crystal; and one may look downward unmeasured fathoms beneath the vessel's keel, but still find no boundary; the sight is lost in one uniform transparent blueness."

9. The sea is the reservoir for all the moisture which circulates through the world. From it arises all the water carried over the land, and to it this water returns through myriads of channels. "We may form some idea of the enormous quantity of water occupying the bed of the ocean, when we find that were the seas emptied, it would require the flow of all the rivers in the world for 40,000 years to refill them."

LESSON III.

THE OCEAN.—II.

1. The ocean is in ceaseless motion; its extended surface rises and falls, as if softly breathing; its movements, gentle or violent, slow or rapid, are determined by the variations of temperature combined with the earth's rotatory motion, by the winds, and by the attraction of the moon and the sun.

2. Heat affects the volume, and consequently the weight, of the water as it expands or contracts. As the temperature falls the water becomes gradually heavier, and descends into the depths until it reaches nearly 40° Fahr., the point of its maximum density, which temperature is found in every clime at the depth of about 550 fathonis. If the water continues to cool, it is lighter than it was at 40° Fahr., and therefore begins to rise; so that freezing, by this wonderful provision, can only take place at the surface. When the temperature is above 40' Fahr. the light and warm water rises to the surface, and the cold water descends to the bottom. If it be under 40° Fahr. the opposite process takes place the cold layers of water rise, and the warm in their turn sink. The former phenomenon takes place in the temperate and tropical zones, the latter in the polar regions; and so it happens,

whether in the warmest or coldest seas, the temperature at great depths is the same. The continual ascent of these warm layers furthers evaporation, by which clouds are formed. The loss the sea thus sustains in the warm latitudes is compensated by currents of cold water which flow in from the poles.

3. Again, the rain formed by the condensation of the clouds is either warmer or colder than the surface of the sea on which it falls; in the first case it will remain on the top, in the second case it will sink beneath. The waters of the rivers have an effect upon the sea by their temperature, their specific gravity, and their impetus.

4. The movements of the air-the winds and stormshave a still more manifest influence upon the waters of the ocean. The force of waves in a severe gale of wind is tremendous. It has been estimated that a wave twenty feet high exercises a pressure of three tons on each square foot of perpendicular wall or rock. Waves vary in height from mere ripples to enormous billows, but their height during storms varies from ten to twenty feet.

5. Currents in the ocean are of three kinds—permanent, caused by the heat of the sun, the earth's daily motion on its axis, and the trade winds; periodical, dependent on monsoons and tides; and temporary, arising from tides, melting ice, and from every lengthened gale.

6. The three most important of the currents are those which flow in the Pacific, the Atlantic, and the Indian Oceans, respectively. These currents are immense rivers in the sea, which cause great alterations in the temperature of many regions. The first has received the name of Humboldt's Current. Starting from the South Pole, it runs along the coast of Chili and Peru. This current is cold.

7. The current of the Atlantic Ocean touches the southern extremity of Africa, when it divides into two. The western portion leaves the coast, and runs parallel with it for some distance; the northern branch follows the eastern coast-line of the continent, and runs from south to north. Reaching the equator it changes its direction, crosses the Atlantic at its widest part from

east to west, and runs up the Brazil coast, where it divides into two parts. The branch current is called the Equinoctial. The main stream continues its northerly course along the coasts of Brazil and Guiana, enters the sea of the Antilles, turns towards the Bay of Honduras, and sweeps round the Gulf of Mexico. Taking then the name of the Gulf Stream, it comes out of the Bahama Channel, and flows-much enlarged and with greater velocity towards the north-east. The rapidity of its current exceeds that of either the Mississippi or the Amazon. It is said that it runs at the rate of five miles an hour. There is not upon earth a more majestic stream of water.

8. The Gulf Stream casts itself into the Atlantic Ocean, and the last of its waters are found on the eastern coast of Spitzbergen. The current is warm. By cleverly steering his vessel along the margin of this marine river, it is said that a seaman can dip one hand into the warm water of the current, and the other into the cold water of the ocean.

The

9. The current of the Indian Ocean flows towards the east until it encounters the coast of New Holland. Part of its waters run along the south of that continent, and joins the current which flows in the Pacific Ocean. other part turns to the north, and, following the equator from east to west, bends to the southward, passes between Africa and Madagascar, rounds the Cape of Good Hope, and enters the current of the Atlantic.

10. And lastly, the combined attraction of the sun and moon draws daily around the globe two immense waves, which, at the periods of the full and new moon, rise to their greatest height, and sweep over parts of the shores which are usually dry. These great sea movements are known by the name of the tides. During one-half of the year the highest tides occur during the day, and in the other half during the night. The tides in the mid-ocean do not rise three feet; but when the tidal wave encounters a continent which opposes its onward roll, it rushes up its shores with the velocity of a torrent, rising to heights which vary from ten to sixty feet. Under the influence

of celestial bodies which are thousands of miles away, the tides preserve, in their periodical return, as strict a mathematical regularity as the movements of those bodies themselves. The enormous volume of water which they raise, and which could sweep down the mightiest barriers, gently subsides at the appointed moment without passing its prescribed limits.

11. All the movements in the "world of waters,' whether brought about by the sun's heat, the earth's motion, or the winds, or resulting from any other cause, tend to equalise the temperature in the ocean, and, as we shall see in future lessons, form an important factor in the climate of the various countries in the world.

1. The atmosphere is an ocean of thinner and lighter material than the waters of the sea. It is colourless and invisible. It has neither taste nor smell, and-except when moved-is insensible to the touch. It stretches far above the summits of the loftiest mountains, forming a layer of great thickness which envelops the earth on every side. Various conjectures have been made as to the height to which the atmosphere extends above the earth's surface. We know that as we ascend the air gets lighter and lighter, or rarer and rarer, and it is tolerably certain that at a height of fifty miles it is so extremely rare as to be inappreciable. Yet this thickness, compared with the size of the globe it covers, is but small. Reduce the earth to the size of a peach, and the down on the skin represents the atmosphere.

2. The action of the sun's heat on the atmosphere is very much the same as that on the waters of the ocean. It causes motion, or currents, in the air, and these we call winds. Let us imagine for a moment the globe to be at rest, and all the land sunk beneath the surface of the

ocean. What would then be the direction of the air currents?

3. We must remember that hot air is lighter than cold; for heat causes air to expand that is to swell and spread its atoms apart-and becomes thinner; while cold causes air to contract-that is, to shrink, and close its atoms together-and thus becomes denser. We must remember further, that, near the equator, in the tropics, it is always hot summer, and that near the poles it is always cold winter. Nor must we forget that as we ascend above the sea-level the colder it becomes, till, at 16,000 feet, even at the equator, the snow never melts completely away.

4. Now in the tropics the hot air must always be swelling and rising, while at the poles it must be always shrinking and falling. And what must happen then? The cold air must always be flowing from the poles to the equator to fill up the space of the warm air which ascends, and more than this, the warm air must flow towards the poles to fill up the space left empty there. You may see the same sort of thing any day. Let the door of a warm room be slightly opened: a candle placed near the top of the opening will have its flame blown outwards; but placed near the bottom, the flame will be blown inwards, showing that there is a flow of cold air into and of warm air out of the room. And just as in the air-currents of the room, the cooler air-in the tropics at least-must be at the bottom, and the warmer air above. In the higher regions, however, the warm air must gradually cool, and as it approaches the temperate zone, begins to sink. Then the regular winds will cease, and there will be constant conflict between the tropical and polar winds, causing currents in all directions, varying even from day to day.

5. The two great causes which modify and even change the direction of these great general currents, and which we have so far left out of consideration, are the rotatory motion of the earth and the existence of vast masses of land. Let us see how the first of these disturbing causes would act alone. At the poles the earth