the sun; while, according to Fourier's celebrated computation, the heat radiated from the earth's surface is only sufficient to melt a layer of ice ten feet thick in one hundred years.

The most conservative of these results may be regarded as showing that our earth is actually losing heat to a perceptible and measurable extent. Neither is the amount of heat.escaping at Paris to be taken as the measure of the reduction of the temperature of the mass of the earth in general. There are three hundred active volcanoes in existence, from the craters of which enormous quantities of heat are permitted to waste. The ocean, too, carries off vastly larger quantities than the land. The floor of the ocean is generally overlaid by a stratum of ice-cold water setting southward from the polar regions. This cold stream is overlaid by a warmer one moving northward from the tropics. Water being a better conductor of heat than atmospheric air, this cold stratum must necessarily abstract terrestrial heat with vastly greater rapidity than the average atmosphere of the temperate zone. Many observations indicate that the temperature of the solid crust beneath the waters of the ocean is much higher than that of continental surfaces, and hence imparts its warmth in larger quantities. Throughout all that part of the Frozen Ocean north of Europe and Asia, the temperature is found to increase at considerable depths, contrary to the well-known laws of hydrostatics. [See Appendix, Note X.] The same phenomenon has been observed on the coast of Australia, in the Adriatic, and Lago Maggiore. Horner asserts that in the deep soundings of the Gulf Stream, off the coast of the United States, the lead, when drawn up," used to be hotter than boiling water."

These facts, with others, seem to demonstrate that our planet is wasting its warmth many times faster than the

It

calculations of the mathematicians would indicate. seems inevitable, therefore, that the earth should have expended sufficient heat in 2500 years to effect a sensible reduction in the length of the day.

Thanks to the mathematicians, they have again come to our aid. The tide-wave is a protuberance of the oceanwaters raised by the moon, and following the moon around the earth from east to west. This motion is contrary to the earth's diurnal rotation, and the friction of the tidal waters against the shore and the standing waters must necessarily tend to retard the rotary motion of the earth. Now it has been calculated that this retardation must have amounted to one sixteenth of a second in 2500 years. If, therefore, no counteracting tendency has been experienced, the sidereal day is one sixteenth of a second longer than it was in the time of Hipparchus. But Laplace has shown that the sidereal day has not varied in length. It follows, therefore, that the shrinkage of the earth from loss of heat has tended to accelerate its rotation to the extent of one sixteenth of a second in twenty-five centuries. Such an acceleration corresponds to a shortening of the diameter about sixty feet, and a reduction of the temperature of the whole mass of the earth one fourteenth of a degree.

When the earth was in its youth, just emerging from a molten state, the loss of heat and consequent contraction must necessarily have been rapid. During this period the sidereal day underwent a much more rapid shortening than at present. In the distant future, on the contrary, the loss of heat will become diminished to an extreme extent, and, as a consequence, the retardation caused by the tide-wave will gain the ascendency, and the day will eventually be lengthened to such an extent that the earth will always turn the same side toward the sun, as the moon always turns the same side toward the earth. The historic period

of our race, as Mayer suggests, occupies consequently the comparatively brief space during which the retarding and accelerating tendencies neutralize each other.

These are the determinations of exact science. Mathematics have demonstrated that the cooling process which geology affirms of the past is certainly in progress in the present. It is immaterial how slow the process may be; the ultimate total refrigeration of the earth is a result which time will accomplish. Time, I say, since after the work is completed eternity will stretch onward as fresh, and inexhaustible, and limitless as when the career of planetary matter began.

This earth, to which our life-long round of labor and care is limited by an inexorable decree, was once a self-luminous orb. Far away in space, where Sirius was gleaming with his silver, or, perchance, his ruddy light, dwelt intelligent beings upon a planet which had already attained a habitable condition. From that abode the astronomer found means to contemplate the fiery globe that was destined to become the dwelling-place of man. Centuries of centuries later, the astronomer upon that distant orb noted the disappearance of a star upon which his predecessors had taken observations. Our planet had become opaque. Mists had gathered about it, and the ocean had descended from the clouds. Never more has this once resplendent orb greeted the eye of the astronomer of other systems; and while now the annals of his science perpetuate the memory of a lost star, that star first becomes a reality to conscious man. But our occupancy of the terrestrial globe is only a phase as evanescent as the self-luminous stage. While we build our cities and recount the achievements of a few generations past, this globe of matter hurries onward in its destined career as rapidly as a million years ago, when merely preparing for the occupancy of Adam's race. Every

year and every day witnesses the dissipation of terrestrial warmth. While we ponder the great fact, the world is growing cold beneath our feet. The current of events is carrying us inevitably to a state of total refrigeration. Perhaps the mountains will have been leveled first, and the continents swallowed up in the sea. Perhaps the volcano will have been first extinguished, and the earthquake will have lain down to its final slumber. Buffon imagined that the final refrigeration of the earth would introduce the rigors of perpetual winter, and render our planet uninhabitable. Though more recent investigators have asserted that that event would only reduce our earth's surface temperature one fortieth of its present amount, it seems difficult to rest upon that conclusion. The interior of the earth is probably half as hot as the sun. The earth's molten core is separated from us by not more than a hundred miles of rocky crust. The glowing sun is a million times farther removed, and yet, it is alleged, yields forty times the warmth which we derive from the nearer heat. In face of the testimony of figures, it is scarcely possible to doubt that the final cooling of our earth will exert a greater influence upon its surface conditions than these philosophers have dreamed.

CHAPTER XXXVIII.

THE SUN COOLING OFF.

E are not driven to the necessity of summoning exaggerated and imaginary agencies to the destruction of the earth. There are hostile powers reserved for the final conflict that will not be content with directing toward us merely "Quaker guns.'

The sun, we say, affords us thirty-nine fortieths of all the warmth which we enjoy, and we feel quite unconcerned about the alleged slow cooling of the earth. To the sun we owe the numberless activities of the organic and inorganic worlds, and we feel quite independent of the waning temperature of this dying ember which we call the

earth.

The amount of heat dispensed by our solar orb is truly something the contemplation of which overpowers the imagination. The rays which fall upon a common burningglass, converged to a focus, speedily ignite a piece of wood. . The heat which is received by a space of ten yards square is sufficient, as Ericsson states, to drive a nine-horse power engine. The amount of heat which falls upon half a Swedish square mile is sufficient to actuate 64,800 engines, each of 100 horse power. The total amount of heat received annually by the earth would melt a layer of ice one hundred feet thick. As the solar heat is radiated equally in all directions, it is easily calculated that the total emission of heat from the sun is 2300 millions of times the whole amount which reaches our earth.

Such an enormous expenditure of heat is sufficient to re