xxviii PREFACE TO PART II. 



and other Assistants at the Observatory of Cambridge ; these tables have 

 also been calculated by Mr Koberts and by Mr T. Wright and other 

 Assistants at the Nautical Almanack Office and have been compared and 

 tested most carefully in a variety of ways for their accuracy. 



A short account of the method and the results of the investigation 

 was given by Professor Adams in the Mathematical Section of the British 

 Association at the Manchester Meeting in 1887, but unfortunately no record 

 was made of this communication and no account of any part of the work 

 Avas at any time written or published. Under these circumstances it has 

 been no easy task to piece together into this connected whole the detached 

 portions of the work which were delivered into my hands, without any 

 explanation as to any part of them or as to their connection with one 

 another, during my brother's last illness, when he was no longer able to 

 give me any hints as to his theory or mode of treatment. 



To the unravelling and the publication of this work, which was begun 

 in 1849, and no part of which was published until 1887, I have devoted 

 whatever leisure I could command during the past eight years, as a tribute 

 to the memory of one whose accuracy of work has probably never been 

 surpassed. 



I desire to acknowledge the very great assistance which I have received 

 from Dr C. Chree, the Superintendent of the Kew Observatory, whom I 

 have consulted on various parts of the work. 



My best thanks are due to the Lords of the Admiralty and to 

 Captain Creak for the permission to use and to reproduce the Charts of 

 the magnetic elements for 1880; from which the observations for that 

 period are taken. There is found to be a remarkable agreement on com- 

 paring the theoretical results for the mean vertical and horizontal forces 

 over the polar areas at the north and south poles with the values 

 deduced by Captain Creak from the observations and given in the polar 

 Charts at the end of this volume. 



W. GRYLLS ADAMS. 



PHYSICAL LABORATORY, 



KING'S COLLEGE, LONDON. 

 February 16t/i, 1900. 



