190 THE PRINCIPLES OF SCIENCE. 



(5) We may indirectly predict or determine the quan- 

 tity of an effect without being able to verify it by experi- 

 ment. 



These various classes of quantitative facts might be 

 illustrated by an almost infinite number of interesting 

 points in the histoiy of physical science. Philosophical 

 prophecies especially serve to show the mastery which is 

 sometimes attained over the secrets of nature, and to 

 convince the least intelligent of the value of theory. 



Empirical Measurements. 



Under the first head of purely empirical measurements, 

 which have not been brought under any theoretical 

 system, may be placed the great bulk of quantitative 

 facts recorded by scientific observers. The tables of nu- 

 merical results which abound in books on chemistry and 

 physics, the huge quartos containing the observations of 

 public observatories, the multitudinous tables of meteoro- 

 logical observations, which are continually being compiled 

 and printed, the more abstruse results concerning terres- 

 trial magnetism such results of measurement, for the 

 most part, remain empirical, either because theory is de- 

 fective, or the labour of calculation and comparison is too 

 formidable. In the Greenwich Observatory, indeed, the 

 salutary practice has been maintained by the present 

 Astronomer Royal, of always reducing the observations, 

 and comparing them with the recognised theories of 

 motion of the several bodies. The divergences from 

 theory thus afford a constant supply of material for the 

 discovery of errors or of new phenomena ; in short, the 

 observations have been turned to the use for which they 

 were intended. But it is to be feared that other establish- 

 ments are too often engaged in merely recording numbers 

 of which no real use is made, because the labour of reduc- 



