the spring-tides do not happen exactly on the day of new and full moon, nor the neap-tides exactly on the days of the quadratures, but from one to two days afterwards. 6. Though high tide occurs in open seas soon after the moon has passed the meridian, yet in other places shoals and channels, capes and headlands, and the diversified form of shores, so retard the progress of the undulation, that high water happens at all distances of the moon from the meridian, and consequently at all hours of the day. 7. Differences of local situation cause great differences as to the height to which the tide rises. In the central parts of the ocean, the height is comparatively small, seldom exceeding three feet, but in rushing up contracted channels, the tide-wave frequently causes a difference of twenty, thirty, and sixty feet between high and low water-mark. 8. Tidal motion is not an advance of the water, but an undulation propagated in the form of a wave, the surface rising and falling, except that near land shoals and the configuration of coasts occasion an actual flow.

142. Currents.-Currents, or real marine streams, differing greatly in their direction, magnitude, and velocity, are the result of a variety of causes, the rotation of the earth, inequalities of temperature, the melting of the polar ice, and permanent winds. 1. Strong and rapid evaporation within the tropics disturbs the equilibrium of the ocean, and occasions a flow of water from either pole towards the equator to restore the balance periodically augmented by the melting of the ice. Thus there is a very marked flow of water southwards from the Arctic Ocean, which appears to originate under the immense masses of ice which surround the North Pole. It descends from Spitzbergen along the east coast of Greenland, is deflected around Cape Farewell into Davis's Strait, and from thence runs along the shores of Labrador to Newfoundland, passes over the great bank, and terminates towards the centre of the North Atlantic. It is by this current that the icebergs are swept off the polar shores, and brought down in summer as far south as latitude 40°, where they perish from collision, or melt away, occurring much nearer to America than to Europe. 2. The equatorial ocean is distinguished by the westward tendency of its waters, to which the eastward rotation of the earth and the trade winds contribute. In the Atlantic,

the equatorial current forms a complete bar across it from the coast of Guinea to Brazil, and considerably interferes with navigation between the southern and northern hemispheres. Upon reaching the easterly projection of South America, this primary current is divided off Cape St. Roque. A derivative stream flows southward along the shores of Brazil, the main branch continuing its westward route by the mouths of the Amazon and Orinoco into the Caribbean Sea, and thence through the channel of Yucatan into the Gulf of Mexico. The waters of the Pacific Ocean, in its equatorial district, have also a very strong and regular westerly flow, so that vessels passing from Acapulco to Manilla rarely have occasion to set their sails. 3. In the Gulf of Mexico, the westerly flow of the Atlantic waters being arrested, they sweep round the shores, and collecting at the Strait of Florida, form a very remarkable current, called the Gulf Stream, from the place of its origin. It flows with great force through the Floridan strait, passes along the coast of the United States, is deflected eastward by the form of the shores, and by the Arctic current from the north, runs across the Atlantic to the Azores, where it bends southwards, and finally rejoins the equatorial current. This stream is distinguished by its saltness, deep blue colour, and great warmth, acquired in the Mexican Gulf. A branch from it, extending to the shores of Britain and Norway, contributes to their milder climate as compared with that of the opposite shores of Labrador and Greenland; and has been known to carry fragments of wreck, and the seeds of tropical plants, from the West Indies to the western isles of Scotland and the neighbourhood of the North Cape. The waters of the Atlantic, by means of the gulf stream and equatorial current, perform a vast whirl of almost 4,000 miles, varying in velocity, and in breadth from 50 to 250 miles. Humboldt has calculated that a drop of water, travelling freely in this current, would return in about two years and ten months to the place from which it departed. A boat not acted upon by the wind, and without any artificial means of impulsion, would go from the Canaries to the coast of Caraccas in thirteen months; in ten months it would make the tour of the Gulf of Mexico, and, in forty or fifty days, would go from Florida to the bank of Newfoundland. 4. Interesting evidence of the existence, force,

and direction of currents has been obtained by means of bottles hermetically sealed and thrown overboard in various parts of the ocean, containing the position of the vessel and the date of the immersion. Sir James Ross set a number afloat off Cape Horn, April 4, 1842,-one of which was picked up about the middle of September 1845, near Cape Liptrap, in the neighbourhood of Port Philip, Australia. Assuming that the bottle had newly reached the strand when discovered, it had accomplished a course of 9,000 miles in 3 years; but it cannot be supposed to have been transported in a perfectly straight line, and an additional thousand miles may be allowed for detours. 5. Whirlpools result from two or more currents meeting together. most celebrated is the whirlpool of Maelstrom, on the coast of Norway, occasioned by the meeting of strong tidal streams round the islands of Logodon and Maskoe. It is a mile and a half in diameter, but is only at certain seasons in full play, when it becomes dangerous to vessels, and may be heard roaring for many leagues. 6. The currents of the ocean serve many important purposes. They tend to keep it salubrious by preventing the accumulation of putrescent matter; and to equalize temperature by transferring the warm water of the equator towards the poles, and the cold water of the poles towards the equator.

The

143. The ocean grandly reflects the omnipotence of its Maker, whether calm or convulsed; and often in relation to its displays of turbulence has human experience answered to the fine description of the inspired writer :

66

They that go down to the sea in ships,

That do business in great waters;

These see the works of the Lord,

And his wonders in the deep.

For he commandeth, and raiseth the stormy wind,

Which lifteth up the waves thereof.

They mount up to the heaven,

They go down again to the depths:

Their soul is melted because of trouble.

They reel to and fro, and stagger like a drunken man,

And are at their wit's end.

Then they cry unto the Lord in their trouble,

And he bringeth them out of their distresses.

He maketh the storm a calm,

So that the waves thereof are still.

Then are they glad because they be quiet;

So he bringeth them unto their desired haven."

"The sea is His, and he made it." He prepared for it a "decreed place, and set bars and doors, and said, Hitherto

shalt thou come, but no further: and here shall thy proud waves be stayed." "He hath compassed the waters with bounds, until the day and night come to an end." Hence those movements, so apparently lawless and uncertain, threatening the invasion of continents and islands, are under a Supreme control, and kept within strictly defined limits as if with bit and bridle. The billows roll against the shores, but everywhere find a barrier which they cannot overleap. The ocean, constantly unstable in relation to itself, has yet a general attribute of stability in relation to the land, a slight intrusion at some points being compensated by a retirement at others. If its mighty tidal swell continued to flow a few hours longer, life and property would be swept away as with a deluge, and the dry ground exchange places with the bed of the sea; but just as regularly as the tides rise, they find a "bound" fixed "by a perpetual decree to their progress, and a reflux commences. Not less strikingly does the vast body of fluid evidence the Divine Wisdom and Beneficence. It is the ever-teeming reservoir from whence that humidity is derived, upon which the fertility of the soil, and the existence of man depend. "The waters fail from the sea." "He calleth for the waters of the sea, and poureth them out upon the face of the earth.' They are exhaled from their bed by a process of marvellous subtilty and energy, in which heat is the principal element. But the invisible vapour formed, rising in the atmosphere, becomes condensed by the cold of the upper aërial regions, and reappears in a thousand fantastic shapes as clouds, which are wafted by the winds over the continents, discharging their moisture in rain upon the surface, feeding innumerable springs and rivers, by which the fluid is returned to the parent ocean, after having qualified the soil to sustain vegetation, and quenched the thirst of man and beast. "All the rivers run into the sea; yet the sea is not full," because it gives in evaporation an equivalent for what it receives, "unto the place from whence the rivers come, thither they return again."

III. THE ATMOSPHERE.

144. The term atmosphere is derived from the Greek, and signifies a sphere of vapour. It denotes a thin, trans

parent, and highly elastic fluid, which everywhere environs the solid and liquid surface of the earth, partaking its diurnal motion and annual revolution, being preserved in connexion with it by the attraction of gravitation. The aërial stratum is necessary to the life of a great portion of organic beings. It is the medium of sound, light, odour, and of all terrestrial action; the vehicle in which moisture is raised and diffused; and the agent by which that diversity of colour is produced so pleasing to the eye in natural objects. This wonderful envelope, of whose existence we are unconscious when comparatively at rest, demonstrates its reality in agitation by a direct appeal to the senses, and compels us to recognise the presence of a mighty unseen power. It consists mainly of two gases, oxygen, and nitrogen, in definite proportions, with a small varying quantity of carbonic acid gas, and a still more varying quantity of the vapour of water. The same proportions of its prime constituents are exhibited at all times, in all countries, and at all elevations, over land and over sea, on the summit of the highest mountains and at their base, at the poles and the equator; for though we speak, and truly so, of a vitiated air and an infected atmosphere, rightly dreading such districts as the "white man's grave on the coast of Africa, the deleterious ingredients intermingling with the air are of too subtle a character to be distinguished by chemical analysis, and are only known by their effects.

145. That the atmosphere must terminate at a certain distance from the surface is susceptible of direct proof, but its actual limit cannot be ascertained, though probably the extreme height is below fifty miles, and the chief portion is known to be comparatively near the general surface, from the rarefaction evident on lofty mountains. About threefourths of it are within four miles of the earth. Its extent is thus so trifling when compared with the terrestrial diameter, that the relation between the two may be represented by that between an artificial globe and its coats of varnish. There must be differences in the height of the atmosphere in different latitudes, for the rotation of the earth upon its axis, and the greater and more direct influence of the solar heat near the equator, will necessarily cause it to be higher in equatorial than in polar regions.

146. The air is ponderable, or has weight. The pressure