capable of easy expression and measurement; and, secondly, it must be so arranged that the expenditure of all the moving force on friction, percussion, &c., be at the spot where it is wanted, or in the vessel into which the motion is to be poured for measure ment. "The great natural source of heat to this earth is the Sun." Solar heat. 566. Without the heat received from this luminary neither animal nor vegetable life could be sustained. Human art can now, by concave mirrors or convex lenses, gather the sunbeams together, so as to produce intense heat in the focus of their meeting. A surface of glass, or any other small mirror, will so reflect the sun's rays as strongly to affect the eye at a distance even of miles; and that the heat accompanies the light is shown by the fact that many mirrors directed towards one point heat intensely. Archimedes is said to have set fire to Roman ships besieging Syracuse, by sun-beams thus converged from many points to one. When the light of a very broad sunbeam is made by a convex glass or lens to converge accurately to one narrow focus, the concentrated heat is sufficient to cause a piece of fusible metal held in the focus to drop like melting wax. Persons, wherever the sun can be seen may conveniently light their fires at the sun, by directing his energies through a burning-glass of sufficient power; and it is by an experiment of this kind that we may estimate the amount of heat which falls upon a given surface of the earth. A lens, two inches and a half in diameter, with a focus of about six inches where receiving the direct rays of a summer's sun, fires gunpowder in a second. The heat required for this is 545°. As a lens does not in any way augment, but merely concentrates the solar rays to a point, its area will correspond to the terrestrial surface which would receive this amount of heat. The area of such a lens would represent about five square inches, and thus it follows, by a simple calculation, that a square foot of the earth's surface would receive nearly twenty-nine times the quantity above mentioned. Faraday has made the curious calculation that the average amount of heat radiated in a summer's day upon each acre of land in the latitude of London is not less than that which would be emitted by the combustion of six tons of coals. Lenses of large dimensions have been formed which have so concentrated solar heat in vessels containing oxygen, as to bring 386 Amount of Solar Radiation. iron, charcoal, and even the diamond, to full redness and cause their rapid combustion. It is found that a heat of about 1200° is required to produce these chemical effects. 567. It is a remarkable fact that in these experiments the lens is not warmed, nor is the atmosphere which the heat-rays traverse, provided it be perfectly clear and transparent. Thus a double convex lens of transparent ice will equally concentrate the rays of heat radiated from the sun without absorbing them, and cause the ignition of phosphorus, a substance which requires a temperature of at least 113° for its ignition in air. The writer has witnessed this experiment as it was performed by Faraday at the Royal Institution. By that ingenuity so characteristic of that great master of science, he made the lens from Wenham ice in a few minutes, and the sun being favourable, used it for the ignition of phosphorus, the burning of paper, &c. As further illustrations of this fact it may be mentioned that, although gun-cotton may be exploded by a heat of about 300°, the same focus of heat-rays which would explode instantaneously gunpowder, &c., will traverse the transparent fibres of gun-cotton without causing ignition until after the lapse of some time. The gunpowder being black absorbs the heat-rays readily. The same focus of rays concentrated in ether contained in a thin glass tube, traverses this volatile and inflammable liquid without heating it, although its boiling point is so low as 96'. If a small quantity of charcoal is added to it, the heat is absorbed, and the ether begins to boil immediately. 568. Reflection on such facts as these, and on the globular form and changing positions of our earth in relation to the sun, will lead to a clear explanation and measure of the differences of climate and of season found in different parts of the earth. All understand that if a small globe be suspended before a fire, the part which is nearest to the fire and receives the rays directly, will be much more heated than the other parts receiving the rays more or less obliquely. So on this earth, which is rotating before the sun, the regions about the equator are the most heated. The sunny side of many a steep hill in England receives the sun's rays in summer as perpendicularly as the plains about the equator; but such hill-sides are not heated like those plains, because the air over them is colder-as very elevated mountain tops, even at the equator, owing to the rarified, and, therefore, cold, air always around them, remain permanently hooded in snow. In England, at the time of Influence of Solar Heat on the Globe. 387 the equinoxes, a level plain receives only about half as much of the sun's light and heat as an equal extent of level surface near the equator; and in the short days of winter, when the beams fall more obliquely, it receives less than a third part of the summer amount. 569. A picture has been designed to assist a student in conceiving clearly the consequences of the different intensities of the sun's influence in different latitudes on the earth. It is an elongated landscape of a strip of the earth's surface, stretching from the equator to the pole, exhibiting the more remarkable objects belonging to the vegetable and animal kingdoms peculiar to the different latitudes. At the equatorial end of this representation appear, amidst the endless variety of broad-leaved palms, such animals as the elephant, tiger, parrot, etc., and men as naked savages, or as the civilized Hindoos, seeking the shade of bungalows with darkened windows, and wetted mats hung round the walls to cool them. Corresponding views are given of the intermediate temperate climates, until at the polar extremity appear the dwellers in barren Greenland or Iceland, where the thermometer stands below zero, and the people are clothed in thick furs of polar animals, and protected by close artificially-warmed huts. Then one may reflect that all the contrasts here referred to, are in existence on the earth at the same moment of time. 570. An interesting evidence of solar influence on our globe is the periodical migration of animals which have their home not in any fixed region on earth, but wherever the sun has for the time the particular degree of influence best suiting them, and which accordingly follow the sun in the changes of season. In England, for instance, we have the swallow in vast numbers coming to visit our isles in the spring, to play over our woods and waters in pursuit of the insects which the heat then breeds to fill the air; and in autumn the same creatures are seen congregating on the shores, to wing their flight in united multitudes back to more southern countries, where, in turn, they find the needed warmth. The same season brings the cuckoo, the nightingale, and many other winged species In the waters of the bays and coasts, too, there appear in their seasons vast shoals of the finny races-the herring, the mackerel, and the beautiful salmon, which last, at stated times, penetrates from the ocean far up the mountain streams, to deposit its spawn in suitable localities. 571. Some animals which do not migrate, and which would be 388 The Source of Solar Heat. unable to resist the cold of winter, pass into a state of apparent death, which is called hybernation. The dormouse, the bat, and the marmot, or mountain rat, furnish instances of hybernation as a result of the withdrawal of solar heat. It has been proved that in these animals the vital functions are not arrested, but simply reduced to their lowest ebb. In the active state of the marmot, the pulsations of the heart are ninety in a minute. In the torpid or hybernating state they are reduced to eight or ten. Such are the effects of solar heat upon living animals. 572. What is the source of Solar heat?-Seeing that the experimeats of Rumford and Davy have demonstrated on a small scale that molar motion, or the motion of masses, is a source of heat (Art. 563), some physicists have endeavoured to account for the enormous amount of solar heat shed over the globe on similar mechanical principles. The difficulties of this problem may be estimated when we consider that spectral analysis proves that iron and other fixed metals exist in vapour in the photosphere surrounding the sun, that an artificial heat of nearly 3000° is required to bring iron to a fluid state, and that the most intense heat which man can apply has failed to vaporize the metal. Dr. Mayer has endeavoured to solve this problem. Assuming that heat is in all cases derived from matter in motion, he has suggested that meteorites having an intense velocity from solar attraction, are continuously falling upon the surface of the sun, and that their suddenly arrested motion is converted into that which we call solar heat. Further, it is assumed that if the earth were suddenly stopped in its course and drawn into the sun by gravitation, the heat generated by the collision would suffice to convert into vapour the earth itself and all that was upon it. This ingenious hypothesis would, however, involve the necessity for an unlimited supply of meteorites, of the existence of which there is no evidence. Thus, in removing one difficulty this hypothesis would create another. There is no reason to believe that the heat of the sun is derived from anything analogous to combustion on the earth, or that it is a product of chemical changes among the elements from which light and heat result. Angström detected in the photosphere of this luminary by spectro-telescopic observation, the vapours of thirteen metals, including those of iron and aluminium. It is remarkable that no lines indicative of oxygen, the most abundant element of our carth, could be detected. Delarue, on the occasion of the eclipse a few years since, was able to detect and measure the incan Terrestrial or Subterranean Heat. 389 descent hydrogen emitted from the edge of the sun's disc; and he calculated that it rose to the height of from sixty to eighty thousand miles above the photosphere. Spectral examination clearly proves that neither the hydrogen nor the metals are in a state of combustion or oxidation. They are simply incandescent, or in the form of glowing vapours intensely heated, but remaining unchanged, or in their elementary state. By what form of energy they are maintained in this state it is impossible at present to speculate. All that ve are entitled to say at present is that neither the theory of motion nor of chemical combustion affords any satisfactory explanation of the source of solar heat. 573. Terrestrial Heat-Physics of the Earth.-Although there are certain facts which show that the interior of our globe below a certain depth is in a heated state, there is nothing to indicate that any of this heat is transmitted to the surface, or that it affects the atmosphere above it. At a certain depth below the surface of the ground, the thermometer undergoes no change (see Art. 599). Below this, it is observed to rise gradually, and in a degree proportioned to the depth at which the observation is made. This statement rests upon experiments carried on in deep mines, and in artificial borings through the soil, as well as on observations of the temperature of the waters of Artesian wells and Thermal springs. It may be stated generally that in English mines at a depth of from 1500 to 1800 feet, the temperature at all periods of the year has been found to be from 70° to 80°. In one of the most recent borings for coal in Sussex (1875)*, at a depth of 1640 feet, the thermometer was found to stand at 72°. Among Artesian wells may be mentioned that of Grenelle, in Paris, the depth of which from the surface is 1794 feet,—the temperature of the water is 82°. In the well of Mondorf, in Germany, which has a depth of 2202 feet, the emperature of the water is 93°. The high temperature of Thermal springs also proves that there are sources of great heat in the interior of the earth itself quite irrespective of solar influence. The temperature of the water as it issues in some localities, as in Iceland and South America, reaches the boiling point, 212°. In the Pyrenees some have a temperature of 152°; in Auvergne they vary from 113° to 176°. The hottest in England is at Bath, of which the temperature is 117°, i.e., 67° higher than the mean temperature of the place. 574. In considering the physics of the earth, we must notice not *The Subwealden Exploration. |