Fig. 17.—Photograph of the surface of the moon, in the vicinity of the crater of Copernicus. Taken at the Yerkes Observatory, Chicago, U. S. A. Scale: Diameter of moon, 0.55 m. = 21.7 in. Owing to the absence of an atmosphere and of atmospheric precipitations, the precipitous walls of the crater and other elevations do not indicate any signs of decay.
If the carbonic acid percentage of the air had kept constant for ages, the percentage of the water would have found time to get into equilibrium with it; but the sea actually absorbs carbonic acid from the air. Thus the sea-water must have been in equilibrium with an atmosphere which contained less carbonic acid than the present atmosphere. Hence the carbonic acid percentage has been increasing of late.
We often hear lamentations that the coal stored up in the earth is wasted by the present generation without any thought of the future, and we are terrified by the awful destruction of life and property which has followed the volcanic eruptions of our days. We may find a kind of consolation in the consideration that here, as in every other case, there is good mixed with the evil. By the influence of the increasing percentage of carbonic acid in the atmosphere, we may hope to enjoy ages with more equable and better climates, especially as regards the colder regions of the earth, ages when the earth will bring forth much more abundant crops than at present, for the benefit of rapidly propagating mankind.
III
RADIATION AND CONSTITUTION OF THE SUN
The question has often been discussed in past ages, and again in the last century, in how far the position of our earth within the solar system may be regarded as secure. One might apprehend two things. Either the distance of the earth from the sun might increase or decrease, or the rotation of the earth about its axis might be arrested; and either of these possibilities would threaten the continuance of life on the earth. The problem of the stability of the solar system has been investigated by the astronomers, and their patrons have offered high prizes for a solution of the problem. If the solar system consisted merely of the sun and the earth, the earth’s existence would be secure for ages; but the other planets exercise a certain, though small, influence upon the movements of the earth. That this influence can only be of slight importance is due to the fact that the total mass of all the planets does not aggregate more than one-seven-hundred-and-fiftieth of the mass of the sun, and, further, to the fact that the planets all move in nearly circular orbits around the centre, the sun, so that they never approach one another closely. The calculations of the astronomers demonstrate that the disturbances of the earth’s orbit are merely periodical, representing long cycles of from 50,000 to 2,000,000 years. Thus the whole effect is limited to a slight vacillation of the orbits of the planets about their mean positions.
So far everything is well and good. But our solar system is traversed by other celestial bodies, mostly of unknown, but certainly not of circular orbits—namely, the comets. The fear of a collision with a comet still alarmed the thinkers of the past century. Experience has, however, taught us that collisions between the earth and comets do not lead to any serious consequence. The earth has several times passed through the tails of comets—for instance, in 1819 and 1861—and it was only the calculating astronomer who became aware of the fact. Once on such an occasion we have thought that we observed a glow like that of an aurora in the sky. When the earth was drawing near the denser parts of the comet, particles fell on the earth in the shape of showers of shooting-stars, without doing any appreciable damage. The mass of comets is too small perceptibly to disturb the paths of the planets.
The rotation of the earth about its axis should slowly be diminished by the effects of the tides, since they act like a brake applied to the surface of the earth. This retardation is, however, so unimportant that the astronomers have not been able to establish it in historical times. The slow shrinkage of the earth somewhat counteracts this effect. Laplace believed that we were able to deduce, from an analysis of the observations of solar eclipses in ancient centuries, that the length of the day had not altered by more than 0.01 second since the year 729 B.C.
We know that the sun, unaccompanied by its planets, is moving in space towards the constellation of Hercules with a velocity of 20 km. (13 miles) per second, which is amazing to our terrestrial conceptions. Possibly the constituents of our solar system might collide with some other unknown celestial body on this journey. But as the celestial bodies are sparsely distributed, we may hope that many billions of years will elapse before such a catastrophe will take place.
In mechanical respects the stability of our system appeared to be well established. Since the modern theory of heat has made its triumphant entry into natural science, however, the aspect of matters has changed. We are convinced that all life and all motion on the earth can be traced back to solar radiation. The tidal motions alone make a rather unimportant exception. We have to ask ourselves: Will not the store of energy in the sun, which goes out, not only to the planets, but to a far greater extent into unknown domains of cold space, come to an end, and will not that be the end of all the joys and sorrows of earthly existence? The position appears desperate when we consider that only one part in 2300 millions of the solar radiation benefits the earth, and perhaps ten times as much the whole system, with all its moons. The solar radiation is so powerful that every gramme of the mass of the sun loses two calories in the course of a year. If, therefore, the specific heat of the sun were the same as that of water, which in this respect surpasses most other substances, the solar temperature would fall by 2° Cent. (3.6° F.) every year. As, now, the temperature of the sun in its outer portion has been estimated at from 6000° to 7000°, the sun should have cooled completely within historical times. And though the interior of the sun most probably has a vastly higher temperature than the outer portions which we can observe, we should, all the same, have to expect that the solar temperature and radiation would noticeably have diminished in historical times. But all the documents from ancient Babylon and Egypt seem to point out that the climate at the dawn of historical times was in those countries nearly the same as at present, and that, therefore, the sun shone over the most ancient representatives of culture in the same way as it shines on their descendants now.
The thesis has frequently been advanced, therefore, that the sun has in its heat balance not only an expenditure side, but also an almost equally substantial income side. The German physician R. Mayer, who has the immortal merit of first having given expression to the conception of a relation between heat and mechanical work, directed his attention also to the household of the sun. He suggested that swarms of meteorites, rushing into the sun with an amazing velocity (of over 600 km. per second), would, when stopped in their motion, generate heat at the rate of 45 million calories per gramme of meteorites. In future ages it would be the turn of the planets to sustain for some time longer the spark of life in the sun, by the sacrifice of their own existences. The sun would therefore, like the god Saturn, have to devour its own children in order to continue its existence. Of how little avail that would be we learn from the consideration that the fall of the earth into the sun would not be able to prolong the heat expenditure of the sun by as many as a hundred years. By their rush into the sun, almost uniformly from all sides, the meteorites would, moreover, long since have put a stop to the rotation of the sun about its axis. Further, by virtue of the increasing mass and the hence augmenting attraction of the sun, the length of our year would have had to diminish by about 2.8 seconds per year, which is in absolute contradiction to the observations of the astronomers. According to Mayer’s thesis, a corresponding number of meteorites would, finally, also have to tumble upon the surface of the earth, and (according to data which will be furnished in Chapter IV.) they should raise the surface temperature to about 800°. The thesis is therefore misleading.