X" PREFACE. 



indifference to fame should be counteracted by a full 

 history of his scientific labours, comparing the state of 

 the science as he found it with that in which he left 

 it. My own personal acquaintance with some of the 

 great men whose history I have ventured to write, 

 enabled me to throw additional light upon it ; and re- 

 specting one, whom of course I could not have known, 

 Mr. Hume, I have obtained information from good 

 sources through the kindness of friends. The mate- 

 rials of his life are, however, chiefly to be sought in 

 his writings, and especially in his letters. The same 

 remark is applicable to the life of Voltaire. Those 

 who have written it, like the Marquis de Condorcet, 

 without ever referring to the fourteen large volumes 

 (containing nine thousand closely-printed pages) of his 

 Correspondence, might just as well have undertaken to 

 give a life of Rousseau without consulting his ' Con- 

 fessions,' or of Hume without reading his ' Autobio- 

 graphy.' I have, besides, had access to valuable ori- 

 ginal documents both of Voltaire, Robertson, and 

 Cavendish ; to some respecting Watt and Simson. 



The course of this work has kept me, for the most 

 part, at a distance from questions touching political 

 affairs, or the constitution and progress of society, but 

 not always. The reader will find that no opportunity 

 has been left unimproved, as far as I was capable of 

 seizing it with any effect, for inculcating or illustrat- 

 ing the great doctrines of peace, freedom, and religious 

 liberty. The observations on historical composition in 



