The Poetry of Science; or, Studies of the Physical Phenomena of Nature

by Robert Hunt

are, by their different colours, prevented from ever having the same temperatures under the same sunshine.

Every plant bears within itself the measure of the heat which is necessary for its well-being, and is endued with functions which mutely determine the relative amount of dew which shall wet its coloured leaves. Some of the terrestrial phenomena of this remarkable principle will still further illustrate the title of this volume.

To commence with the most familiar illustrations, let us consider the consequences of change of temperature. However slight the additional heat may be to which a body is subjected, it expands under its influence; consequently, every atom which goes to form the mass of the earth moves under the excitation, and the first heat ray of the morning which touches the earth’s surface, sets up a vibration which is continued as a tremor to its very centre. The differences between the temperature of day and night are considerable; therefore all bodies expand under the influence of the higher, and contract under that of the lower temperature. During the day, any cloud obscuring the sun produces, in every solid, fluid, or aëriform body, within the range of solar influence, a check: the particles which had been expanding under the force of heat suddenly contract. Thus there must of necessity be, during the hours of sunshine, a tendency in all bodies to dilate, and during the hours of night they must be resuming their original conditions.

Not only do dissimilar bodies radiate heat in different degrees, but they conduct it also with constantly varying rates. Heat passes along silver or copper with readiness, compared to its progress through platinum. It is conducted by glass but slowly, and still more slowly by wood and charcoal. We receive some important intimations of the molecular structure of matter, from those experiments which prove that heat is conducted more readily along some lines than others. In some planes, wood and other substances are better conductors than in others. The metallic oxides or earths are bad conductors of heat, by which provision the caloric absorbed by the sun’s rays is not carried away from the surface of this planet so rapidly as it would have been had it been of metal, but is retained in the superficial crust to produce the due temperature for healthful germination and vegetable growth. The wool and hair of animals are still inferior conductors, and thus, under changes of climate and of seasons, the beasts of the field are secured against those violent transitions from heat to cold which would be fatal to them. Hair is a better conductor than wool: hence, by nature’s alchemy, hair changed into wool in the animals of some countries on the approach of winter, and feathers into down.

It is therefore evident that the rate at which solar heat is conducted into the crust of the earth must alter with the condition of the surface upon which it falls. The conducting power of all the rocks which have been examined is found to vary in some degree.[60]

It follows, as a natural consequence of the position of the sun to the earth, that the parts near the equator become more heated than those remote from it. As this heat is conducted into the interior of the mass, it has a tendency to move to the colder portions of it, and thus the heat absorbed at the equator flows towards the poles, and from these parts is carried off by the atmosphere, or radiated into space. Owing to this, there is a certain depth beneath the surface of our globe at which an equal temperature prevails, the depth increasing as we travel north or south from the equator, and conforming to the contour of the earth’s surface, the line sinking under the valleys and rising under the hills.[61]

A question of great interest, in a scientific point of view, is the temperature of the centre of the earth. We are, of course, without the means of solving this problem; but we advance a little way onwards in the inquiry by a careful examination of subterranean temperature at such depths as the enterprise of man enables us to reach. These researches show us, that where the mean temperature of the climate is 50°, the temperature of the rock at 59 fathoms from the surface is 60°; at 132 fathoms it is 70°; at 239 fathoms it is 80°: being an increase of 10° at 59 fathoms deep, or 1° in 35·4 feet; of 10° more at 73 fathoms deeper, or 1° in 43·8 feet; and of 10° more at 114 fathoms still deeper, or 1° in 64·2 feet.[62]

Although this would indicate an increase to a certain depth of about one degree in every fifty feet, yet it would appear that the rate of increase diminishes with the depth. It appears therefore probable, that the heat of the earth, so far as man can examine it, is due to the absorption of the solar rays by the surface. The evidences of intense igneous action at a great depth cannot be denied, but the doctrine of a cooling mass, and of the existence of an incandescent mass, at the earth’s centre, remains but one of those guesses which active minds delight in. The mean annual temperature of this planet is subject to variations, which are probably dependent upon some physical changes in the sun himself, or in the atmospheric envelope by which that orb is surrounded. The variations over the earth’s surface are great. At the equator we may regard the temperature as uniformly existing at 80°, while at the poles it is below the freezing point of water; and as far as observations have been made, the subterranean temperatures bear a close relation to the thermic condition of the climate of the surface. The circulation of water through faults or fissures in the strata is, without doubt, one means of carrying heat downwards much quicker than it would be conducted by the rocks themselves. It is not, however, found that the quantity of water increases with the depth. In the mines of Cornwall, unless where the ground is very loose, miners find that, after about 150 fathoms (900 feet), the quantity of water rapidly diminishes. That water must ascend from very much greater depths is certain, from the high temperatures at which many springs flow out at the surface. In the United Mines in Cornwall, water rises from one part of the lode at 90°; and one of the levels in these workings is so hot that, notwithstanding a stream of cold water is purposely brought into it to reduce the temperature, the miners work nearly naked, and will bathe in water at 80° to cool themselves. At the bottom of Tresavean Mine, in the same county, about 320 fathoms from the surface, the temperature is 100°.

One cause of the great heat of many of our deep mines, which appears to have been entirely lost sight of, is the chemical action going on upon large masses of pyritic matter in their vicinity. The heat, which is so oppressive in the United Mines, is, without doubt, due to the decomposition of immense quantities of the sulphurets of iron and copper known to be in this condition at a short distance from these mineral works.