23

TABLE.

TIME to be ADDED to the RIGHT ASCENSION of a STAR, to find the TIME of its PASSING the MERIDIAN on any day of the YEAR.

Water Spouts.

Remarkable Water-spout in France in 1823.—In the arrondissemens of Dreux and of Mantes, about 3 o'clock of the 26th of August, 1823, a storm came on from the S.W., accompanied with a sudden and powerful heat. A water spout was seen not far from the village of Boncourt, having its broad base resting on the ground, and its summit lost in the clouds. It consisted of a thick and blackish vapour, in the middle of which were often seen flames in several directions. Advancing along with the storm, it broke or tore up by the roots, in the space of a league, seven or eight hundred trees of different sizes, and at last burst with great violence in the village of Marchepoy, one half of the houses of which were instantly destroyed. The walls, overturned to their foundations, rolled down on all sides; the roofs, when carried off, broke in pieces, and the debris were dragged to the distance of half a league by the force of this aërial torrent. Some of the inhabitants were crushed to pieces, or wounded by the fall of their houses, and those who were occupied in the labours of the field, were overthrown or blown away by the whirlwind. Hailstones as large as the fist, and stones and other foreign bodies carried off by the wind, injured several individuals. Carts heavily loaded were broken in pieces, and their loads dispersed. Their axle-trees were broken, and the wheels were found at the distance of 200 or 300 paces from the spot where they were overturned. One of these carts, which had been carried off almost bodily, was pitched above a tilekiln, which had been beaten down, and some of the materials of which had been carried to a considerable distance. A spire, several hamlets, and different insulated houses, were overthrown. Several villages were considerably injured. The lower part of the waterspout is supposed to have been about 100 toises in diameter.-See the Moniteur of the 31st October, where the account is signed by M. Foucault, and the Bibl. Univers. Oct. 1823, p. 133.

Water-spout near Genoa in 1823.-In the communes of Quigliano and Valeggia, in the province of Savona, a heavy rain fell on the 16th September, at 5 o'clock in the morning. It increased to such a degree, that at 9 o'clock in the morning the country was inundated. Towards noon there issued from a mountain situated in the parish of Valeggia, a whirlwind of black smoke and fire. It first carried off the roof of a house, in which two children were crushed to pieces, and the parents wounded. The water-spout then advanced to the opposite side of the mountain, called Magliolo; crossed the river, the waters of which it heaped up in an instant, though they were much swelled;-carried off the roofs of two inhabited houses, and advanced along the same mountain in the district of Quigliano, where it dissipated itself near the Convent of Capuchins, situated in the village. It tore up many large trees of all kinds, and committed ravages, the extent of which is not yet known. The preceding account was sent by the commandant of the province of Savona to the governor of Genoa, in a letter, part of which is published in the Moniteur of the 1st of October, and in the Bibl. Univers. Nov. 1823, p. 135.

Of the Rainbow.

THE phenomena of the rainbow consists, as every person knows, of two bows, or arches, stretching across the sky, and tinged with all the colours of the prismatic spectrum. The internal or principal rainbow, which is often seen without the other, has the violet rays innermost, and the red rays outermost. The external, or secondary rainbow, which is much fainter than the other, has the violet colour outermost, and the red colour innermost. Sometimes supernumerary bows are seen to accompany the principal bows.

As the rainbow is never seen unless when the sun shines, and when rain is falling, it has been universally ascribed to the decomposition of white light by the refraction of the drops of rain, and their reflection within the drops. The production of rainbows by the spray of water-falls, or by drops of water scattered by a brush or syringe, is an experimental proof of their origin.

Let an observer be placed with his back to the sun, and his eye directed through a shower of rain to the part of the sky opposite to the sun. As the drops of rain are spherical particles of water, they will reflect and refract the sun's rays, according to the usual laws of refraction and reflection. Thus, in the following figure, where ssss represent the sun's rays, and A the place of a spectator, in the centre of the two bows (the planes of which are supposed to be perpendicular to his view), the drops a and b produce part of the inner bow by two refractions and one reflection;

and the drops c and d part of the exterior bow, by two refractions and one reflection.

This holds good at whatever height the sun may chance to be in a shower of rain; if high, the rainbow must be low; if the sun be low, the rainbow is high and if a shower happen in a vale when a spectator is on a mountain, he often sees the bow completed to a circle below him. So, in the spray of the sea, or a cascade, a circular rain

g

bow is often seen; and it is but the interposition of the earth that prevents a circular spectrum from being seen at all times, the eye being the vertex of a cone, whose base (the bow) is in part cut off by the earth.

It is only necessary, for the formation of a rainbow, that the sun should shine on a dense cloud, or a shower of rain, in a proper situation, or even on a number of minute drops of water, scattered by a brush or by a syringe, so that the the light may reach the eye after having undergone a certain angular deviation, by means of various refractions and reflections, as already stated. The light which is reflected by the external surface of a sphere, is scattered almost equally in all directions, setting aside the difference arising from the greater efficacy of oblique reflection: but when it first enters the drop, and is there reflected by its posterior surface, its deviation never exceeds a certain angle, which depends on the degree of refrangibility, and is, therefore, different for light of different colours: and the density of the light being the greatest at the angle of greatest deviation, the appearance of a luminous arch is produced by the rays of each colour at its appropriate distance. The rays which never enter the drops produce no other effect, than to cause a brightness, or haziness, round the sun, where the reflection is the most oblique: those which are once reflected within the drop, exhibit the common internal or primary rainbow, at the distance of about 41 degrees from the point opposite to the sun : those which are twice reflected, the external or secondary rainbow, of 52 degrees; and if the effect of the light, three times reflected, were sufficiently powerful, it would appear at the distance of about 42 degrees from the sun. colours of both rainbows encroach considerably on each other; for each point of the sun may be considered as affording a distinct arch of each colour, and the whole disc, as producing an arch about half a degree in breadth, for each kind of light; so that the arrangement nearly resembles that of the common mixed spectrum.

The

A lunar rainbow is much more rarely seen than a solar one; but its colours differ little, except in intensity, from those of the common rainbow.

The appearance of a rainbow may be produced at any time, when the sun shines, as follows; opposite to a window, into which the sun shines, suspend a glass globe, filled with clear water, in such a manner as to be able to raise it or lower it at pleasure, in order that the sun's rays may strike upon it. Raise the globe gradually, and when it gets to the altitude of forty degrees, a person standing in a proper situation, will perceive a purple colour in the glass, and upon raising it higher the other prismatic colours, blue, green, yellow, orange, and red, will successively appear. After this, the colours will disappear, till the globe be raised to about fifty degrees, when they will again be seen, but in an inverted order; the red appearing first, and the blue, or violet, last. Upon raising the globe to about fifty-four degrees, the colours will totally vanish.

In the highest northern latitudes, where the air is commonly loaded with frozen particles, the sun and moon usually appear surrounded by halos, or coloured circles, at the distances of about 22

and 46 degrees from their centres. Several new forms of halos and paraselenae, or mock-moons, have been described by Captain Ross and Captain Parry. And Captain Scoresby, in his account of the Arctic Regions, has delineated an immense number of particles of snow, which assume the most beautiful and varied crystallizations, all depending more or less on six-sided combinations of minute particles of ice.

When particles of such forms are floating or descending in the air, there can be no difficulty in deriving from them those various and intricate forms which are occasionally met with among this class of phenomena.

Halos are frequently observed in other climates, as well as in the northern regions of the globe, especially in the colder months, and in the light clouds which float in the highest regions of the air. The halos are usually attended by a horizontal white circle, with brighter spots, or parhelia, near their intersections with this circle, and with portions of inverted arches of various curvatures; the horizontal circle has also sometimes anthelia, or bright spots nearly opposite to the sun. These phenomena have usually been attributed to the effect of spherical particles of hail, each having a central opaque portion of a certain magnitude, mixed with oblong particles, of a determinate form, and floating with a certain constant obliquity to the horizon. But all these arbitrary suppositions, which were imagined by Huygens, are in themselves extremely complicated and improbable. A much simpler, and more natural, as well as more accurate explanation, which was suggested at an earlier period by Mariotte, had long been wholly forgotten, till the same idea occurred to Dr. Young. The explanation given by the last mentioned philosophers is, that water has a tendency to congeal or crystallize in the form of a prism, and that the rays of light passing through these prisms (which are disposed in various positions,) by their own weight, are so refracted as to produce the different appearances which halos and parhelia have been observed to assume.

The colours which these phenomena exhibit, are nearly the same as the rainbow, but less distinct; the red being nearest to the luminary, and the whole halo being very ill defined on the exterior side. Sometimes the figures of halos and parhelia are so complicated, as to defy all attempts to account for the formation of their different parts; but if the various forms and appearances which the flakes of snow assume, be considered, there will be no reason to think them inadequate to the production of all these appearances.

On the Line of Perpetual Congelation.

In consequence of the diminution of temperature which is experienced as we ascend in the atmosphere, it is evident that in every climate a point of elevation may be reached where it will be continually freezing. The altitude of the point above the surface of the earth, will depend partly on the temperature of the lower regions of the atmosphere, and partly on the decrement of heat belonging to the column at the period of observation, Thus, near the equator, it was observed by Bouguer, that it began to freeze on the sides of