PREFACE. ix 



Mr. Rudler has entirely fulfilled my expectations, and has 

 made many valuable suggestions and additions. I have 

 entirely re-written such parts of the work as I. thought I 

 could improve ; I have added to others ; and I have 

 carefully revised the proofs of every chapter. 



I trust therefore that the book maybe useful to both learners 

 and teachers ; but, I am most concerned, that the latter 

 should find in it the ground-work of an introduction to the 

 study of nature, on which their practical experience will 

 enable them to erect a- far better superstructure than that 

 which I have been able to raise. 



T. H. H. 



London, 



Nm'cmber, 1877. 



