PHYSICAL GEOGRAPHY. 



numberless data of composition, structure, inclina- 

 tion, and fossil contents, geologists have been able 

 to form a definite list of the various rock-formations 

 from the earliest to the most recent, arranged in 

 the order of time. They have divided the whole 

 of the rocks composing the crust of the earth into 

 sections called 'systems,' and have subdivided 

 these again into ' groups/ in a certain well-defined 

 order. So that when a rock is presented to their 

 observation in any part of the globe, they can 

 state, with more or less certainty, the system to 

 which it belongs, and the period in the past his- 

 tory of the earth at which it was deposited. See 

 GEOLOGY. 



The constituents of all these rocks or strata, as 

 well as those of plants and animals, are com- 

 pounded of about sixty-three elementary sub- 

 stances; of which, at the ordinary pressure and 

 temperature of the atmosphere, five are gaseous, 

 and the rest mostly solid, by far the greater 

 number being metals. See CHEMISTRY. 



Of the constitution of the ocean, or watery por- 

 tion of the earth's superficies, chemical research 

 affords us precise data. When pure, water is 

 composed of i part of hydrogen and 8 oxygen by 

 weight, or of 2 hydrogen and I oxygen by volume. 

 In nature, however, water is generally found to con- 

 tain many impurities such as clay, sand, animal 

 and vegetable matter, &c. which, if left at rest, 

 by their own weight soon fall to the bottom. Such 

 substances are said to be mechanically suspended, 

 and when deposited at the bottom, form sediment. 

 Besides impurities of this description, water may 

 contain matter which will not fall down, and which 

 is said to be held in chemical solution. The saline 

 matter in sea-water amounts to about 3^ per cent, 

 of its weight, or nearly half an ounce to the pound. 

 It consists chiefly of common salt, sulphate of 

 soda, carbonate of lime, chloride of magnesia, and 

 silica, although more than twenty other substances 

 have been detected in minute quantity. The salt- 

 ness of the sea, however, is not quite uniform. In 

 general, it is greater in the trade-wind regions 

 than in extra-tropical ; or wherever evaporation 

 exceeds precipitation. A rainless sea, like the Red 

 Sea, is considerably salter than the ocean gener- 

 ally ; the Baltic, again, which receives a great 

 many rivers, and is besides shallow, contains only 

 half as much salt as the Atlantic. It is often found 

 that while a comparatively fresh current is flowing 

 one way on the surface, a salter undercurrent is 

 setting in the opposite direction. A knowledge of 

 the constitution of the ocean is necessary to the 

 explanation of numerous facts in geology and 

 biology. 



The atmosphere, the next great constituent of 

 the globe, plays an equally important part in the 

 organic and inorganic economy. As it presses 

 with a weight of about fifteen pounds upon every 

 square inch at the ordinary sea-level, and dimin- 

 ishes in density upwards in geometrical progres- 

 sion, it is evident that animals and plants fitted to 

 live at small elevations will die if removed to great 

 heights a circumstance corroborated by the fact, 

 that travellers experience difficulty in respiration 

 on very high mountains. Notwithstanding its 

 transparency, the air intercepts and reflects the 

 sun's rays, and propagates them by an infinity 

 of repercussions ; and were it not for this pro- 

 perty, objects would never be illuminated unless 

 exposed to the direct light of the sun. The air 



and more especially the watery vapour forming a 

 part is also the recipient and retainer of the heat 

 radiated from the earth ; and were it not so con- 

 stituted, the heat derived from the sun's rays would 

 be returned to space, and an excessive cold con- 

 tinually prevail. See METEOROLOGY. Chemically 

 speaking, the atmosphere is a mixture of two gases, 

 nitrogen and oxygen, in the proportion of 77 of the 

 former to 23 of the latter, with about one part 

 in 2000 of carbonic acid. In addition to these, 

 which are its permanent constituents, there are 

 always traces of ammonia and a certain amount 

 of aqueous vapour, amounting from I to i'8 per 

 cent. ; in certain localities, also minute quantities 

 of other ingredients are to be occasionally detected. 

 The atmosphere may therefore be regarded as the 

 laboratory in which clouds, rain, snow, and other 

 vapours are formed the medium through which 

 the light and heat of the sun are diffused and 

 equalised an element without which animal and 

 vegetable life could not exist, for both incessantly 

 inhale and exhale its elements ; and an agent 

 indispensable to those innumerable physical oper- 

 ations which constitute the progressive history of 

 our planet. 



Thus assisted by the determinations of as- 

 tronomy, geology, chemistry, and meteorology, as 

 regards the general constitution of the globe, phy- 

 sical geography proceeds to describe and explain 

 its superficial appearance and conditions, and 

 those, again, as influencing the life and distribu- 

 tion of the plants and animals by which it is 

 peopled. Before entering, however, upon these 

 interesting but complicated details, it will be 

 necessary to explain the principal terms and tech- 

 nicalities usually employed by geographers. 



GEOGRAPHICAL TERMS. 



The direction from which the earth moves in its 

 daily rotation is called the West; that towards 

 which it moves, the East; the point which is on 

 the right hand of one standing with his back to 

 the east is called the North; that on the left hand, 

 the South. The imaginary line on which the earth 

 turns is called the Axis; its termination towards 

 the north is known as the North Pole; that to- 

 wards the south, the South Pole. The early culti- 

 vators of geography, dwelling on a part of the 

 earth nearer the north than the south pole, sup- 

 posed the former to be uppermost, though, in 

 reality, such ideas as upper and under do not 

 belong to astronomy; and it is for this reason 

 that in globes and maps the northern part is 

 always placed at the top, the east being towards 

 the right, and the west towards the left hand, with 

 the south at the bottom. Hence also the term 

 high latitudes, applied to places lying far north. 

 Exactly between the two poles, and consequently 

 dividing the earth into two equal portions, is a line 

 called the Equator; all north and south of which 

 are respectively called Northern and Southern 

 Hemispheres or Half-spheres. In the same way, 

 an encircling line, at right angles to the equator, 

 divides it into Eastern and Western Hemispheres. 

 The circuit of the earth, both in its girth between 

 east and west, and between north and south, is 

 divided into 360 parts, called degrees, each degree 

 being equal to about 69^ British miles. At the dis- 

 tance of 23$ oi these degrees from the equator, m 

 both directions, are two parallel lines called the 



