barometer rises, the thermometer falls, the relative humidity diminishes, and the tension of aqueous vapor increases.

VIII.-FRACTO-CUMULUS, (POËY.)

Wind-cloud, (illustrations Nos. XIII and XIV.)-The clouds which I have denominated fracto-cumulus are isolated fragments of cumulus, more or less considerable, without determinate form, jagged at the edges, the lowest and most rapid of all, and whitish, grayish, or slate-colored, according to their density. As soon as an invisible storm has broken out in the distance, we see them moving with great rapidity, almost grazing the highest buildings and tallest trees; their borders are greatly torn and white, contrasting strongly with the grayish stratum of superior pallio-cumulus. They are visible day and night, and often traverse the firmament from northeast to southwest without interruption for many days, while the sky above and in the intermediate space is perfectly clear. In winter we see them alone under a blue sky, sending in their passage to the zenith interrupted showers of rain, accompanied by strong gusts of wind, which occasion a very slight elevation and oscillation in the barometric column. In the Antilles these clouds produce the disagreeable winter rains, and in Europe the March storms. They generally follow the direction of the wind prevailing at the surface of the earth. When this wind is contrary to the direction of fracto-cumulus, it soon takes the same course. A little before a storm or tempest arises, there appear a series of very small fracto-cumuli, which move rapidly, almost to two-thirds of their height, along a considerable mass of cumulus, which is stationed very often, as if immobile, near the southern horizon. Soon these fractocumuli become more abundant, less rapid, and form a horizontal band, which cuts the cumuli near their summit. This appearance is a warning sign for sailors, as it announces a squall. In fact, the fracto-cumuli become more and more developed; an exchange of opposite electricities takes place between them, and the storm quickly arises. It is, therefore, the same little cloud, of which I have spoken above, that, returning then from the combat, comes now to offer new battle. The aggregation of fracto-cumulus forms the pallio-cumulus, and increases and constantly maintains this stratum.

NATURE OF THE CLOUDS DEDUCED FROM THE FORMATION OF HALOS, CORONAS, AND RAINBOWS.

We can further distinguish the nature of clouds by the optical phenomena to which they give rise, according as their intimate constitution is more or less connected with a certain degree of aqueous vapor, in the state of snowy or glacial congelations of the strata corresponding to the formation of each type. The following are some facts which I have observed at Havana, which it is important to verify in other regions: Generally speaking, cirrus, more especially pallio-cirrus, gives rise to a large solar and lunar halo of 220 radius. When it is produced by the

sun, it sometimes presents the seven colors of the spectrum, although usually there is only a single internal tint of orange, terminating at times in a little red. On the contrary, the great halo produced by the moon is almost always white, and seldom exhibits the tint of orange without the red.

The cirro-cumuli produce the lunar halo of about 20 to 40 radius, which may be triple or formed of sixteen prismatic rings with an internal tint of red. This halo is still more brilliant when it takes place, rarely enough, with cirro-stratus.

The fracto-cumuli are the only clouds which do not produce halos, but they do produce complete coronas or segments of ares, according to the extent of the fragments which traverse the lunar disk. These coronas are also prismatic, having a blue internal tint.

The pallio-cumulus and the cumulus form neither halos nor coronas, but solar and lunar rainbows. In fine, aqueous vapor extremely dissolved, elastic, uniformly distributed in the higher regions of the atmosphere, without much altering the transparency of the air, gives rise to the formation of a little halo. Their unique coloration, in brown or russet, light or dark, as well as their size, is intimately connected either with the density of aqueous or elastic vapors or their altitude; their dimensions may vary from the borders themselves of the lunar disk up to 20 radius. We find them in every lunation.

QUANTITY OF CLOUDS.

We measure by the eye the blue vault of the sky, or the quantity of visible clouds, which can then be determined according to a conventional scale in decimal fractions from zero (0) to unity, (1.) But it is preferable to take directly the quantity of clouds, and to repeat this calculation for each quadrant, upon each stratum and upon each type, instead of restricting ourselves to the ensemble of the sky, paying no attention to their nature, as has been hitherto everywhere done.

Here is the mode of proceeding: We explore the first quadrant, and if we find three different types of clouds-for example, the cirrus elevated, cumulus at the horizon, and fracto-cumulus low and isolated— we judge one after the other, according to their extent in height and breadth, the space which they occupy relatively to the 90° comprised from north to east, and from the horizon to the zenith of this quadrant. We then write for example in the corresponding columns 0.4 cirrus, 0.3 cumulus, and 0.2 fracto-cumulus. If the quadrant examined is completely covered by a single cloud, we mark unity, (1,) and its corresponding type. If, on the contrary, there is no cloud, we place 0. We repeat in turn the same operation in the three other quadrants, from southeast, from southwest, and from northwest.

At times the quantity of clouds associated with others of a different nature is so small, consisting merely of fragments, that it would be extremely difficult to make a just calculation, and then we mark isolated