INTRODUCTION. 9 



may be when they draw from such sources the 

 materials on which the mind of the philosopher 

 employs itself; the history of those branches of 

 knowledge for the last three hundred years abun- 

 dantly teaches us. 



Accordingly, the existence of clear Ideas applied 

 to distinct Facts will be discernible in the History 

 of Science, whenever any marked advance takes 

 place. And, in tracing the progress of the various 

 provinces of knowledge which come under our 

 survey, it will be important for us to see, that, at 

 all such epochs, such a combination has occurred; 

 that whenever any material step in general know- 

 ledge has been made, whenever any philosophical 

 discovery arrests our attention; some man or 

 men come before us, who have possessed, in an 

 eminent degree, a clearness of the ideas which be- 

 long to the subject in question, and who have 

 applied such ideas in a vigorous and distinct man- 

 ner to ascertained facts and exact observations. We 

 shall never proceed through any considerable range 

 of our narrative, without having occasion to remind 

 the reader of this reflection. 



Successive Steps in Science. But there is an- 

 other remark which we must also make. Such 

 sciences as we have here to do with, are, com- 

 monly, not formed by a single act; they are not 

 completed by the discovery of one great prin- 

 ciple. On the contrary, they consist in a long-con- 

 tinued advance ; a series of changes ; a repeated 



