the coasts, the water assumes a lighter shade. This apparent color of the sea may be explained upon the same principle as that of the azure blue of the atmosphere. Both fluids are colorless when in a glass; the air reflects chiefly the most refrangible rays of light, viz. the violet, indigo, and blue, and therefore usually appears of an azure color, the result of a mixture of these: but the sea, from its density and depth, is able to reflect not only many blue and violet, but also some of the less refrangible rays in sufficient proportion to compose a greenish blue. The other shades in the color of seawater depend on illusory or local causes. The green and yellow shades of the sea arise from marine plants; a distinct shade is often communicated to its surface by the presence of myriads of minute insects: and in shallow water, the light reflected from the sand at the bottom often gives a reddish hue to the surface. In the West Indies, where "The floor is of sand like the mountain-drift;

And the pearl-shells spangle the fifty snow,"

the waters of the ocean are often so beautifully transparent, as to exhibit the minutest object they contain or cover at the depth of several fathoms. In the Gulf of Guinea the sea is white; and around the Maldive islands it is black.

A very curious and magnificent spectacle is often presented at night by the luminous appearance of the sea,-a phenomenon which seamen generally regard as the precursor of blowing weather. It is of most frequent occurrence in summer and autumn. Three species are generally distinguished. The first is generally seen close to a ship when sailing before a fresh wind, and forms a tail of light in the wake of the ship; at other times, during stormy weather, it spreads over the whole surface of the sea, clothing it apparently in a sheet of fire. This species is ascribed to electricity. The second kind of marine phosphorescence, penetrates beneath the surface; and when a quantity of the illuminated water is put into a vessel, it retains the brilliance as long as it is kept agitated, but loses it as soon as the agitation subsides. This species occurs during dead calms or in very hot weather, and seems to be a true phosphoric light, emanating from particles of putrid animal matter suspended in the water. The third species exceeds the two former in intensity of brilliance; and it is supposed that the appearance is occasioned by innumerable minute animals of a round shape, moving rapidly through the water in all directions, like so many luminous sparks.

The sea is subject to various motions, arising from differe t causes. Even when unruffled by the winds, it is agitated by the rotation of the earth, and the attraction of the moon and the sun. These three causes produce a threefold motion, viz. the motion of the waves, that of the currents, and that of the tides.

The most wonderful and important motion of the sea is that of

high and low tide, or that regular ebb and flow of the sea whic:. occur every day at a certain interval. The sea rises to its greatest height in about 6 hours, and remains stationary for about 6 minutes; after which it recedes for other 6 hours, and having remained stationary at its lowest tide for a few minutes, begins to rise again. In the Baltic and the Black Sea there is no tide; and almost none in the Mediterranean.

Besides these motions of the ocean, there is another not so easily accounted for. There is felt in the open sea between the Tropics, and as far as the 30th degree of latitude, a constant motion from east to west, which manifests itself in the quick sailing of vessels moving in that direction. The most celebrated of these currents is the Gulf-stream, which rises in the Gulf of Mexico, between Florida and the Bahama islands, and sets in a bended and expanded flow north-easterly, along the coasts of North America, till it reaches Norway, whence repulsed by the Scandinavian coasts, it turns N. W. towards Greenland. The current is known by the beautiful blue color of its waters. When two or several currents meet each other, or cross at angles, violent circular motions of the sea are produced, which attract every thing coming within their vortex, and whirling it round in decreasing gyrations, finally ingulf it in their bosoms. These motions of the sea are called whirlpools. Some naturalists believe that they mark the situation of profound abysses in the bottom of the sea, into which the water precipitating itself produces this dangerous suction. Among the most remarkable whirlpools is that of Chalcis in the Euripus, near the coast of Smreee, which alternately absorbs and rejects the water seven times every twenty-four hours. Charybdis, near the Strait of Sicily, rejects and absorbs the water thrice in twenty-four hours. The largest known whirlpool is the Maelstrom in the Norwegian sea, the circumference of which exceeds 20 leagues.

CLASSIFICATION OF CLOUDS.

The clouds are aqueous vapors, which hover at a considerable height above the surface of the earth. They differ from fogs only Ly their height and less degree of transparency. The distance of the clouds from the surface of the earth is very different. Thin and light clouds are higher than the highest mountains; thick and heavy clouds on the contrary, touch low mountains, steeples, and even trees. The average height of the clouds is calculated to be two miles and a half. Innumerable as the forms of clouds

may appear to be, correct observers have stated that they may be all comprised in seven modifications.

Fig.
1.

5.

6

7.

8.

These following modifications are arranged in the order of their ordinary elevation, but which is very frequently deranged. We give the names both in Latin and English; the former are perhaps most generally used. The figures refer to the above engraving.

FIG. 1, CIRRUS

2, CIRROCUMULUS

Curl-Cloud.

Sonder-Cloud.

Fig. 1. The curling and flexuous forms of this cloud constitute its most obvious external character, and from these it derives its name. It may be distinguished from all others by the lightness of its appearance, its fibrous texture, and the great and perpetually changing variety of figures which it presents to the eye. It is generally the most elevated, occupying the highest regions of the atmosphere. The comoid cirrus cloud, vulgarly called the mare's tail, is the proper cirrus. It has, as represented in the engraving, somewhat the appearance of a distended lock of white hair, or of a bunch of wool pulled out into fine pointed ends. This variety is an accompaniment of a variable state of weather, and forebodes wind and rain.

Fig. 2. This consists of extensive beds of a number of little, well-defined, orbicular masses of clouds, or small cumuli, in close horizontal opposition; but at the same time lying quite asunder (sonder-cloud), or separate from one another. Their picturesque appearance in summer often presents, as Bloomfield expresses it,

The beauteous semblance of a flock at rest.

This variety is commonly a forerunner of storms, and has been remarked as such by the poets.

Fig. 3, 4, 5, 6, 7. This cloud is distinguishable by its flatness, and great horizontal extension in proportion to its perpendicular height. Under all its various forms, it preserves this characteristic. As it is generally changing its figure, and slowly subsiding, it has received the name of wane-cloud. Sometimes this cloud is disposed in wavy bars or streaks, in close horizontal opposition, and these bars vary infinitely in size and color, generally blended in the middle, but distinct towards its edges, fig. 4. A variety not unlike this, is the mackerel-back sky of summer evenings. It is often very high in the atmosphere. Another common variety appears like a long streak, thickest in the middle, and wasting away at its edges. This, when viewed in the horizon, has the appearance of fig. 7. Another principal variety of the cirrostratus is one which consists of small rows of little clouds, curved in a peculiar manner; it is from this curvature called cymoid. fig. 5. This cloud is a sure indication of stormy weather. Fig. 6 is the representation of a similar one, less perfectly formed, having more of the character of the cirrocumulus, and is often produced when a large cumulus passes under the variety marked fig. 7. Another remarkable developement of this varying genus is, that extensive and shallow sort of cloud, which occurs particularly in the evening ad during night, through which the sun and moon but faintly

appear. It is in this cloud that those peculiar refractions of the light of those bodies, called halos, mock suns, &c. usually appear. This variety is the surest prognostic we are acquainted with, of an impending fall of rain or snow.

Fig. 9. The base of this modification is generally flat, and lies on the surface of an atmospheric stratum, the superstructure resembling a bulky cumulus overhanging its base in large fleecy protuberances, or rising into the forms of rocky mountains. Considerable masses of these frequently are grouped upon a common stratum or base, from which it has been named cumulostratus. It derives the other appellation, twain-cloud, from the frequently visible coalescence of two other modifications, as, for example, the cirrus and the cumulus. Cumulostratus often evaporates, sometimes changes to cumulus, but, in general, it ends in nimbus, and falls in rain. In long ranges of these clouds it has been observed that part has changed into nimbus, and the rest remained unchanged.

Fig. 9. This cloud is easily known by its irregular hemispherical or heaped superstructure, hence its name cumulus, a heap or pile. It has usually a flattened base. The mode of its formation is by the gathering together of detached clouds, which then appear stacked into one large and elevated mass, or stackencloud. The best time for viewing its progressive formation is in fine settled weather. It may be called the cloud of day, as it usually exists only during that period.

Fig. 11. This is not a modification depending upon a distinct change of form, but rather from increase of density and deepening of shade in the cumulostratus, indicating a change of structure, which is always followed by the fall of rain. This has been, therefore, called nimbus, (a rainy black cloud.) Any one of the preceding six modifications may increase so much as to obscure the sky, and, without falling in rain, "dissolve," and "leave not a rack behind." But when cumulostratus has been formed, it sometimes goes on to increase in density, and assume a black and portentous darkness. Shortly afterwards the intensity of this blackness yields to a more gray obscurity, which is an evidence that a new arrangement has taken place in the aqueous particles of the cloud; the nimbus is formed, and rain begins to fall.

Fig. 11. This kind of cloud rests upon the surface of the globe. It is of variable extent and thickness, and is called stratus, a bed or covering. It is generally formed by the subsidence of vapor in the atmosphere, and has, therefore, been denominated fall-cloud. This genus includes all fogs, and those creeping mists, which in summer evenings fill the valleys, remain during the night, and disappear in the morning. This cloud arrives at its density about midnight, or between that time and morning, and it generally disappears about sunrise. It is, for this reason, called by some, the cloud of night.