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CORRIGENDA IN THE PLATES OF POLYEDRA IN VOL. XXXII.

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CHANGE OF CLIMATE-SECULAR, AND CAUSED BY HUMAN AGENCY.

BY RICHMOND LEIGH, M.R.C.S.

THE climate of the earth is not immutable-at least, when considered in limited localities-but has experienced many and varied changes. Taken in its entirety, no climatic change may have occurred at any period; there may have always been the same average mean temperature, the same amount of evaporation and of condensation, as rain, snow, etc., as well as the same relative succession of the seasons.

Indeed, it is difficult to conceive, except on one theory, of any cause of change from without, except to such slight extent as might be accounted for by the attraction of aerolites and similar bodies, or by eccentricity of the earth's orbit. This latter cause might produce some considerable alteration of climate, but, from its nature, it would tend to self-rectification.

There is one particular theory that presumes, and, if correct, that would account for great alteration in climate, viz., that of the secular cooling of the earth. But geological research has tended to reduce the probability of this theory to a minimum, and it has been, in consequence, almost discarded.

It would be a most interesting and valuable point to discover whether the mean temperature of the whole globe varies or not, but the facts available at the present time for such an inquiry are greatly insufficient for such a purpose.

Whether the sum total of the earth's climate be changed or not, its distribution is very greatly altered.

The causes which have contributed to this alteration are either general or local..

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The general causes are those which act on the globe as a whole, and which, consequently, affect to a greater or lesser extent the entire surface of the earth. They have left many traces of their action in prehistoric times, and many theories have been invented to explain this action.

Different mathematicians who have of late gone into the question of a change in the geographical position of the earth's axis of rotation, agree in its theoretical possibility by redistribution of its superficial matter; but they do not agree on the amount of such change. Sir W. Thomson has asserted, as highly probable, that the poles may, in ancient times, have been very far from their present position, and may have gradually moved through ten, twenty, forty, or more degrees, without the occurrence at any time of any perceptible sudden disturbance of land or water.

Croll considers the chief cause of secular change of temperature to be the deflection of ocean currents by alterations in their beds. It is thus that he considers the gulf stream to have been produced, which he assumes to have changed the climate of England from its antecedent glacial character.

There is, at the present time, an unequal distribution of temperature in the two hemispheres of the earth, north and south, the north being much the warmest, to account for which various explanations have been offered, besides those already adduced. But whatever other causes may have assisted to produce this result, a more direct one appears in the fact that the summer period of the northern hemisphere actually exceeds that of the south by seven or eight days, or about one hundred and seventy-eight hours. This is owing to the unequal speed of the earth in different parts of its orbit.

The hours of daylight in the north also exceed the hours of night, in consequence of the longer time the Arctic pole is inclined to the sun. A very gradual change, however, is

in constant action, through the precession of the equinoxes, which will in due time equalise the mean temperature of both hemispheres.

The March equinox is every year twenty minutes before the last. Therefore, in twelve thousand nine hundred years the conditions of the two hemispheres will be changed, and the south will be the warmest.

These general causes of climatic change are beyond the range of human influence, and can but be viewed as a part of the general cosmic economy. The local changes, on the contrary, are nearly all the direct result of human action, and as such of great practical importance to mankind.

When civilised man first appeared, the surface of the earth, with few exceptions, was most probably covered with forest.

The two Americas were so clothed at their discovery, and we may reasonably infer that other portions of the globe were in like condition. This forest-clad state is evidently the normal and natural one; for when land is abandoned after cultivation-the climate remaining suitable for forest growth -and being, at the same time, free from browsing animals, it almost invariably returns to that primitive condition.

In all probability, for a considerable space of time, the earth's surface and local climate underwent slight alteration. But it is evident that at an early period a partial denudation of forest, and deterioration of soil, produced a corresponding unfruitfulness; and that the lesson learnt so recently and dearly, of the folly of disturbing nature without equivalent counteraction, was learnt previously by the ancients. The existence of works of irrigation, canals and reservoirs, indicate a deficient natural supply of water, or rainfall, when they were constructed; and since their decay, the lands they fertilised have sunk into either partial or complete sterility.

There are numerous facts which can only be understood

by considering that the climatic conditions of certain countries, in the earlier historical times, were very different to their present states. In many places where, at the present time, a scattered people obtain a bare subsistence, in former times a dense population lived luxuriously. Syria, Persia, Northern Africa, and many other countries, present notable instances of fertility, and consequent great habitability, within the earlier historical period. The Roman Empire also, throughout a very great portion of its extent, was endowed with a much greater fertility than it has now, according to the accounts of the old historians.

Marsh* concludes that " more than half their extentnot excluding the provinces most celebrated for the profusion and variety of their spontaneous and cultivated products, and for the wealth and social advancement of their inhabitantsis either deserted by civilised man, and surrendered to hopeless desolation, or, at least, greatly reduced in both productiveness and population. Vast forests have disappeared, the vegetable earth accumulated beneath the trees, by the decay of leaves and fallen trunks, is washed away; meadows, once fertilised by irrigation, are unproductive, because the cisterns, reservoirs, and canals are broken or destroyed; rivers have shrunk to rivulets, and rivulets become the mere outflows of the rainy season, and other scarcely less important changes in the physical condition of the inhabited earth have taken place."

The regions thus affected include Persia, Syria, Mesopotamia, Armenia, much of Asia Minor, Arabia in great part, Northern Africa, Greece, and parts of even Italy, Spain, and Sicily, etc.

This decadence of highly-developed countries forms a most interesting and instructive study, which is well illus* An able American author, upon whose work, The Earth as Modified by Human Action, I have largely drawn.

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