450 Formation of Glaciers. height, and above that height the air is at the time too dry and rare to have clouds. Very lofty summits are seen from a distance projecting much above the clouds, and the admirer of such scenery who climbs towards them, may have to contemplate the grand phenomena of the thunderstorm far beneath his feet. Teneriffe soars so sublimely, that the distant sailor not unfrequently mistakes the line of clouds hanging around its sides for the white streak which elsewhere indicates the cliffs and waves of the sea-shore. 651. When the elevation to which moist air is suddenly carried is very great, the fall of temperature is proportional, and the separating water becomes snow instead of rain. This phenomenon is remark. ably illustrated by a great Hero's fountain, established in one of the salt mines of Hungary; during the play of which, the confined air in one place is so compressed, that on being suddenly released, it expands and cools enough to cause the moisture contained in it, to come out, even in summer, as a shower of snow. Glaciers and Icebergs. 652. We may here consider the striking phenomenon of what are called glaciers, formed among mountains which rise above the snow-level of the region. When snow, falling on these mountains, as it does, through both summer and winter, accumulates on their sides beyond a certain degree, it breaks loose and slides down in masses called avalanches into the hollows and valleys below, and there, owing to the great pressure and partial meltings, it soon becomes solid ice, called a glacier, often hundreds of feet in thickness or depth. One of the singular facts connected with these masses is, that they have a slow onward motion toward the lower country, as if they were of semi-fluid or viscous substance, like soft pitch or yielding clay. It had been noted by the inhabitants around that huge pieces of rock, fallen from bordering heights to the surface of the ice, seemed to have a gradual onward motion towards the lower country, but only of late have scientific men ascertained that the whole mass of the glacier has such motion, carrying the rocky fragments on its surface. Then experiment shows that if a block of ice is crushed and broken by great pressure, and the pressure is continued afterwards, the fragments all perfectly re-unite to become as solid a mass as before, and in any new form which the resisting sides of the containing space may give. Now glacier-ice in a sloping valley is undergoing, by the pressure of its weight, a constant bruising and general internal fracture of its substance as it is Icebergs. 451 forced along in its irregular channel, and the displaced paits arc being as constantly re-united into a solid mass. Glaciers from smaller valleys meet and join with those in the larger, just as smaller streams of water meet to form large rivers. It is interesting to observe, that the constantly falling fragments of rock and earth from the elevations around the moving glacier, form continuous lines on the surface along the margins to the termination of the glacier, where the increasing warmth of the low country is causing the glacier ice to melt and disappear; and the earthy loads there dcposited form ridges or embankments, called moraines, of vast magnitude, crossing the mouth of the valley, the stupendous gatherings of bygone ages. The continual and rapid waste of glaciers going on below the level of the snow-line confines those of temperate regions to high positions among the mountains, but in colder regions towards the poles, glaciers extend down not merely to the sea-shores, but often project far into the sea. When such projections break off, they become the so-called icebergs met floating away from the places of their origin. 653. Icebergs.-Some remarks have already been made on these enormous masses of floating ice (see note, p. 409). Those met with in the North Atlantic are supposed to be derived from the great glaciers or ice deposits in the rocks on the coasts of Greenland, Spitzbergen, and other Arctic localities. Floating southwards towards the Mid-Atlantic, they make their presence known by cooling the temperature of the air and the sea-water for a considerable distance around them. Their proximity may in general be discovered by the thermometer, the sea-water indicating a much lower temperature than usual. Owing to the specific gravity of ice being less than that of seawater, about 12th of the bulk of the ice is above, and 11ths are below the sea-level. Ice-fields are icebergs of low altitude. An officer of the Alexandra measured one which had an average height of 51 feet, and a length and breadth of about two miles each way. A floe signifies a detached portion of one of these fields. Pack-ice means a number of floes so closely wedged together as to prevent the passage of a ship; and drift-ice implies that the floes are not welded together. 654. Although the proofs are not at once apparent, the line of congelation exists as truly everywhere in the open sky, over sea and plains, as where there are mountain heights to wear its livery; and considerably below the line, the cold, aided probably by electrical agency, is sufficient to produce, in the form of mist or clouds, a 452 Influence of Elevation on Climate. copious separation from the air of the watery vapour contained in it. There is thus in nature an admirable system at work to shade the surface of the earth at times from the too powerful rays of the sun, and to supply rain as wanted, without the transparency of the inferior regions of the atmosphere being much affected. As the watery evaporation rising from sea and lake, and invisibly diffused in the atmosphere, can reach only to the height where the cold is intense enough to condense it, the clouds may in general be regarded as the high stratum of that atmosphere of watery vapour or aëriform water, which is always mixed more or less with the atmosphere of mere air; and as the quantity of watery vapour which can exist invisibly in a given space depends altogether on the amount of heat present, the clouds in a cold or a humid atmosphere will generally be low, and in a warm or a dry atmosphere will be high, or there may be none. An aëronaut mounting in his balloon through a clear sky often enters a dense cloudy stratum, and for a time is surrounded by the gloom almost of night, the face of the earth below being completely hidden from him, while the heavenly bodies are equally veiled from him above; but rising still higher, he again emerges to brightness, and looks down upon the fleecy ocean rolling beneath, as a climber to a very lofty peak looks down from the pure atmosphere around it on the inferior region of clouds and storms. 655. The diminished temperature of air in the higher regions of the atmosphere often enables the natives of temperate climates, when obliged to reside in hot countries inimical to their health, to find near at hand, on some mountain height, the congenial temperature of their wished-for homes. The interiors of many tropical lands have localities of great extent, high table-lands which combine, as above described, the advantages of tropical situation and temperate climate, and which might well be inhabited by European colonists. Much of the central land of South America, is similarly circumstanced (Art. 639). It is not uncommon, where the ascent to such land is rapid, to find near the bottom, towns with their markets stored only with the productions of the equator, while higher up are seen also what belong to the temperate skies of Europe. ́ In the province of Valencia, in Spain, invalids needing temperate climate can find it near the sea-level during winter, and in summer can climb the hills to enjoy the cool atmosphere which befits them. 656. The facts detailed in the preceding paragraphs illustrate the subject of the relatior of volume in a body to the capacity for heat, by the change of capacity produced in the same quantity of air Density in relation to Capacity for Heat. 453 according as it is more or less either dilated or compressed. We have now to speak of the effect of permanent density in the same respect. It might be anticipated that a dense body, or one in which the constituent particles may be supposed to fill more completely the space occupied by it than the particles of a rarer body, would have a smaller capacity for heat, in proportion to the smaller space left vacant in its mass; and in a general comparison of the capa. cities of equal bulks of different substances, such anticipation is partly verified. The relation, however, is by no means universal, nor at all in proportion to the differences of density, for water, which is denser than oil, has twice as much capacity for heat. 657. The relation, then, between various substances and heat, which we call capacity for heat, depends much more on the nature of the ultimate particles of the substances than either on the absolute bulk or comparative density of the masses. It has been ascertained that all material substances are composed of extremely minute unchangeable atoms, of which, in the different substances, the comparative weights have been determined (Art. 42), although not the absolute weights; that is to say, for example, the atom of gold is known to weigh nearly six times as much as the atom of iron, although we do not know how many thousands or millions of atoms are required to form a grain of either. And recent researches prove that the capacities for heat, or the specific heats of simple bodies, are inversely as their atomic weights, the two numbers multiplied together, therefore, being a constant quantity. Instead of the term capacity for heat used in the preceding pages, with respect to particular substances, that of specific heat has by some authors been preferred; but as the latter gives to a commencing student, the idea rather of kinds of heat than of quantities, the term capacity has been here retained. "Each substance in nature, for a given change of temperature, undergoes expansion in a degree proper to itself, the expansion generally increasing more rapidly than temperature, being remarkably greater therefore in liquids than in solids, and in gases than in liquids, the rate being quickened, moreover, near the points of change." 658. The following table, containing the names of some common substances, solid, liquid, and aëriform, shows approximately, by the figures following each, how much the substance increases in bulk by 454 Rule of Expansion by Heat. having its temperature raised from that of freezing to that of boiling water. A lump of glass, for instance, would gain one cubic inch for every 416 cubic inches contained in it; while a mass of water would gain one inch for twenty-one, dilating thus for the same range of temperature eighteen times more than the glass. 659. We have to warn readers here not to confound the increase by heat of the general bulk of a solid body with the increase of its length. The latter is only one-third as great as the former. This will be understood by considering that the increase of bulk is made up of increase in the length, breadth, and depth (or thickness). Il the substance of a metallic square rod or wire be dilated, by heat, a one-hundredth part of its bulk, it does not gain all that hundredth at its end, becoming 101 inches long instead of 100; but every part becomes deeper and broader in the same proportion as it becomes longer, and the rod gains in length only the third part of an inch. A fluid enclosed in a tube unchangeable by heat (if such tube there were) would show its whole dilatation in an increase of length, because there could be no swelling laterally, and its extremity, |