PREFACE. 



Teachers in science are nearly equally divided into 

 two classes ; — those who know too much, and those who 

 know too Uttle. Those of the first class, overloaded 

 with science, cannot admit the possibihty of meeting 

 with readers who have none ; and, therefore, their 

 essays and introductions are so worded that it requires 

 a tolerable proficiency to understand them. The teach- 

 ers of the second class fall into the opposite eiTor; 

 they curtail, garble, and popularize the writings of 

 others without understanding them, forgetful that it 

 requires a consummate knowledge of any science to 

 abridge a work which treats of it ably and at large. 

 The author submits, with much humility, that both 

 classes are in error : he submits also that introductory 

 works should be wiitten for those who know nothing 

 of the subject on wliich they read, and hy those who 

 possess, in themselves, some practical knowledge of 

 the subject on which they write. 



