io2 POPULAR SCIENCE MONTHLY 



referred to and expressed it correctly was a German physician, J. E. 

 Mayer, of Heilbronn, in the year 1842." Then he says, " I myself, 

 without being acquainted with Mayer or Colding, and having first 

 made the acquaintance of Joule's experiments at the end of my investi- 

 gations, followed the same path." 14 



We have now briefly sketched the birth of the principle of the con- 

 servation of energy in the minds of Mayer, Joule and Helmholtz. 

 After examining the facts we are convinced that these great physicists 

 were independent discoverers. Lack of time prevents us from making 

 reference to forerunners like Count Rum ford, 15 Sadi Carnot, 16 Seguin, 17 

 Mohr 18 and to the Dane, named Colding, 19 who in 1843 gave utterance 

 to the law before the Academy in Copenhagen. We pass by the 

 researches of Rankine, to whom we owe the expression, " conservation 

 of energy," as well as William Thomson's doctrine of the " dissipation 

 of energy." 



We come now to the second part of this paper — the application of 

 the conservation of energy and other principles of physics to the exam- 

 ination of the age of the sun and of the earth. The two problems are 

 closely interrelated; the earth-age is measurable by the sun-age. 



Before the time of the Scotch geologist, James Hutton, some 6,000 

 years was believed to indicate the age of the earth, and, indeed, of the 

 entire universe. The advent of the uniformitarian school of geologists 

 marks a radical departure from the old estimates. The pendulum 

 swings from one extreme to the other. Boundless distances of time 

 were now drawn upon. So great an antiquity of the earth seemed to 

 reveal itself to geologists, as to defy all attempts at measurement. In 

 the further pursuit of Hutton' s line of investigation, Play fair and 

 Lyell were unable to discover among the records of the earth and in 

 planetary motion either a beginning or an end of the present order of 

 things. They found no indication of infancy or decaying old age. 20 



This convenient doctrine of infinite durability came to be rudely 

 attacked by the physicists. Here, as in the history of the conservation 

 of energy, the earliest investigator is Robert Mayer. To be sure, he 



""Popular Lectures," by H. Helmholtz (transl. by E. Atkinson), New 

 York, 1897, p. 1G7. 



15 "The Complete Works of Count Rumford," published by the American 

 Academy of Arts and Sciences, Boston, Vol. I., pp. 481-488. 



16 " Reflexions sur la puissance motrice du feu," 1824, reprinted in Ostwalds 

 Klassiker, No. 37; English translation by R. H. Thurston appeared in 1890. 



17 "De L'influence des Chemins de Fer," Paris, 1839, pp. 378-397. 



18 " Allgem. Theorie der Bewegung," Braunschweig, 18G9, pp. 80-84. 



19 A. Colding, Det. Kongel. dansk vidensk. selsk. naturv. ogmath. afh. (5), 

 II., 1843, p. 121, 167. 



20 Sir Archibald Geikie, presidential address before British Association, in 

 Report, British Association for the Advancement of Science, 1892, Vol. 62, pp. 

 3-26; Smithsonian Report, 1892, p. 124. 



