atmost fury, to apparently return with the wind in quite the op posite direction, after an interval of 15 or 20 minutes of perfect calm. The cause of these calms and the course of the air in the focus of a circular storm, is represented in the diagram.

We have scarcely room to add a description of the famous Rodriguez hurricane of April 1843. Such a fleet of wrecks from one storm never was seen before, having 14 or 15 vessels entangled its stormy circuit, some on its skirts, others crossing its path and following its wake, some rushing into the very focus and scudding around the vortex till rendered perfectly unmanageable by the fury of the sea and wind; its course could be tracked for 1500 miles, demonstrating beyond a doubt, that it was a vast whirlwind. We We give a diagram of its position on the 4th of April, when its centre had just passed over the island of Rodriguez. The arrows represent the positions of the vessels and the direction of the wind, as gathered from the log-books. Though some of these vessels were but a few hundred miles from each other, yet they had the wind from opposite quarters.

Besides the winds which we have mentioned we may enumerate among local winds, or winds which affect only particular sections of country, the Sirrocco, a hot wind, moist and relaxing, which visits Naples and the south of Italy, blowing from the opposite shores of the Mediteranean. This wind is very unhealthy; during its continuance all nature seems to languish, the vegetation droops and dies, and the animal spirits are too much exhausted to per

mit any exertion. The Harmattan is a cold dry wind of very

parching quality, frequent in Africa; it is a periodical wind of uncertain continuance, and is attended by a thick fog or haze, which either wholly obscures the sun or makes it appear a faint red easily borne by the eye. It is so dry that the eyes, lips, palate &c., are parched and painful, causing the skin to shrink and crack, and sometimes to peel off. It is however considered as an effectual cure for some diseases. The Samiel or Simoon, is a burning, pestilential blast, extremely arid, which springs up at times in the vast deserts of Africa, and rushes with tremendous fury, involving whole pillars of sand. It produces instantaneous death, and mortifies the body so effectually, that the limbs may be separated without difficulty.

The camels seem to have an almost instinctive

notice of its approach, and are so well aware of the noxious quali ties arising from its extreme dryness, that they bury their noses in the sand to avoid breathing it. So impetuous is this wind that its fury is past in a few moments.

We have now briefly described the more prominent winds which blow over the surface of our globe. We shall, when considering the changes which have modified, or entirely altered the face of the various continents and islands, which at present, form the "dry land" of our planet, perceive that they have acted a most important part. No force can resist the perpetual assault of the winds and waves. The sea, lashed into fury by the careering tempest, forces its way through barriers of porphyry and granite, undermines the rocky cliffs, and piles immense dunes or sandhills along the low shores. Beneath the fine sands of the boundless eastern deserts, the monuments of ancient Egypt, her statues and sculptured temples, have lain hidden for ages, but the dry impalpable powder has fallen harmless upon them, and now, when the curious traveler from the western world, from lands unknown to the hierophants, uncovers with careful hands the buried tombs of kings, he beholds the color glowing upon the walls as bright, and perfect, as when first laid on by the Egyptian artist 3000 years ago; and the winged globe and heads of Isis as sharply outlined as though chiseled but yesterday.

CLOUDS.

137

CHAPTER III.

Formation of Clouds and Dew.

"The clouds consign their treasures to the fields,
And, softly shaking on the dimpled pool
Prelusive drops, let all their moisture flow
In large effusion o'er the freshen'd world."

Thomson.

THE relations of the atmosphere to water are very important and numerous. The various winds as they sweep over large tracts of country, or the ocean, become charged with moisture, which they bear with them to the higher regions, to be there employed in the formation of clouds, or rain. The formation and dissolution of clouds, produces all the varied train of meteorological phenomena. The humidity suspended in the atmosphere is derived by exhalation partly from the land, but ultimately from the vast expanse of the ocean. The moisture, deposited by the air is in the form of minute globules, which remain suspended, or subside slowly, constituting a cloud. When it comes near us, whether it hovers on the tops of the hills, or spreads over the valleys, it receives the name of a fog; when deposited from the air in a clear night, upon the surface of the ground, or bodies exposed to the air, it is called dew.

In order to explain more clearly the formation of clouds and also the deposition of dew, let us consider in what manner the capacity of air for moisture will be affected either by heat or cold. By capacity, we mean power of stowing away so as to render invisible. As a general rule, we find the capacity of air for moisture increased by heating, and dimished by cooling, it being capable of taking up and holding in an invisible state, a greater quantity of water when heated, than it can retain when cooled. Although the capacity of air for moisture is increased by heating, yet it is not proportional to the heat, but increases in a faster ratio.

For instance, an increase of temperature 10°, of air already heated to 70°, will increase its capacity for water much more than the same increase to air heated to only 40°. The converse of this is also true, the cooling of hot air diminishes its capacity for heat, much faster than the cooling of air already cold. Air in mounting upwards becomes colder, and since every increase of cold is attended with a dimunition of capacity for moisture, it becomes proportionally damper, and thus the middle regions of the atmosphere become soon charged with moisture, and were it not for a conservative principle which we will now mention, the heavens would be perpetually shrouded with clouds; this principle is, air in expanding has its capacity for moisture increased, and therefore, as the air which is ascending, continually expands from the dimunition of pressure, it becomes consequently drier and drier.

Clouds are formed either by a watery vapor rising so high as to reach a degree of cold sufficient to condense it, or from a mixture of warm air with cold, the moisture being derived from the warmer portion. Fog is nothing more than a cloud formed upon, or near to, the surface of the earth, and is due to a mixture of warm and cold air. Thus in a cold morning we see moisture deposited from the warm breath of animals, when it comes in contact with the air. The course of that remarkable body of water which is called the Gulf Stream, and which flows in a warm current from the Gulf of Mexico as far up as the banks of Newfoundland, is marked by a fog, produced by the colder air sweeping over it. Fogs are not common in hot climates, the air being too warm near the surface of the earth to condense the moisture sufficiently to form a cloud.

Dew is formed when air charged with moisture comes in contact with a surface in a certain degree colder than itself. The formation of dew may be very prettily illustrated by bringing a tumbler of cold water into a warm room, the outside of the tumbler will soon be covered with a coat of moisture, deposited from the warm air which has come in contact with it. In order to have a copious deposite of dew it is essential that the ground should be considerably colder than the air above it, and, it is actually found, that upon those nights when the most copious dews