ART. XXXIV.-Terrestrial Climate as influenced by the Distribu tion of Land and Water at different geological epochs; by HENRY HENNESSY, F.R.S., M.R.I.A., Professor of Natural Philosophy in the Catholic University of Ireland.*

EVERY point on the earth's surface is continually gaining and losing heat, and its actual temperature at any given moment depends on the difference between its gains and its losses. If the outer coating of the earth were exclusively composed of solid materials, terrestrial climate would depend principally on the heat gained from sunshine and the heat radiated into space. But as the earth is completely enveloped by an atmosphere, and partly surrounded by a liquid, its thermal conditions must be greatly influenced by the physical properties of these fluid coverings. While the heating or cooling of a solid follows the clearly defined and comparative well understood laws of conduction and radiation, the heating or cooling of gases and liquids is further greatly modified by the mobility of their particles. The changes of state which frequently take place in fluids, whether by evaporation or condensation, freezing or liquefaction, introduce agencies which still further complicate the study of their thermal relations.

When we study the thermal conditions of a liquid distributed over the terrestrial spheroid, it becomes manifest, that these conditions are influenced by the area, configuration, and physical structure of such portions of the solid earth as rise above the ocean and come in contact with the atmosphere, so as to constitute the surface of the dry land. Upon this matter I propose to develop certain views which are closely connected with those I have already published relative to the distribution of heat over such solid surfaces.+

2. When a surface, covered with ordinary soil, receives the rays of the sun, the heat thus acquired passes downwards, but on arriving at a very small depth its intensity rapidly diminishes. The solar heat which is thus received by the ground may, therefore, be considered as confined almost entirely to a thin superficial stratum. The air in contact with the soil becomes heated, expands, and tends to ascend: a circulation thus follows between the upper and lower strata of the atmosphere situated above the heated ground. During the night a different process takes place; for then the radiation of the soil causes its temperature

*Cited from the Atlantis, for January, 1859.

On the Distribution of Heat over Islands, etc., Atlantis, No. ii, p. 896. See also the Note on the Laws that Regulate the Distribution of Isothermal Lines, No. üii, p. 201.

to fall below that of the superincumbent air; the coldest stratum of the lower portions of the atmosphere being in contact with the ground, the equilibrium of those above is not so much disturbed. Yet, even in this case, causes exist which tend to produce a series of actions and reactions between the upper and lower strata of air, by which a process of convection will be ultimately developed. These actions will be rendered especially remarkable if the soil is not bare, but covered with vegetation in the manner of the greater part of the dry land. This question has been fully treated by Melloni,* in his memoir on the nocturnal cooling of bodies. His general proposition, that "a body exposed during the night to the influence of a sky of equal clearness and calmness, is always cooled to the same extent, whatever may be the temperature of the air," is fruitful in im portant results. Thus is explained the great differences between the temperature of the day and night on land in the torrid zone. The intense cold observed during the night by Denham in trav ersing the great Desert of Sahara, the process of artificial freezing at Bengal, and the rain-like dews observed by Humboldt in the forests of South America, are all necessary consequences of the energy of the actions and reactions by which the outer coating of the earth loses the warmth it has acquired from sunshine during the day. Conversely, the almost constant temperature of the sea in tropical regions, by day and night, and the nearly total absence of dew on the rigging of vessels far removed from the land, clearly show the peculiar retentiveness of heat possessed by the water, and that, unlike the land, it does not readily part with whatever warmth it may have acquired from sunshine during the day. The cold southerly breezes sometimes observed in Egypt during the winter months, when the air has passed over immense surfaces of sandy desert, present a striking contrast to the southwesterly winds which at the same season traverse the ocean and visit our shores. It appears, from a communication in the Times newspaper, dated Melbourne, November 15, 1858, that in South Australia, the coldest winds during the winter months, are those blowing from the northerly and tropical regions, while the warmest are those blowing from the pole. The former pass over extensive surfaces of heat-radiating, and therefore heat-losing land, while the latter traverse the heat-retaining ocean. In the summer (at least by day) the opposite phenomena are observed, of warm winds from the north and cold from the south. Combined observations on the wind, and on temperature, by day and night, would further elucidate a problem which, in the words of the writer, "cannot be solved

*Taylor's Scientific Memoirs, vol. v, pp. 453 and 530; and Annales de Chimie et de Physique, for February and April, 1848.

+ Kaemtz Metéorologie, French edit., p. 45.

without greatly adding to the stock of our knowledge." While the feeble conducting power of the solid portion of the earth's coating, allows but a small portion of the sun's heat to pass beneath the surface, so that whatever warmth is thus received on that surface during the day is readily radiated into space during the night, a liquid mass, similarly exposed to sunshine and subsequent nocturnal radiation, possesses peculiar properties which greatly influence the differences between its thermal losses and gains. The most important of these properties are, (1) the great capacity of water for heat, by which it gradually accumulates and slowly parts with whatever warmth it has received; and (2) the intermobility of its particles, by which exchanges of temperature in different parts of the liquid mass are essentially promoted.

Let us consider the effect of the sun's rays on a globe covered with water, and we shall soon perceive that a more energetic process than that of conduction accompanies the exchanges of temperature between the different portions of the fluid. The water which receives the vertical rays of the sun will be more heated than the waters which receive its rays at more oblique inclinations. Not only the amount of warmth received over a given area, but also the depth to which the rays of heat penetrate below the surface, depends upon the angles made by these rays with the vertical. Inequalities of surface temperature, depending on the latitude, the hour angle, and the sun's longitude, should thus result. The more heated waters would expand, and tend to spread over the cooler waters in other regions. Currents should arise from the mutual actions and reactions of the unequally heated portions of the fluid. The colder currents would usually tend to flow beneath the warmer, unless at temperatures approaching that of the maximum density of water, and thus a process of circulation would be established by which the temperature acquired by the superficial strata of the water should be ultimately propagated to a certain depth below the surface. Evaporation would also take place, and by the condensation of vapor a certain portion of the heat received by the water would be imparted, in the formation of clouds, to the superincumbent atmosphere.

If, as in the existing oceans, this water be salt, the inequalities of temperature producing inequalities of evaporation, will also produce diversities in the density of the water in different regions, and thus additional energy will be imparted to the process of circulation. The salter and heavier surface water will tend to sink into the colder liquid which lies beneath, and which shall naturally tend to take its place, by ascending upwards. The process of evaporation would cool the surface of the water; but, unlike that of radiation, it is not altogether a losing process

as far as the entire surface of the earth is considered; for it is sooner or later followed by condensation, whereby the greater part of the absorbed heat is again returned. When a piece of land or water parts with its heat by radiation into space, that warmth can never be restored to any part of the earth's surface; but whatever heat the water loses by evaporation, becomes latent in the vapor so produced, and is ultimately transferred by condensation to some other part of the globe; and hence evaporation does not constitute an agent in causing a diminution of general terrestrial temperature. Let us now suppose a sheet of water at the equator nearly surrounded by fixed boundaries, so as to form a species of immense lagoon. Its temperature, from the causes here referred to, will rapidly augment. The heat which it has acquired during the day shall have penetrated so deeply as to be incapable of being radiated backwards into space during the night, with the same facility as on the surface of a sandy plain or from the summits of a mass of vegetation. Its temperature should thus continue to accumulate up to a certain limit imposed by the conditions of evaporation, and it might ultimately attain a mean temperature superior to any which is now met at the surface of intertropical seas.

3. These views are strikingly illustrated by the phenomena accompanying the origin of the Gulf Stream. The mass of water which rushes into the Gulf of Mexico, along the southern shores of the Carribbean Sea, has already acquired a certain elevated temperature from the action of sunshine in the southern torrid zone in its passage from Cape St. Roque. In moving around the Caribbean Sea and the Mexican Gulf, these waters still continue under the influence of a tropical sun, and are constantly increasing in temperature. The islands and coasts which they happen to bathe, have no part in directly promoting this augmentation. On looking over the isothermal chart of the Caribbean Sea and Gulf of Mexico, prepared by Mr. Charles Deville, it becomes manifest that in general the temperature decreases in going towards the land. In some places the mean annual temperature of the water close to the land is 24°-5 Centigrade; further out at sea it is 25°, and still further from the land it is 25°5. In other places it gradually augments from 26°, in going from the land, up to 27°4. These results are unconnected with the influence of latitude, and they are still less explicable by the influence of centrifugal force, in driving the cooler and heavier waters towards the edges of the great current, in its semi-rotatory movement around the gulf. For in this case the law of decrease of temperature in going from the

*

Annuaire de la Société Météorologique de la France, tom i, p. 160.

+ Reduced to degrees of Fahrenheit's scale, these numbers, arranged in the same order as in the text, are 76°-1, 770-0, 770.9, 780-8, 810.3.

land, should not hold on approaching the coasts of large islands situated towards the centre of the moving mass of waters. But, in such instances, it is also manifested; for on the north and south coasts of the Island of Cuba we find the isothermal lines of 26°-2 and 26°5, while the isothermals of 26°-7 and 26°-8 are situated outside them respectively. In Mr. Deville's chart these are closed isothermals, similar to those which I have indicated on the surface of the British Islands; but as the lowest isothermals in my map are the most remote from the sea, those in his chart which exhibit the highest temperature are farthest from the land. It is thus apparent that the intertropical sea may become a storehouse of heat, by retaining much of what it receives from the sun, which, but for the physical properties of water, it would, like the intertropical land, lose by radiation into space. It is important to bear this conclusion in mind in any inquiries respecting the influence of the distribution of land and water on general climate, especially as the influence of the land seems to have been hitherto principally considered as a calorific agent.

The heating action of intertropical land has been so often discussed by writers on climate, that it is unnecessary to do more than to point out its principal agency in the production of aerial currents, by which exchanges of temperature may be promoted between different parts of the earth's surface.

In contrasting the mean temperature of the sea with that of the land in tropical climates, the want of nocturnal observations, as referred to by Melloni, is peculiarly felt. While the temperature of the one is nearly constant, that of the other is liable to considerable fluctuations; and, as our records are principally derived from diurnal observations, the results are probably too favorable to an excess of land temperature. This conclusion is confirmed by the results exhibited in Mr. Deville's map, and, in some measure, by the fact of the higher mean temperature of the entire oceanic covering of our planet compared to its atmospheric coating.

In comparing the calorific influence of the land on distant regions with the agency of the sea, it should therefore be remembered, that while the latter stores up heat and acts by night as well as by day, the action of the land is effective only as long as the sun's rays are impinging upon it.

4. Let us endeavor to apply these conclusions to the question of the influence of the distribution of land and water upon general terrestrial temperature. As the amount of solar heat received by any point on the earth's surface is a function of the latitude, it follows that the distribution of land and water at different latitudes must be studied in order to obtain its influence

* Equivalent respectively to 790-16, 79°7, 80°06, and 80°-24 of Fahrenheit's scale.