A Century of Science 27 



ghens, whom we should also remember as the dis- 

 coverer of Saturn's rings and the inventor of the 

 pendulum clock. But Huyghens was in advance 

 of his age, and the overshadowing authority of 

 Newton, who maintained a rival hypothesis, pre- 

 vented due attention being paid to the undulatory 

 theory until the beginning of the present century, 

 when it was again taken up and demonstrated by 

 Fresnel and Thomas Young. About the same 

 time, our fellow countryman, Count Rumford, was 

 taking the lead in that series of researches which 

 culminated in the discovery of the mechanical 

 equivalent of heat by Dr. Joule in 1843. One of 

 Priestley's earliest books, the one which made him 

 a doctor of laws and a fellow of the Royal Society, 

 was a treatise on electricity, published in 1767. 

 It was a long step from that book to the one in 

 which the Danish physicist Oersted, in 1820, de- 

 monstrated the intimate correlation between elec- 

 tricity and magnetism, thus preparing the way for 

 Faraday's great discovery of magneto-electric in- 

 duction in 1831. By the middle of our century 

 the work in these various departments of physics 

 had led to the detection of the deepest truth in 

 science, the law of correlation and conservation, 

 which we owe chiefly to Helmholtz, Mayer, and 

 Grove. It was proved that light and heat, and 



