180 TERRESTRIAL MAGNETISM. 



days tbe ])hotograpliic traces by which tlie history of a niajjnetic storm 

 is mapped. Is it possible for Greenwich and Paris also to agree in 

 their choice of calm days for the calculation of the diurnal variation 

 so that a precise similarity of method may obtain not only between 

 the English observatories but between England and France? 



The importance of cooperation between institutions engaged on the 

 same tasks having been illustrated, I am glad to be able to announce 

 that another step is about to be taken in the same direction. For some 

 years, in spite, I believe, of great linancial difficulties, the Cornwall 

 Royal Polytechnic Society has maintained a magnetic observatory at 

 Falmouth. The results of the observations have hitherto been printed 

 in the journal of the society only, but the Royal Society has now con- 

 sented to iHiblish them in the Proceedings. Before long, therefore, the 

 Kew and Falmouth records, which are already worked up in the same 

 way, will be given to the world side by side. Is it too much to hope 

 that this may be the first step toward the production of a British 

 Magnetic Year Book, in Avhich observations whose chief interest lies in 

 their comparison may be so published as to be easily compared? 



We owe to private enterprise another advance of the same kind. 

 The managers of the new journal Science Progress have made arrange- 

 ments with the Kew committee for the yearly publication of a table 

 showing the mean annual values of the magnetic elements as deter- 

 mined at the various magnetic observatories of the world. It will 

 therefore in future be possible to get a general idea of the rate of 

 secular change in different localities without searching through a 

 number of reports in ditferent languages, which can only be consulted 

 in the rooms of the few societies or institutions to which they are 

 annually sent. The present state of our knowledge of the secular 

 change in the magnetic elements afibrds, indeed, very strong support 

 to the arguments I have already adduced in favor of a comparison 

 between the instruments of our magnetic observatories. 



The whole question of the cause of this phenomenon has entered on 

 a new stage. It has long been recognized that the earth is not a simple 

 magnet, but that there are in each hemisphere one pole or point at which 

 the dip needle is vertical and two foci of maximum intensity. A com- 

 parison of earlier with later magnetic observations led to the conclusion 

 that one or both of the foci in each hemisphere is in motion, and that to 

 this motion, however caused, the secular change in the values of the 

 magnetic element is due. Thus the late Prof. Balfour Stewart, writing 

 in 1883, says: "While there is no well-established evidence to show that 

 either the pole of verticity or the center of force to the north of Amer- 

 ica has perceptibly changed its place, there is on the other hand very 

 strong evidence to show that we have a change of iilace on the part of 

 the Siberian focus." ^ The facts in favor of this conclusion are there dis- 

 cussed. The arguments are based, not on the results of any actual 



' Encyclopajtlia Brit., 9tli ed., art. "Meteorology — Terrestrial magnetism." 



