As to progressive changes in the amount of the solar heat, it can be said, however, that there is no evidence of anything of the sort since the beginning of authentic records. There have been no such changes in the distribution of plants and animals within the last two thousand years, as must have occurred if there had been within this period any appreciable alteration in the heat received from the sun. So far as can be made out, with few and slight exceptions, the vine and olive grow just where they did in classic days, and the same is true of the cereals and the forest-trees. In the remoter past there have been undoubtedly great changes in the earth's temperature, evidenced by geological records; carboniferous epochs, when the temperature was tropical in almost arctic latitudes; and glacial periods, when our now temperate zones were cased in sheets of solid ice, as northern Greenland is at present. Even as to these changes, however, it is not yet certain whether they are to be traced to variations in the amount of heat emitted by the sun, or to changes in the earth herself or in her orbit. So far as observation goes, we can only say that the outpouring of the solar heat, amazing as it is, appears to have gone on unchanged through all the centuries of human history.
What, then, maintains the fire? It is quite certain, in the first place, that it is not a case of mere combustion. As has been said only a few Images back, it has been shown that even if the sun were made of solid coal, burning in pure oxygen, it could only last about six thousand years—it would have been nearly one third consumed since the beginning of the Christian era. Nor can the source of its heat lie simply in the cooling of its incandescent mass. Huge as it is, its temperature must have fallen more than perceptibly within a thousand years if this were the case.
Two different theories have been proposed, which are probably both true to some extent. One of them finds the chief source of the solar heat in the impact of meteoric matter, the other in the slow contraction of the sun. As to the first, it is quite certain that some of the solar heat is produced in that way; but the question is, whether the supply of meteoric matter can be sufficient to account for any great proportion of the whole. As to the second, on the other hand, there is no question as to the adequacy of the hypothesis to account for the whole supply of solar heat; but there is as yet no direct evidence whatever that the sun is really shrinking.
The basis of the meteoric theory is simply this: If a moving body be stopped, either suddenly or gradually, a quantity of heat is generated, which may be expressed, in calories, by the formula mv2850' in which m is the mass of the body in kilogrammes, and v its velocity in metres per second: a body weighing 850 kilogrammes and moving one metre per second would, if stopped, develop just one calory of heat—i. e., enough to heat one kilogramme of water from freezing-point to 1° centi-
