Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/225

Rh PHYSICAL] information may be looked for. In the ﬁrst three parts of the article GEOLOGY, a large section of what is usually included under physical geography will be found. 1. The Earth in its Cosmical I2elations.—From astronomy we learn the shape and size of the earth, its motions of rotation round its axis and of revolution in an elliptical orbit ro11nd the sun, the origin of day and night, and of the seasons. Speculating on the original condition of the whole sol-ar system, we may regard it as having been in the condition of a nebula, gradually contracting, con- densing, and leaving behind successive rings, which on disruption and reaggregation formed planets. Hence the primitive condition of our globe as a separate mass must have been gaseous or ﬂuid. Since that time the earth has been cooling and contracting, but still retains a high resi- dual temperature in its interior. This original condition, and the internal heat of the earth, must be constantly kept in view as an explanation of many of the features of its outer surface. See GEoLoGv, part i.; ASTRONOMY, chapter i.; G1;o(:n.rnv (.I.'nEu-'r1cAL) ; GEoDEsY. 2. The .-ll/nospherc or Gaseous 15'-Iwelope of the Ea7'th.—— The solid planet is covered by two envelopes, one of gas which completely surrounds it, and one of water, which occupies about three—fourths of its surface. In studying the atmosphere we have to consider its height, its composition, its temperature, its moisture, and its pressure (see ATMO- sPuEnE, lIE'rEonoL0GY). Its height must be at least 40 or 50 miles. This deep gaseous ocean consists of a mixture of the two gases, oxygen (:31 parts by weight) and nitrogen (79 parts), with a minute proportion of carbonic acid ('O04) and of aqueous vapour. The physical geographer takes note of the manifold importance of oxygen, not only in supporting animal life, but in the general oxidation of the earth’s outer crust. He recognizes the atmospheric car- bonic acid as the source of the carbon built up into the structure of plants. He cannot contemplate without ever- increasing wonder and delight the coming and going of the water-vapour in the air, as it rises incessantly from every sea and land, and after condensation into visible form courses over the laml as rain, brooks, rivers, and glaciers (see GEOLOGY, part iii.). The consideration of the tempera- ture of the atmosphere elicits the facts that temperature falls as we rise above the sea-level, and as we recede from the equitor to the poles, and that it is profoundly affected by the relative positions of sea and land. The want of strict dependence upon latitude in this distribution of temperature is strikingly brought out by the contrast be- tween the mean temperature of Labrador and Ireland on the same parallels (see CLIMATE, IsoTHEP.us). In dealing with the moisture of the air we have to consider the phenomena of evaporation and condensation, the formation of dew, clouds, rain, snow, and hail, the distribution of rain, the position of the snow-line, the occurrence of deserts, «he. (see )IErE0noLoGY). The study of the. pressure of the atmo- sphere, which appears to vary with variations in temperature and amount of vapour, brings before 11s the cause of the constant aerial movements. The law has now been well established that air always flows o11t fro1n tracts where the barometric pressure is high into those where it is low. A knowledge of the distribution of pressure over the globe furnishes the key to the great movements of the atmo- spheric circulation. The trade winds, for example, blow constantly from a belt of high pressure towards the equator, where the pressure remains low. Periodic winds, like the monsoons and land and sea breezes, shift with the changes in atmospheric pressure. Thus Asia during winter is a vast region of high pressure; the winds round its margin therefore flow out towards the sea. In summer, on the other hand, it becomes a region of low pressure, and the winds consequently blow inland from the sea. Sudden and GEOGRAPHY 211 violent atmospheric movements, such as tempests and hurri- canes, 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). 3. The Ocean or ll'ate7'-Envelope of the Earth, from the point of view of physical geography, presents for considera- tion 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 cosmical 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 Govern- ment. 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 ((1.2).), 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° F ahr.) 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 vi as obtained giving even atemperature 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. 4. The Laml.—We have to consider the distribution of the land over the face of the globe, the grouping of the con- tinents, 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 re- lation of the continents to each otheras 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 con- densed into clouds and rain, falls in large measure upon the land, and courses over its surface fro1n mountain to shore in brooks and rivers, which again have their own distinguish- ing 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 oscilla- tions in the level of the land, so1ne 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 ter- restrial movements that we owe the existence of mountain ranges and even the dry land itself (see GEOLOGY, part 111.,