Snow flakes are made up of crystals, arranged in star-like forms with three or six branches, differently arranged, but always remarkable for their regularity and beauty. When snow falls, the temperature of the air is near 32° F. If the temperature is much lower, the snow is less abundant, because the amount of vapor in the air is less.

The quantity of snow that falls in any place is generally the greater as the place is nearer the pole, or as it is higher above the level of the ocean. At the poles, and on the summits of high mountains in all latitudes, snow remains through the entire year. As we approach the equator, the region of perpetual snow rises higher and higher above the level of the ocean. In the Andes, under the equator, the limit of perpetual snow is between 15,000 and 16,000 feet above the level of the ocean; in the Alps it is only 10,500 feet above the level of the ocean; towards the northern extremity of Norway it is but 3,000 feet above the ocean level.

HAIL is composed of layers of compact ice, arranged concentrically about nuclei of snow. Its formation is undoubtedly of electrical origin, and will be again treated of under the head of electricity.

Winds.

250. WINDS are currents of air, moving with greater or less rapidity. They are generally named from the quarter whence they blow; thus a wind that blows from the east is called an east wind, and so for other winds. Winds are sometimes named from some local peculiarity. Thus we have trade winds, monsoons, siroccos, and the like. The prevailing directions of the wind are different in different countries, for reasons that will be explained hereafter.

Causes of Winds.

251. Winds are caused by variations of temperature in the atmosphere; these variations produce expansions and

Describe a snow flake. What law governs the fall of snow? What is Hail? (250.) What are Winds? How named? (251.) What are the causes of winds?

contractions, thus disturbing the equilibrium of the atmosphere, causing currents. These currents are winds. For example, if the air is more heated over one country than over the neighboring countries, it dilates and rises, its place being supplied by the colder air which flows in from the surrounding regions. The surplus of air thus brought in flows over at the top of the ascending column. Hence there is a current near the earth in one direction, whilst at a higher elevation there is a current flowing in a contrary direction.

Regular, Periodic, and Variable Winds.

252. Winds are divided into three classes: REGULAR WINDS, PERIODIC WINDS, and VARIABLE WINDS.

1. Regular winds.-Regular winds are those which blow throughout the year in the same direction. They occur in the neighborhood of the equator, extending on each side about 30 degrees. From their advantage to commerce they are called trade winds. On the north side of the equator they blow from the north-east, on the south side they blow from the south-east.

The trade winds arise from currents of air flowing from the polar regions towards the equator; the velocity of the earth about its axis being greater as we approach the equator, these winds lag behind as it were, and become inclined to the westward, giving north-east winds on the northside, and south-east ones on the south side of the equator.

2. Periodic winds.-Periodic winds are those which at regular intervals of time blow from opposite directions. Such are the monsoons that prevail in the Indian ocean,

(5.) How are winds divided? What are regular winds? Where do they occur? What are they called? What is their direction on the north side of the equator? On the south side? Explain the causes of the trade winds? What are periodic winds?

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blowing one half of the year from north-east to south-west, and the other half in the opposite direction. When the sun is on the north of the equator, the southern portion of the Asiatic continent is warmer than the southern part of Africa, and the winds blow from south-west to north-east; when the sun is on the south side of the equator, the reverse is the case.

3. Variable winds.-Variable winds are those which blow sometimes in one direction and sometimes in another, with. out any apparent law of change. The further we recede from the equatorial regions, the more variable are the winds in their character.

The Simoon.-The Sirocco.

253. The SIMOON is a hot wind that blows from the deserts of Africa. It is felt in the northern and northeastern parts of the African continent. During its prevalence the thermometer often rises to 120° F. In the desert this wind becomes suffocating from its heat and dryness. Travellers exposed to it cover their faces with thick cloths, and their camels turn their backs to escape its injurious effects.

The SIROCCO is a hot wind that sometimes is felt in Italy. When it blows, people remain in their houses, taking care to close every door and window. Some suppose this to be a continuation of the simoon from the African desert, others think that it has its origin in Sicily.

Velocity of Winds.

The

254. The velocity of winds is very variable. velocity is measured by instruments called anemometers.

Explain the cause of the monsoons. What are variable winds? When are they most variable? (253.) What is the Simoon? Explain. What is the Sirocco? Explain. (254.) What is an anemometer?

These consist of a species of windmill attached to a train of wheel-work, by means of which the number of revolutions per minute can be registered. From the number of revolu tions the velocity can be computed.

The velocity of the gentlest breeze, or zephyr, is not more than one mile per hour; a moderate wind travels at the rate of 4 to 5 miles per hour, a brisk wind 20 miles per hour, a tempest 40 to 50 miles per hour, and a hurricane from 90 to 100 miles per hour.

X. SOURCES OF HEAT AND COLD.

Sources of Heat.

255.

The principal sources of heat, are: the sun, elec tricity, chemical combination and combustion, pressure and percussion, and friction.

1. The sun.-The sun is the most abundant source of heat. We are ignorant of the cause of heat in the sun's rays.

It has been computed that the heat received from the sun by the earth in a year is sufficient to melt a layer of ice extending over the entire globe, and 100 feet in thickness. Yet on account of the great distance of the earth from the sun, and its comparatively small size, it can receive only 'the minutest portion of the heat which the sun radiates in all directions.

2. Electricity. The subject of heat due to electricity will be treated of under the head of Electricity.

3. Chemical combination and combustion. - Chemical combinations are generally accompanied by a disengagement of heat. When they take place slowly, the heat is inappre

Describe it. What are the velocities of some of the winds? (285.) What are the principal sources of heat? What is the most abundant source? What is the amount of heat received by the earth from the sun in a year? Explain chemical combination as a source of heat.

ciable, but when they take place rapidly, there is often produced an intense heat, and sometimes a development of light.

Combustion is one form of chemical combination. The forms of combustion exhibited in our fire-places and our lamps, is a combination of the carbon and hydrogen of the wood and oil with the oxygen of the air. The products of such forms of combustion are watery vapor, carbonic acid, with gases and volatile products that appear under the form of smoke. Combustion is a decomposition of certain substances, accompanied by a composition of new products. In this change, no element is lost, simply a change of form takes place.

The flame produced in combustion, is a mixture of gaseous and volatile matters, heated red hot by the heat disengaged in the process of combustion.

The process of respiration is a species of slow combustion, in which the carbon and other matter of the blood unites with the oxygen of the air. This species of combustion gives rise to the heat of the body of men and animals. This heat is called animal heat. Fermentation is a chemical process that gives rise to heat.

4. Pressure and percussion.-Whenever a body is compressed so as to reduce its volume, heat is developed. The greater the compression, the greater the amount of heat developed. If gas be suddenly and violently compressed, the heat generated is sufficient to set fire to inflammable bodies. This subject was referred to in the article on Compressibility, in which the instrument used for inflaming tinder is figured. (See Fig. 4.)

Rercussion is a source of heat. If a body, like a piece of metal, for example, be hammered, it soon becomes hot. It is percussion that causes the heat when a flint is struck against a piece of steel. In this case there is a piece of the steel detached and rendered red hot by the collision.

Explain the phenomena of combustion. What is flame? What is respiration! What kind of heat comes from respiration? What is fermentation? Explain compression as a source of heat, Illustrate. Explain percussion as a cause of heat.