420 Heat spreads by Convection or gunpowder in blasting the coal. The lengthening of the flame should serve as the first indication of approaching danger, and the miner should remove from the spot, and not allow the wire cage to become overheated. In some of the mines of Lancashire it was the custom not only to lock the wire cage to the lamp, but to suspend above the flame an extinguisher by means of fusible metal, so that when overheated, this was melted by conduction, and the flame was extinguished. 66 Heat spreads in fluids chiefly by convection* or the trans- 610. The reason why the hand judges a liquid like water to be cold, is, that from the mobility of the liquid particles among themselves, those in contact with the hand are constantly changing; they quickly give place to others, so that there is a constant renewal of particles in contact with the skin. If a finger held motionless in water feels cold, it will feel colder still when moved about: and a man in the air of a calm frosty morning, does not experience a sensation nearly so sharp as if with the same temperature there be wind, or if he himself is constantly changing his position. A finger held up in the wind discovers the direction in which the wind blows by the greater cold felt on the windward side where the shifting of particles is greatest, the effect being still more remarkable, if the finger be wetted and moved about; the cooling effect of evaporation is then added. If a person in a room with a mercurial thermometer, were with a fan or bellows to blow the air against it, he would not thereby lower the mercury, because it had already the same temperature as the air, yet the air blown against his own body would appear colder than when at rest, because, being colder than his body, the motion would supply heat-absorbing particles more quickly. In like manner, if a fan or bellows were used against a thermometer hanging in a furnace or hot-house, the thermometer would suffer no change, but the hot air blown against the hand of a person would be distressingly painful, like the blasting sirocco of the sandy deserts of Africa. If two similar pieces of ice be placed in a room somewhat warmer than ice, one of them may be made to melt much sooner than the other, by blowing on it with a bellows; -as we see the accumulated ice and snows of winter so rapidly *Convection, from the Latin, convectio, signifying the act of carrying or transporting. in Gases and Liquids. 421 melting when the warm south winds of spring begin to blow upon them. In dry air, at rest, the human body can resist a temperature of 250° (Graham). Cold air, considerably below the zero of Fahrenheit, can be easily borne, provided it is not put in motion by winds or currents. In such cases the heat or cold becomes unbearable. All gases resist the passage of heat by conduction, but the lighter the gas the greater the rapidity of convection and the transportation of heat. 611. Owing to the mobility among themselves of fluid particles, heat entering a fluid anywhere below the surface, by dilating and rendering specifically lighter the portion heated, allows the denser fluid around to sink down and force up the rarer; and the continued currents so established, diffuse the heat through the mass much more quickly than heat spreads by conduction in any solid, hence all liquids should be heated at the lower part. Perhaps the best experimental illustration of this subject is obtained by dipping a tall glass jar, filled with water in which small particles of sawdust or any other light substance are diffused to show its movements, first in a warm bath, and then in a cold bath. In the warm water the sawdust near the outside of the jar, where it is heated, will exhibit a rapid upward current, while in the centre of the jar, where it still remains cool, it will form an opposite or descending current. In the second case, or when the jar is placed in a cold bath, the direction of the currents will be reversed. Count Rumford's experiments led him at first to conclude erroneously that liquids, but for this carrying process of the particles changing their place, were absolutely obstructive of heat. The following experiments will, however, show that water is a very bad conductor. If a thermometer is placed in a jar of water, and a red-hot copper ball is gradually dipped into the upper stratum to cause it to boil violently, the bulb of the thermometer a few inches below is quite unaffected. If ether is poured on the surface of the water and inflamed, the thermometer is unchanged. No heat is conducted downwards so as to affect it. The most striking proof is to freeze in the lower part of a thin glass tube a quantity of water, and then, grasping the tube with flannel, to boil the water above the ice in a spirit flame. It soon boils, without conveying its heat downwards, for the ice remains unchanged. From this it may be inferred that in the equatorial regions the heat of a tropical sun penetrates only to a slight depth. 422 Warming by Hot Water. Mercury does conduct downwards to some extent; but as this is a metal, it possesses a high conducting power as such. A hot poker, plunged into water, heats only the portion of water which it touches, or which is immediately around it. If plunged into mercury, the whole becomes warm, the heat being rapidly spread by conduction. The internal currents or circulation produced by heat in fluid masses, and of which there are so many important instances in nature, have been already explained. Changes of temperature are the active causes of winds and other atmospheric phenomena. 612. Warming buildings by the circulation of hot water.-Tne heating of buildings by warm water distributed through iron pipes, depends on this kind of circulation or distribution. The water is heated by a stove placed in the basement of the building, B, and one of the most convenient forms is a welded iron boiler, a (fig. 166), in the shape of a stove, the heating material being thus in the centre. e B b and so placed as to impart all its heat to the water through the inner casing. A wrought-iron tube, bi, of great strength, and secured to the boiler by a water-tight junction, rises from the top of it and is connected on the different floors with a system of cast-iron pipes, ccc, four inches in diameter (a convenient size, as every foot in length then corresponds to a square foot in area), the number of which must depend on the space to be warmed. From the lower part of these pipes, which are arranged so as to favour a flow downwards, there is another wrought-iron tube, which is carried down so as to enter the boiler at the bottom. At the highest point is a narrow pipe for carrying off any steam which may be produced. The boiler itself is connected with a small iron cistern, d, placed above the level of all the pipes. This continually supplies water to the boiler by the descending pipe, ee. When the water is heated, it rises through the ascending or flow-pipe, b b, and gradually heats the water in the cast-iron pipes, ccc. As the pipes cool by radia Fig. 166. Oceanic Currents. The Gulf Stream. 423 tion and convection, the cooled water descends through the return pipe, e e, and enters the boiler at the bottom, to be again heated and again circulated. The writer has for many years used an apparatus of this kind, the water rising to the height of thirty feet from the boiler, and equally warming the water in 160 feet of iron pipes. The highest temperature observed in the water distributed through the pipes was 150°. This system is well adapted for conservatories, halls, and any large spaces which require to be heated to a temperature between 50° and 60o. As the supply cistern is freely exposed to the atmosphere, the temperature of the water in it can never exceed 212°. 613. Oceanic Currents. The Gulf Stream.-The distribution of heat in the oceanic waters is a further illustration of these principles. In the Atlantic equatorial regions the water becomes strongly heated, and currents of enormous extent are thus set up, which serve by their circulation to equalize temperature. One remarkable current of this kind issues from the Gulf of Mexico, and is well known as the Gulf Stream. Passing round the peninsula of Florida, it takes a north-easterly direction, and spreads in a vast stratum to the west of the Azores. It is thus found to traverse 3000 miles in about seventy-eight days. The temperature varies according to the latitude, but it is generally 8° or 10° above the temperature of the surrounding sea. It discharges large quantities of heat in its progress over the Atlantic, and its influence in conveying warmth is felt on the coasts of England, Ireland, Scotland, and Norway, and it is even said to be perceived as far north as Spitzbergen. There is a counter-current of cold Arctic water on each side of it, which, according to Dr. Carpenter, buoys it up from beneath. This writer describes the Gulf Stream as a river of superheated water, which widens and becomes more shallow as it proceeds northwards. Off Sandy Hook it is sixty miles in breadth, and it has a depth of 600 feet. Below this there is a considerable stratum of water at a temperature of from 60° to 65°, upon which this warm oceanic river rests. Below this, again, is a deep stratum, 2000 fathoms or 12,000 feet, in thickness, of which the temperature was found to be from 35° to 40°.* * Dr. Carpenter estimates from the numerous soundings which have been taken that the general depth of the Atlantic does not exceed three miles. The most remarkable of these, taken in the Challenger, gave a depth of 3800 fathoms, or 22,800 feet. As there are 880 fathoms to a mile this is equivalent to 4 miles. This sounding was apparently in a deep hole about 100 miles north of St. Thomas, while the Challenger was on her way to 424 Currents in the Atlantic. This is about the degree at which water reaches its greatest density. The researches of Dr. Carpenter lead to the conclusion that these cold strata in the deep Atlantic, are vast polar currents flowing from the Arctic and Antarctic regions, and supporting upon their surface, and indeed upbearing, the heated waters which issue from the Gulf of Mexico and the equatorial region.* The great Arctic current which underlies the Gulf Stream has a definite movement southwards. This is proved by various facts. Icebergs occasionally cross the Gulf Stream off the Banks of Newfoundland, and are carried to the south of it. This can arise only from the southerly movement of the deeper stratum in which the lower part of the vast mass of the iceberg is immersed, which carries it along against the counteraction of the upper or warm current. A remarkable fact mentioned by Dr. Carpenter confirms this view. The buoy which was attached to the broken end of the Atlantic cable of 1865, having got adrift, was found to have travelled due south nearly a distance of 600 nautical miles in seventy-six days, in opposition to the Gulf Stream, presumably by the action of the underflow upon the long buoy-rope suspended in it. The dense fogs which are well known to surround Newfoundland, are attributed by some physicists to the meeting of the warm waters of the Gulf Stream with the cold Arctic current proceeding from the north. There are other currents of vast extent in the Atlantic, Pacific, and Indian Oceans, depending on changes of temperature in the water, and the transfer of heat by convection or circulation to restore the equilibrium. These have been well represented by Messrs. Johnston on a hydrographic map of the world: they convey an impressive idea of the vast changes produced by heat in the circulation of oceanic waters. Bermuda. The crushing of thermometers which had been already used at a depth of 2600 fathoms, proved that the instruments had gone to a much greater depth. Some of the loftiest mountains on the globe might be sunk in this depression without showing any portion above the sea-level. * In Dr. Carpenter's view, the Gulf Stream issuing through the Florida Channel is entirely a product of the action of the trade winds on the equatorial Atlantic, driving it into the Carribean Sea and the Gulf of Mexico, and then forcing on the current through the Florida Channel, to come out as what is called the Gulf Stream. It has a certain superficial spread, but is a mere surface current.-' On the Deep Sea Bottom.` |