PREFACE vil 



and his sense of propriety come into conflict, by 

 no means to the advantage of the former. 



I had little notion of the real magnitude of 

 these difficulties when I set about my task ; but I 

 am consoled for my pains and anxiety by observing 

 that none of the multitudinous criticisms with 

 which I have been favoured and, often, instructed, 

 find fault with me on the score of having strayed 

 out of bounds. 



Among my critics there are not a few to whom 

 I feel deeply indebted for the careful attention 

 which they have given to the exposition thus 

 hampered ; and further weakened, I am afraid, by 

 my forgetfulness of a maxim touching lectures of 

 a popular character, which has descended to me 

 from that prince of lecturers, Mr. Faraday. He 

 was once asked by a beginner, called upon to 

 address a highly select and cultivated audience, 

 what he might suppose his hearers to know 

 already. Whereupon the past master of the art of 

 exposition emphatically replied " Nothing ! " 



To my shame as a retired veteran, who has all 

 his life profited by this great precept of lec- 

 turing strategy, I forgot all about it just when 

 it would have been most useful. I was fatuous 

 enough] to imagine that a number of propositions, 

 which I. thought established, and which, in fact, I 

 had advanced without challenge on former oc- 

 casions, needed no repetition. 



I have endeavoured to repair my error by 



