OF SNOW AND HAIL.

58. Snow consists of those vapours which are frozen while the particles are small; for, if these stick together after they are frozen, the mass formed out of them will be of a loose texture, in little flakes or fleeces, of a white substance, somewhat heavier than the air: they therefore descend in a slow and gentle manner in snow. But you will observe that snow is formed by this process of regular crystallization among minute frozen particles of water floating in the air. It is remarkable, that previous to, and during, the fall of snow in quantity, the temperature continues about 32°.

The structure of a flake of snow proves that a drop of rain is also composed of a great number of smaller drops. When these come together in the act of freezing suddenly, they form a nucleus of white spongy ice, which by its extreme coldness, becoming incrusted with clear ice, from the water it collects in its descent, constitutes hail as we usually see. Hail has been likewise observed perfectly transparent, and having the form of an oblate spheriod, shewing that it consisted of drops which had been frozen entire in falling with a rotatory motion.

Hail, which is a more compact mass of frozen water, consists of such vapours as are united into drops, and are frozen while they are falling. They assume various figures, being sometimes round, at other times pyramidal, cuneated, angular, thin, and flat, and sometimes stellated, with six radii like the small crystals of snow; and natural historians furnish us with various accounts of surprising showers of hail, in which the hailstones were of extraordinary magnitude.

OF HALOS, PARHELIA, &c.

59. A halo is an extensive luminous ring, including a circular area, in the centre of which the sun or moon appears; whose light, passing through an intervening cloud, gives rise to the phenomenon. Those about the moon are most common.

Of Corona. When the sun or moon is seen through a thin cloud, a portion of the cloud, round the sun or moon, ap

pears lighter than the rest; and this luminous disc is called a corona. They are of various sizes, but they seldom exceed 10°. in diameter: they are generally faintly coloured at their edges. Frequently when a halo encircles the moon, a corona surrounds it.

Parhelia or mock suns vary considerably in general appearance: sometimes the sun is encircled by a large halo, in the circumference of which the mock suns usually appear: which have often small halos round them.

The paraselene, the parhelion, and the several kinds of halo and corona, all appear to result from the intervention of clouds between the spectator and the sun or moon, through which the light passes.

OF METEORS.

60. The most common sort of igneous meteors, are those very small meteors which are prevalent in clear frosty winter nights, and in summer also, when there are dry easterly winds with a clear sky. They leave little or no train behind them, and shoot along in straight lines, generally oblique downwards.

The falling stars, and other fiery meteors, which are frequently seen at a considerable height in the atmosphere, and which have received different names according to the variety of their figure and size, arise from the fermentation of the effluvia of acid and alkaline bodies, which float in the atmosphere. When the more subtile parts of the effluvia are burnt away, the viscous and earthy parts become too heavy for the air to support, and by their gravity fall to the earth.

The disappearance of fiery meteors is frequently accompanied by a loud explosion like a clap of thunder, and heavy stony bodies have been observed to fall from them to the earth. Dr. Thomson has given a table of thirty-six showers of stones, with the places where they fell, the dates and the testimonies annexed.

These stony bodies when found, are always hot, and their size differs from a few ounces to several tons. They are usually round, and always covered with a black crust. When broken, they appear of an ash-grey colour, and of a granu

lar texture like coarse sand-stone. These substances are probably concretions actually formed in the atmosphere, but in what manner no rational account has yet been given.

61. Of the Ignis Fatuus, commonly called Willwith-a-Wisp, or Jack with a Lantern. This meteor, like most others, has not failed to attract the attention of philosophical inquirers. Sir Isaac Newton, in his Optical Queries, calls it a vapour shining without heat. Various accounts of it may be seen in the Philosophical Transactions. The most probable opinion is, that it consists of inflammable air, or oleaginous matter, emitted from a putrefaction and decomposition of vegetable substances, in marshy grounds; which being kindled. by some electric spark, or other cause unknown to us, will continue to burn or reflect a kind of thin flame in the dark, without any sensible degree of heat, till the matter which composes the vapour is consumed. This meteor never appears on elevated grounds, because they do not sufficiently abound with moisture to produce the inflammable air, which is supposed to issue from bogs and marshy places. It is often observed flying by the sides of hedges, or following the course of rivers: the reason of which is obvious, for the current of air is greater in these places than elsewhere.

These meteors are very common in Italy and in Spain. Dr. Shaw has described a remarkable Ignis Fatuus, which he saw in the Holy Land, when the atmosphere was so uncommonly thick and hazy, that the dew on the horses' bridles was remarkably clammy and unctuous. This meteor was sometimes globular, then in the form of the flame of a candle, presently afterwards it spread itself so much as to involve the whole company in a pale harmless light, and then it would contract itself again, and suddenly disappear; but, in less than a minute, it would become visible as before, and running along from one place to another, with a swift progressive motion, would again expand itself, and cover a considerable space of ground.

62. OF THE AURORA BOREALIS, or NORTHERN LIGHTS. There have been various opinions and conjectures respecting the cause and properties of these extraordinary phenomena; and the most probable opinion is, that they arise from exhalations, and are produced by a combustion of inflamable air, caused by electricity.

This inflammable air is generated particularly between the tropics, by many natural operations, such as the putrefaction of animal and vegetable substances, volcanoes, &c.; and being lighter than any other, ascends to the upper regions of the atmosphere, and, by the motion of the earth, is urged towards the poles; for it has been proved by experiments that, whatever is lighter, or swims on a fluid which revolves on an axis, is urged towards the extreme points of that axis : hence these inflammable particles continually accumulate at the poles, and by meeting with heterogeneous matter take fire, and cause those luminous apparences frequently seen towards the polar regions.

We have very few accounts of the Aurora Australis, or Southern Lights, owing perhaps to the want of observations in those remote parts of the globe, and a proper channel of information. Captain Cook, in his second voyage towards the south pole, says: (February 17th, 1773,) We observed a beautiful phenomenon in the heavens, consisting of long columns of clear white light, shooting up from the heavens to the eastward, almost to the zenith, and gradually spreading over the whole southern part of the sky. Though these columns were in most respects similar to the Aurora Borealis, yet they seemed to differ from them in being always of a whitish colour. The stars are sometimes hid by, and sometimes faintly to be seen through the substance of these Aurora Australes. The sky was generally clear when they appeared, and the air sharp and cold, the thermometer standing at the freezing point; the ship being in latitude 58° south.

In high latitudes the Aurora Boreales appear with the greatest lustre, and extend over the greater part of the hemisphere, varying their colours from all the tints of yellow to the most obscure russet. In the north-east parts of Siberia, Hudson's Bay, &c. they are attended by a continued hissing and cracking noise through the air, similar to that produced by fire-works.

63. OF THE RAINBOW. The rainbow is the most beautiful meteor with which we are acquaint

ed; it is never seen but in rainy weather, where the sun illuminates the falling rain, and when the spectator turns his back to the sun. There are frequently two bows seen, the interior and exterior bow. The interior bow is the brightest, being formed by the rays of light falling on the upper parts of the drops of rain; for a ray of light entering the upper part of a drop of rain will, by refraction, be thrown upon the inner part of the spherical surface of that drop, whence it will be reflected to the lower part of the drop, where undergoing a second refraction, it will be bent towards the eye of the spectator, hence the rays which fall upon the interior bow come to the eye after two refractions and one reflection, and the colours of this bow from the upper part are red, orange, yellow, green, blue, indigo, and violet. The exterior bow is formed by the rays of light falling on the lower parts of the drops of rain; these rays, like the former, undergo two refractions, viz. one when they enter the drops, and another when they emerge from the drops 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 orer, viz. from the under part they are red, orange, yellow, green, blue, indigo, and violet.

To illustrate this by experiment, suspend a glass globe filled with water in the sun-shine, turn your back to the sun, and view the globe at such a distance that the part of it the 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, and another person lower the glass globe gradually, the orange, yellow, green, &c. colours, will appear in succession, as in the interior bow. Again, if the glass globe be elevated, so that the side nearest to the sun may appear red, the rays which come from the globe to the eye will make an angle of about 50 degrees; then, if another person gradually raise the glass globe, while the