INTRODUCTION. 13 



tions have agreed in admiring. But, when we 

 examine more clearly the history of such disco- 

 veries, we find that these epochs have not occurred 

 suddenly and without preparation. They have been 

 preceded by a period, which we may call their 

 Prelude, during which the ideas and facts on which 

 they turned were called into action; were gra- 

 dually evolved into clearness and connexion, per- 

 manency and certainty; till at last the discovery 

 which marks the Epoch, seized and fixed for ever 

 the truth which had till then been obscurely and 

 doubtfully discerned. And again, when this step 

 has been made by the principal discoverers, there 

 may generally be observed another period, which 

 we may call the Sequel of the Epoch, during which 

 the discovery has acquired a more perfect certainty 

 and a more complete developement among the 

 leaders of the advance; has been diffused to the 

 wider throng of the secondary cultivators of such 

 knowledge, and traced into its distant consequences. 

 This is a work, always of time and labour, often of 

 difficulty and conflict. To distribute the History of 

 science into such Epochs, with their Preludes and 

 Sequels, if successfully attempted, must needs make 

 the series and connexion of its occurrences more 

 distinct and intelligible. Such periods form rest- 

 ing-places, where we pause till the dust of the con- 

 fused march is laid, and the prospect of the path 

 is clear. 



Inductive Charts. Since the advance of science 



