104 PROGRESS OF SCIENCE. 



water follows all the sinuosities of the ground, and the motion? 

 is complicated, to all appearances, by foreign causes." And 

 from chapter xxxii. to xlii. (of his great work) every passage,, 

 almost, alludes to attraction and gravitation. If Kepler was- 

 a MATHEMATICIAN chiefly he should also be remembered as 

 an astronomer, A PHYSICIST, an optician of a high order. 



We may suitably complete this brief chronicle of the 

 advance made in the Experimental School during the Revival, 

 by quoting what Leonardo da Vinci said about experi- 

 ment, verification, and the Inductive Process, to show 

 what a powerful and precise instructor and advocate he was- 

 of those very principles which Roger Bacon had already 

 proclaimed as the indispensable requirement of scientific 

 progress. He says 



" Theory is the general and practice the soldiers. 



" Experiment is the interpreter of the artifices of nature. 

 It never deceives us; it is our judgment itself which some- 

 times leads us astray, because we expect from it effects 

 which are contrary to experiment. We must consult experi- 

 ment by varying the circumstances till we have deduced 

 from it general laws ; for it is it which furnishes true laws. 



" In the study of the sciences which depend on mathe- 

 matics, those who do not consult nature, but authors, are 

 not the children of nature ; they are only her grandchildren. 

 Nature alone is the master of true genius. 



" In treating any particular subject, I would first of all 

 make some experiment, because my design is first to refer to- 

 experiment, and then to demonstrate why bodies are con- 

 strained to act in such a manner. This is the method which 

 we ought to follow in investigating the phenomena of nature. 

 It is very true that nature begins by reasoning and ends with 

 experiment; but it matters not; we must take the opposite 

 course ; as I have said, we must begin by experiment, and en- 

 deavour by its means to discover general principles. 



"Experience has been the mistress of those who have 

 written sensibly, and I shall in any case proclaim her my 

 mistress. Truth is the daughter of time alone, as wisdom is 

 the daughter of experience. 



" Sound judgment is derived from sound understanding; 



