Take an example in nature, of an island situated alone in the midst of the ocean. The land absorbing the solar heat more readily than the water, the island becomes much warmer than the neighboring sea-an ascending current is established, and the fresh air of the sea flows from all directions under the form of a sea-breeze. In the absence of the solar heat it is the reverse. During the night the island loses heat by radiation, and cools quicker than the sea. The atmosphere, having become heavier, runs into that of the sea under the form of a land-breeze. Thus a constant interchange is established. These phenomena are observed almost daily on nearly all the seaboards.

222. What takes place here on a small scale during a day and a night is exhibited on a majestic scale, in a permanent manner, between the continents and the oceans, from one season to another; between the tropical regions and the temperate and polar regions.

ture.

223. All atmospheric currents are the result of difference of tempera The difference of temperature between day and night gives birth to the diurnal breezes; the difference of temperature between extreme seasons gives rise to the season-breezes or monsoons; the difference of temperature between the tropics and the poles is the cause of the great annual breeze, the trade winds, though modified by the rotary motion of the earth.

224. Classification of Winds.-Permanent breezes prevail within the tropics, called Trade Winds, which maintain nearly the same direction and rate throughout the year. They are termed N. E. in the northern hemisphere from their direction, and S. E. in the southern hemisphere; but both blow more decidedly from the east as they approach the equator. The limits of the N. E. trades are the parallels of 80 and 23° N. lat.; of the S. E. are the parallels of 3 N. and 28° S. Between them, from 3° to 9 N. lat. is a zone styled the region of calms, in which thick foggy air prevails, with frequent, sudden and copious rains, attended by thunder and lightning. (See Physical map.).

225. The trade winds may be readily explained: The regions bordering on the equator are the hottest on the earth. In consequence of rarefaction the air there ascends, and flows over the colder masses on either side towards the poles, from which a colder atmosphere moves to supply its place. Thus two currents are created in each hemisphere, an upper and a lower, but flowing in op

posite directions; and if the earth did not rotate upon its axis, the direction of the lower current in the northern hemisphere would be from N. to S., and in the southern hemisphere from S. to N. The earth, however, rotates from W. to E., and the atmosphere surrounding it partakes of this rotary motion. (See Fig. 5.)

226. In proceeding from the poles to the equator, the masses of air flow from regions where the rotary motion of the surface is less, to where it is greater; and unable to acquire the new velocity at once, in virtue of a law

of inertia, they are deflected towards the west, just in proportion as they do not keep up with the surface towards the east. Hence, instead of being north and south winds, which they would be if the earth were at rest, they become north-east and south-east winds.

227. When these winds approach each other, they tend to produce a purely eastern breeze, but this is not perceptible, because the horizontal motion of the air is neutralized by the vertical motion consequent upon excessive heat and rarefaction. Here is the region of calms, in which there would be an almost perfect calm, but for the great evaporation and violent rains which disturb the equilibrium of the atmosphere, and occasion sudden storms and squalls, attended by thunder and lightning. This zone of calms is entirely north of the equator, extending about 6o in width

at a mean.

228. Periodical Winds, or those which prevail at a certain time of the year or day, belong to various districts of the globe. The monsoons of the Indian Ocean, the estesian winds of the Mediterranean, and the land and sea-breezes are of this class.

229. Monsoons. The winds of the Indian Ocean blow in one direction for five months; after an interval of one month, during which calms, light breezes, gales, hurricanes, and thunder storms prevail, the wind changes and blows in the opposite direction for the same time, and so on alternating with great regularity. From April to September, the south-west monsoon prevails; from October to March, the north-east. The cause of these seasonal breezes is easily understood.

230. During the southern summer months of December, January, and February, Southern Africa receives the vertical rays of the sun, while Southern Asia experiences the low temperature of winter. In virtue of that law by which all aerial currents are established, the air rushes from the colder regions of Southern Asia towards the warmer regions of Africa, and a north-east wind blows as long as the difference of temperature lasts. The reverse takes place when Southern Asia is heated by the sun of the northern summer, and Africa is cooled by the southern winter. The air now flows from the colder region of Africa to the warmer region of Asia, and a south-west wind prevails. The direction of the monsoons, like the trade winds, is modified by the rotary motion of the earth.

231. The Estesian Winds, which blow strongly from the north in the Mediterranean in summer, are caused by the high temperature produced by the vertical rays of the sun upon the sandy and flinty soil of the immense desert of Sahara.

232. Variable Winds prevail in mean and high latitudes. The same wind blows but a few days in succession. Between the parallels of 24" and 50o N. lat., the south-west wind blows more frequently than any other throughout the year. Between the same paraliels, S. lat., the north-west wind is the prevailing current.

233. The winds of these regions blow from every direction and are very complicated. The endeavor to arrive at causes is in a great measure attended with unsatisfactory results.

234. The Winds of the Deserts. The Samoon or Samiel of Arabia, Nubia, and Persia, the Khamsin of Egypt, the Harmattan of Seriegambia

and Guinea, the Sirocco of the Sahara, the Salano of Spain, are caused usually by the difference of temperature in countries subject to extreme heat and drought, and generally partake of that character.

235. The Pampas (or south-west winds) of Buenos Ayres, South America, which originate among the snows of the Andes, are cold winds, and sometimes rush with such incredible velocity over the level pampas, and so suddenly set in, that persons bathing in the La Plata have been drowned, finding it impossible to regain the shore.

236. Hurricanes. Frequently in serene hot weather, in this latitude, a whirling motion of the air takes place, usually indicated by sand, dust, and leaves ascending in spiral columns. Larger and stronger whirls carry up water, animals, and even buildings lying in their course. These whirlwinds are generally caused by the struggle of two winds meeting at an angle in the same manner as whirlpools are formed in water by two currents being abruptly impelled against each other.

237. There are three well known hurricane regions-the West Indies, the Indian Ocean, and the Chinese Sea.

The general course of the West Indian hurricanes is from the Leeward Islands W. N. W., passing around the shores of the Gulf of Mexico, or across it, then following the Gulf Stream and terminating in the Atlantic, or exhausting their fury in the United States. They prevail from June to October. They are the most frequent in August.

The hurricanes of the Indian Ocean come from the N. E, near Sumatra and Java, and travel to the S. E. towards Rodriguez and the Mauritius Islands. They occur chiefly from December .to April, the hot season in that hemisphere.

In the Chinese Sea, the hurricanes (there called typhoons) range from the shore to about 1500 E. long. They occur from June to November, after an interval of three or four years.

238. Water-Spouts. These singular phenomena, so frequently observed on the ocean, result when two winds of different temperature meet. The winds condense the vapor and give it a whirling motion, so that it descends tapering towards the sea below, and causes the surface of the water

to ascend in a pointed spiral form till it joins that from above, assuming the appearance of an upright and an inverted cone, being thinner in the centre than either above or below.

239. When a water-spout has a progressive motion, the upper and under part must move in the same direction, and with equal velocity, otherwise it breaks, which frequently happens, when a noise is heard resembling the falling of a cataract. The following is a general description:

240. Under a dense cloud the sea becomes agitated with violent commotions, the waves dash rapidly towards the centre of the agitated waters, on arriving at which, they rise whirling round in a spiral direction towards the clouds. This conical ascending column is met by another descending column, which moves towards the water and joins it. In many instances the lower column is from 18 to 30 rods in diameter near the base. Each column, however, diminishes towards the centre, where they often unite, their diameters being no more than 4 or 5 feet. If a ball is discharged from a gun or cannon into the centre, it causes the water-spout to break, and its progressive motion is arrested. Lightning is frequently seen to issue from the centre and sides of the water-spout, particularly when it breaks, but no thunder is heard.

The winds perform a grand and important part in the economy of nature. They moderate the severity of the northern climates by bringing to them the heat of the south-promote the fecunda, tion of flowers by agitating the branches of plants, at the same time diffusing the productions of the vegetable kingdom by the transport of pollen and seeds to great distances; and but for the aerial currents, rain would be confined to maritime countries, the interior of continents becoming arid deserts. They serve also to renew the air of cities, where causes of vitiation largely operate, and prevent that stagnation of the atmosphere which tends physically and mentally to deteriorate the human race.

241. Rain. If a vessel of water be exposed in open air on a hot day, the quantity of fluid soon sensibly diminishes, or it evaporates, being converted by the heat into invisible vapor, and diffused through the air. In like manner evaporation transpires upon a grand scale from the great collections of water on the earth's surface as well as from the moist ground-subject to diurnal and annual variations, because depending mainly upon temperature.

242. A determinate volume of air, at a given temperature, is capable of receiving a certain quantity of vapor in an invisible state. When it contains all the vapor it is capable of receiving, it is said to be saturated. If the temperature be increased, its capacity for receiving vapor is also increased. On the contrary, if the temperature be reduced, its capacity for vapor is diminished. Like a sponge saturated with water, reduce its volume by pressure, and a part of the water will run out. Decrease of temperature takes the place of pressure in air saturated with moisture, and precipitation or rain is the result.

243. Take an example in nature: A warm, moist south-west wind blowing from the tropics comes in contact with the colder air of the temperate regions, its temperature is lowered, it can no longer contain its full quantity of vapor. A portion of its moisture is soon condensed into clouds, and then falls in rain. On the contrary, if a wind bearing clouds meet a warmer and dryer air, as is the case in the burning deserts, the heated air, having a much greater capacity for moisture, dissipates all the clouds, and the threatening storm suddenly disappears.

244. If a wind bearing moisture encounter an elevated mountain range, forced to ascend high into the cold regions of the atmosphere, its vapors

are condensed, and rain falls on the windward slope. The wind passes over to the other side dry and cool, deprived of most of its moisture. This phenomenon is common to all elevated mountain ranges.

245. Distribution of Rain. Since heat is the cause of evaporation, rain is very unequally distributed. The following are general laws relative to the distribution of rain.

246.-1. It decreases from the equator to the poles, because heat, the cause of vapor, diminishes. 2. It decreases from the coast to the interior of a continent, because the land supplies a less quantity of vapor than the sea. 3. In the temperate zones there is less rain on the eastern coasts than on the western, because the latter are first exposed to the western winds which blow from the ocean and discharge their moisture upon them, but within the tropics more rain falls on the eastern than the western coasts, because of their exposure to the trade winds. 4. More rain falls in mountainous regions than in level districts, because mountains arrest the course of the clouds, and a condensation of vapor ensues from collision with their cold summits.

247. Local Causes. The winds being the bearers of the clouds and vapors, consequently any circumstance that modifies the course or temperature of the winds, affects the condition of precipitation: 1. In Europe, if the wind always blew from the north-east, it would never rain, because it would blow over a great extent of continent; on the contrary, if the wind always blew from the south-west, the rain would be incessant, because it would come loaded with vapors from the Atlantic. 2. If the great mountain barriers of Asia were removed, those immense arid deserts would soon smile with a luxuriant vegetation, because abundant rains from the south and east would be permitted to visit them. Again, suppose the Andes were transferred from the western to the eastern coast of South America, preventing the trade winds from bearing the vapors of the ocean into the interior of the continent, those excessively fertile valleys of the Amazon and Paraguay would soon become a sterile desert.

248. There are extensive tracts of country in which rain is unknown; in some districts it falls periodically, and in others rain may be said to be constant. (See Hyetographic map.)

249. The rainless regions of the New World comprise portions of California and Guatemala, the Mexican table-land, and the coast line of Peru; those of the Old World comprehend an immense territory, stretching from Morocco through the Sahara, a part of Egypt, Arabia, Persia, into Beloochistan, with another great zone commencing north of the Hindoo-Koosh and Himalayas, including the table-land of Thibet, the desert of Gobi and a portion of Mongolia. In these tracts there is either no rain at all or only a very small quantity-so seldom occurring as to be quite a phenomena.

250. The regions of periodical rain are within the tropics, and have seasons of extreme humidity, alternating with excessive drought. The length of time of the rainy seasons differs in different districts, but lasting generally from three to five months. The rains follow the sun: when the sun is north of the equator, the rains prevail in the northern tropic; when south of the line, in the southern tropic, except in India, where the rainy and dry seasons are regulated by the monsoons.