12 PHEFACE 



I am under the deepest obligations to nay wife, whose help 

 and indefatigable enthusiasm have made the completion of the 

 manuscript possible. 



The illustrations were taken largelj^ from books and mono- 

 graphs, to the authors of which I offer my gratitude. Due ac- 

 knowledgment of the source is made in each instance. Illustra- 

 tions of apparatus were kindly furnished by manufacturers or 

 dealers in such articles. Finally I wish to thank the publishers 

 for their interest and courtesy in all dealings. 



P. G. Heineman. 

 WooDWORTH, Wisconsin, 

 July, 1919. 



