The size of the earth, its average density, the velocities of its diurnal rotation, and of its annual course, the size and weight of its satellite-these and similar facts belonging to astronomy, supply considerations which lie at the root of physical geography. We are continually, and, on the whole, equally affected by one class of astronomical facts, such as the recurrence of night and day, summer and winter, lunar action on the tides, and many others; while another class of astronomical facts relate to changes very slowly effected, and only producing important results after the lapse of long periods of time. We, or the people of any given generation, are of course most immediately influenced by the quickly-recurring changes; but, looking to the laws of succession and continuity, manifested throughout the operations of nature, it will be seen that our physical structure, our methods of life, and even our habits of thought, are, to a certain and large extent, the results of previous conditions of our planet, dating back even to its origin in the remote abysses of the past.

A very important consideration, well adduced by Professor Ansted, arises out of the astronomical fact that the eccentricity of the earth's orbit is the subject of secular changes. "When the eccentricity is greatest, the greatest distance of the sun from the earth may amount to 102,256,873 miles, and its nearest distance will then only be 87,503,039 miles, showing the very considerable difference of 14,753,834 miles, or more than one-seventh of the larger semi-diameter. This is a very important fact; for, as the amount of heat received from the sun varies as the square of the distance, it follows that the quantity of heat received in the former position, compared with that in the latter, will be as 19 to 26."

It will take twenty-four thousand years for the minimum of eccentricity to be reached, so that the change does not immediately concern the present inhabitants of the globe; but in past periods it may have materially affected the creatures then living upon it, and likewise produced modifications of form and climate, the influence of which has descended to our own times.

At present the earth comes nearest to the sun in winter; but "if, when the eccentricity is greatest, the case is reversed, the land of the northern hemisphere would be warmed only to the very minimum extent in winter, and heated to a maximum in summer, the difference would then equal one-fifth of the whole. There would then be far more extreme climates on the earth than there are now." In his "Outlines of Astronomy," Sir John Herschel, commenting on speculations of this nature, observes that about four thousand years before the Christian era, the place of the perihelion must have coincided with the

vernal equinox (or have been situated in long. 0°), and in long. 90° about A.D. 1250. In A.D. 11700, he adds that "the extreme summer and winter of the southern hemisphere will be transferred to the northern." Such changes must have taken place thousands of times in geological eras, and may have partially accounted for those changes of climate which geology and palæontology show to have occurred.

Terrestrial climate is still more strongly affected by the disposition of masses of land and water, the height and direction of mountain-chains, the character of the soil, its being covered with trees or left bare, etc. Of the whole surface of the earth "only a fourth part, or about fifty-two millions of square miles, rises above the uniform level of the water and form land," and it is remarkable that a great part of this land " is grouped round one hemisphere, so that not more than one twentyseventh part has land opposed to it on the opposite hemisphere. Thus, if a person stationed vertically over Falmouth in England could see half the globe, he would see more than forty-nine out of the fifty-one millions of square miles of land, or about an equal surface of land and water. If, however, he were perched equally high above New Zealand, he would see ninetysix and a half millions of square miles of water, and less than two millions of square miles of land."

Any important change in the relative proportions and dispositions of land and water would materially modify the earth's climate. Supposing the quantity of each within a moderate number of miles from the surface to remain the same, but the shallow seas to be considerably deepened and at the same time narrowed, and the bulk of continuous continents very much augmented, there would be a diminution of evaporation, and an addition to excessive or continental climate. On the other hand, rendering deep seas shallower, and diminishing the area of land by a corresponding change in its level would produce opposite effects, and the violence of the alteration would depend partly on the average extent to which levels were modified, still more on the position of the high lands.

The laws which determine the relative disposition of land and water are very little known; and hence there is room for two opposite suppositions, according to one of which the present arrangement depends on permanent causes of very ancient date; and, according to the other, great changes may have taken place within comparatively moderate periods, and equally important alterations may still be going on, though their rate may be very slow. Professor Ansted refers to a speculation of Mr. James Yates, that the centre of gravity of the earth would not be coincident with the centre of magnitude without the protuberance of water on one side, and that the inequality of land

in the two hemispheres is permanent. We have not read the paper in which Mr. Yates put forward this ingenious speculation, but we doubt there being sufficient evidence to raise it above the level of a conjecture, and when we look to the action of aqueous, igneous, and aerial causes of modification, we are not disposed to attach very great importance to a difference of average specific gravity, if it exist, in the land of the two hemispheres, through a great metalliferous deposit in one, and a more cavernous texture in the other.

The great continental mass ranges from north-east to southeast, thus crossing the earth's diurnal motion from west to east, but what was the direction of former continents we do not know; it may at some periods have been widely different from what it is now.

In

Passing for the moment from the consideration of the disposition of land and water masses as affecting climate, and through that acting upon civilization, another important influence of such arrangements must be noticed, and that is the facility they afford, or the difficulties they oppose to, intercommunication and commerce. Hitherto lands much indented by seas and navigable rivers have had an enormous advantage in this respect over massive continents; and the advance of Europe, as compared with Asia, has, in no small degree, been occasioned by the extent and sinuosities of its coast lines. Europe there is a mile of coast for every hundred and eightyseven square miles of surface; in Asia the proportion is one mile of coast to five hundred and twenty-eight square miles of surface; in Africa one to seven hundred and thirty-eight; in North America one to two hundred and sixty-six; in South America one to three hundred and thirty. But we now live in an epoch in which human invention modifies the value of these conditions, great continents are being traversed by railways, even before roads are constructed, and ultimately this may more than compensate for their deficiency in coast.

The physical characteristics of the different continents are peculiarly interesting:-" Europe is, on the whole, a hilly and mountainous country. Asia contains the loftiest mountain groups of the globe, and also vast plateaux. Africa has its lofty mountains and elevated plains. Deserts very little above the sea level accompany the plateaux; but are small in comparison. America, notwithstanding the important and lofty chains of the Andes and Rocky Mountains, is mostly characterized by its plains. The high lands are a fringe of plateaux, and the gigantic valleys, of which those of the Mississippi, the Amazons, and La Plata are examples, are low plains. Mountains, except in the great chain of the Andes, in Central America and on the west coast of the Andes, are subordinate,

the important cultivation and vegetation of the continent being dependent on the broad, rich, level tracts that reach for thousands of miles in successive terraces, almost unbroken by hills, through a large part of both divisions of America."

Europe is the most complicated of continents. America and Australia are both simple, the one with its great mountain system in an almost continuous line, and its chief drainage in one direction; and the other with its prosaic destitution of great features, either mountain, lake, or river.

Variety of conditions and influences being essential to the progress of civilization, it is interesting to observe the tendency of different regions of the globe to assist or to hinder human advancement. Monotonous climates and monotonous countries tend to induce a stationary and conservative condition of the human mind, and had it not been for the warlike irruptions made from time to time by more impetuous mountain races, the inhabitants of great plains would have sunk into a state of vegetative repetition of the same forms of life, and of the same habits. The existence of variety to stimulate, and of difficulty to render industry indispensable, both of which arise, to a great extent, out of conditions of physical geography, exert the most powerful influence on the education of the human race; and if we understand the structure and position of different countries, we can easily see where civilization can be an early, and where it must be a late condition of the inhabitants.

A school of unphilosophers has lately been trying to grow up amongst us, who caricature the doctrine of race, and who think themselves wiser than other persons, because they are unable to perceive that the mobility of certain races and the stationary character of others are intimately connected with the conditions under which they have existed. Such shallow reasoners feel no scruples in laying down the law that certain races must be extirpated, and that certain others must be enslaved, and they raise a sort of hyena laugh against those who believe that, by applying principles of justice, benevolence, and industry, all the great branches of the human family may be at last raised to a participation of the blessings of liberty and civilization. The unchangeable habits of savages are paralleled by the unchangeable habits of the pauper and criminal class in European countries, and yet there is no distinction of race between the tramp and the tradesman, the hereditary pickpocket and the thriving artizan. If a large portion of Africa is pointed out as the abode of races who do not improve, we ask, why should they? When have they been subjected to the operation of causes similar to those which have led Europeans in their progressive career? And if we look a

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little farther, we shall see that the physical geography of Africa has opposed difficulties to the advance of civilization which we do not yet know how to overcome. In like manner immense tracts of country on the borders of the Amazon cannot, from considerations of physical geography, become the home of a numerous and cultivated population, until gigantic means can be employed to cut away its rampant vegetation, and drain its enormous swamps. In dealing with Africa, the difficulties are physical and moral, and the beneficent influence of the European mind upon the African mind can only be exerted successfully by a combination of powerful countries like England, America, and France, to compel the cessation of the slave trade, and establish honest, commercial settlements at the most convenient points.

Professor Ansted has an interesting chapter on the efforts of human agency in modifying the earth's surface, and there is every reason to believe that, within a few generations, the conditions of prodigious tracts of country will be greatly improved. India is undergoing an enormous change, through the restoration and extension of works of irrigation, and the impulse given to cultivation by the augmentation of means of transport. The slave states of America are passing from barbarism to civilization, and in a generation or two, millions of educated men will replace millions brought up in a condition little above that of domestic animals, swamps will be drained, waste land reclaimed, barren land cultivated, and thus many thousands of square miles will experience a considerable modification of climate as well as of aspect. When sandy wastes are sufficiently near centres of civilization to be worth the cost of modifying, we find them gradually converted into pasture and forest. First, certain plants are grown which give stability to the shifting mass, and other plants succeed them, and, in time, trees with their power of attracting moisture and protecting the soil from evaporation, rear their heads. Even in certain extensive deserts, it is believed that water for their fertilization might be obtained by artesian wells, and the constant demands of commerce for new markets and new routes of transit, tend to bring into importance tracts of country that have for ages been stationary, because the world's industry and invention had left them alone.

Apparently trifling circumstances may lead to considerable changes, as when the accidental introduction of a new plant or a new insect interferes with pre-existing arrangements and modifies the vegetation of a country or a district, thereby leading to changes in its supply of moisture, amount of evaporation, etc., etc.

It is, however, when civilization is most active, that man's