G HISTORICAL \<>TK. 



original curves. The measurements were received by the Hydrographic Department of the Admiralty 

 who had undertaken the " term " day observations along with the absolute observations only a short 

 time before the printing of the volume of "Physical Observations" was completed. Under these 

 circumstances the Hydrographer and Captain CHKT\VV\I> agreed that the material had better be handed 

 over to me, to be discussed in the present volume. The discussion will be found in Appendix A. 



In view of the absence of reference in the text to the recently published ' The Norwegian Aurora Polaris 

 Kxpcdition, 1902-1903,' vol. 1, of Professor Kit. MniKr.i. VM>, I should explain that Professor BIRKELAXD'S 

 volume did not appear until the present work had been practically completed. The disturbed curves had 

 bri-n selected and copies had been made and sent to the engraver, and all the mathematical calculations of 

 Chapters IX and X had been carried out and the discussion written. The preparation of the present 

 volume had already taken much longer time than was originally anticipated, and to have deferred the 

 printing, pending an examination of Professor BlRKELAND'S large volume, did not appear advisable. 



The text has thus boon left unaltered, and any conclusions or theoretical views which it may contain 

 are absolutely independent of any similar or conflicting results which Professor BIRKELAND has reached. 



On studying Professor BIRKELAND'S volume, however, I found that almost all the disturbances which 

 he had selected for discussion were represented in the Antarctic. The opportunity for comparing Arctic 

 and Antarctic results appeared so unique that it was decided that a special Appendix, B, should be written 

 dealing with the subject. It is hoped that the interest attaching to the results will be deemed sufficient 

 justification for the three or four months' delay which the preparation of the Appendix has entailed in the 

 appearance of the present volume. 



My part of the work has had to bo carried out with due regard to the claims of official duties, which 

 at times leave very little unoccupied leisure. 



A great many difficulties had to be dealt with, some of them calling for very delicate discrimination. 

 Under these circumstances the ordinary tendency of humanity to err is pretty certain to have asserted 

 itself, but, at all events, no pains have been spared to aim at that measure of accuracy which it is given 

 to erring mortals to secure. 



Observatory Department of the 



National Physical Laboratory, 



Richmond, Surrey, 



July, 1909. 



