there to form a similar protuberance or high wave. In the one case, the water is drawn directly up or towards the moon (M); in the other, the water is M E feet! Inland expanses of water, like the Baltic, Mediterranean, and Caspian Seas, and the lakes of North America, have no perceptible tides. Besides being affected by the regular motion of the tides, the ocean is pervaded by a system of currents, mostly constant, which has been compared to the circulation of the blood. These currents play a most important part in modifying the climates of different regions. As respects the causes of oceanic currents, much remains to be cleared up; but some of the leading causes in some shape left behind by the land being pulled seem well established. The two prime movers away from it. In both a similar effect is produced: are differences of temperature and prevalent two tides (t, t) are caused at opposite extremities winds. Sea-water does not freeze until it is of the earth. Where the higher part of either of cooled down to about 28°; and, unlike fresh these great billows strikes our coasts, we have the water, it continues to grow heavier down to that phenomenon of high-water; and when the lower point. The effect of the intense cold of the polar touches us, it is low-water. Each of the waves is regions is thus to cause a constant sinking down brought over any given place in the circumference of the surface, and to establish a current of iceof the earth in twenty-four hours, so as to cause cold water along the bottom towards the equator; high-water twice a day. The sun exerts a far while, to supply the place of what sinks down, an greater attractive influence on the earth than the in-draught or northward flow takes place on the moon does; but from the great distance of that surface, which brings the warm water of the luminary, the difference of that attractive force on temperate and tropical regions towards the poles. different parts of the globe is much less, and there- This is the general theory of the vertical circufore the effect in raising tides is comparatively lation of the ocean-a circulation which might small. But when this minor influence of the sun almost be assumed from the well-known laws of coincides with that of the moon, or acts in the same the flow of liquids, and which recent observations line of attraction (Mt), we perceive a marked in- have established as a fact. The general prevalence crease in the tides; on such occasions we have what of cold currents along the bed of the ocean from are called spring or large tides. When the solar the poles to the equator is now beyond dispute. and lunar attractions act in opposition, we have neap Motion once thus begun, however, is differently or small tides. The spring-tides happen twice a modified in each locality by the shape of the coasts, month, when the moon is at full and change; and by prevalent winds, and other circumstances. But the neap when the moon is in the middle of its orbit one cause which modifies all currents that tend between those two points. A tide requires six hours either north or south, is the daily rotation of the to rise-which it does by small impulses or ripplings earth. At the equator, any spot on the surface is of the water on the shore-and six hours to ebb or moving eastward at the rate of 1000 miles an hour; fall; but every successive high-water is from twenty at 60° north latitude, the velocity is only one half. to twenty-seven minutes later than the preceding, or, Thus, the water of a current starting from the on an average, about fifty minutes for two tides, in equator northward, is constantly coming to places consequence of the earth requiring that time above where the bottom under it has less and less eastthe twenty-four hours to bring any given point ward velocity. But, by the law of inertia, the again beneath the moon. The tides are thus re-water tends to retain the same velocity eastward tarded by the same reason that makes the moon with which it started, and thus it moves to the east rise fifty minutes later every day. It is evident of north-shooting ahead, as it were, of the bottom that the tides will be greatest at that point of the over which it is flowing, as a rider does whose earth's surface which is nearest to the moon, or horse slackens his pace. The contrary happens where the latter is vertical. She is so between the to a stream flowing from north to south. In this tropics; and accordingly the tides are there great-case, the eastward motion or motal inertia of the est, and they diminish as we approach either pole. It is further to be remarked that the moon does not anywhere draw up the tides immediately. In consequence of the law of inertia and of fluid friction, the tidal wave lags behind the moon. Moreover, in consequence of all the great seas and oceans forming, as we have seen, only one sheet of water variously distributed, the ebb and flow in each depend not on its own proper tide, but are the result of the combination of that tide with currents mingling with it from tides of other seas-a result depending upon inequalities of sea-bottom, the configuration of its coasts, their inclination under water, the size and direction of the channel which connects it with other seas, and occasionally upon winds and currents which are not tidal. So much do these circumstances affect the astronomical or primary tidal wave, that while it rises in the expanse of the Pacific to one or two feet only, the derived wave often rises in confined or obstructed seas to elevations of thirty, fifty, or even a hundred water is too slow for the parts of the bottom to which it successively comes; the bottom slips in a manner from under it, and it falls to west of south. This, in combination with the action of opposing coasts, accounts for the circular sweep which many of the currents make, returning partly into themselves. Different in origin from this vertical circulation, though partly mixed up with it, is the horizontal circulation caused by prevalent winds. The best example of this is the Equatorial Current, which sets from the west coast of Africa to the east coast of Brazil, and which is owing to the action of the trade-winds. Currents caused by winds are always shallow, and their rate of motion seldom exceeds half a mile an hour: they are called 'drift-currents,' in opposition to the deeperseated 'stream-currents.' In order to feed this westerly equatorial current, there spring up two in-draught currents-the one from the north along the west coast of Portugal and Morocco, the other from the Cape of Good Hope along the | ameliorated by having its shores washed by waters west coast of Africa as far as the Gulf of Guinea. When the equatorial current reaches the coast of Brazil, it divides into two branches. One proceeds southwards, turning gradually eastwards across the Atlantic until it falls in with the northern in-draught from the Cape of Good Hope. The other branch is deflected northwards into the Caribbean Sea and the Gulf of Mexico. The water thus driven into this pent-up sea now rushes with accumulated momentum through the strait or gulf between Florida and the Bahamas, and forms the famous Gulf Stream. The Gulf Stream, after issuing from the Florida Strait, proceeds at first northward, parallel to the American coast; but between the parallels of 35° and 37°, it turns gradually eastward, passing over the southern extremity of the Bank of Newfoundland, and all the while expanding in breadth and becoming shallower. The temperature of the stream, when it starts, is from 83° in summer to 77° in winter, and even after travelling 3000 miles to the north, as high as the Banks, there is a difference in a winter day between its water and that of the surrounding ocean of 20° to 30°. Along its whole course a cold arctic current underlies it; and this arctic current intervenes between the western border of the stream and the coasts of Florida, Georgia, and the Carolinas, the line of demarcation between the two being so abrupt that it is known as the 'cold wall.' At the bow of a ship entering the Gulf Stream the temperature has been found to be 70°, while it was only 40° at the stern. The velocity of the stream, at its outset, is from 50 to 60 miles a day; but this velocity becomes greatly reduced as it proceeds. from warm seas, and by the moist and warm southwesterly winds that predominate, the corresponding coast of America is, at least, as much depressed by a current from the Greenland seas which flows southward along the shores of Labrador, carrying with it immense fields of polar ice, and accompanied by dry and piercing winds from the north and north-west. This arctic current intervenes between the coast of America and the Gulf Stream, as already mentioned, and flows under it into the Gulf of Mexico. The currents of the Pacific Ocean are little known; but the Indian Ocean, exposed to a tropical sun and hemmed in on the north, sends out several large currents of warm water. One is the Mozambique current; another escapes through the Strait of Malacca, and flows past China and Japan into the Pacific, making for the north-west coast of America. This current resembles in many respects the Gulf Stream. Two currents of equal force, but of different directions, meeting in a narrow passage or gut, will cause a whirlpool, a phenomenon which has ignorantly been said to be produced by subterranean rivers, gulfs, chasms, &c., but essentially is only an eddy. Charybdis, in the Strait of Sicily, and the Maëlstrom, on the coast of Norway, are eddies of this kind, alternately absorbing and casting up again whatever approaches them. Being an elastic and mobile fluid, water is readily acted upon by winds; and thus waves are produced, varying in height and velocity according to the force and continuity of the wind, extent of uninterrupted surface, depth of the ocean, contending currents, and the like. The common cause It has usually been held that the Gulf Stream of waves is the friction of the wind upon the extends across the Atlantic to the shores of North- surface of the water. Little ridges or elevations ern Europe, and is the cause of the mild and first appear, which, by continuance of the force, moist climate enjoyed by the western parts of that gradually increase until they become the rolling continent. The opinion, however, is beginning to mountains seen where the winds sweep over a prevail that, as a distinct current, the Gulf Stream great extent of water. The velocity of waves is in ceases in the middle of the North Atlantic, its proportion to the square root of their length. The waters being by this time thinned out to a mere large waves just spoken of proceed at the rate film, and its initial velocity and distinctive heat of from thirty to forty miles an hour. It is a having been dissipated. That warm waters from vulgar belief that the water itself advances with tropical seas are brought to the coasts of Britain, the speed of the wave; but in fact the form only and even into the polar seas beyond, is proved by advances, while the substance, except a little spray drift-wood, seeds, and fruits from the West Indies above, remains rising and falling almost in the being frequently cast ashore on the Hebrides, the same place with the regularity of a pendulum. A north of Norway, and Spitzbergen. But this is wave of water, in this respect, is imitated by the accounted for by the general flow of the surface- wave running along a stretched rope when one water towards the poles, forming part of the ver- end is shaken. But when a wave reaches a tical oceanic circulation; a flow which receives a shallow bank or beach, the water becomes really westward deflection as it proceeds northwards progressive; for then, as it cannot sink directly in the way above explained. This general set downward, it falls over and forward, seeking the of the surface-water is further promoted by level. Sailors and others speak of waves running the prevalence of south-westerly winds or re-mountains high;' but, according to Scoresby, 43 turn-trades, which maintain a pretty constant feet is about the utmost difference of level between north-east drift over the whole surface of the crest and trough in ocean-waves. north-eastern portion of the Atlantic. In this way, although the Gulf Stream may have lost its original impetus, a large portion of the superheated water which it brings into the centre of the Atlantic, must be carried to the shores of Europe and into the Arctic Sea. Before, however, the Gulf Stream loses its force as a distinct current, it sends off a branch southwards by the Azores which re-enters the equatorial current before described. While the climate of the west of Europe is thus LAKES AND RIVERS. Lakes are inland bodies of water not connected with the ocean or any of its branches: they are generally fresh, but are occasionally brackish, or even decidedly salt. They are classified according as they are fresh or saline, and according to the manner in which they receive and discharge their waters—namely, those that both receive and discharge running water; those that receive waters, but have no visible outlet, as the Caspian Sea; those which receive no running water (being fed by springs), but have an outlet; and such as neither receive nor discharge running water. Lakes are distributed over the globe according to the inequalities of surface; and all tend to annihilation, partly by silting up their basins, and partly by deepening their outlets, thereby effecting an entire drainage of their waters. The most gigantic are those of North America-such as Superior, Huron, Michigan, Erie, and Ontario, which respectively Occupy 32,000, 20,000, 16,000, 10,000, and 7200 square miles. Next in order are the lakes of Africa, three of which-Victoria Nyanza, Albert Nyanza, and Tanganyika—are estimated at 29,900, 25,400, and 10,400 respectively. The largest lakes in Asia are Aral and Baikal; the surface of the former is estimated at 23,000, and the latter at 15,000 square miles. Europe can boast of a vast number, which, though generally small, give beauty and diversity to her landscapes. Those of Ladoga and Onega, in Russia, are the largest ; the former having a surface of 6330, and the latter of 3280 square miles. A comparative estimate of the extent of these vast sheets may be formed when we mention that the area of Lake Geneva does not exceed 340 square miles. Lakes subserve important purposes in the economy of nature. They serve as reservoirs for the waters which rivers would too speedily carry away from the land; they are the tanks, as it were, in which the impurities of streams subside; they refresh and enliven the landscape; and as they all tend to silt up their own sites, these sites become in time tracts of fertile alluvium, and such has been the origin of some of our finest plains. Rivers, streams, springs-whether flowing with a volume several miles in breadth, or trickling in a tiny rill which a child's hand might obstructconstitute a class of the most valuable agencies in the physical history of our globe. They are the irrigators of its surface, adding alike to the beauty of the landscape and the fertility of the soil; they carry off impurities and every sort of waste débris, to be deposited in the ocean as the strata of future continents; and when of sufficient volume, they form the most available of all channels of communication with the interior of continents. Rivers originate in the rain and snow which descend from the sky (see METEOROLOGY). Falling on the surface, the water percolates the soil, finds its way through the rents, fissures, and pores of the rocky strata, and ultimately escapes at some lower level in the form of springs. Some of these springs are perennial, others temporary or intermittent some are limpid, and almost absolutely pure; others are impregnated with metallic, earthy, and saline ingredients, according to the nature of the strata through which they have percolated: some are cold, others tepid; while many issue, with bubbling and steam, near the ordinary boiling-point of water. Springs, naturally tending to lower levels, unite and form streams; and these, again, falling still lower, conjoin in valleys, and form rivers-creating in their course rapids, cataracts, and waterfalls, ravines and dells, lakes, swamps, and marshes, alluvial plains, and low terminating deltas. The valley in which a river flows is usually termed its basin; and its drainage is that portion of country drained by its streams or tributaries; the terms are often used syn The climatology of the globe relates to the degree of heat and cold to which its respective countries are subject, the dryness and moisture of the air, and its salubrity or insalubrity as influenced by these and other causes. As yet the minutiae of climate are but imperfectly determined; the following general causes, however, have been sufficiently ascertained: 1. The action of the sun upon the soil and atmosphere; 2. The internal heat of the globe; 3. The height of the place above the sea; 4. The general exposure of the region; 5. The direction of its mountains relatively to the cardinal points; 6. The neighbourhood of the sea, and its relative position; 7. The geological character of the soil; 8. The degree of cultivation which it has received; and 9. The prevalent winds. These causes, acting together or separately, determine the character of a climate as moist and warm, moist and cold, dry and warm, dry and cold, &c.; and this climatic character is the main influence which determines the nature and amount of vegetable and animal development. The several subjects belonging to this section are treated in detail in the number on METEOROLOGY. DISTRIBUTION OF PLANTS AND ANIMALS. The life of the globe—that is, its vegetable and animal productions-constitutes its most important and exalted feature as a creation. All the varied materials of which it is composed, all the | jected to somewhat similar laws of distribution. complicated actions, reactions, and mutations to Some are strictly tropical, others confined to the which they are subject, are humble phenomena compared with the production of the lowliest organism. This life is everywhere: the waters teem with it, the dry land from pole to pole is clad with it; nay, there is life within life, and perhaps there exists not a single plant or animal but becomes in turn an abode for others of more diminutive dimensions. Speculations as to the origin and generic classification of vegetable and animal life belong not to our subject. Geography views them simply as they exist, and endeavours to determine the laws which regulate their distri- | bution. Vegetables are regulated in their terrestrial distribution by conditions of soil, heat, moisture, light, height of situation, and various other causes; in the waters, by depth, heat, light, nature of bottom, and the presence of mineral and saline ingredients. Were it not for these causes, there is no reason why the tribes and genera of one region should not be identical with those of another-why the palms of India should not flourish alongside the oaks of England, the oaks of England with the pines of Norway, or these again with the dwarf birches of the arctic regions. As it is, the tropics have genera unknown to the temperate zone, and every advance poleward brings us in contact with new and peculiar species. Temperature in this case seems to be the grand regulating condition; and as this is effected by elevation, as well as by increase of latitude, we find the mountainranges near the equator presenting all the features of a tropical, temperate, and even arctic vegetation. Thus palms and plantains may luxuriate at their bases; then appear oranges and limes; next succeed fields of maize and wheat; and still higher, commences the series of plants peculiar to temperate regions. In temperate latitudes, though the variety of vegetation be less, similar phenomena present themselves. Besides these great climatic effects, there are others depending on soil, moisture, light, &c., which, though limited, are not less imperative. Thus, the southern slope of a hill is generally clothed with species distinct from those on the north; a limestone district presents a carpet of vegetation widely different from that of the clayey moorland: some tribes flourish in the moist valley, which would die on the open plain; some tribes thrive in the marsh, others on the dry upland; some luxuriate under the influence of the sea-spray, which would be instant destruction to others. But whilst most species are subject to these laws, there exists in the constitution of many a certain degree of elasticity which admits of their adaptation to a wider range-a beneficent arrangement, which permits man to extend through cultivation those grains and fruits upon which his subsistence so essentially depends. (For further and more minute information respecting the laws which regulate the dispersion and distribution of plants, see VEGETABLE PHYSIOLOGY.) The animals which people the globe are sub 61 temperate zone; while not a few are destined to find their subsistence wholly within the polar circles. Besides this general distribution, we find a more particular restriction to certain continents and tracts where peculiarities of soil, climate, and food seem to be the governing conditions. Thus, the elephant roams only in India, Burmah, and Africa; the ostrich in Africa; the rhea in the pampas of South America; the kangaroo in Australia; the reindeer within the arctic circle; the polar bear amid the snows of Greenland and Labrador; and so on, as will be more minutely shewn under ZOOLOGY. Similar laws are impressed on the life of the ocean. The 'right' whale, as it is termed, of the northern hemisphere, is a different animal from that of the southern; for the tropical regions of the ocean are to him as a sea of fire, through which he cannot pass, and into which he never enters;' while the sperm whale delights in warm water. The herring finds its chosen habitat in the Northern Sea; the oyster clings to a peculiar bottom, at a certain depth; the cod inhabits the same banks and shoals for ages; and a few fathoms of greater or less depth would be more fatal to many species of shell-fish than the dredge of the fisherman. As on plants, so on animals, altitude exerts a very decided influence; and we do not exaggerate when we affirm that a lofty mountain-range presents a more impassable barrier to vital distribution than the widest expanse of ocean. Though presenting a close analogy in the manner of their distribution, plants and animals differ in this respect, that many tribes of the latter-—birds, fishes, and mammalia-make periodical migrations of vast extent; food and proper breeding-places being the objects of their search. These migrations must not be confounded with that adaptability of constitution which fits the horse, the dog, the ox, the sheep, the pig, and other domestic animals, to be the companions and supports of man in his onward possession of the globe. The one is but a change of place in search of food, under a congenial temperature; the other amounts to a constitutional change, irrespective of climatic influence. Man, of all animals, has the widest geographical distribution. This he enjoys not only from the greater adaptability of his constitution, but from that superior intelligence which enables him to counteract the effects of climate by clothing, houses, fire, and the storing of provisions. It may be justly affirmed, therefore, that there is no region where man may not exist and carry on the purposes of life in a higher or lower degree of civilisation. Though generally regarded as a single species of a single genus, naturalists have divided mankind into several varieties, according to their more prominent physical features; and ethnologists, extending the subject according to minor features, language, and so forth, have subdivided these varieties into branches, tribes, and families. See ETHNOLOGY. THE VEGETABLE PHYSIOLOGY. HE science which embraces the study and investigation of the vegetable kingdom, is known by the name of BOTANY, from the Greek word botané, meaning an herb or grass. That department of the subject which explains the organisation and vital functions of plants, is called Vegetable Physiology; and that which recognises their arrangement into orders, tribes, genera, and species, according to their respective forms and qualities, Systematic Botany. The one relates to functions which are common to all vegetables, the other takes notice only of those structural peculiarities which serve to distinguish one species from another, and to enable the botanist to form these into natural and artificial groups. It is to the former of these departments that we now direct attention. GENERAL ECONOMY OF VEGETATION. Nature and Functions of Plants.-The simplest forms of life are observable in certain plants and animals, whose economy is limited to the absorption and assimilation of nutriment, and the power of reproduction; and the difference between these inferior forms is so trifling, that in them the animal and vegetable kingdoms seem to pass into each other. The absolute differences between plants and animals are indeed difficult to define, when they are to be applied to all plants and to all animals. In many cases, form and structure afford no decisive characters whereby we may separate the two kingdoms from each other; while phenomena usually regarded as pertaining to the animal kingdom are prevalent in the lower forms of plants, and indicate that no reliance can be placed upon their mode of life. In like manner, the chemical distinctions upon which much dependence has hitherto been placed, give way before increased knowledge; for cellulose and starch, long considered as peculiarly vegetable products, are now known to occur in animal structures. The locomotive power of many of the lower algæ is greater than that of many animal organisms; and even the spores, or seeds, of some algae of more complex organisation, move about when freed from their parent, with an activity which appears truly animal, by means of the cilia with which they are provided. When the spore finds a suitable resting-place, its movements cease; and having thus exchanged its animal-like mode of life for one of a less erratic character, it becomes developed into a beautiful alga in all respects resembling its parent. The two classes, plants and animals, seem, as it were, to start from a common point at the base, the inferior forms bearing a certain similarity in structure and functions, which gradually disappears as we ascend in the scale of develop ment. Plants derive their food partly from the soil and partly from the air; and whatever they take must either be reduced to a liquid or to a gaseous state. The ultimate elements of which plants are composed are-carbon, oxygen, hydrogen, and nitrogen. Of these, carbon, which is a solid substance, is the principal; and as it is insoluble in water, it must be combined with oxygen, so as to form carbonic acid gas, before it can be taken up by plants. Oxygen is the next in abundance, and it is absorbed principally when combined with nitrogen, in the form of atmospheric air. Hydrogen is not found in a free state in the atmosphere, and therefore it can only be taken up by plants when combined with oxygen, in the form of water, or with nitrogen, as ammonia, in which last form it exists in animal manure. Nitrogen, though found in very small quantities in plants, is an important element, as it constitutes the principal ingredient in the gluten, which is the most nutritive part of corn and other seeds, and which is essential to the germination and nourishment of young seedling plants. Nitrogen also appears to be a principal agent in the production of colour in leaves and flowers, especially when they first expand. As oxygen is imbibed by plants in combination with all the other elements of which they are composed, it is not surprising that the plant takes up more of this gas than it requires; and, consequently, it has been furnished with a remarkable apparatus in the leaves, to enable it to decompose the carbonic acid and other gases which it has absorbed, and to part with the superfluous oxygen. Plants are thus found to improve the air by the removal of carbonic acid, which is injurious to animal life, and by the restoration of oxygen, which is favourable to it; and so to maintain a necessary equilibrium in the atmosphere, as animals are continually absorbing oxygen, and giving out carbonic acid. Light being essential to the decomposition of carbonic acid gas in the leaves, oxygen is not exhaled by plants during the night; but, on the contrary, a small quantity of carbonic acid gas escapes, and oxygen is absorbed. These processes have been called the respiration of plants; but they are very different from the respiration of animals. Development of Vegetable Life. This depends upon the concurrence of certain agents, the principal of which are-heat, air, moisture, light, and soil. No seed can germinate without the concurrence of the three agents of heat, air, and moisture; but in the growth of most plants, the agency of soil and light is also necessary. Every perfect seed contains the germ or embryo of a new plant of the same kind as the parent, and a portion of concentrated carbon and nitrogen, in the form of starch and gluten, laid up to serve as nutriment for the young plant, till its organs are sufficiently developed to enable it to seek food for itself. This nutrient matter is either contained in the tissues of the embryo's cotyledons, or laid up beside it in the seed, in the form of separate albumen. The seed is generally furnished with a hardened covering, in order to preserve it in an inert state as long as may be necessary. The common bean will afford a familiar example of the process of germination. As soon as it is put into the ground, it is acted upon by the influence of heat and moisture, which distend its tissues, so as to burst the external integument. The agency of the air is next required to |