same kind of remarkable exception in regard to the gravitation of the gases as the peculiar law of the freezing of water makes in regard to the comparative bulk and weight of ice. It resists a law which everywhere else governs material things; and does so with most signally wise and beneficent results. It sends aloft the heavy carbonic acid, which would suffocate all animals if allowed to rest on the ground. It keeps down the lighter nitrogen as a necessary diluent of the too-exciting oxygen. It suffuses high and low the light vapour which is needed high and low and all between for the many purposes of weather. And, in general, it secures to the atmosphere that uniformity of character, throughout. all its volume, amid all its commotions, and at all hours and seasons, which is essential to its million connections with light, electricity, chemical action, sound, land and sea, and all organic nature.

"Few perhaps of our readers," remarks a writer in the British Quarterly Review, "have considered how, but for this force, rain and dew would long ago have ceased to fall, and the green earth have been parched and dried up like a desert 'All the rivers run into the sea, yet is the sea not full. From the place whence the rivers came, thither they return again!' And why is it so? Even because this force of diffusion, when assisted by the sun, is able to lift up the ocean itself, and to make it thin air. We have all watched with delight a drop of dew lying in the cup of a flower; but few marvel at the fact, that the little drop returns to the air whence it came.

its flower-cup for ever?

Why should it not lie in

A pearl lies at the bottom

of the sea, and makes no effort to float up to the sur

face; and yet the difference in density between the pearl and the sea, is much less than that between the dewdrop and the air. A globule of quicksilver let fall into the ocean rests in its bed for ever, yet it is only some eleven times heavier than the water above it. The dewdrop is 815 times more dense than the air, and there are hundreds of tons of the latter pressing on it; but no sooner does the sun arise, than it brightens and exhales to heaven. It bounds up like a bird into the blue sky. The air opens its arms for it, and lifts it into its bosom, and by and by spreads it from pole to pole, and it encircles the world. The atmosphere thus solicits and encourages-nay, compels the rise of vapour, and keeps undiminished an embryo store of refreshing dews and warm showers for the earth, and so it ever holds good that the 'clouds come after the rain.""

"But for this force," says the same writer, "all other contrivances for maintaining the life of animals would have totally failed to secure that end; for respiration would have been impossible. To sentient beings, the atmosphere would have been as useless as the most dainty and nutritious food is to one who has not the power to swallow. There is this perplexing problem to be solved in the case of respiration. An animal

has not two sets of air-tubes, as it has two kinds of blood-vessels along one of which (the arteries) the blood goes, whilst by the others (the veins) it returns. There is only one windpipe in animals, by which the oxygen may travel to reach the blood, and the carbonic acid return to reach the air. By the same channel we must constantly cause two counter or reverse currents to pass,—a stream of oxygen from the outer

The

air to dissolve in the blood,-a stream of carbonic acid from the blood to dissipate into the air. breathing tube of an animal is thus like a railway tunnel, through which trains are constantly passing in opposite directions, and yet there is but one pair of rails. There is no mechanical or vital device for effecting the transference of the opposing aerial currents, -no living alternating pump like the heart, which should this moment suck oxygen into the blood, and the next moment suck carbonic acid out of it. The muscles of the chest, by their action, alternately fill and empty the larger wind-tubes, or what we may call the lobbies of the air-galleries. It is only in the narrow passages and distant corridors, that the blood and air meet and act on each other. There, however, the pantings and heavings of the chest have no direct effect in filling or emptying the air-channels. It is all occasioned by the power of diffusion. The issuing carbonic acid acts like a vacuum to the entering oxygen, or, at most, neither gas resists the passage of the other, more than the pebbles in the bed of a stream do the water flowing over them. They glide past each other, impelled by an irresistible force which obliges them to change places; so that a certain volume of the one cannot by possibility travel in one direction, without permitting, nay, without compelling, a certain volume of the other to pass in the opposite one. gases entering and leaving the blood are like weights hanging at opposite ends of a string suspended over a pulley, or like the buckets in a well. The one cannot sink without causing the other to ascend, or either move in one way without causing the other to move in the reverse one. There are animals in which the

The

air-tubes are as rigid as iron, so that they cannot expand or contract to carry air to or from the blood. In these the force of diffusion alone maintains respiration; but without that force it could not go on in any class of terrestrial beings."

CHAPTER VIII.

CONTINENTS AND ISLANDS.

LAND-HEMISPHERE AND WATER-HEMISPHERE-PROJECTIONS OF COASTLINE THE OLD WORLD AND THE NEW WORLD-CHARACTERISTICS OF AFRICA, ASIA, AND EUROPE-THE GREAT MOUNTAIN CHAINS-THE CONTOUR OF CONTINENTS-THE FEATURES AND INFLUENCES OF ALPINE RANGES-THE SCENERY OF MOUNTAINS-THE ANTARCTIC CONTINENT-SINGLE MOUNTAIN MASSES GREAT PLATEAUS AND GREAT THE PAMPAS, SILVAS, LLANOS, AND PRAIRIES OF AMERICA -FLAT LANDSCAPES-VALLEYS-MOUNTAIN PASSES-GLENS AND

PLAINS

GORGES-EARTH-CHASMS-GROTTOES

COLONNADES.

AND

CAVERNS-BASALTIC

A GREAT circle drawn through the coast of Peru and the southern extremity of Asia, cuts the world into a land hemisphere and a water hemisphere. The former has Britain for its centre, and contains nine-tenths or so of all the inhabited land in the world; and the latter has its centre not far from New Zealand, and contains little other land of any kind than Australia, the islands of the Pacific, the southern point of America, and the recently discovered antarctic continent. Thus, in the grandest sense, is Britain the middle of the peopled earth.

All the great masses of land send off bold, highpointed projections to the south. All the great peninsulas, and even most of the smaller ones, look to the south, and have either lofty promontories or rocky headlands. And all the great continents taper to the