PREFACE. 



15 



given in different language, style, and with different illustra- 

 tions. Those who have attended more 'than once the same 

 lectures delivered by me, will recognise the truth of this ob- 

 servation. 



But a written didactic discourse ought to differ materially 

 from an oral discussion of the same subject. A reader and a 

 hearer are placed under very different conditions. The one 

 can proceed with such deliberation as the readiness or slow- 

 ness of his capacity and the greater or less abstruseness of 

 the subject may require. He may retrace his steps as often 

 as he may find necessary, returning again and again on the 

 same sentences. The other must catch the spirit and sense 

 as fast as the words fall from the lips of the speaker. The 

 style of a written essay is like that of a cabinet picture, that of 

 an oral discourse like scene painting. The effect of the one is 

 produced by elaborate finish, that of the other by bold and 

 rough lines which seize the most inattentive and unskilled 

 eye. 



These distinctions, however true and important, are rarely 

 attended to by those on whom the duty of public instruction 

 devolves. Lectures accordingly, even when they proceed 

 from those who by acquirement are most competent to in- 

 struct, are often either nothing more than demonstrations of 

 scientific propositions and principles, or written discourses, 

 generally read from the manuscript, or, as much more rarely 

 happens, committed to memory, and delivered verbatim as 

 written. 



The qualifications of a good public lecturer for popular audi- 

 ences are seldom found combined in the same person, although 

 none of them can be regarded as very exalted intellectual 

 gifts. Such a teacher must above all things possess a knowl- 

 edge of his subject much more profound than that which he 



