two; and we should thus have the paradox of warming the entire globe by modelling its warmer hemisphere after its colder. Unless the influence of Victoria Land as a refrigerator of the southern hemisphere should be greater than that of the immense masses of land in the northern parts of the new and old continents, this paradox would seem inexplicable on the theory under consideration. But it can be in some measure explained, if the agency of oceanic currents in storing up and transporting the heat acquired from sunshine be fully admitted. In the actual state of the earth's surface, the form of the basin of the South Atlantic Ocean, combined with other physical conditions, seems to determine the transfer of a great volume of heated water from the southern intertropical regions to the northern hemisphere, which, passing subsequently through the Caribbean Sea and Gulf of Mexico, acquires a still higher temperature, and ultimately confers its warmth on regions in high northern latitudes. From the direction of the currents of the Pacific, as laid down on some of Maury's charts, it is probable that a similar transfer northwards of heated southern intertropical water is effected in that great ocean as well as in the Atlantic. The general result is, that the southern hemisphere is not only deprived of a certain amount of the solar heat absorbed by its waters, but that the temperature of the northern hemisphere is augmented to a corresponding amount. But although the paradox alluded to may be thus explained, this result shows the danger of underestimating the agency of aqueous currents in connection with any theory of the distribution of land and water that may be proposed in order to explain vicissitudes of terrestrial climate. 7. In examining the consequences resulting from the suppression of the Gulf Stream on the climate of western Europe, with reference to the question of glacial action at former geological epochs, as has been done by Mr. Hopkins,* we need only direct our attention to what actually takes place at corresponding latitudes in the southern hemisphere. In these regions, there is not only an absence of such an active calorific agent, but even an abstraction of some of the heat due to them from the sunshine which falls upon a portion of their oceans, which heat we have seen is transferred to the northern hemisphere. Glaciers consequently descend to the sea, not only about the latitude of 54° S., as observed by Captain Cook, but even so close to the equator as 48° 30' S., where they were noticed in great abundance on the western coast of South America by Mr. Darwin.t He even observed one instance of a glacier reaching the sea in the latitude of 46° 40', which corresponds to that of Napoleon Vendée, in the west of France. The existence of glacial action in the * Quarterly Journal of the Geological Society, 1852, p. 85. Voyage of Adventure and Beagle, iii, p. 282. southern latitudes, equivalent to those of the temperate regions of western Europe, suggests the possibility, that by an inversion of the operating causes, the southern hemisphere might have enjoyed a milder climate at the same geological period when glacial phenomena were most completely developed north of the equator. 8. The results of our inquiry may be thus recapitulated: (1.) The physical properties of water appear upon the whole more favorable than those of the land, to the accumulation, retention, and distribution of solar heat throughout the matter composing the external coating of the earth. (2.) Phenomena presented by intertropical seas at the present day, confirm and illustrate this conclusion. (3.) The distribution of land and water most favorable to a general increase of terrestrial mean temperature, should, therefore, be such as would imply the existence of great intertropical seas and of groups of islands evenly distributed both within the tropics and in extratropical regions. (4.) Such a distribution of land and water at former geological epochs, seems to be indicated by the results of observation. (5.) The superior mean temperature of the northern compared to the southern hemisphere is probably due, not to the direct influence of the greater proportion of land in the former, but to currents which determine the transfer towards the north of a portion of the solar heat absorbed south of the equator. 9. While fully acknowledging the important influence which changes in the distribution of land and water may exercise on terrestrial climate, we are not precluded from studying the action of other causes, and of giving to them such weight as the evidences in their favor may render advisable. If, from the results of astronomical as well as of geological testimony, we are induced to believe that the earth has been for ages slowly cooling from a state of former incandescence, its climate during the earlier epochs of its physical history must have been more or less influenced by the heat thus passing outwards through its crust. However efficient, as applied to recent phenomena, we may find the theory of geological climates that explains the variations of the earth's superficial temperature by changes in the distribution of the liquid and solid portions of its outward coating, it seems by itself incompetent to rationally and consistently account for the very high temperature which must have prevailed during remote epochs of the earth's history. If we reject the evidence on which it has been concluded that the earth has slowly cooled from a fluid incandescent state into its observed condition, and admit that the earth's spheroidal shape was due to gradual and even existing causes, and not to the mechanical consequences of its primitive and universal fluidity, we shall arrive at a conclu * sion which, on the supposition of the complete adequacy of superficial causes to explain all changes of climate, would lead to the inference that, from very remote epochs, the mean temperature of the globe should be increasing instead of diminishing. By rejecting the former fluid condition of the earth, we are compelled to account for its oblateness in the way attempted by Playfair, that is, by appealing to the influence of certain superficial actions coexisting with the phenomena of geological changes. But I have proved, that if, from superficial causes, the earth's figure became gradually more oblate, the extent of polar dry land would gradually diminish, while that of equatorial dry land would, at the same time, tend to augment. Hence the very operations required to mould the earth's figure into the shape now observed, would, on this theory, point to a gradual increase in the efficiency of the physical conditions required for an augmentation of terrestrial temperature in proceeding from the most remote to the most recent geological epochs. But this is the very reverse of the conclusions deduced from the entire mass of geological inquiries; hence, as far as observation enables us to judge, we cannot explain by superficial actions alone, the twofold conditions of the spheroidal shape in the earth's figure, and the gradual diminution of its surface temperature from the earliest periods of geological history up to the most recent. ART. XXXIV.-Note on the Laws which Regulate the Distribution of Isothermal Lines; by HENRY HENNESSY, F.R.S., M.R.I.A., Professor of Natural Philosophy in the Catholic University of Ireland.t IN my essay on the Distribution of Heat over Islands, I referred to another mode of treating the general problem of isothermal lines, by which similar conclusions are derived. As these conclusions are not only obtained by a method somewhat different from that already published, but as they are accompa nied with a few additional remarks relative to their connection with the climatology of the globe, I may be permitted to present the following investigation as a development of a portion of the former inquiry. 2. The general problem, whose solution is here attempted, is to find the influence exercised by the physical structure and hydrographical relations of an island on the temperature at its surface. Let us consider an island having a certain definite figure, * Proc. Royal Irish Academy, vol. iv, p. 333, and Journal of the Geological Society of Dublin, March, 1849. Atlantis, vol. i, p. 899. Cited from the Atlantis for January, 1859. and surrounded by an ocean so warm that we may at first neglect the influence on its climate of the difference of latitude of the several parts of its surface. Let it be supposed perfectly free from hills or mountains, land breezes will tend to blow from the interior, and sea breezes from every point on the coast. The disturbing action of other winds would sometimes greatly modify the directions and intensities of the land and sea breezes; but, abstracting for a moment the effect of such general winds, it is evident that the temperature at any point of the island due to the action of the warm air flowing in from the ocean, and of the cold air flowing from the interior, will be some function of its distance from the coast, and, consequently, the forms of the isothermal lines should have some relation to the coast line. If no other winds blew over the surface of the island but land and sea breezes, and these with uniform intensity and frequency at every point, the isothermal lines should be similar to the coast line. Let us now superimpose on the island a series of elevations sufficiently considerable to offer impediments to the currents of wind: the forms of the isothermal lines will undergo important changes. If these eminences are scattered around the coast, their influence shall be greater than if they were all concentrated towards the interior of the island; for, in the former instance, they will present a kind of barrier, more or less broken, between the air resting on the central plains and the air outside covering the ocean. The sea breezes will no longer exercise the same effect on the portions of the interior situated behind the mountains, while their influence will remain unchanged, or be even increased, on the portions still unscreened from the ocean. A corresponding change must, therefore, take place in the forms of the isothermal lines. They should approach the coast at the parts screened by the mountains, while they should remain stationary, or sometimes recede towards the interior, at the intervals between the mountains. If the interior of the island does not consist entirely of dry plains, but is covered with lakes and considerable areas of undrained marshy land, such evaporating surfaces will cool the surrounding air. If the evaporating surfaces be concentrated chiefly about the centre of the island, their influence will not be much felt at the coast, and thus, although they may produce some local changes in the forms of the isothermal lines in their neighborhood, their most important effect will be to render still more decisive the differences of temperature on a line drawn from the coldest region at the centre to the coast; in other words, to contract or enlarge the dimensions of some of the isothermals. 3. If the influence of the differences of latitude of the surface of the island be now considered, it can be demonstrated that its SECOND SERIES, VOL. XXVII, No. 81.-MAY, 1859. tendency will be to transport the centres of the isothermals towards the pole, in whatever hemisphere the island may be situated, and that the isothermals at the centre shall be more affected from this cause than those at the coast. Let us suppose, for precision, the island in the northern hemisphere. Let us at first abstract the effect of all other sources of terrestrial temperature but solar radiation, and consider the proportions of heat that may be received by two elements of the surface of the island included between two adjacent isothermal lines. It will suffice to determine the quantities for the spaces included between each of their northern and each of their southern extremities respectively. From the great distance of the sun, its rays may be supposed nearly parallel, and from the limited area we are considering, the earth's figure may be supposed perfectly spherical. By the laws of radiant heat, the amount of heat received by an element s of the surface of the earth, will be represented by* 8 G cos q G being a coefficient, independent of the state of the earth's surface, and expressing the amount of heat that passes from the sun to a unit of surface placed perpendicularly to the direction of the sun's rays at a certain unit of distance, the inclination of the sun's rays to a perpendicular to the plane of the elements of the earth's surface, and R the radius of the earth's orbit. But where is the sun's declination, and an angle depending on the hour of the day, being included between the meridian of the element and that of the sun. The problem now before us, being connected with the proportional quantities of sunshine received by different elements and not with the absolute amounts, we may in a first approximation consider these quantities as proportional to the amount received at noon; consequently for a limited area of the sphere the quantity of heat received in the time dt is proportional to * Poisson Théorie Mathématique de la Chaleur, No. 210. |