These results show that on the Arctic Circle there is a difference of temperature of 28° between the western and eastern parts of the Continent of Europe and Asia; and a difference of 22° between the western and eastern parts of North America.

Third Law of Climate.—In latitudes above 30o N. and 20° S. the range of temperature from summer to winter, and from day to night, in places on the same parallel of latitude, is least on the west coast, and greatest on the east coast of a continent.

The westerly winds carrying vapour on the west coasts of continents, in depositing their vapour as rain, raise the mean temperature of the entire year, but at the same time lessen the heat of summer and the cold of winter; and these same winds, arriving as dry air on the east coasts of continents, have lost their controlling power, and the ranges of temperature become greater than where winds laden with vapour are prevalent. It is customary to call climates with a small range of temperature, Insular or Marine climates, and climates with a great range of temperature, Continental climates. The following examples illustrate the third law of climate:

EXAMPLE 1.-If we take, as before, Lisbon and Pekin, for comparison, we find the following results:

These figures show that the range of temperature, from summer to winter, at Pekin is double the corresponding range at Lisbon.

EXAMPLE 2.-The mean annual temperature of Dublin is the same as that of New York and Philadelphia, although it lies 13° or

14° to the north of those cities; yet the range from summer to winter in Ireland is only half the range on the east coast of America.

On the east coast of Asia, the Straits of Sangar (which separate the islands of Niphon and Yesso), have a mean annual temperature of 50°, the same as that of Dublin and New York; but the range of temperature, in Japan, from summer to winter, amounts to 56°, being nearly three times as great as the corresponding range in Ireland.

The following table shows in a striking manner the influence of the third law of climate, as we pass from west to east across a great continent :

MEAN RANGE OF TEMPERATURE FROM SUMMER TO WINTER.

The important difference between marine and continental climates mainly depends upon the amount of moisture in the air carried by the wind. If the air be moist, it will by its clouds in summer prevent the sun from greatly heating the ground, and in winter it will protect the ground from excessive loss of heat by nocturnal radiation. If, on the contrary, the air be dry, the sky will be comparatively devoid of clouds, the bright clear sky will promote the heating effect of the sun by day and summer; and at night and winter, the cloudless sky will permit the radiation of heat into space, and lower the temperature of the ground.

It is obvious that the difference between a marine and continental climate must produce most important influences on the vegetable and animal life of various places.

Thus, the mean temperature of New York is the same as that of Dublin, while the range from summer to winter is twice as great as that of Dublin. Hence we find the summer heat of Dublin insufficient to ripen Indian corn; and, on the other hand, the ivy which flourishes luxuriantly in Dublin, cannot outlive the cold of the winters in New York. Again, the mean temperature of Pesth-Buda is the same as that of Dublin, and its range of temperature the same as

that of New York, or double that of Dublin. Hence we find that while the grape cannot ripen in Dublin, some of the finest wines in Europe are produced in Hungary.

2o. THE DISTRIBUTION OF MOISTURE ON THE SURFACE OF THE EARTH.

The distribution of moisture on the surface of the earth depends mainly on the latitude of the place, on the direction of the prevailing winds, the distribution of land and water, and the arrangement of the land as to vertical height.

The vapour contained in the air is produced by the evaporation of the ocean and other bodies of water under the influence of the solar heat, and as rain is merely the condensation of that vapour produced by a sudden loss of heat, we should expect to find both the quantity of vapour in the air and the amount of yearly rainfall greatest at the equator, and diminishing as the latitude increases. The following table, which represents the mean results of many thousands of observations, shows that this expectation is fully justified :

MEAN ANNUAL RAINFALL DEPENDING ON LATITUDE.

The proximity of large masses of water increases the rainfall, especially on the west coasts of continents in the temperate climates, because the prevailing westerly winds arrive laden with moisture. The influence of the distribution of land and water may be illustrated by one or two examples.

EXAMPLE 1.-If we pass across the Continent of Europe and Asia, on the band lying between 50° and 55° N. latitude, we find the following mean rainfall :

1. Ireland, 34.74 inches. 3. European Russia, 18.51 inches. 2. Prussia, 4. Siberia,

19.50

12.69

The steady diminution of the rainfall, in going eastward, is to be explained by the increasing dryness of the prevailing south-westerly wind, which deposits its superfluous moisture on the western portion of the continent.

EXAMPLE 2.-If we take a similar band in North America, lying

between 45° and 50° of latitude, we find the rainfall to be as follows:

1. West Coast, 48.11 inches. 2. Central States, 31.44

3. East Coast, 39.33 inches.

We see here the influence both of the Pacific and Atlantic Oceans, that of the Pacific being the greatest, in consequence of the S.W. winds being preponderant. The rainfall of the west coast is mainly supplied by the westerly winds, while the rainfall of the east coast depends chiefly on easterly winds. On the Atlantic coast of North America it is found that 72 per cent. of rainfalls occur with easterly winds, whereas in Europe it is found that 75 per cent. of rainfalls accompany westerly winds. The elevation of the place of observation above the sea level exerts a most important influence upon the amount of rainfall. The cause of rain is the sudden cooling down of a mass of air containing vapour, so that when a wind blowing from the sea meets with land rising to a considerable height, the air is compelled to ascend, and its temperature undergoes a reduction of 1° for each 240 feet of ascent. Hence we find that the most remarkable rainfalls in the world are connected with the foregoing conditions, and when these occur in the tropics, we have all the conditions necessary for a maximum rainfall. The following examples illustrate the influence of elevation upon rain. fall:

EXAMPLE 1.-At Cherapoonjee, in the Southern Himalayahs, 4,500 feet above the sea level, and 300 miles north of Calcutta, the fall of rain in 1861 was 610 inches.

EXAMPLE 2.-At Bombay, on the west side of the Western Ghauts of India, the average rainfall is 78 inches; at an elevation of 4,500 feet on the Ghauts, the average rainfall is 254 inches; and at Poonah, on the eastern side of the mountains, the rainfall is reduced to 23 inches. This remarkable result is due to the fact that the rainfall occurs with the S.W. monsoon, making Bombay a wetter place than Poonah.

EXAMPLE 3.-In the Island of Guadaloupe, in 1828, the rainfall at the summit of a mountain 5,000 feet in height was 292 inches, while near the base of the mountain it was only 127 inches.

EXAMPLE 4.-At Vera Cruz, the average rainfall is 185 inches, and is double the average rainfall of the Gulf of Mexico. This is to be ascribed to the high mountains near Vera Cruz, up whose slopes the warm moist air from the Gulf of Mexico is carried by the easterly trade winds, and deprived of its vapour by the cold due to the elevation./

EXAMPLE 5.-At the latitude of 60° on the north-west coast of North America, the mean rainfall amounts to 90 inches, which is more than four times the average for that latitude all over the globe. This effect is due to the S.W. wind on a west coast, and to the lofty range of the Rocky Mountains.

EXAMPLE 6.-At the same latitude on the coast of Norway, the rainfall amounts to 80 inches, and is produced by the S.W. winds from the Atlantic, whose moisture is condensed by the Norwegian chain of mountains.

Having mentioned some of the localities in which the rainfall exceeds the average, we may now notice the localities in which rain is almost unknown.

In the northern portion of Africa, between 20° and 30° of latitude, there is a band of rainless country stretching eastwards into Arabia and Persia, and embracing a great part of Egypt. The great African desert, the Sahara, owes its origin to the absence of rain, and in many parts of it carbonate of soda is found in large deposits, and huts are constructed out of blocks of this material which would be washed away by a few heavy showers of rain.

To the north-east of the Himalayah Mountains, there is a large oval district, corresponding with the Desert of Gobi, in which rain is hardly ever known to fall. This desert is characterized by the occurrence of large quantities of saltpetre, or nitrate of potash, which is of great value in the manufacture of gunpowder.

In Peru there is a tract of country running north and south for many miles, but having only a small width, east and west, in which no rain ever falls. This district is remarkable for the occurrence of nitrate of soda, and deposits of borax.

Another rainless district of small extent occurs on the confines of Mexico and California.

The absence of rain in these deserts can be readily explained on the principles already laid down. In the African and Arabian Desert the prevailing wind is N.E., and it is evident from the map that this wind does not pass over any large body of water, and it therefore is comparatively a dry wind. Again, Arabia and North Central Africa are low flat countries, without elevated mountain ranges to condense whatever vapour exists in the air.

The Gobi Desert is caused by the Himalayah Mountains,

This is the Natron or Nitre of the Bible.