PREFACE TO THE SECOND EDITION. 



WHILE revising for the press, the edition now presented to the public, I 

 gladly availed myself of the opportunity of correcting a few trivial errors, 

 both of the press and the pen. More than this I have scarcely ventured 

 to do, as teachers of great experience have informed me, that much 

 inconvenience is occasionally felt in the school-room, from differences 

 between successive editions of the same book. Guided by their advice, I 

 shall endeavour in future editions, if such should be called for, to keep 

 pace with the progress of science, by means of an appendix, or of foot 

 notes, rather than by changes in the text. I have only further to add, 

 that a few wood-cuts of a superior kind, have been introduced instead of 

 some of the illustrations which formerly appeared. 



R. P. 



Belfast, 3, College Square North, 

 3rd September, 1849. 



