This bool: is copyrighted. Xo 2^ortion. of it may be 

 reproduced by any process without written pertiiission. 

 Enquiries should be addressed to the p^iblishers. 



Published 1947 



Second Edition 194S 



Third Edition 1951 



f'HINTED IS (.RKAT HUriATV RY UNIVK.RSITY TUTORIAL PRESS 

 SKAR CAMRRIDOE 



