No observed changes. Combustion. The meteoric theory. warm their bodies in ancient times just as now, and were oppressed by the heat of midsummer as they are to-day. There is no trustworthy human record of any great migration of animals, which might be due to changes of temperature. The rocks, to be sure, tell of great changes in the remote past, epochs when high northern latitudes experienced tropical temperatures, and other epochs when the temperate zones were encased in ice. But no one knows whether these conditions were due to variations in the earth's distance from the sun or to changes in the intensity of the solar heat or to a combination of both causes. Amazing as is the daily outpour of solar heat, there is no evidence from observation that it has changed in quantity or quality since human history began. The supply cannot be infinite; how, then, is the radiation maintained? Not by combustion, for in that case the solar fires would have burned out ages ago. If the sun were a mass of the best hard coal, burning in oxygen, it would be consumed in sixty centuries. If combustion is excluded from the list of possible causes, what shall we say about the impact of bodies from without? If a projectile from a rifled gun strikes the armorplate of an ironclad, the shot is not only deformed but heated. If the earth should fall to the sun from its present distance, as much heat would be developed by the impact as the sun radiates in ninety-five years. The fall of giant Jupiter would cause an accession of heat equal to the amount now given off in over 30,000 years. Why may it not be that meteoric bodies fall upon it in sufficient numbers to keep up the supply of heat? We reply that if there were any such aggregation of meteors in the sun's vicinity it ought to have a marked effect upon the motion of some comets which come near the sun, and would encounter it. Doubtless the sun receives some heat from such a source as this, but only a fraction of its heat can be thus accounted for. The theory generally accepted is called the contraction The contraction theory. When a body falls from any elevation to the theory. earth's surface, heat is produced when it strikes. If the same body be attached to a rope and made to turn a machine with badly oiled bearings, at least a portion of the energy of the descending body is converted into heat. In the first case energy is converted into heat suddenly, at the instant when the body strikes; in the second case a portion of the energy of the descending weight is being gradually converted into heat. Without going more deeply into details we may say that if the sun be slowly contracting in size, so that the particles of matter which compose it are falling toward the center, heat is being produced by this contraction. If the sun's diameter diminishes five feet a week the total radiation of the sun is explained. Such a shrinkage is so slight that it would not be certainly detected by our present means of astronomical measurement in 10,000 years. The contraction theory is considered the most. reasonable which has yet been advanced. The sun's past If all the heat which the sun gives out comes from its contraction, and if the amount of heat radiated yearly is and future. practically constant from age to age, it is possible to reason backward to a time when the sun was inconceivably vast, and to reason forward to a time when it will probably cease to give sufficient heat to maintain human life on the earth. Upon these hypotheses the sun would consume 18,000,000 years in radiating away the heat which would be developed by its contraction from a size inconceivably great to its present dimensions. Five mil Contraction. Amusing speculations. lion years hence it will, upon this hypothesis, have only half its present diameter, and the matter composing it will be crowded into one eighth the space now occupied. The compression will probably turn most of it into a liquid or solid form. Further contraction being then very difficult, the temperature of the sun will probably fall so rapidly that its function as a life-giver to the earth will cease before another 5,000,000 years have rolled away. Our reasoning has been based upon unverifiable hypotheses and the conclusions may be far astray. They simply represent the best guessing that scientists can make with reference to the past and future of the sun. There is at any rate no reason for alarm at present. Mark Twain has well satirized scientific speculations which involve millions of years in the following passage: Now, if I wanted to be one of those ponderous scientific people, and "let on" to prove what had occurred in the remote past by what had occurred in a given time in the recent past, or what will occur in the far future by what has occurred in late years, what an opportunity is here! Geology never had such a chance, nor such exact data to argue from! Nor "development of species," either! Glacial periods are great things, but they are vague, vague. Please observe: In the space of 176 years the Lower Mississippi has shortened itself 242 miles. That is an average of a trifle over one and one third miles per year. Therefore, any calm person, who is not blind or idiotic, can see that in the old Oölitic Silurian period, just a million years ago next November, the Lower Mississippi was upwards of 1,300,000 miles long, and stuck out over the Gulf of Mexico like a fishing rod, and by the same token any person can see that 742 years from now the Lower Mississippi will be only a mile and three quarters long, and Cairo and New Orleans will have joined their streets together, and be plodding comfortably along under a single mayor and a mutual board of aldermen. There is something fascinating in science. One gets such wholesome results of conjecture out of such a trifling investment of fact. CHAPTER XII. THE MOON AND ECLIPSES. "That orbed maiden, with white fire laden, Glides glimmering o'er my fleece-like floor, -Shelley. Tides retard rotation. THOSE who speculate about the origin of celestial bodies have a fine field of thought in connection with the earth's the moon. It is an undoubted fact that the moon raises tides in our oceans. The wash of the tides against continents and islands tends to retard the rotation of the earth by a trifling amount. If this retardation is not offset by other causes, as, for instance, a shrinking of the earth from its progressive cooling, the length of the day must be gradually increasing. The increase must be very slow, because it has not yet been brought to light by observation. The action of the moon upon the earth is accompanied by a reaction of the earth, which expresses itself in allowing the moon gradually to move farther and farther away. Looking Reversing the process, we look back through geologic ages to a time when the earth whirled much faster than backward. at present, and the moon was close to its surface, both bodies being hotter than now. How did these bodies come to be in such close companionship? Does it not seem probable that they were originally one? A grindstone which rotates too rapidly bursts asunder. Is it not then entirely possible that when a mass of heated Looking forward. The moon's rotation. matter in a fluid state rotates rapidly, a piece of it may fly off? If we have hit upon a correct theory of the moon's origin, let us follow up the clue. The moon has disengaged itself from the earth, but is still held in check by the attraction of gravity, so that it is describing an orbit about the earth. Both bodies are in a fluid condition and rotating. They are so close together that the attraction of each raises large tides on the other. The tide on the earth checks the swiftness of its spinning. If the moon is rotating swiftly its tides put a brake upon it. If, on the other hand, it is rotating very slowly, the friction of the tides quickens its rotation. As previously mentioned, one result of this tidal action is that the two bodies separate. They grow cooler and more rigid; the powerful tides raised upon the moon by the earth, keeping it continually eggshaped, have had such an effect upon the original rotation that the moon has now solidified as a slightly elongated body, the longest axis of which points toward the earth. So it has come to pass that the moon keeps the same face turned toward the earth. If this be true, does it rotate at all? Certainly; while it is making one revolution about the earth it also makes one complete rotation on its axis. illustrated very simply. This may be In the center of a circle one hundred yards in diameter a man is standing; he watches a boy who runs at a uniform rate around the circle; the boy keeps the left side of his head continually toward the man. At one instant the boy is facing the north; in a few seconds he has run one fourth of the way around the circle, and faces westward; in a few seconds more he faces southward, then eastward, and finally northward again, when |