ual decrease in the elevation of the snow-line above the level of the sea, takes place as we approach the poles.

By examining the diagram, it will be perceived that the limit of the snow-line, within the tropics, is from 15,000 to about 20,000 feet above sea-level; and that this line, at about latitude 80°, descends to the level of the sea.

It will also be perceived that the snow-line is higher at the distance of 10 to 20 degrees from the equator than at the equator itself.

It must be borne in mind, however, that, at the equator, the sun is never more than 12 hours above the horizon; whereas, near the tropics, the longest days are 13 hours in length; as the sun at that period is vertical, or nearly so, the summer heat, on which the line of perpetual snow depends, is greater than directly under the equator.

GLACIERS.

Glaciers.-Glaciers are immense masses or fields of ice formed on the sides of snow-capped mountains. They are found chiefly in the temperate and frigid zones; and of the greatest extent among the Alps, and the Norwegian mountains, and on the coasts of Greenland and the Polar regions.

"They sometimes descend so far below the snow-line that their termination is surrounded by full-grown trees, cultivated fields, and orchards. The extent of a large glacier depends partly on the size and formation of the valley and partly on the extent of the snow-mountain of which it is a branch. The peasants in Switzerland say a lean snow-mountain cannot produce a fat glacier."

The Alps, between Mont Blanc and the Tyrol, contain about 400 glaciers, some of which are 15 to 20 miles long.

Avalanches.—Avalanches are masses of snow or ice which, breaking off from the snows and glaciers of lofty mountains, glide down into the valleys at their base. They sometimes occasion much damage in the destruction of life and property.

Hail.-Sometimes rain-drops are formed in the upper portions of the atmosphere, and in their descent fall through a stratum of air at the freezing point, when they assume the form of hail; if they freeze only partially, they form sleet.

A sudden and partial congelation of currents of the atmosphere may happen even in summer, and thus hail-showers occur frequently at this season.

CHAPTER XIV.

METEOROLOGICAL PHENOMENA-CONTINUED.

Climate. The term climate is used to express the combination of temperature and moisture which prevails in any particular region.

The sun is the great agent in diffusing heat over the surface of the globe. Whenever the sun is above the horizon of any place, that place is receiving heat; when below, parting with it, by the process of radiation.

The division of the earth into five zones, the torrid, the two temperate, and the two frigid, conveys but a general idea of the temperature of these respective portions of the earth's surface, because the power of the sun's rays is greatly modified by other circumstances; hence, we find great varieties of temperature in the same latitude.

The most prominent causes that affect or determine the climate of a place, are as follows:

1st. THE LATITUDE OF THE PLACE.-This has considerable influence in determining its climate; for it may be regarded as a general law that the greatest degree of heat prevails near the equator, and diminishes as the latitude (or distance from the equator) increases.

2d. THE HEIGHT OF THE PLACE ABOVE THE LEVEL OF THE SEA.—' -This materially affects its climate. We may travel several miles from the equator toward the poles, along the level surface of the earth, before we become sensible of a diminished temperature; but immediately we begin to ascend above the level, a rapid change of temperature takes place.

An elevation of only about 180 feet causes the same variation of temperature as the removal of one degree (691 miles) from the equator toward the poles. At the height of about 15,000 feet above the level of the sea, at the equator, we attain the region of perpetual snow, which corresponds to about the 70th parallel of latitude.

3d. THE POSITION AND DIRECTION OF THE MOUNTAIN-CHAINS.-These produce considerable effects on the climate of any region, more especially if they extend from east to west, and thus, according to their relative position with regard to the lowland districts, either screen them from the north or leave them unsheltered in that direction.

Russian Poland, situated to the north of the Carpathian Mountains and having no elevated land to protect it from the piercing blasts which sweep across the plains of Northern Europe, suffers in winter from a climate which is nearly as severe as that of Sweden. Hun

gary, on the contrary, sheltered by the same range from the cold blasts of the north, enjoys a milder climate than any part of Germany.

The inhospitable climate of Siberia arises from its being unprotected on the north from the cold winds of the Arctic, or Frozen Ocean, while on the south a vast chain of mountains intercepts the warm southerly winds whose access would tend to mitigate the rigor of the atmosphere.

The central and southern parts of European Russia are colder than their latitude would lead us to expect, owing to the absence of any chain of mountains to protect them from the influence of cold northerly winds.

4th. THE PROXIMITY OF A COUNTRY TO, OR ITS REMOTENESS FROM, THE SEA.-This is an important element in determining its climate. The waters of the ocean, being of a more uniform temperature than the land, have a tendency to preserve a similar uniformity wherever their influence extends, moderating alike both heat and cold.

A cold wind in passing over the sea becomes warmed, while a hot wind, on the contrary, becomes cooled. Hence, we find that the climate of islands and of countries bordering on the sea, differs from that of countries situated in the interior or remote from the sea; the former being characterized by milder winters and more temperate summers.

5th. THE SLOPE OF THE COUNTRY, OR THE ASPECT IT PRESENTS TO THE SUN'S COURSE.-This has considerable influence upon its climate. The angle at which the sun's rays strike the ground, and, consequently, the power of those rays in heating it, vary with the exposure of the soil relatively to that luminary.

When the sun is elevated on the meridian 45° above the horizon, his rays fall perpendicularly on the side of a hill facing the south at an equal angle, while the plain below receives them at an angle of 45°.

6th. THE GEOLOGICAL CHARACTER OF THE SOIL.-The character of the soil of a country produces marked effects on its climate, which is owing principally to the greater or less power any soil possesses of radiating heat.

Sandy soils become rapidly and intensely heated, and when the sun's rays are withdrawn they readily radiate their heat and thus impart to the atmosphere an increase of temperature.

Clayey soils, on the other hand, become slowly heated, and as slowly part with heat. Marshy ground chills the air, and extensive forest tracts have a similar effect.

7th. THE DEGREE OF CULTIVATION WHICH THE COUNTRY HAS RE

CEIVED. This affects the climate to some extent; for if marshes are drained, or forests cleared, the temperature will be raised. Such has been the case in some parts of our country, where the clearing of the forests and the progress of cultivation have improved the climate and rendered the winters milder.

"The destruction of forests may, however, be carried to a pernicious extent, either by depriving a country of shelter from particular winds or by lessening too much the quantity of moisture; it being well known that there is a great evaporation from the leaves of vegetables."

8th. THE PREVALENT WINDS OF THE COUNTRY.-The winds of a country produce considerable effect on its climate. The character of a wind, as we have before shown, depends upon the region whence it comes and the nature of the surface over which it passes.

9th. THE ANNUAL QUANTITY OF RAIN THAT FALLS.-The quantity of rain that falls in a country affects the climate by imparting to it a greater or less degree of humidity. In general, more rain falls on islands and on sea-coasts than in inland districts, among mountains than in level regions, and within the tropics than in either of the other zones.

Kinds of Climate.-The climate of a country is said to be excessive or continental when the contrast between the summer and the winter temperature is very great; and insular, when the difference of temperature between these seasons is comparatively small.

For example, the mean summer temperature of England is about 63°, and the mean winter about 37°, making a difference of only 26°; whilst, at Pekin, the mean summer temperature is 79°, and the mean winter 23°, making a difference of 56°. The former has an insular, and the latter an excessive, climate.

ISOTHERMAL LINES.

Isothermal Lines (lines of equal heat) are lines drawn upon a map, connecting places on the surface of the globe that have the same mean annual temperature. All places situated on the same isothermal line, therefore, have the same mean annual temperature; but this does not necessarily imply that their climates correspond. In some the winters may be mild and the summers only moderately warm, whilst in others the extremes of heat and cold may be experienced.

The Warmth Equator, or line of greatest mean annual heat, extends for the most part north of the equator; which is owing to the position of the great land masses of the earth.