PUBLISHER'S ANNOUNCEMENT 



When Professor H. U. Williams was requested to undertake 

 the sixth revision of this book he expressed a wish to be relieved 

 of it and of all further obligations in respect thereto because oi 

 the demands that were being made upon his time by largei 

 duties in connection with the University. 



As this manual is a popular one we did not feel warranted in 

 letting it go out of print as suggested by Dr. Williams, therefore 

 we arranged with Professor Ward J. MacNeal to continue it with 

 the approval of Professor Williams. The wisdom of this choice 

 should be evident in the following pages. 



100 



