PREFACE. vii 



I cannot say how much I am indebted to the con- 

 tinued help of those friends who assisted me in former 

 editions ; and I have also to acknowledge with gratitude 

 the aid afforded me by Prof. C. S. Boy, to whose kindness 

 I owe several of the new illustrations. 



The appendix on chemical matters, as in former 

 editions, has been under the care of Mr Sheridan Lea; 

 in this, which stands on a somewhat different footing 

 from the rest of the work, references and names of authors 

 have been retained. 



TRINITY COLLEGE, CAMBRIDGE, 

 February, 1883. 



