410 Conducting Power of the Earth. frequently preserves the lives of travellers, and even of whole flocks of sheep, where the keen north wind, finding them unprotected, would soon extinguish life. Conducting Power of the Earth. Stratum of fixed Temperature. 599. It is because the strata of the earth conduct heat slowly, that the intense frosts of winter and the direct sunbeams of summer penetrate but a short way into it. The temperature of the ground, a few feet below its surface, is nearly the same all the year round, for the extent to which the solar heat penetrates is exceedingly small. This is easily determined by burying thermometers in the earth, or fixing them at different depths in mines and other excavations. The experiments hitherto made, lead to the following interesting results. Diurnal variations of temperature are not perceived beyond two or three feet. Variations depending on the months or seasons extend somewhat lower; and annual variations are entirely lost at a depth of from 60 to 100 feet, varying in different localities. Hence it follows that the maximum depth at which the changes in the thermometer are perceptible, amounts to only the 400,000th part of the earth's diameter. Upon the alternate heating and cooling of this film, which does not exceed the 9,000,000th of an inch in a globe of three feet in diameter, depend all the vicissitudes of temperature in climates, seasons, and cycles of years. The depth at which a thermometer undergoes no change has been accurately determined for these latitudes by experiments made at Paris. In July, 1783, a very delicate thermometer was placed by Lavoisier, an eminent philosopher who fell a victim to the great French Revolution, in an excavation beneath the Observatory of Paris, at a depth of 90 feet below the surface of the ground. This thermometer was so sensitive to changes of temperature as to allow of the measurement of what would be equivalent to the 100th of a degree on Fahrenheit's scale. From observations made by Cassini, Bouvard and others, extending over a period of fifty years, this thermometer remained stationary at a point corresponding to 53° F., which is about a degree and a half above the mean temperature of Paris. On one occasion only in seventeen years, it was observed to rise a quarter of a degree. This movement was attributed to the effect of currents of air from some excavations made in quarries adjoining the Observatory. Hence in Paris the position of the invariable stratum of temperature is fixed at from 80 to 100 feet below Temperature at different Depths. 4II the surface. In no other place in the world has so accurate a series of observations been made; but from a few data, and from theoretical considerations, the stratum has been considered by Humboldt to exist throughout Europe, between the parallels of 48° and 52°, at from 55 to 60 feet below the surface. Professor Thomson considers its depth in England to be from 30 to 60 feet, and the experiments performed by Mr. Fox in the mines of Cornwall render it probable that in that county it is situated at from 60 to 75 feet.* M. Baer found that at Yakutsk, in Eastern Siberia, the frozen ground was thawed during the short summer to the depth of only 3 feet. Below that, at all periods of the year, there is a band of ice or frozen soil which has been perforated to the depth of 382 fect, but without entirely traversing it. The solar influence, therefore, in this desolate region, scarcely extends in the course of seasons beyond 3 feet from the surface. This may explain the fact mentioned by Erman, that the body of Prince Menschikof, one of the favourites of Peter I., which had been buried in this frozen soil at Beresov, was exhumed in 1821, and was found to have undergone but little change, although ninety-two years had elapsed since the burial! As a general rule, it is considered that the mean temperature of the locality corresponds to the temperature of the invariable stratum. A thermometer carried below it continues to rise in proportion to the depth, although there is no regular rate of increase. 600. As further illustrations of the imperfectly conducting power of the earth may be mentioned the following facts :-Water in pipes which are laid two or three feet under ground, as in the streets of cities, does not freeze, although it may be frozen in all the smaller branches exposed above. Hence, again, springs of water do not freeze, and therefore often become remarkable features in a snowcovered country; the living water, after issuing from the bowels of the earth, is seen running a considerable way through fringes of green, before the frost can arrest it. A spring at the bottom of a frozen pond or lake may cause the ice to be so thin over it, that a skater arriving there may break through. The spring water, which appears warm in winter, is deemed cold in summer, although really of the same * In excavating great masses of earth, even above the sea-level, a high temperature is commonly found. Thus in the tunnel of the Mont Cenis, about halfway through, and nearly four miles from the Italian side, the temperature of the air was found to be 862°. At this point the height of rock above was 5280 feet, or one mile. This portion of the tunnel is 4250 feet above the sea level, and the total height of the mountain is 9530 feet. 412 Retention of Heat. temperature, because in summer it is compared with the warmer atmosphere and objects around it, and in winter with the colder. In proportion as buildings are vast and massive, they acquire more of the quality of uniform temperature here spoken of. Many of the Gothic halls and cathedrals are called cool in summer and warm in winter, as are also old-fashioned houses or castles with thick walls, embayed windows, and deep cellars. Natural caves in the mountains or by the sea-shores furnish other examples of a similar kind. 601. When in the arts it is desired to prevent the passage of heat out of or into any body or situation, a covering of a slow-conducting substance is employed. Thus, to prevent waste of heat from a smelting or other furnace, it is lined with fire-bricks, and thickly covered with a kindred material. A furnace so guarded may be touched on the outside by the hand with impunity, even while having within it melted iron. To prevent, during the winter, the freezing of water in pipes, by which occurrence the pipes would be burst, it is common to cover them with straw-bands or coarse flannel, or to enclose each in a larger outer pipe, the interval between the two being filled up with dry charcoal, sawdust, spent tan, or chaff. If a pipe, on the contrary, be for the conveyance of steam or other warm fluid, the heat in it is retained, and therefore saved by the very same means. Ice-houses are generally made with double walls, between which dry straw is placed, or sawdust, or air, to prevent the passage of heat. Pails for carrying ice in summer, or intended to serve as winecoolers, are guarded on the same principle. A flannel covering keeps a man warm in winter-it is also powerful to keep ice from melting in summer. In the cylinder of a steam-engine, and in the body of a locomotive the heat is preserved by surrounding them with wooden casings, with a space between to receive powdered charcoal, sawdust, cotton, or some light material which operates as a non-conducting medium, and prevents the passage of heat outwards. The "cosies" made for covering teapots consist of cotton bags stuffed with cottonwool. The heat is thus retained for a considerable time. Fire-proof safes and refrigerators used for preserving ice are constructed on a similar principle. Each safe is provided with a double casing filled with some non-conducting material, and in the refrigerator there is a wide space for air. Heat does not readily penetrate from the outside, owing to this non-conducting space. Documents placed in a fire-proof safe may be thus preserved from entire de Action of Heat on Glass. 413 struction, and the ice in a refrigerator may be kept for a long time without melting. Urns for hot water, tea-pots, coffee-pots, &c., are sometimes made with wooden or ivory handles, because, if metal alone were used, it would conduct the heat so readily that the hand could not bear to touch them. 602. It is because brittle substances like glass and earthenware do not allow a ready passage to heat, that vessels made of such materials are so frequently broken by sudden changes of temperature. On pouring boiling water into such a vessel, the internal part is so much heated and expanded before the external part has felt the influence by conduction, that the inner portion is riven or cracked by its connection with it. A red-hot rod of iron drawn along a pane of glass will divide it almost like a diamond knife. Even cast iron, as in the backs of grates, iron pots, &c., although conducting more readily than glass, is often, owing to its brittleness, cracked by unequal heating or cooling, as from pouring cold water on it when hot. Pouring cold water into a heated glass, or boiling water into a cold glass, will produce a similar effect. Hence glass vessels intended to be exposed to sudden changes of temperature, as retorts for distillation, flasks for boiling liquids, &c., are made very thin, so that the heat may pervade the whole substance almost instantly and therefore safely. Action of heat on glass.—Annealing.—Tempering. 603. Any glass, if cooled suddenly when first made, remains very brittle, for the reason now explained. What is called the Bologna phial is a very thick small tube, cooled rapidly, which is broken into fragments when a grain of sand or a piece of flint is allowed to fall into it. The process of annealing, to render glass-ware more tough and durable, is merely the allowing it to cool very slowly in an oven, so that the whole may lose its heat nearly at the same rate. Toughened glass.-A new process has been recently discovered by which the brittleness of glass is in a great measure removed. This is called toughening. The glass after manufacture is heated up to a certain degree, and plunged while so heated into an oil-bath at a temperature short of the boiling point (650°). It is dipped into the heated oil, and instantly withdrawn. The glass preserves its transparency, but undergoes a remarkable molecular change. Its hardness as well as its cohesion is increased, and it appears to be brought in some respects to the condition of the glass in a Rupert's 414 Toughened Glass. Tempering of Metals. drop, or of the Bologna phial above mentioned. It is not affected by sudden changes of temperature like ordinary glass. It may be thrown with some violence on a deal floor or against a wall without being broken; and from the experiments of the inventor, M. de la Bastie, it will bear from 80 to 100 times the strain of ordinary glass without breaking. A brass weight, allowed to fall on a square of ordinary glass from a height of two feet, broke it into several fragments. With a thinner piece of toughened glass, no impression was made in allowing the same weight to fall upon it from a height ranging from two to ten feet. The brass weight simply rebounded from it. When the weight fell from a height of six feet, the glass broke, but, unlike ordinary glass, the whole mass fell into a fine powder. Like Rupert's drop at the larger end, this glass possesses enormous cohesive force and may be struck with a mallet without breaking; but if the equilibrium of the mass is once disturbed at any one point, a general disintegration takes place throughout the whole mass. Water may be boiled in a vessel of toughened glass over a naked fire, and the vessel suddenly cooled without fracture. It cannot, like other glass, be cut with a diamond, which shows the existence of a distinct molecular structure, a fact otherwise proved by its action on polarized light (to be afterwards explained). It admits of engraving in the ordinary way, or by the action of hydrofluoric acid. As yet it has scarcely come into general use. The effect of heat in the tempering of metals is a well-known process, having some relation to that now described. (See Art. 53, p. 21.) It depends on molecular changes produced by heating and suddenly cooling the metal. Steel thus suddenly cooled by plunging it into a cold liquid becomes intensely hard. On the other hand, bronzé, an alloy of copper and tin, treated in the same manner, becomes very soft. 604. It is the difference of conducting power which is the cause of a very common error made by persons in estimating the temperature of bodies by the touch. In a room without a fire all the articles of furniture soon acquire the same temperature; but if in winter a person move a bare foot from the carpet to the wooden floor, from this to the hearthstone, or from the stone to the steel fender, his sensation deems each of these objects in succession colder than the preceding; the truth being, that although all have the same temperature, but inferior to that of the living body, the best conductor, when in contact with the |