INSTANCES OF REDISCOVERY IN SCIENCE. 299 



sent a copy, informed me that some of the chief facts I had 

 discovered had been found by him twelve years before and 

 published in 6 PoggendorfFs Annalen,' under the wider 

 title of 'Magnetische Untersuchungen.' This title did 

 not indicate the special character of the phenomenon, and 

 the research had not been referred to in any of the papers 

 on kindred subjects which I had previously examined. 



Many cases have occurred of the rediscovery of the same 

 truths, through want 'of knowledge of their previous dis- 

 covery; as an instance may be mentioned that of the 

 analogy between the mathematical theory of conduction of 

 heat in solid bodies and that of electric and magnetic 

 attraction. The known mathematical theorems regard- 

 ing the conduction of heat were first applied by George 

 Green, of Nottingham, to establish some of the most im- 

 portant theorems in the mathematical theories of electricity 

 and magnetism, and published by him in a most general 

 and complete form in the year 1828. They were subse- 

 quently rediscovered by Gauss, of Gottingen ; then inde- 

 pendently by Charles; and then again by Sir William 

 Thompson in the year 1842. 



As it is frequently almost impossible, in consequence 

 of absence of sufficient records or testimony, to ascertain 

 or infer with certainty the exact circumstances which led 

 to, or formed the chief cause of a given discovery, it is 

 probable that some of the following instances are not really 

 ones of rediscovery. The variation of the moon was dis- 

 covered by an Arabian astronomer of the tenth century, 

 and was rediscovered by Tycho-Brahe, six centuries later. 1 

 The Chinese appear to have noticed the variation of the 

 magnet in the year 1111, long before Columbus redis- 

 covered it. 2 The Arabians, about the year 1000 of the 



Vlt 



1 Whewell, History of the Inductive Sciences, vol. i. 3rd it. p. 175. 



2 Davis, Chinese, pp. 277, 278. 



