History of the Intellectual Development of Europe, Volume II (of 2) / Revised Edition

by John William Draper

If any sudden visible effect took place in the sun, [299] we should not see it at the absolute moment of its occurrence, but about eight minutes and thirteen seconds later, this being the time required for light to cross the intervening distance. All phenomena take place in reality anterior to the moment at which we observe them by a time longer in proportion as the distance to be travelled is greater.

There are objects in the heavens so distant that it would take many hundreds of thousands of years for their light to reach us. Then it necessarily follows, since we can see them, that they must have been created and must have been shining so long.

The velocity with which light moves was first determined by the Danish astronomer Römer from the eclipses of Jupiter's satellites, November, 1675. It was, therefore, a determination of the rate for reflected solar light in a vacuum, and gave 198,000 miles in a second. In 1727, Bradley determined it for direct stellar light by his great discovery of the aberration of the fixed stars. More recently, the experiments of M. Foucault and those of M. Fizeau, by the aid of rotating mirrors or wheels, have confirmed these astronomical observations, Fizeau's determination of the velocity approaching that of Römer. Probably, however, the most correct is that of Struve, 191,515 miles per second.

Investigation of the age of the earth through the phenomena of heat.This astronomical argument, which serves as a general introduction, is strengthened by numerous physical and physiological facts. But of the different methods by which the age of the earth may be elucidated, I shall prefer that which approaches it through the phenomena of heat. Such a manner of viewing the problem has led to its determination in the minds of many thinking men.

Astronomical heat alone on the earth's surface.As correct astronomical ideas began to prevail, it was perceived that all the heat now on the surface of our planet is derived from the sun. Through the circumstance of the inclination of her axis of rotation to the plane of her annual motion, or through the fact of her globular form occasioning the presentation of different parts of her surface, according to their latitudes, with more or less obliquity, and hence [300] the reception of less or more of the rays, there may be local and temporary variations. But these do not affect the general principle that the quantity of heat thus received must be the same from year to year.

The equilibrium of interior heat.This thermometric equilibrium not only holds good for the surface, it may also be demonstrated for the whole mass of the planet. The day has not shortened by the ¹/²⁰⁰ of a second since the time of Hipparchus, and therefore the decrease of heat can not have been so much as the ¹/³⁰⁰ of a Fahrenheit degree, on the hypothesis that the mean dilatation of all terrestrial substances is equal to that of glass, ¹/¹⁸⁰⁰⁰⁰ for one degree. If a decline had taken place in the intrinsic heat of the earth, there must have been a diminution in her size, and, as a necessary consequence, the length of the day must have become less. The earth has therefore reached a condition of equilibrium as respects temperature.

Its ancient decline.A vast body of evidence has, however, come into prominence, establishing with equal certainty that there was in ancient times a far higher temperature in the planet; not a temperature concerned with a fraction of a degree, but ranging beyond the limits of our thermometric scale. The mathematical figure of the earth offers a resistless argument for its ancient liquefied condition—that is, for its originally high temperature. But how is this to be co-ordinated with the conclusion just mentioned? Simply by the admission that there have elapsed prodigious, it might almost be said limitless, periods. Necessity for a long time. As thus the true state of affairs began to take on shape, it was perceived that the age of the earth is not a question of authority, not a question of tradition, but a mathematical problem sharply defined: to determine the time of cooling of a globe of known diameter and of given conductibility by radiation in a vacuum.

In such a state of things, what could be more unwise than to attempt to force opinion by the exercise of authority? How unspeakably mischievous had proved to be a like course as respects the globular form of the earth, which did not long remain a mere mathematical abstraction, but was abruptly brought to a practical issue by the voyage of Magellan's ship. And on this question of [301] the age of the earth it would have been equally unwise to become entangled with or committed to the errors of patristicism—errors arising from well-meant moral considerations, but which can never exert any influence on the solution of a scientific problem.

Indications of the interior heat of the earth.One fact after another bearing upon the question gradually emerged into view. It was shown that the diurnal variations of temperature—that is, those connected with night and day—extend but a few inches beneath the surface, the seasonal ones, connected with winter and summer, to many feet; but beyond this was discovered a stratum of invariable temperature, beneath which, if we descend, the heat increases at the rate of 1° Fahr. for every fifty or seventy feet. The uniformity of this rate seemed to imply that, at depths quite insignificant, a very high temperature must exist. This was illustrated by such facts that the water which rushes up from a depth of 1794 feet in the Artesian well of Grenelle has a temperature of 82° Fahr. The mean temperature of Paris being about 51° Fahr., these numbers give a rate of 1° for every fifty-eight feet. If, then, the increase of heat is only 100° per mile, at a depth of less than ten miles every thing must be red hot, and at thirty or forty in a melted state. It was by all admitted that the rise of temperature with the depth is not at all local, but occurs in whatever part of the earth the observation may be made. The general conclusion thus furnished was re-enforced by the evidence of volcanoes, which could no longer be regarded as merely local, depending on restricted areas for the supply of melted material, since they are found all over the land and under the sea, in the interior of continents and near the shores, beneath the equator and in the polar regions. It had been estimated that there are probably two thousand aerial or subaqueous eruptions every century. Some volcanoes, as Ætna, have for thousands of years poured forth their lavas, and still there is an unexhausted supply. Everywhere a common source is indicated by the rudely uniform materials ejected. The fact that the lines of volcanic activity shift pointed to a deep source; the periodic increments and decrements of force bore the same [302] interpretation. They far transcend the range of history. The volcanoes of central France date from the Eocene period; their power increased in the Miocene, and continued through the Pliocene; those of Catalonia belong to the Pliocene, probably. Coupled with volcanoes, earthquakes, with their vertical, horizontal, and rotary vibrations, having a linear velocity of from twenty to thirty miles per minute, indicated a profound focus of action. The great earthquake of Lisbon was felt from Norway to Morocco, from Algiers to the West Indies, from Thuringia to the Canadian lakes. It absolutely lifted the whole bed of the North Atlantic Ocean. Its origin was in no superficial point.

Proof from the mean density.A still more universal proof of a high temperature affecting the whole mass of the interior of the globe was believed to be presented in the small mean density of the earth, a density not more than 5·66 times that of water, the mean density of the solid surface being 2·7, and that of the solid and sea-surface together 1·6. But this is not a density answering to that which the earth should have in virtue of the attraction of her own parts. It implied some agent capable of rarefying and dilating, and the only such agent is heat. Although the law of the increase of density from the upper surface to the centre is unknown, yet a comparison of the earth's compression with her velocity of rotation demonstrated that there is an increasing density in the strata as we descend. The great fact, however, which stands prominently forth is the interior heat.