elevation as they extend towards the E. These mountains, except the loftier summits, are, for the most part, covered with thick forests of pine, oak, cork, white poplar, wild olive, and other trees. The inferior ranges seem to be principally composed of Secondary limestone, which, at a greater elevation, is succeeded by micaceous schist and quartz-rock ; and the higher chains are said to consist of granite, gneiss, mica-slate, and clay-slate. The Secondary and Tertiary formations are frequently disturbed and upraised by trap-rocks of comparatively modern date. Lead iron copper, antimony, sulphur, and rock-salt occur frequently ; and in the Marocco portion of the range gold and silver are said to exist, In the Algerian division are mines of copper, lead, silver, and antimony. The lion, hyena, boar, and bear are common throughout the moun tains. None of the rivers which take their rise in the sys tem are of any great importance. The Tafilet is absorbed in the sands ; the Tensift and Draa flow into the Atlantic; j and about five or six find their way to the Mediterranean. Dr Hooker has explored the botany of many parts of the range, and the travels of Rohlfs have added largely to our general knowledge of it.
ATMOSPHERE
ATMOSPHERE is the name applied to the invisible elas tic envelope which surrounds the earth, the gaseous matter of which it is composed being usually distinguished by the name of air. Storms and weather generally, solar and terrestrial radiation, the disintegration of rocks, animal and vegetable life, twilight, and the propagation of sound, are some of the more striking phenomena which are either to a large extent or altogether dependent on the atmo sphere. That air possesses weight may be shown by the simple experiment of taking a hollow globe filled with air and weighing it; then removing the contained air by means of an air-pump, and again weighing the globe, when it will be found to weigh less than at first. The difference of the two results is the weight of the air which has been removed. From Regnault s experiments, 100 cubic inches of dry air, or air containing no aqueous vapour, under a pressure of 30 Euglish inches of mercury, and at a temperature of 60 Fahr., weigh 31 03529 grains; and since 100 cubic inches of distilled water at the same pressure and temperature weigh 25,252^ grains, it follows that air is 813 67 times lighter than water. Air as an elastic fluid exerts pressure upon the earth or any substance on which it rests, the action of a boy s sucker and of a water-pump being familiar instances showing the pressure of the atmosphere. When air is removed from a water-pump, the water rises in the pump only to a certain height ; for as soon as the water has risen to such a height that the weight of the column of water in the pump above the level of the surface of the water in the well just balances the pressure exerted by the atmosphere on the surface of the well, it ceases to rise. If the pressure of the atmosphere be increased, the water will rise higher in the pump ; but if diminished, the level of the water will sink. The height to which the water rises within the pump thus varies with the pressure of the atmosphere, the height being generally about 34 feet. Since a given volume of mercury weighed in vacua at a temperature of 62 Fahr. is 13 5 69 times heavier than the same volume of water, it follows that a column of mercury will rise in vacuo to a height 13 5 69 times less than a column of water, or about 30 inches. If we suppose, then, the height of the mercurial column to be 30 inches, which is probably near the average height of the barometer at sea-level, and its base equal to a square inch, it will contain 30 cubic inches of mercury ; and since one cubic inch of mercury contains 34267 grains, the weight of 30 cubic inches will be nearly 147304 ft> avoirdupois. Thus the pressure of the atmosphere is generally, at least in these latitudes, at sea-level equal to 147304 5> on each square inch of the earth s surface. Sir John Herschel has calculated that the total weight of an atmosphere averaging 30 inches of pressure is about 11 trillions of pounds; and that, making allowance for the space occupied by the land above the sea, the mass of such an atmosphere is about that of the earth itself. This enormous pressure is exerted on the human frame in common with all objects on the earth s surface, and it is calculated that a man of the ordinary size sustains a pressure of about 14 tons ; but as the pressure is exerted equally in all direc tions, and permeates the whole body, no inconvenience arises in consequence of it. A pressure agreeing approximately with the average atmospheric pressure at sea-level is often used as a unit of pressure. This unit is called an atmosphere, and is employed in measuring pressures in steam-engines and boilers. The value of this unit which has been adopted, in the metrical system, is the pressure of 760 millimetres (29-922 Eng. inches) of the mercurial column at C. (32 Fahr.) at Paris, which amounts in that latitude to 1 033 kilogrammes on the square centimetre. In the English system, an atmosphere is the pressure due to 29 905 inches of the mercurial column at 32 Fahr. at London, amounting there to nearly 14 ft) weight on the square inch. The latter atmosphere is thus 99968 of that of the metrical system. As regards the distribution of atmospheric pressure over the globe, there was little beyond conjecture, drawn from theoretical considerations and for the most part erroneous, till the publication in 1868 of Buchan s memoir "On the Mean Pressure of the Atmosphere and the Prevailing Winds over the Globe." 1 By the monthly isobaric charts and copious tables which accompanied the memoir, this important physical problem was first approximately solved. Since then the British Admiralty has published charts showing the mean pressure of the atmosphere over the ocean. 2 The more important general conclusions regarding the geographical distribution of atmospheric pressure are the following : There are two regions of high pressure, the one north and the other south of the equator, passing completely round the globe as broad belts of high pressure. They enclose between them the low pressure of tropical regions, through the centre of which runs a narrower belt of still lower pressure, towards which the north and south trades blow. The southern belt of high pressure lies nearly parallel to the equator, and is of nearly uniform breadth throughout ; but the belt north of the equator has a very irregular outline, and great differences in its breadth and in its inclination to the equator, these irregularities being due to the unequal distribution of land and water in the northern hemisphere. Taking abroad view of the subject, there are only three regions of low pressure, one round each pole, bounded by or contained within the belts of high pressure just referred to, and the equatorial belt of low pressure. The most remarkable of these, in so far as yet known, is the region of lovr pressure surrounding the south pole, which appears to remain pretty constant 1 Trans. Roy. Soc. Edin., vol. xxv. p. 575. 2 Physical Charts of the Pacific, Atlantic, and Indian Oceans, Lend.
1872.
