4 i8 THE WORLD MACHINE 



needful to develop its annual expenditure of heat can be cal- 

 culated. Relative to its mass this amount is small ; two or 

 three hundred feet per year, a few miles per century, would 

 suffice. It would be long before such a contraction would be 

 evident to any human measures. In six thousand years it 

 would lessen the apparent size of the sun to us by less than 

 a second of arc that is to say, by less than an iSooth part of 

 its present angular size. 



This is slow; but it is unceasing. Upon the calculations of 

 von Helmholtz and others who followed him, it could scarce 

 have been going on, at its present intensity, for more than 

 eighteen or twenty millions of years. Moreover and this was 

 the startling feature the process is nearing an end. If the 

 total available supply which the contraction of the sun's mass 

 could produce were divided into 454 parts, Helmholtz cal- 

 culated that 453 of these parts had been spent. He pictured 

 the solar mechanism as a kind of clock which was run down. 

 Some day or other it would cease on present-day calculations, 

 within a period of something like seven millions of years. 



It is needless to say that these estimates excited the liveliest 

 interest, even among the laity. By some twist of chance, 

 von Helmholtz's conclusions were made public in a popular 

 lecture delivered at Konigsberg, the home of Kant, whither he 

 had gone to occupy a chair of physiology. They were presented 

 as an incident in an exposition of the new theories of energy ; 

 the lecture was entitled " On the Interaction of Natural Forces." 



Von Helmholtz's ideas were vehemently attacked. The con- 

 clusions of the geologists demanded, even for the events upon 

 our little earth, a bank account of time compared with which 

 seventeen million years was a bagatelle. The intrusion of the 

 physicist was hotly resented. The new ideas found a valiant 

 exponent in the sturdy figure of young William Thomson, now 

 our distinguished Lord Kelvin. He has done many a battle in 

 their defence. In later years Lord Kelvin has shown an in- 

 clination to modify that is to say, enlarge the estimated 

 allowance. Even stretching it to the utmost, it still seems 

 insufficient. The explanation of one mystery seemed only to 

 substitute another hardly less acute. It is possible that, within 

 the last year or two, the faint beginnings of our knowledge of 

 radium and radio-activity offer a gleam of explanation and 

 perhaps conciliation. It is too soon to be sure. 



