AUTHOR S PREFACE XI 



cial branches of the several departments of science, whose 

 mutual connection is indicated in the beginning of the work. 

 Wherever it has been possible to do so, I have adduced the au- 

 thorities from whence I derived my facts, with a view of afford- 

 ing testimony both to the accuracy of my statements and to the 

 value of the observations to which reference was made. In 

 those instances where I have quoted from my own writings 

 (the facts contained in which being, from their very nature, scat- 

 tered through different portions of my works), I have always 

 referred to the original editions, owing to the importance of 

 accuracy with regard to numerical relations, and to my own 

 distrust of the care and correctness of translators. In the few 

 cases where I have extracted short passages from the works 

 of my friends, I have indicated them by marks of quotation ; 

 and, in imitation of the practice of the ancients, I have inva- 

 riably preferred the repetition of the same w(^ds to any arbi- 

 traiy substitution of my own paraphrases. The much-con- 

 tested question of priority of claim to a first discovery, which 

 it is so dangerous to treat of in a work of this uncontroversial 

 kind, has rarely been touched upon. Where I have occasion- 

 ally referred to classical antiquity, and to that happy period 

 of transition which has rendered the sixteenth and seventeenth 

 centuries so celebrated, owing to the great geographical dis- 

 coveries by which the age was characterized, I have been sim- 

 ply led to adopt this mode of treatment, from the desire we 

 experience from time to time, when considering the general 

 views of nature, to escape from the circle of more strictly dog- 

 matical modern opinions, and enter the free and fanciful do 

 main of earlier presentiments. 



It has frequently been regarded as a subject of discouraging 

 consideration, that while purely literary products of intellect 

 ual activity are rooted in the depths of feeUng, and interwoven 

 with the creative force of imagination, all works treating of 

 empirical knowledge, and of the connection of natural phe- 

 nomena and physical laws, are subject to the most marked 

 modifications of form in the lapse of short periods of time, both 



