that the earth's axis had been for ages perpendicular to the plane of the ecliptic, so that there was a perpetual equinox, and uniformity of seasons throughout the year; that the planet enjoyed this paradisiacal' state until the era of the great flood; but in that catastrophe, whether by the shock of a comet, or some other convulsion, it lost its equipoise, and hence the obliquity of its axis, and with that the varied seasons of the temperate zone, and the long nights and days of the polar circles.

When the progress of astronomical science had exploded this theory, it was assumed, that the earth at its creation was in a state of igneous fluidity, and that, ever since that era, it had been cooling down, contracting its dimensions, and acquiring a solid crust. It was also taken for granted that this original crust was the same as that which we are now studying, and which contains the monuments of a long series of revolutions in the animate world. This notion, however arbitrary, was well calculated for lasting popularity, because it referred the mind directly to the beginning of things, and required no support from any ulterior hypothesis. But the progress of geological investigation gradually dissipated the idea, at first universally entertained, that the granite or crystalline foundations of the earth's crust were of older date than all the fossiliferous strata. It has now been demonstrated that this opinion is so far from the truth that it is difficult to point to a single mass of volcanic or plutonic rock which is more ancient than the oldest known organic remains. Such being the case, the question of original fluidity, although a matter of legitimate speculation to the physicist, is one with which the geologist is but little concerned. It may relate to a state of things which preceded our earliest records by a lapse of ages many times greater than the entire series of geological epochs with which we are acquainted.

If, instead of indulging in conjectures as to the state of the planet at the era of its creation, we fix our thoughts steadily on the connection at present existing between climate and the distribution of land and sea, and then consider what influence former fluctuations in the physical geography of the globe must have had on superficial temperature, we may

make a near approximation to a true theory. But the effect of former variations in the heat and cold of the different seasons in the year, caused by the precession of the equinoxes, combined with the revolution of the apsides, and still more by variations in the excentricity of the earth's orbit, will have to be taken into account, as subsidiary to the more dominant influence of geographical conditions. Should doubts and obscurities still remain, they should be ascribed to our limited acquaintance with the laws of Nature, not to revolutions in her economy. They should stimulate us to farther research, not tempt us to indulge our fancies respecting imaginary changes of internal temperature, or the unsettled state of the surface of a planet before it was prepared for the habitation of living beings.

Diffusion of heat over the globe.-In considering the laws which regulate the diffusion of heat over the globe, we must be careful, as Humboldt well remarks, not to regard the climate of Europe as a type of the temperature which all countries placed under the same latitude enjoy. The physical sciences, observes this philosopher, always bear the impress of the places where they began to be cultivated; and as, in geology, an attempt was at first made to liken all the volcanic phenomena to those of Italy, so in meteorology, a small part of the old world, the centre of the primitive civilisation of Europe, was for a long time considered a type to which the climate of all corresponding latitudes might be referred. But this region, constituting only one-seventh of the whole globe, proved eventually to be the exception to the general rule. For the same reason, we may warn the geologist to be on his guard, and not hastily to assume that the temperature of the earth in the present era is a type of that which most usually obtains, since he contemplates far mightier alterations in the position of land and sea, at different epochs, than those which now cause the climate of Europe to differ from that of other countries in the same parallels of latitude.

It is now well ascertained that zones of equal warmth, both in the atmosphere and in the waters of the ocean, are neither

parallel to the equator nor to each other.* It is also known that the mean annual temperature may be the same in two places which enjoy very different climates, for the seasons may be nearly uniform, or violently contrasted, so that the lines of equal winter temperature do not coincide with those of equal annual heat or isothermal lines. The deviations of all these lines from the same parallel of latitude are determined by a multitude of circumstances, among the principal of which are the position, direction, and elevation of the continents and islands, the position and depths of the sea, and the direction of winds and currents.

On comparing the two continents of Europe and America, it is found that places in the same latitudes have sometimes a mean difference of temperature amounting to 11°, or even in a few cases to 17° Fahr.; and some places on the two continents, which have the same mean temperature, differ from 7° to 17° in latitude. Thus, Cumberland House, in North America, see fig. 9, having the same latitude (54° N.) as the city of York in England, stands on the isothermal line of 32°, which we have to seek in Europe at the North Cape, in lat. 71°, but its summer heat exceeds that of Brussels or Paris.† The principal cause, says Humboldt, of the greater intensity of cold in corresponding latitudes of North America, as contrasted with Europe, is the connection of America with the polar circle, by a large tract of land, some of which is from three to five thousand feet in height; and, on the other hand, the separation of Europe from the arctic circle by an ocean. The ocean has a tendency to preserve

*We are indebted to Alex. von Humboldt for having first collected together the scattered data on which he founded an approximation to a true theory of the distribution of heat over the globe. Many of these data were derived from the author's own observations, and many from the works of M. Pierre Prévost, of Geneva, on the radiation of heat, and from other writers. -See Humboldt on Isothermal Lines, Mémoires d'Arcueil, tom. iii. translated in the Edin. Phil. Journ. vol. iii. July, 1820.

The map of Isothermal Lines, pub

lished by Humboldt and Dove in 1848, (re-edited by Dove in 1853, from which fig. 9, is extracted), supplies a large body of well-established data for such investigations, of which Mr. Hopkins availed himself in an able essay On the Causes which may have produced Changes in the Earth's Superficial Temperature.'-Q. Journ. Geol. Soc. 1852, p. 56.

Sir J. Richardson's Appendix to Sir G. Bach's Journal, 1843-1845, p. 478.

everywhere a mean temperature, which it communicates to the contiguous land, so that it tempers the climate, moderating alike an excess of heat or cold. The elevated land, on the other hand, rising to the colder regions of the atmosphere, becomes a great reservoir of ice and snow, arrests, condenses, and congeals vapour, and communicates its cold to the adjoining country. For this reason, Greenland, forming part of a continent which stretches northward to the 82nd degree of latitude, experiences under the 60th parallel a more rigorous climate than Lapland under the 72nd parallel.

In addition, however, to the cause here assigned by Humboldt, it must be borne in mind that the eastern coast of Greenland is skirted for a thousand miles by the cold waters of the Greenland current flowing from the North Pole, while Lapland is warmed by the waters of the Gulf-stream flowing from the south.

But if land be situated between the 45th parallel and the equator, it produces, unless it be of great height, exactly the opposite effect; for it then warms the tracts of land or sea that intervene between it and the polar circle. For the surface being in this case exposed to the vertical or steeply sloping rays of the sun, absorbs a large quantity of heat, and raises the temperature of the atmosphere which is in contact with it. For this reason, the western parts of the old continent derive warmth from Africa, which, like an immense furnace, distributes its heat to Arabia, to Turkey in Asia, and to Europe.'* The north-eastern extremity of Asia, on the contrary, experiences in the same latitude extreme cold; for it has the land of Siberia on the north between the 65th and 70th parallel, while to the south it is separated from the equator by the Pacific Ocean.

In consequence of the more equal temperature of the waters of the ocean, the climate of islands and of coasts differs essentially from that of the interior of continents, the more maritime climates being characterised by mild winters and more temperate summers; for the sea breezes moderate the cold of winter, as well as the heat of summer. When, therefore, we trace round the globe those belts in which the mean * Malte-Brun, Phys. Geol. book xvii.