the less will the atmosphere be illuminated; and when he has got eighteen degrees under the horizon, cease to be illuminated, and then all that part of the heavens which is over the place will become dark: for the place will then be turned too far from the sun, and his rays will strike too high on the atmosphere to be refracted or bent downwards at that place. In consequence of the refractive power of the atmosphere, all the heavenly bodies appear higher than they really are; for, on account of the variation in the density of the air, a ray of light in passing through it will be refracted at every instant, and consequently the path of the ray will be a curve. And as an object is always seen in the direction in which the rays of light proceeding from it enter the eye, it is evident every celestial body will appear more elevated above the horizon than it actually is, by a quantity equal to the refraction which a ray of light suffers in passing from it through the atmosphere to the eye

of the observer.

When the object is in the zenith, the refraction is quite insensible; but it increases as the altitude of the object diminishes, till it reaches the horizon, and then the refraction is greatest; for the rays which proceed from the object, in that situation, enter the atmosphere more obliquely than in any other, and consequently are more turned out of their course.

figure.

This will be evident from an inspection of the following

Where BOD represents the surface of the earth, O the place of an observer, and FGH the surrounding atmosphere. A ray of light proceeding from a body, Z, in the zenith, is not refracted; but if it proceed from a body at A, it will enter the eye at O, and appear in the direction Oa; if the body be in the horizon, as at E, the rays proceeding from it will enter the eye at O, and appear to come in the direction e O.

On some occasions, the horizontal refraction amounts to 36 or 37 minutes, and, generally, to about 33 minutes, which is equal to the diameter of the sun or moon; and therefore the whole disc of the sun or moon will appear above the horizon, both at rising and setting, although actually below. This is the reason 'that the full moon has sometimes been seen above the horizon before the sun was set. A remarkable instance of this kind was observed at Paris, on the 19th of July, 1750, when the moon appeared visibly eclipsed, while the sun was distinctly to be seen above the horizon.

At some seasons of the year the sun appears ten minutes sooner above the horizon in the morning, and continues as much longer above