PREFATORY NOTE 



I AM indebted to the Koyal Society of Edinburgh for per- 

 mission to reproduce the Frontispiece and Plate I., which 

 have already appeared in the Proceedings of the Society 

 as Illustrations to papers of my own, 



I desire, also, to thank Sir Herbert Maxwell for valued 

 criticism with regard to certain views I have ventured to 

 express in this little book, and Mr. H. W. Johnston for 

 many kindnesses in facilitating the experimental work in 

 which we have been much associated. 



W. L. C. 



Cabr Bridge, 



Avg'ust 15, 1907. 



