GEOGRAPHY (MATHEMATICAL) ; GEODESY.

violent atmospheric movements, such as tempests and linrricanes, are illustrations of the same law, the force of the wind being always proportional to the shortness of the space between great extremes of pressure (see ATMOSPHERE)., The Ocean or Wate•-Envelope of the Earth, from the point of view of physical geography, presents for consideration the form of the basins in which it is contained, the shape and nature of their bottom, their submarine ridges and islands, the density and composition of the water, the distribution of marine temperature, the ice of the sea, and the movements of the ocean due to comical causes as in the tides, to the effects of winds as in surface drifts, currents, and waves, and to differences of temperature. The largest additions in recent years to our knowledge of the earth have been made in the ocean, notably by the different expeditions and cruises equipped for the purpose by the British Government. The climates of the sea have been systematically determined, and the extraordinary fact has been brought to light that the great mass of the ocean water is cold, or below 40° Fahr. Even in the equatorial parts of the ATLANTIC and PACIFIC OCEANS (q.v.), though the upper layers of water partake in the heat of the intertropical latitudes, a temperature of 40° is found within 300 fathoms of the surface, while at the bottom, at depths of 2500 or 3000 fathoms, the temperature (32•4 to 33° Fahr.) is very little above that of the freezing-point of fresh water. It has been proved that the bottom temperature of every ocean in free communication with the poles has a temperature little different from that of the water in polar latitudes. Between Scotland and the Faroe Islands a sounding was obtained giving even a temperature of 29°•6, or 2.4 degrees below the freezing-point of fresh water, and very little above that of salt water. These observations warrant the conclusion that a vast system of circulation takes place in the ocean. The cold heavy polar water creeps slowly towards the equator under the upper lighter water, which moves away towards the poles.

Time Land. - We have to consider the distribution of the land over the face of the globe, the grouping of the continents, the forms and trend of the great terrestrial ridges, the relation of coast-line to superficial area, the contours of the land, as mountains, table-lands, valleys, and plains, the relation of the continents to each other as regards general mass (see GEOLOGY, part ii.; AFRICA, AMERICA, ASIA, EUROPE). Over this framework of land there is a ceaseless circulation of water. The vapour raised by the sun's heat from every ocean and surface of water on the land, after being condensed into clouds and rain, falls in large measure upon the land, and courses over its surface from mountain to shore in brooks and rivers, which again have their own distinguishing phenomena, such as the formation of terraces, deltas, &c. Part of the water performs an underground circulation and returns to the surface in springs. Another portion falls as snow upon the mountains and descends into valleys in the form of glaciers. In this ceaseless flow of water from the summits to the sea we must recognize one of the great agencies by which the present contour of the land has been moulded (see GEOLOGY, part iii., section ii.).

The physical geographer collects, moreover, data which show the reaction of the earth's interior upon its surface, - proofs from bores and mines of a progressive increase of temperature downwards, the evidence of hot springs, and of earthquakes and volcanoes. He finds proofs of oscillations in the level of the land, seine regions having been raised and others depressed within the times of human history. From the geologist he learns that such instability has characterized the outer crust of the planet from very ancient times, and that indeed it is to the results of terrestrial movements that we owe the existence of mountain ranges and even the dry land itself (see GEOLOGY, part in., section L, and part vii.). He perceives that the present area of land on the earth's surface is the result of the balance of two antagonistic processes - the destruction caused by superficial agents on every portion of land exposed to their influence, and the periodic elevation, by subterranean action, of the land so wasted, or of new land from beneath the sea.