536 THE PRINCIPLES OF SCIENCE. [CHAP. 



result had the right application of the formulae occurred 

 to any one. 1 Giffard's injector for supplying steam hoilers 

 with water by the force of their own steam, was, I 

 believe, accidentally discovered, but no new principles of 

 mechanics are involved in it, so that it might have been 

 theoretically invented. The same may be said of the 

 curious experiment in which a stream of air or steam 

 issuing from a pipe is made to hold a free disc upon the 

 end of the pipe and thus obstruct its own outlet. The 

 possession then of a true theory does not by any means 

 imply the foreseeing of all the results. The effects of even 

 a few simple laws may be manifold, and some of the 

 most curious and useful effects may remain undetected 

 until accidental observation brings them to our notice. 



Predicted Discoveries. 



The most interesting of the four classes of facts specified 

 in p. 525, is probably the third, containing those the 

 occurrence of which has been first predicted by theory and 

 then verified by observation. There is no more convincing 

 proof of the soundness of knowledge than that it confers 

 the gift of foresight. Auguste Comte said that " Prevision 

 is the test of true theory ; " I should say that it is one test 

 of true theory, and that which is most likely to strike 

 the public attention. Coincidence with fact is the test of 

 true theory, but when the result of theory is announced 

 before-hand, there can be no doubt as to the unprejudiced 

 spirit in which the theorist interprets the results of his 

 own theory. 



The earliest instance of scientific prophecy is naturally 

 furnished by the science of Astronomy, which was the 

 earliest in development. Herodotus 2 narrates that, in 

 the midst of a battle between the Medes and Lydians, the 

 day was suddenly turned into night, and the event had 

 been foretold by Thales, the Father of Philosophy. A 

 cessation of the combat and peace confirmed by marriages 

 were the consequences of this happy scientific effort. 

 Much controversy has taken place concerning the date of 



1 Airy, On Tides and Waves, Encyclopaedia Metropolitana, p. 348*. 



2 Lib. i. cap. 74. 



