IN THE LAST HALF-CENTUEY. 37 



absolutely true, in the sense that a mathe- 

 matical proposition is so ; but, if its errors 

 can become apparent only outside the 

 limits of practicable observation, it may 

 be just as usefully adopted for one of the 

 symbols of that algebra by which we in- 

 terpret nature, as if it were absolutely 

 true. 



The development of every branch of 

 physical knowledge presents three stages 

 which, in their logical relation, are suc- 

 cessive. The first is the determination of 

 the sensible character and order of the 

 phenomena. This is Natural History, in 

 the original sense of the term, and here 

 nothing but observation and experiment 

 avail us. The second is the determination 

 of the constant relations of the phenomena 

 thus defined, and their expression in rules 

 or laws. The third is the explication of 

 these particular laws by deduction from 

 the most general laws of matter and mo- 

 tion. The last two stages constitute Nat- 



