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Climatic effects of Oceanic Currents.

The currents produced by heat and cold in the ocean, and in great masses of water generally, maintain in and over them a comparatively uniform temperate freshness, while the rocks and soil on the shores around, may be either parched under a burning sun, or bound up in frost.

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614. A keen frost chills, and soon hardens in its icy grasp the surface of the ground; but of water similarly exposed, the part first cooled descends to the bottom by its increased density, and forces up warmer water to take its place and to diffuse heat: this in its turn is cooled and descends, and a continued circulation is established, so that the surface cannot become ice until the whole mass, of whatever depth, has been cooled down to a considerable density. Hence the very deep sea is not frozen, even in the coldest climates; and in temperate climates the severest winter freezes only superficially lakes of ordinary depth. During this internal movement in the water, that which ascends to the surface to be cooled, by losing one degree of its heat, warms more than 500 times its bulk of air one degree, and thus tempers remarkably the air passing over it. Hence, places in the vicinity of the sea and of lakes are warmer in winter than places farther inland, although nearer to the equator. England is much warmer in winter than Central Germany, which lies south of England; and the coasts of Scotland and the north of Ireland may be warmer than London :snow never lies long upon these coasts. As continental or inland countries have thus in winter an extreme of cold, so have they in summer an extreme of heat. Water admits the rays of the sun, and absorbs the heat into the thickness of its mass, and therefore is warmed very slowly; but the dry earth, because a slow conductor, retains all the heat near its surface, and is therefore soon heated to

excess.

615. The ordinary ventilation of our dwellings and places of assembly is owing to the motion produced by the changed specific gravity of air when heated. The air which is within the house, owing to fires, the respiration of inmates, &c., becomes warmer than the external air, and the latter then presses in at every opening or crevice, to displace or force up the other. The ventilation of the person by the slow passage of air through the texture of our clothing is a phenomenon of the same kind; and thicker clothing keeps a person warm chiefly by diminishing the rapidity of this passage. Hence an oiled-silk or other air-tight covering laid on a bed bas greater

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Transference of Heat by Liquids.

influence in preserving warmth than one or two additional blankets, and is not generally used, only because it prevents ventilation, and, by shutting in the insensible perspiration, soon produces dampness. From the part of the bed-clothes immediately over the person there is a constant outward oozing of warm air, and there is an oozing inward of cold air in lower situations around.

616. The power of fluids to diffuse heat being due thus to their power of carrying, and not of conducting it, the consequence should follow, that any circumstance which impedes the internal motion of the fluid particles would diminish the diffusing power. Accordingly, we find that fluids in general transfer heat less readily in proportion as they are more viscid. Water, for instance, transfers less quickly than alcohol; oil than water; molasses or syrup than oil and water thickened with starch or other substances dissolved or suspended in it, or which has its internal motion mechanically impeded by feathers or thread immersed in it, undergoes circulation much less quickly than where it is pure and at liberty. Cooling being merely a motion the reverse of heating, is influenced by the same law. Hence the reason why thick soups, pies, puddings, preserves, and all semi-fluid masses, retain their heat so long-so much longer than equal bulks of water, although they are cool on the surface. The same law affords explanation of the facts, that very porous masses and powders, such as charcoal, metal filings, sawdust, sand, &c., conduct heat more slowly than denser masses,—their interstices being filled with air, which searcely conducts heat, and which, by the structure of the substance, has no freedom of motion or circulation by which it might carry the heat.

The Heat Transferrer.

617. In reflecting upon the fact that heat is diffused in masses of fluid, not by simple conduction, as in solid bodies, but chiefly by motions or currents among the particles, produced, as above described, by changed specific gravity, and knowing that two equal measures of water having different temperatures, when completely mixeċ, become a double quantity of an exactly intermediate temperature. the writer perceived the possibility (Art. 331) of causing equal quantities of boiling and freezing water to run past each other, in perfectly distinct although touching channels, in such a manner, that instead of the two becoming of the same middle temperature, the lately boiling should become nearly freezing, and the lately freezing nearly boiling. It seemed strange that this casy operation

Heat Transferrers.

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was not in common use and employed for many important purposes. But it was not unnatural for persons to think that when a measure of hot water had given up half its heat to an equal measure of cold, and had thereby rendered that as warm as itself, no further change could occur. At first one might not think of the consequences of making the currents run in contrary directions and in vertical channels. The adjoining engravings will explain the process.*

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Let H K (fig. 167), be a long tube formed of very thin metal, with a funnel, H, at the top, into which boiling water can be poured, to issue again below through the regulating cock, K. Let B G be a larger tube surrounding the other, and just so much larger as to allow an equal current of nearly freezing water coming from the

*The reader will find another form of apparatus for a similar purpose described at p. 197.

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Cooling of Liquids.

funnel, C, by the tube, C G, to enter below at G, and to rise around the smaller tube, to be discharged through the cock, E. Evidently then, if the tubes be of sufficient length, the lately boiling water descending in the internal tube, H K, must be losing every instant, and at every point in its course, part of its heat, to the colder ascending current around it, until it has lost the whole of its excess, which being all gained by the equal ascending current, the transference becomes complete. It might at first be supposed that great length of tubes would be required in the experiment; but in truth a length of about one yard suffices to prove conclusively the efficacy of the arrangement.

Fig. 168 is only a more capacious repetition of fig. 167. In it the channel of the descending hot water, instead of being a single cylindrical tube of narrow surface, through which the heat has to pass from one fluid to the other, has a very extensive surface compressed into small bulk by being corrugated or folded longitudinally upon itself. It so forms many thin flat channels in which the hot fluid descends, while the rising colder fluid occupies similar flat spaces between these; and no particles of the hot fluid can avoid being always close to passing particles of the colder. Fig. 169 is a section of the parallel columns as viewed from above.

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Fig. 169.

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The dark spaces mark the hot liquid descending, the light spaces the colder liquid rising. In the larger apparatus, the water which has been cooled is caused to rise from the bottom in the tube, ID, to be discharged conveniently at D, nearly at the level of the other inlets and outlets. Among the purposes which the heat transferrer may serve, are the following :—

1. Cooling at once the hot wort of brewers, instead of having to pump it up as of old, to spread on wide expanded cooling floors, where it loses aroma, and may suffer other damage.

2. To lessen the expense of warm-bathing establishments, and of public wash-houses.

3. To facilitate in winter the complete warm ventilation of dwellings, churches, manufactories, halls of assembly, &c., without danger of cold draught, and with small expense of fuel. transfer of heat takes place between currents of air as it does from liquid to liquid

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4. To allow the formation of a perfect breath-warmer, important to persons of delicate health, in cold weather.

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5. To utilize the warmth of smoke in various ways. Engineers having familiarity with the principle may suggest many forms and applications.

"Heat spreads also, partly, by being radiated or transferred from one body to another, through transparent media or empty space, with a readiness which is modified by the material and the state of the giving and receiving surfaces."

618. If a heated ball of metal be suspended in the air, a hand brought in any direction near to it will experience the sensation of heat and beneath it the sensation would be as strong as on the sides, but that the heat has to meet a current of cool air approaching from below, to rise from it, as explained in a preceding section. A delicate thermometer substituted for the hand will equally detect the spreading heat, and if held at different distances, will prove it to diminish in the same ratio as light diminishes in spreading from any luminous centre, viz., to be only a fourth part as intense at a áouble distance, and in a corresponding proportion for other distances. If the heated body be enclosed in a perfect vacuum, a thermometer placed near it will still be affected in the same manner; hence no apparent medium is required for the transference of the heat. If a screen be interposed between the body and the thermometer, the latter will not be affected at all, proving that the heat spreads in rays or straight lines. Heat when diffusing itself in this way, is called radiant heat, to distinguish it from heat passing by contact or communication, as described in the last section, and, like it, decreasing in intensity according to the square of the distance of the heated solid, ¿.e., at two feet its intensity is reduced to one-fourth, and at three feet to one-ninth. Fig. 1, page 9, will serve to render this law of decrease intelligible.

619. Radiant heat resembles light in other respects. It rapidly permeates certain substances, such as rock salt, and its course suffers in them the kind of bending termed by opticians refraction. It is reflected from polished surfaces, just as light is reflected from a common mirror; and many such surfaces directed to one point or centre, or a single concave surface having its one centre or focus, will concentrate the heat with the light (Art. 566). Its motion in the sunbeam is so rapid, that for any distance at which men can try the experiment, it appears instantaneous; and the rays of heat from hot iron or burning charcoal concentrated at great dis

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