" Regula Tertia Philosophandi." 33 



governed by mathematical formulaB. This necessary prelimi- 

 nary department of physical philosophy, which has been 

 handled with admirable skill and exactness in those treatises, 

 evidently differs in its character from Newton's mathematical 

 Principles of Natural Philosophy. French writers, with their 

 usual attention to logical accuracy, name that department 

 La Physique (Physics), and carefully distinguish between 

 ascertaining laws by such means, and referring them to a 

 priori principles. The deduction of laws from experiment and 

 observation, and expressing them by mathematical formulae, 

 is often called theory ; and such it is in a subordinate sense. 

 But theory in its most exact and complete sense consists in 

 giving reasons for experimental laws by means of mathema- 

 tical reasoning founded on intelligible primordial principles. 

 This is the department I have taken up, in accordance with 

 Newton's example and anticipations, being of opinion that 

 his Principia has established the presumption that laws, just 

 because they are laws, are proper subjects of human research 

 and demonstration. 



My physical investigations may be classed under two heads : 

 — those relating to the principles and processes of Hydrody- 

 namics, which I commenced in the Philosophical Magazine as 

 far back as the year 1829, and have carried on, not always 

 successfully, up to the present time ; and those which treat 

 theoretically of the laws of the physical forces on the basis of 

 the Newtonian hypotheses, beginning with a Mathematical 

 Theory of Heat contained in the Philosophical Magazine for 

 March 1859. Among this class of researches I beg to call 

 attention more particularly to the article on Newton's ^Foun- 

 dation of all Philosophy/' in the Philosophical Magazine for 

 October 1863, p. 280. As I intend the present communica- 

 tion to be a kind of resume of the portions of my productions 

 which I consider to be most conducive to the progress of theo- 

 retical physics, and as probably I may be unable to make any 

 further efforts of the same kind, I take this opportunity for 

 expressing the opinion, which I have long entertained, that 

 injustice has been done to Newton's scientific fame by the 

 persistent neglect, and even opposition, with which modern 

 physicists have treated the parts of his Book III. which have 

 reference to the future of theoretical philosophy, and may 

 be considered to justify his naming that Book Mundi Sy sterna, 

 those parts being, according to my judgment, among the most 

 remarkable proofs of the greatness of his genius. 

 Cambridge, November 17, 1879. 



Postscript, November 26, 1879. — The theoretical rate of 

 Phil Mag. S. 5. Vol. 9. No. 53. Jan. 1880. D 



