xii PREFACE. 
them, I may hope to have added something to human knowledge 
which the professional scientist may utilize. 
I must here acknowledge my indebtedness to many friends 
for encouragement and assistance in the preparation of this 
work, some of whose names will be found in the text. 
The work has been carefully revised for this second edition, 
and important alterations and additions have been made, which it 
is hoped will add to its value. 
Orrawa, ILL., January, 1881. 
