them in its upper strata. When the latter has attained a certain degree of thickness the further formation of the lower strata progresses only very slowly. The addition of fresh ice-crystals from below goes on regularly, but in their formation the salt is almost all carried downwards into the sea.'

206. Icebergs.-But besides the various forms of marine ice just described, there are found in the Arctic seas huge masses of ice called icebergs. These have been derived from the surface of the land in the Arctic regions, and, having been pushed down to the sea, have floated away. Nearly the whole surface of Greenland is covered with an icesheet which advances towards the sea, the rate of motion of the Greenland glaciers being much greater than that of Alpine glaciers, nearly 100 feet a day in summer and 30 to 35 feet a day in winter. Here immense fragments are broken off and carried by the waves and currents towards the south. The ice breaks off from one of these Arctic glaciers either

by being pushed along the bottom of the sea till it reaches such a depth that, being lighter than water, it is broken away by the upward pressure of the water, or by coming at once into deep water, when the mass is snapped off by its own weight. Fig. 179 explains the origin of icebergs by the extension of a polar glacier seaward.

Some of the largest icebergs are found in Davis Straits, and are often of enormous size. They carry boulders and smaller pieces of rock derived from the land over which they have passed, and floating southwards they often reach as far as the coast of Newfoundland. Here they melt away and deposit their load of stony rubbish on the bottom of the sea. At times icebergs from the west coast of Greenland have been known to reach latitude 36° N. before they were melted, but owing to the warm waters of the Gulf Stream the bergs from the east coast of Greenland do not reach nearly so far south. It must be noticed that an ice-floe, being a mass of

marine formation, bears no transported blocks, as icebergs do, except it be derived from the ice-foot. These floating mountains of ice drifted by polar currents constitute a source of great danger to the steamers approaching the American shores from England. Owing to the coldness of the air

FIG. 178.-A scene in the Arctic regions, showing a floating iceberg.
(By permission of Messrs. Barne & Co.)

produced by so great a mass of ice, the vapour in this air is often condensed into a fog, so that the berg is only visible at a short distance.

Not only do icebergs scatter boulders and gravel over the floor of the sea, but they often ground in shallow water, furrowing and grinding up the sea bed. In this way large masses of seaweed may be loosened and sent

The Antarctic Ocean.

253

floating away. The large number of grounded bergs on the coast of Labrador gives rise to almost continuous chill fogs during the summer. The visible portion of a berg is only a fraction of what is below water. The specific gravity of ice is about 92, and the specific gravity of sea water nearly 103. The ice being thus lighter than water floats, but being only a little lighter the greater portion is submerged, only about one-ninth of the bulk being above water. As, however, the ice often encloses bubbles of air, a greater fraction than this may often be allowed. The actual height above and depth below water depend to some extent on the shape of the berg. The Arctic bergs are of all sizes, and often of the most fantastic shapes. Dr. Hayes measured one which stood 315 feet out of water, and was over three-quarters of a mile in length. Assuming that only oneseventh was above the water, such a berg would have gone aground at a depth of about half a mile. Being continually acted on by the waves and

the warmth, the floating bergs often fall to pieces, or, becoming top-heavy, fall over, causing great turmoil in the sea.

207. Antarctic Ocean.-The Antarctic Ocean includes the great body of water within the Antarctic Circle, and is a continuation to the south of the Atlantic, Pacific, and Indian Oceans. Compared with the district round the north pole, the south polar regions are but little known. This is partly owing to the great severity of the climate, for its temperature is generally lower than that of the corresponding latitudes of Arctic regions. South of 62 S. lat. the temperature of both air and water is almost constantly below freezing-point even in summer. The greatest known tract of land is the line of coast stretching between the parallels of 70° and 78° S., and lying between the meridians of 160° and 167° E. long. It was called Victoria Land in 1841 by its discoverer, Sir James Ross. Ross succeeded in reaching the highest latitude yet attained in the southern seas, 78° 10' S. His progress was then stopped by an icy barrier nearly 200 feet high,

along which he sailed for 300 miles. On this land a mountain chain was seen, in which an active volcano, 12,000 feet high, was observed. To this volcano the name "Erebus " was given, after one of the ships; while an extinct volcano to the east of this was called "Terror," after the second ship. Another explorer, named Wilkes, discovered land in 1839 between the parallels of 65° and 67° S., and extending from the 100th to the 160th meridian of east longitude. Other portions have been sighted by various navigators.

How far the land stretches is not known; but from the swarms of iccbergs that are found, from the continental character of the deposits found on the sea floor, and from the diminution in the depth of the sea bed on going further south, it has been inferred that the parts observed are merely the outer portions of a large island continent surrounding the pole. Between 64° and 66° S. lat., the most southern latitude reached by the Challenger, depths of 1675, 1800, and 1300 fathoms were obtained; but Ross found a depth of only 260 fathoms further south, near the icy cliffs of the barrier. This Antarctic land, however, is almost inaccessible, as it is completely icebound. Only two explorers have actually landed within the Antarctic Circle, though many have seen land. But the ice in the vicinity so blocks up all approach to the coast and hides the shore, that it is difficult to say where the land begins. In some parts a line of icy cliffs 150 to 2c0 feet high, and called the "Ice Barrier," runs along the coast, rendering it impossible to land, whilst in other places there stretches a solid mass of ice pushed off the land, rising 5 or 6 feet above the surface, and going probably to a depth of about 40 or 45 feet below. Ross saw both these kinds of ice, the ice cliffs and the land ice, and he states that the ice cliffs ("Ice Barrier ") are not found where the land is high and mountainous.

208. Antarctic Icebergs-Most of the icebergs met with in the south polar region are tabular flat-topped bergs, and are probably derived from the icy barrier already mentioned. They are often bounded by almost perpendicular sides, showing one or two stories of upright cliffs with crevasses, and having little hillocks of drifted snow on their flat tops. The entire mass shows a well-marked stratification, being composed of alternate layers of white opaque-looking, and blue more compact and transparent ice. The general mass is described as having the appearance of loaf sugar, with a slight bluish tint, excepting where fresh snow resting on the top and ledges is absolutely white.

The colouring of the crevasses, caves, and hollows is said to be a pure azure blue, so that these southern bergs are magnificent sights. The Antarctic bergs also differ from those given off by Arctic glaciers in seldom bearing any visible rocks, stones, or dirt on their surface, though it is probable that, in the lower parts of the bergs under water, gravel and other débris may be present. They are also met with several degrees nearer the equator than the north polar icebergs, being carried by the flow called the Antarctic Drift Current. Generally speaking, the limits are 35° S. lat. and 40° N. lat. (See Map X.).

CHAPTER XIX.

EVAPORATION AND CONDENSATION-DEW, MIST, FOG, RAIN, AND SNOW.

209. Evaporation is the process by which a liquid is changed into a state of vapour, and this is one of the most important effects of heat. During the process a considerable quantity of sensible heat passes into the latent or insensible state (par. 79), and hence evaporation has a cooling effect on the body from which the vapour rises, and the cooling is greater in proportion as the evaporation is more rapid. This heat makes its appearance again when the vapour is condensed or re-converted into a liquid. Water evaporates at all temperatures, though most rapidly during the process of boiling or ebullition (par. 77). But even snow and ice give off vapour from their surface. A piece of ice placed in the balance-pan of the scales and carefully weighed will be found to diminish in weight slowly, though the air may be below freezing point. Whatever, therefore, may be the temperature of the air, it is almost constantly receiving aqueous vapour from the surface of the water and moist ground. This evaporation is due to the heat of the sun, and is therefore most active in the equatorial regions of the earth, where the sun's rays are most powerful. This vapour rises in an invisible form and diffuses itself through the atmosphere. It really exists in the spaces between the air-particles, though it is convenient to speak of it as contained in the air. A certain quantity of air, say a cubic foot, is only capable of receiving a certain quantity of vapour at a certain temperature and pressure. If we increase the temperature or diminish the pressure, the air becomes capable of holding more vapour. At the freezing-point air is only capable of holding about one-tenth the quantity it holds