176 TERRESTRIAL MAGNETISM. 



Couuected also with the vast increase of scientific literatnre is a 

 growing necessity for the publication of volumes of abstracts, in which 

 the main results of recent investigations are presented in a concen- 

 trated form, English chemists have long been supplied with these by 

 the Chemical Society. The Physical Society, thougii far less wealthy 

 than its elder sister, has determined to undertake a similar task. We 

 are compelled to begin cautiously, but in January next the first number 

 of a monthly pamphlet will be issued containing abstracts of all the 

 papers which appear in the principal foreign journals of physics. In 

 this venture the society will incur grave responsibilities, and I avail 

 myself of this opportunity to appeal to all British physicists to sup- 

 port us in a work the scope of which will be rapidly extended if our 

 first efforts succeed. 



From this brief glance at what has been or is about to be done to 

 promote the study of physics I must now turn to the discussion of nar- 

 rower but more definite problems, and I j)resume that I shall be most 

 likely to deserve your attention if I select a subject in which I am 

 myself especially interested. 



During the last ten years my friend Dr. Thorp and I have been 

 eu^gaged upon a minute magnetic survey of the United Kingdom. The 

 main conclusions at which we have arrived are about to be published, 

 and I do not proi)ose to recount them now. It is, however, impossible 

 to give so long a time to a single research without having one's atten- 

 tion drawn to a number of points which require further investigation, 

 and I shall perhaps be making the best use of this opportunity if I 

 bring to your notice some matters in the practical and theoretical study 

 of terrestrial magnetism which deserve a fuller consideration than has 

 yet been given to them. 



In the first place, then, there is little doubt that the instruments at 

 I)resent used for measuring declination and horizontal force are affected 

 with errors far greater than the error of observation. 



We employed four magnetometers by Elliott Brothers, which were 

 frequently compared with the standard instrument at Kew. These 

 measurements proved that the instrumental differences which affect the 

 accuracy of the declination and horizontal force measurements are from 

 five to ten times as great as the error of a single field observation. The 

 dip circle which two generations ago was so untrustworthy, is, in our 

 experience, the most satisfactory of the absolute instruments. 



In most cases these comparisons extended over several days, but the 

 astronomer royal has described in his recent report observations made 

 at Greenwich for two years and a half with two horizontal force instru- 

 ments. These differ between themselves, and the discrepancy is of the 

 same order of magnitude as those we have detected. 



If such diflerences exist between instruments of the Kew pattern, it 

 is probable that they will be still greater when the magnetometers 

 under investi^-atioij ar§ of different types. 



