EDITOR'S INTRODUCTION 



6 International Scientific Series ' and 1 Nature 

 Series 9 are no doubt useful and excellent, and 

 some of the volumes contained in them would 

 well carry out the ideas of the Publishers, but, 

 as a rule, they are somewhat more technical 

 and go into minuter details. 



The names of the Authors are a sufficient 

 guarantee that the subjects will be treated in 

 an interesting and thoroughly scientific manner. 



High Elms, Farnbokough: 

 November, 1891. 



