is stationary; at the equator it is travelling at the rate of over 1,000 miles an hour. The cold air which starts from the poles, therefore, has no inclination to go either east or west, but as it travels towards the equator it finds the earth under it flying faster and faster. The earth, in its motion from west to east, strikes against the air, and we feel it as a wind coming in the opposite directionthat is from the east; just as in a railway train, we always feel the wind coming from the direction in which the train is going. It is not quite correct, however, to say from the east, because there are the original directions from the north and south poles to take into account; and so the winds blow from the north-east in the northern hemisphere, and from the south-east in the southern hemisphere. These are the trade winds.

6. The warm winds from the tropics blow in exactly opposite directions-viz., from the south-west and northwest-from exactly contrary reasons, and form the return trade winds; and although these winds become cool and sink, and cause the variable winds of the temperate zone, yet in the open ocean the south-west wind prevails in the northern hemisphere, and the north-west in the southern hemisphere.

7. The continents form the second great disturbing cause. Land is heated much more readily than the sea, but on the other hand it cools much more rapidly; hence, in the day-time, the sea-board of most tropical countries has a sea-breeze blowing inland, the colder air over the sea rushing in to supply the place of the more heated and lighter air over the land. In the night-time the land becomes cooled more quickly than the sea, and the colder air over the land displacing the warmer air over the sea causes a land-breeze. These daily currents of air are known as land and sea-breezes.

8. The same kind of interchange takes place also on a grander scale between the great continents and oceans, forming the periodical winds called the monsoons, or season winds. The best example of these winds is that of the monsoons of Southern Asia, the northern half of the Indian Ocean, and Southern Africa. When the

southern part of Asia receives the vertical rays of the summer's sun, Southern Africa is experiencing the low temperature of winter, and the winds blow from the south-west-this is the south-west monsoon for India. The reverse takes place when Southern Africa and the neighbouring seas become heated, and India experiences the winter or north-east monsoon.

9. In the polar regions, as in the temperate, the winds are variable, but the prevailing air-currents are from the poles, and hence cold. Among the many minor causes which tend to change the direction of air-currents are mountain ranges, and the proximity of cold or warm ocean currents.

LESSON V.

RAINS.

1. The air, even when it is pure, transparent, and azure, is an immense reservoir of water. From the surface of all waters, from tiny pools as well as from the great oceans, from the moist earth after every shower of rain, and even from plants and animals, water is constantly being emitted into the air in the shape of an invisible vapour.

2. The presence of this vapour in the air is constant, but the amount is in variable proportions, seldom exceeding five parts in a thousand; yet it is almost incredible how much this slight element of watery vapour affects the life and well-being of all animated creation. It acts as a transparent fleecy mantle covering the whole earth to protect it from the too fierce rays of the sun; or, when the sun has disappeared below the horizon, to prevent the too rapid radiation of heat from the earth into celestial space. It is the source and origin of rain and snow and hail, as well as of the sparkling dew. It forms one link

in the great water-circulation of the globe.

3. A determinate volume of air at a fixed temperature possesses the property of absorbing a certain quantity of

B

vapour. When it contains all the moisture it is capable of receiving, it is said to be saturated. If, however, the temperature be raised, its capacity for holding water becomes greater, if the temperature is diminished, its capacity for vapour is diminished. If, then, air saturated with moisture be cooled, a portion of the moisture will be condensed, and deposited in small drops of water. It is on this property of air that the formation and, to a great extent, the distribution of rains depend. If warm moisture-laden winds be carried to colder climates, or if by any means they are forced upward into the colder regions above, the moisture is condensed, and rain falls. If on the other hand the wind blows from a colder to warmer zone, its power of absorbing and retaining moisture

increases.

4. On the waters in the tropical regions the sun constantly pours its burning rays, and enormous quantities of invisible vapour with the lighter heated air, soar into the higher regions of the atmosphere. Here they meet with cooler layers of air, partial condensation follows, clouds are formed, and prodigious quantities of rain fall back again into the tropical ocean whence they came. A large part of the vapour drawn from the equatorial oceans is carried by the winds over the continents, where they fall in abundant rains to fertilise the ground. "The winds are thus the carriers of the waters, which renovate unceasingly the face of the land, and sustain their beauty. Unhappy are the countries to which they cannot come before they have lost a great part of their precious burden. The inhabitants of the desert alone can adequately tell us what price we should set upon this treasure.'

5. In the tropical regions where the temperature and the winds are regular, rainy seasons and dry seasons follow each other in regular succession. When the trade-winds blow with their wonted regularity, and especially when the sun is over the opposite hemisphere, the atmosphere is cloudless, and the sky of a deep clear blue. As the sun approaches to the zenith of a place, a change takes place; the air becomes so much heated that the ascending currents disturb the ordinary course of the

trade-wind, and the vapours are carried into the upper regions, where they become condensed and fall in a deluge of rain. As the sun passes and re-passes from one tropic to the other, it is in the zenith of intermediate places twice in each year, and so there are two rainy seasons in a year, separated by varying periods of time, according to the distance from the tropics.

6. This course of the wet and dry seasons is, how ever, by no means uniform. We have seen in the preceding lesson that the great masses of land in the tropics become intensely heated in the summer, so the cooler air from the ocean is drawn in as it were from all sides, and that the winds blow in the contrary direction in the winter. Where the winds take this course as in the south of Asia and the Indian Ocean there are only two seasons in the year, the wet and the dry.

7. We will now trace generally the course of the trade-winds, with some of their variations, and the greater or less amount of rainfall dependent on them. The northeast and the south-east trade-winds of the Atlantic strike across from the coast of Africa, and gathering moisture as they proceed, converge on Central America and the northern half of South America, yielding an abundant rainfall. Arriving at the great barrier of the Andes, the winds are deprived of whatever moisture remains, so that they pass over the barrier as dry winds, and the rainless deserts on the Pacific coast of Lower California, Peru, and Chili, are the result. From the Southern Indian Ocean the south-east trade-wind carries a copious supply of moisture to equatorial Africa, and for six months of the year the north-west monsoon assists in the supply. India is watered by the south-west monsoon, and the eastern shores, to a lesser extent, by the north-east mon soon from the Bay of Bengal.

8. The great Sahara in the north of Africa, and many of the great table-lands of the world, receive little or no rain. How is this? In North Africa north-east winds prevail, and these have not only passed over a vast expanse of land, but are journeying from a colder to a warmer climate. A cloud approaching from the north

may seem to the thirsty traveller the harbinger of a storm, but it is quickly dissipated by the thirsty air of the desert

9. Again, the great plateaus are usually supported and hemmed in by lofty mountain chains, and on the external slopes of these mountains all the moisture is condensed, leaving only a dry wind to pass over to the other side.

LESSON VI.

EFFECTS OF CLIMATE ON ANIMAL AND VEGETABLE LIFE.

1. We have shown in the preceding volume* that the first element in the formation of climate is the distribution of the sun's heat, the length of the day and night, and the duration of the seasons, and that these all depend on the inclination of the earth's axis, and on latitudethat is, distance-north or south from the equator. In the preceding lessons we have passed in review the chief causes which operate to modify the climate of a place. These are-height above the sea-level, proximity to the ocean or continental situation, direction of mountain chains, direction of wind currents, direction of ocean currents, and the duration and quantity of the rainfall. Besides these there are some minor causes which we have not dwelt upon, such as the slope of the country and the presence of large forests. It remains for us to further illustrate the operation of these various influences, and to show their effects on the vegetable and animal life of the great continents.

2. It is clear that elevation above the sea-level exercises a great effect on the climate of a place, because many mountains even in the tropics are covered with perpetual snow. A traveller may start from his snow-hut in the Andes in the early morning, and, descending, pass through every variation of temperature, and find himself at noon in the sweltering heat of the tropics. In fact, an

* Book IV., Appendix.