VI PREFACE. 



instructive developments was not communicated to the Royal Irish Academy 

 until 1890. The twelfth and latest memoir of the series containing the 

 solution of an important problem which had been under consideration 

 for twenty-five years did not appear until 1898. 



It therefore seemed that the time had now come when an attempt 

 should be made to set forth the Theory of Screws as it stands at 

 present. The present work is the result. I have endeavoured to include 

 in these pages every essential part of the Theory as contained in the 

 twelve memoirs and many other papers. But the whole subject has 

 been revised and rearranged and indeed largely rewritten, many of the 

 earlier parts have been recast with improvements derived from later 

 researches, and I should also add that I have found it necessary to 

 introduce much that has not been previously published. 



The pleasant duty remains of expressing my thanks for the help that 

 I have received from friends in preparing this book. I have received 

 most useful aid from Prof. W. Burnside, Mr A. Y. G. Campbell, 

 Mr G. Chawner. Mr A. W. Panton, Mr H. W. Richmond, Mr R. Russell, 

 and Dr G. Johnstone Stoney. In the labour of revising the press I have 

 been aided by Mr A. Berry, Mr A. N. Whitehead, and lastly by Professor 

 C. J. Joly, who it will be seen has contributed several valuable notes. 



Finally, I must express my hearty thanks to the Cambridge Univer 

 sity Press for the liberality with which they undertook the publication 

 of this book and for the willing consent with which they have met 

 all my wishes. 



ROBERT S. BALL. 



OBSERVATORY, 

 CAMBRIDGE, 17 May, 1900. 



