142 



DISCOVERY 



the Malthus problem of reproduction and food-supply 

 production will become acute ; although, as Dean 

 Inge points out, it is the supply of food-stuff and the 

 wherewithal to live that really limits the population 

 of the world. 



^ :): 4: ^ ^ 



While our sun lasts, the using up of the earth's 

 energy stores, while producing tremendous changes in 

 the distribution of the world's population, will not 

 gravely threaten man's existence, even in great 

 numbers. While the winds blow and the waterfalls 

 last there will always be great possibilities of energy 

 production. And when the cooling sun tempers the 

 vigour of the storms, and by the diminution of 

 evaporation dries the water-courses, the tides can 

 still be harnessed, and used, as they once were near 

 Newhaven, in Sussex, for mechanical power. When 

 we are well on in our hundred thousands, though still 

 in the prime of our immortality, the seas will all 

 freeze over — and where shall we then go for our sources 

 of energy ? There will be no further store inex- 

 hausted ; forests and coal-fields, oil-wells and peat 

 supplies will be long-forgotten luxuries. Perhaps we 

 shall have learnt by then how to use the energy of the 

 atom — how to chain the force of gravitation. Perhaps 

 we shall be independent of the sun. At least we may 

 hope so ; is not necessity tlie mother of invention ? 

 ***** 



Meanwhile, what keeps our sun hot ? Perhaps 

 most people think of the sun, in a dim kind of way, 

 as a gigantic bonfire, which will in time turn into a 

 vast celestial ash heap. That is, however, certainly 

 not true, because the highest known temperature of 

 combustion is about 3,000 degrees, and the sun has 

 a temperature of 6,000 degrees ; and, moreover, it has 

 been calculated that no bonfire could give more than 

 2,500 years of heat. The number of years during 

 which the sun has been radiating heat energy is a 

 difficult problem to solve ; but estimates are usually 

 ■expressed in hundreds of millions. Alex. Veronnet, 

 astronomer at the University of Strasburg, who dis- 

 ■cusses the problem in the Revue Generate des Sciences 

 (March 30, 1923), considers most estimates, which are 

 based on such facts as the forrriation of rocks and the 

 increasing saltness of the sea, to be too large. They 

 assume that things took as long to happen millions of 

 years ago as they do now ; whereas, with a hotter 

 sun the solution of substances in rivers, which causes 

 the saltness of the sea, must have been a more rapid 

 and complete process. Still, our earth and our sun 

 have a long and distinguished history, and an explana- 

 tion of the sun's heat must satisfactorily take into 

 •account these tremendous ages. Other theories have 

 been suggested. Robert Mayer considered that the 



sun was fed by showers of meteorites. But this would 

 involve an increase in the mass of the sun, and therefore 

 an acceleration of the earth in its orbit and a shortened 

 year. Even if meteorites within the orbit of the 

 earth were to feed the sun, the alteration in the orbit 

 of Mercury would be perceptible. There are many 

 reasons, also, which render a theory of radio-activi*y 

 as a source of sun energy inadmissible. 



***** 



The theory which Lord Kelvin adopted and which 

 Alex. Veronnet selects as most plausible was originally 

 advanced by Helmholtz. His opinion was that the 

 sun's heat was the result of the energy of its gradual 

 contraction. Those who wish to investigate the basis 

 of this theory cannot do better than refer to Monsieur 

 Veronnet's article. The deduction from this theory is 

 that in 100,000 years the mean temperature of this 

 earth will be five degrees lower. In a million years, 

 the temperature will be below zero, and the wholi 

 earth will be frozen over. Imagination dare not 

 picture what the life of man, in that eternal arctic 

 winter, will be. The change will have come so gradu- 

 ally that men will be reconciled to it. But it is a 

 dismal picture. As one sits by a coal fire in winter, 

 or lazes in the summer sun in these halcyon days of 

 the sun's gracious middle age, perhaps we ought to 

 be grateful that we have been born in what is probably 

 the most bountiful and luxurious age the earth has 

 known or ever will know. And we may think again 

 before we regret that we may not hope to see the end 

 of the story. 



***** 



There is, however, another theory of the sun's heat 

 which Monsieur Veronnet does not accept, but which 

 English authorities prefer. They do not believe that 

 the contraction theory accounts for more than a five- 

 hundredth part of the energy of the sun. The alterna- 

 tive theory is based on the supposition that elements 

 are formed from hydrogen. If that is so, the mass of 

 their atoms ought all to be exact multiples of the mass 

 of the hydrogen atom. In point of fact, their masses 

 are in general a little less than the calculated figure. 

 The discovery of isotopes by Aston, accounts, in a 

 sense, for this discrepancy ; but it is also possible, 

 using the arguments which Einstein first brought 

 forward, to explain the sun's energy by assuming that 

 the extra mass has been turned into radiant energy. 

 And such a theory postulates a far older sun, and a 

 much greater lease of life on its present scale, than the 

 contraction theory of Helmholtz. 



W'e have received the following bulletin among tln' 

 admirable series which Harvard College Observatory 

 circulates : 



