22 THE POPULAR SCIENCE MONTHLY. 



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 undoubt- 

 edly 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 emit- 

 ted 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 begin- 

 ning 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 pei'ceptibly 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 contrac- 

 tion 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 sufiScient 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 gener- 



mv'' . 

 ated, which may be expressed, in calories, by the formula -, m 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- 



