PHILOSOPHICAL 
TRANSACTIONS 
OF THE 
ROYAL SOCIETY OF LONDON. 
Series A. 
CONTAINING PAPERS OF A MATHEMATICAL OR PHYSICAL CHARACTER. 
VOL. 221. 
Printed and Published for the ROYAL SOCIETY by 
HARRISON AND SONS, LTD., 44-47, ST. MARTIN'S LANE, LONDON. W.C. 2. 
printers in <®rbtnarg to pis $ftajestg. 
March, 1921. 
