This may be taken as a general rule, subject to the following modification; that the south-west and northwest monsoons occur later in the season, according as the regions over which they prevail are farther removed from the equator.

Thus, in India, at Anjengo, on the Malabar coast, 8° 30′ N. Lat., the south-west monsoon commences as early as the 8th of April; at Bombay, 19° N. Lat., about the 15th of May. In Arabia, it begins a month later than on the shores of Africa, and in the northern part of Ceylon, fifteen or twenty days earlier than on the Coromandel coast.

106. ORIGIN. The cause of these regular changes is to be sought in the effect produced by the sun, during his apparent annual progress from one tropic to the other. In the Indian ocean, for example, as this luminary advances towards the north, the zone of greatest rarefaction recedes from the equator, and the northeast monsoon (which is nothing more than the trade wind) then subsides, and is succeeded by calms and variable winds; but as the summer approaches, and the sun arrives at the northern tropic, the southern portions of the Asiatic continent become hotter than the ocean, and the humid air from the equatorial seas flows northward to the land. South-west winds will therefore arise, (Art. 99,) which prevail from the peninsula of India to the Arabian gulf, until the time of the autumnal equinox. During the same period, the south-east monsoon, in the southern hemisphere, tempers the heat of Lower Guinea, and brings rain to the shores of Brazil.

107. The motions of the atmosphere, however, are reversed, as the sun crosses the equator and approaches the southern tropic. Pouring his fervid rays upon Southern Africa, the vast tract of New Holland, and the splendid clime of Brazil, the air flows in from the north and north-west, towards these highly heated regions, and winds from these quarters prevail for several

What modifies the general rule? Give examples.

How are the monsoons caused?

months: the monsoon extending along the coast of Brazil, from Cape St. Augustine to the Isle of St. Catherine. But now the influence of the sun is partially withdrawn from Southern Asia; it glows no longer be neath his vertical rays, and over the cooled earth the north-east monsoon resumes its wonted course.

108. LAND AND SEA BREEZES. On the coasts and islands within the tropics a sea breeze daily occurs, about nine o'clock in the morning; at first, gently blowing towards the shore, but gradually increasing in force until the middle of the day, when it becomes a brisk gale; after two or three o'clock it begins to subside, and is succeeded at evening by the land breeze, which blows freshly off the coast during the night, dying away in the morning, when the sea breeze recommences.

The extent of these winds is variable; in some places they are scarcely noticed beyond the rocks that line the beach; at others they are perceptible three or four leagues from land; while such is their strength on the Malabar coast, that their effects are felt at the distance of twenty leagues from shore.

These breezes are occasionally met with in every latitude. They are perceived upon the coasts of the Mediterranean, are sometimes felt at Bergen, in Norway, and even faintly discerned on the shores of Greenland.

109. ORIGIN. During the day, the islands of the tropics acquire a far more elevated temperature than the adjacent ocean; (Art. 91,) the atmosphere above them partakes of their warmth, and currents of rarefied air ascend from the interior of the land. To supply the partial void thus created, the cool, dense air of the ocean flows in from every quarter towards the shore, and the sea breeze then prevails.

About mid-day the sea breeze is strongest, since the velocity and force of the ascending current is then at its height, for the sun now acts with its greatest energy;

Describe the land and sea breezes. Where do they prevail?
How do they originate?

but as this luminary descends in the heavens, and sinks beneath the waves, the land rapidly loses its heat by radiation, while the temperature of the ocean at its surface is scarcely changed. This is owing to the fact already stated, that the rays warm only the surface of the earth, but are diffused through the water to a considerable depth; and besides, whenever the upper stratum of the fluid is cooled, it becomes heavier and sinks; and a warmer stratum rising to the top the surface thus maintains an almost unvarying temperature. For these reasons, the land, at length, becomes colder than the sea, while the air above it, acquiring its temperature, is condensed, and flowing off in every direction to the warm and lighter atmosphere that floats above the ocean, gives rise to the land breeze, which prevails throughout the night.

110. VARIABLE WINDS. From the extreme mobility of the air, the direction of the wind is affected by a countless variety of causes, such as the nature of the soil, the inequalities of its surface, the vicinity of the ocean and of lakes; and the temperature, course and proximity of mountains.

These local influences are, for the most part, controlled, where the great aërial currents exist in all their power; but in the extra-tropical regions, where the force of the latter is diminished, a perpetual contest occurs between the permanent and temporary currents, giving rise to constant fluctuations in the strength and direction of the wind.

111. It appears from observations made at Toronto, and at Hudson, Ohio, that although the wind blows from every point of the compass during the year, yet, such is the force of the northerly gales, that, in these latitudes, there is a general motion of the atmosphere from N. W. to S. E. In England, on the contrary, from the hourly observations made at Plymouth, there seems

In what regions do variable winds prevail? What appears to be the general course of the atmosphere at Toronto and Hudson? What in England?

to be an annual movement of the atmosphere, from the S. S. E. towards the N. N. W.

112. PHYSICAL NATURE OF WINDS. Winds are hot, cold, dry or moist, according to the direction whence they blow, and the kind of surface over which they pass. In Europe the westerly winds are warm and moist, and the north-easterly cold and dry; for the former come over the sea from the lower latitudes, and the latter sweep across the land from the polar climes: in our own climate, a north-easterly wind is cold and moist.

A south wind in the northern hemisphere is warm and humid, since it comes from warmer regions, and its capacity for moisture is constantly diminishing in its northward progress; from opposite causes a north wind is keen and dry.

In the southern hemisphere the nature of these winds would be interchanged.

113. PUNA WINDS. In Peru, between the Cordilleras and the Andes, at the height of 12,000 feet, are vast tracts of desolate table-land, known by the name of the Puna. These regions are swept, for four months in the year, by a piercingly cold wind from the snowy peaks of the Cordilleras, which is so extremely dry, and absorbs with such rapidity the moisture of animal bodies, that it prevents putridity. If a mule happens to die upon these plains, it is converted, in the course of a few days, into a mummy, even the entrails being free from the slightest evidence of decay.

According to Prescott, the ancient Peruvians preserved the bodies of their dead for ages, by simply ex posing them to the dry, cold, and rarefied atmosphere of the mountains.

114. SIMOOM. Upon the arid plains of Asia, and es pecially on the vast deserts of Africa, an intensely hot

Whence arise the differences in the properties of wind?

What is the nature of a south wind in the northern hemisphere, and why? Of a north wind, and why?

Why would their properties be reversed in the southern hemisphere ? Describe the Puna winds. What fact is stated by Prescott?

wind occasionally prevails. In Arabia and Syria, it is known by the name of the simoom, from the Arabic word samma, signifying at once hot and poisonous. In Egypt it is termed chamsin, fifty, because it usually continues fifty days; while in the western parts of the great Zahara, along the Senegal, and upon the coast of Guinea, it receives the name of harmattan.

The stories of the Arabs, and the accounts of the earlier travelers, in regard to this wind, are clothed with marvelous fictions. It is described as a poisonous, fiery blast, that instantly destroys life; none ever surviving the effects of its deadly influence, if once inhaled. But these fables are now exploded, and the simoom is known to possess no other properties than those which naturally belong to an exceedingly hot and parching wind.

115. CAUSE. Its origin is to be sought in the peculiarities of the soil, and the geographical position of the countries over which it reigns.

The surface of the Asiatic and African deserts is composed of dry quartz sand, which the powerful, vertical rays of the sun render burning to the touch. The heat of these regions is insupportable, and their atmosphere like the breath of a furnace. In June, 1813, at Esné, in Upper Egypt, the thermometer of Burckardt rose to 120° Fah. beneath the roof of a tent, and in 1841, the British embassy to the king of Shoa, while advancing from Tajura to Abyssinia, suffered under a temperature of 126° Fah. in the shade.

When, under such circumstances, the wind rises and sweeps these burning wastes, it is at the same time hot, and extremely destitute of moisture; and, as it bears aloft the fine particles of sand, the atmosphere is tinged with a reddish, or purple haze, the sure precursor of the simoom.

116. Though the blast of the simoom inflicts not instant death, it is yet a dreadful visitant to the traveler

What is the simoom? Where does it prevail?

By what other names is it known? What is the truth respecting it? How does it originate?