PREFACE TO THE THIRD EDITION 



I HAVE carefully revised all parts of the work, and, in addition 

 to introducing a number of minor alterations and some new 

 illustrations, have re-written certain portions, notably in Sections 

 VI., VIII. and X. Professor W. N. Parker has again rendered 

 invaluable help in the revision of the proofs and has made a number 

 of useful suggestions. 



W. A. HASWELL. 

 SYDNEY, 

 March 2M, 1921. 



