zo THE POPULAR SCIENCE MONTHLY. 



thousand years ; but for all that there may have been variations of 

 quite as much as five or ten per cent, as we may judge from consider- 

 ing that the intensity of the solar radiation to the earth is six and a 

 half per cent greater in January than in July ; and neither at the 

 equator nor in the northern or southern hemispheres has this difference 

 been discovered by experience or general observation of any kind. 

 But as for the mere age of the sun, irrespective of the question of 

 uniformity, we have proof of something vastly more than three thou- 

 sand years in geological history, with its irrefragable evidence of con- 

 tinuity of life on the earth in time past for tens of thousands, and 

 probably for millions of years. 



Here, then, we have a splendid subject for contemplation and re- 

 search in natural philosophy, or physics, the science of dead matter. 

 The sun, a mere piece of matter of the moderate dimensions which Ave 

 know it to have, bounded all round by cold ether, has been doing 

 work at the rate of four hundred and seventy-six thousand million 

 million million horse-power for three thousand years and at possibly 

 more, and certainly not much less, than that for a few million years. 

 How is this to be explained ? Natural philosophy can not evade the 

 question, and no physicist who is not engaged in trying to answer it 

 can have any other justification than that his whole working time is 

 occupied with work on some other subject or subjects of his province 

 by which he has more hope of being able to advance science. 



I suppose I may assume that every person present knows as an 

 established result of scientific inquiry that the sun is not a burning 

 fire, and is merely a fluid mass cooling, with some little accession of 

 fresh energy by meteors occasionally falling in, of very small account 

 in comparison with the whole energy of heat which he gives out from 

 year to year. You are also perfectly familiar with Helmholtz's form 

 of the meteoric theory, and accept it as having the highest degree of 

 scientific probability that can be assigned to any assumption regarding 

 actions of prehistoric times. You understand, then, that the essential 

 principle of the explanation is this : at some period of time, long past, 

 the sun's initial heat was generated by the collision of pieces of matter 

 gravitationally attracted together from distant space to build up his 

 present mass ; and shrinkage due to cooling gives, through the work 

 done by the mutual gravitation of all parts of the shrinking mass, 

 the vast thermal capacity in virtue of which the cooling has been, and 

 continues to be, so slow. I assume that you have not been misled by 

 any of your teachers who may have told you, or by any of your books 

 in which you may have read, that the sun is becoming hotter because 

 a gaseous mass, shrinking because it is becoming colder, becomes hotter 

 because it shrinks. 



An essential detail of Helmholtz's theory of solar heat is that the 

 sun must be fluid, because even though given at any moment hot 

 enough from the surface to any depth, however great, inward, to be 



