# Physical Geography of the Sea and its Meteorology/Chapter 22

CHAPTER XXII.
§ 881-895.—THE ACTINOMETRY OF THE SEA.
881. A new field.'—One of the columns in the man-of-war log of the Brussels Conference calls for the temperature of the water below as well as at the surface of the sea. Only a few entries have been made in this column; but these, as far as they go, seem to indicate that the warmest water, especially in tropical ​seas, is not to be found at the top, but in a stratum a little way down. What is the depth of this stratum, and what may be the thermal difference between its waters and those of the surface, are questions for future observations to settle. Indeed, this subject opens a new field of inquiry; it is one from which much useful and instructive information is doubtless to be obtained by any one of our co-operators who will enter upon the investigation patiently and with diligence.
882. The warmest waters in the sea—where are they? at or below the surface?—The observations that we possess do not prove that the warmest water of inter-tropical seas is not at the surface: they go no farther than to show that it is sometimes not at the surface, and to suggest that, in all probability, it is generally below, especially in "blue water." Reason suggests it also. Supposing that, as a rule, the hottest water is below the surface, we may, in order to stimulate research, encourage investigation, and insure true progress, propound a theory in explanation of the phenomenon, looking to future observations to show how far it may hold good.
​883. The annual supply of solar heat uniform.—The flow of heat from the sun is held to be uniform, and the quantity that is annually impressed upon the earth is considered as a constant. The sun spots may make this "constant" a variable, but the amount annually received by the earth is. so nearly uniform, that our best instruments have not been able to show us any variation in its uniformity Some maintain that climates are undergoing a gradual change as to temperature. However this may be as to certain localities. Baron Fourier, after a long and laborious calculation, claims to have shown that if the earth had been once heated, and after having been brought to any given temperature, if it had then been plunged into a colder medium, it would not in the space of 1,280,000 years be reduced in temperature more than would a 12-inch globe of like materials in one second of time if placed under like conditions. It may be assumed that for the whole earth, there has not been since the invention of the thermometer any appreciable change in the temperature of the crust of our planet.
884. Quantity of heat daily impressed upon the earth.—The earth receives from the sun heat enough daily, it has been said (§ 271), to melt a quantity of ice sufficient to incase it in a film 1½ inch thick. What becomes of this heat after it is so impressed, how is it dispersed by the land? how by the sea? Let us inquire.
885. How far below the surface does the heat of the sun penetrate?—The solar ray penetrates the solid parts of the earth's crust only to the depth of a few inches, but striking its fluid parts with its light and heat, it penetrates the sea to depths more or less profound, according to the transparency of the waters. Let us, in imagination, divide these depths, whatever they may be, into any number of stratifications or layers of equal thickness. The direct heat of the sun is supposed to be extinguished in the lowest layer; the bottom layer, then, will receive and absorb the minimum amount of heat, the top the maximum; consequently, each layer, as we go from the top to the bottom, will receive less and less of the sun's heat.
886. The stratum of warmest water.—Now, which will retain most heat and reach the highest temperature? Not the top layer, or that to which most heat is imparted, because by evaporation heat is carried off from the surface of the sea almost as fast as by the sun it is impressed upon the surface of the sea; ​not the bottom layer, because that receives a minimum, which, though it cannot escape by evaporation, may nevertheless fail to make any marked change in temperature—fail, not by reason of no evaporation, but by the ever-changing movements which, considering the length of time required to heat the lower stratum by such slow and gradual accumulation of heat, would alter its place and vary its condition, and indeed removing it beyond the reach of the observer.
887. Its position.—The layer, therefore, which accumulates most heat and becomes warmest, should be neither at the bottom nor at the top, but intermediate, the exact temperature and depth of which it is for observation to determine. To encourage such determination and the investigations which it suggests is the main object of this chapter.
888. The different subjects for observation.—In conducting such investigations, several questions are to be considered, such as the transparency and specific gravity of the water, its phosphorescence; the face of the sky, whether clear or cloudy; the state of the sea, whether rough or smooth; the condition of the weather, whether calm or windy. Then the temperature should be tried, at various depths and at various hours of the night and day, in order to ascertain not only the maximum temperature and average depth of the warmest stratum in the day, but the difference in its temperature and position by day and by night. These observations will afford the data, also, for computing the amount of solar heat that penetrates the bosom of the sea, as well as the amount that is radiated thence again. They will reveal to us knowledge concerning its actinometry in other aspects. We shall learn how absorption by, as well as radiation from, the under strata is affected by a rough sea, as when the waves are leaping and tossing their white caps, and how by its glassy surface, as when the winds are hushed and the sea smooth.
889. Expected discoveries.—Here we are reminded, also, to anticipate the discovery of new beauties and fresh charms among the wonders of the sea. We have seen (§ 366) that while the heat of the sun is impressed alike upon sea and land, nevertheless the solid part of the earth's crust radiates much more freely than the fluid. On the land the direct heat of the sun operates only upon a mere shell a few inches in thickness; at sea it penetrates into the depths below, and operates upon a layer of water many feet thick. The solid land-crust has its ​temperature raised high by day and cooled low down by night; but the most powerful sun, after beating down all day with its fiercest intensity upon this liquid covering, has not power to raise its temperature more than three or four degrees. This covering serves as a reservoir for the solar heat. In the depths below it is concealed from the powers of intense radiation, and held by the obedient ocean in readiness to be brought to the surface from time to time, and as the winds and the clouds call for it. Here it is rendered latent by the forces of evaporation, and in this form, having fulfilled its office in the economy of the ocean, it passes off into the air, there to enter, in mysterious ways, upon the performance of its manifold tasks.
890. Actinic processes.—As evaporation goes on by day or night, the upper stratum is rendered heavier by reason of both the heat and the fresh water borne away by evaporation; the upper water having been thus rendered both Salter and cooler, has its specific gravity increased so much the more. On the other hand, the strata below, receiving more heat by day than they dispense again by radiation day and night, grow actually warmer and specifically lighter; and thus, by unseen hands and the "clapping of the waves," the waters below are brought to the surface, and those on the surface carried down to unknown depths; and thus, also, we discover new and strange processes which have been ordained for the waters of the ocean in their system of vertical circulation.
891. The reservoirs of heat.—Thus we arrive at the conclusion that the ocean is the great reservoir of sensible as the clouds are of latent heat. That in those two chambers it is innocuously stored, thence to be dispensed by processes as marvellous as they are benignant and wise, to perform its manifold offices in the economy of our planet; it is this heat which gives "his circuits" to the winds and circulation to the sea; it is it that fetches from the ocean the clouds that make "the earth soft with showers." Stored away in the depths of inter-tropical seas, it is conveyed along by "secret paths" to northern climes, there to be brought to the surface in due season, given to the winds, and borne away to temper the climates of western Europe, clothing the British Islands as they go, in green, and causing them to smile under the genial warmth even in the dead of winter.
892. An office for waves in the sea.—Thus perhaps we discover a new office for the waves in the physical economy of the ocean. ​Is it not to them that has been assigned the task of bringing up by their agitation of the surface the layers of warm water that are spread out below; and are they not concerned also, as they draw up the genial waters, in regulating the supply of heat for the winds by night, as well as in cold or cloudy days, for the purposes of evaporation? Thus even the waves of the sea are made by this beautiful study to present themselves as parts, important parts, in the terrestrial machinery. We now view them as it were, like balance-wheels in the complicated system of mechanism by which the climates of the earth are governed. If the waves did not stir up the heated waters from below (§ 881), the winds would evaporate slowly by night, for the want of adequate supplies of caloric; the consequence would be less precipitation and a more scanty supply of latent heat for liberation in the cloud region. As a consequence of this, the winds would have less motive power, and the whole climatic arrangements of our planet would be different from what they are.
893. The radiating powers of earth, air, and water compared.—may note also another peculiarity as to the difference in the direct heat-absorbing and radiating properties of sea, land, and air: it is one which presents the atmosphere in the light of a regulator between the land on one hand, and the heating powers of the sun on the other. It is suggestive also of other benign compensations and lovely offices in the physical machinery of our planet: both land and water receive more heat from the sun than they radiate again; but the atmosphere receives less heat direct from the sun than it radiates off again into space: as the heat comes from the sun, part of it is absorbed by the atmosphere; but the largest portion of it is impressed upon the land and water from them a portion passes off into the atmosphere by conduction, while another portion is radiated directly off into the realms of space. What becomes of the remainder? Let us inquire, for there is a remainder, and unless means for its escape were provided, the land and water, especially the latter, would continue to grow warmer and warmer, and so produce confusion in the terrestrial economy. The remainder of this heat, being that which is neither radiated by sea and land directly off into space, nor imparted to the air by conduction from them, is absorbed in the processes of evaporation; it is then delivered to the atmosphere latent in the vesicles of vapour, to be set free in the cloud region, rendered sensible and imparted to the upper air, whence it is sent ​off by radiation into the "emptiness of space." Thus the air with its actinometry presents itself in the light of a thermal adjustment, by which the land and sea are prevented from becoming seething hot; and by which they are enabled to perform their wonderful offices with certainty and regularity.
894. A reflection concerning heat.—It is curious to think that this heat which we have been contemplating, now as latent in the clouds above, now as sensible in the waters below, comes from the same source whence originally came the heat which has been packed away in scams of coal and stored in the bowels of the earth for ages and ages, to be called forth by man at will for his own comfort, pleasure, and convenience; that this protean thing is the agent which controls sea and winds, and they it; that it is it which has lifted up the mountains; which clothes the world with beauty, and keeps the stupendous fabric of the universe in motion; and that after all, this mighty agent is only that gentle thing that "warms in the sun!"
895. Probable relation between the actinism of the sea and its depth.—Pursing this subject, the philosophical mariner, as he sails along and records observations for these purposes, may fancy—and perhaps rightly—that he has traced to the actinometry of the sea one of the physical conditions which, when the depths of the ocean were laid, had its weight with the Almighty Architect.
LONDON: PRINTED BY WILLIAM CLOWES AND SONS, STAMFORD STREET AND CHARING CROSS.