PREFACE. 



Teachers in science are nearly equally 

 divided into two classes ; those who know too 

 much, and those who know too little. Those 

 of the first class, overloaded with science, 

 cannot admit the possibility of meeting with 

 readers who have none ; and, therefore, their 

 essays and introductions are so worded that 

 it requires a tolerable proficiency to under- 

 stand them. The teachers of the second 

 class fall into the opposite error ; they curtail, 

 garble, and popularize the writings of others 

 Avithout understanding them, forgetful that it 

 requires a consummate knowledge of any 

 science to abridge a work which treats of it 



