Report on Terrestrial Magnetism. 321 



repeated at settled intervals and with similar instruments, obser- 

 vations for the determination of numerical elements. 



Travellers, remarks M. de Humboldt, who traverse a country 

 in a single direction and at a single epoch, furnish only the first 

 preparations for labours which ought to embrace the complete 

 course of the lines of no variation ; the progressive displacement 

 of the nodes of the magnetic and terrestrial equators ; the 

 changes in the forms of the isogonal and isodynam-ic lines ; and 

 the influence which, unquestionably, the configuration and arti- 

 culation of the continents exert upon the slow or rapid march 

 of these curves. He will, he considers, be fortunate if the 

 isolated attempts of travellers, whose cause he has to plead, have 

 contributed to vivify a species of research which must be the 

 work of centuries, and which requires at once the co-operation 

 of many observers, distributed in accordance with a well-digested 

 plan, and a direction emanating from many great scientific 

 centres of Europe ; this direction, however, not being for ever 

 restricted by the same instructions, but varying them according 

 to the progressive state of physical knowledge and the improve- 

 ments which may have been made in instruments and the me- 

 thods of observation. 



In begging His Royal Highness the President to communicate 

 this letter to the Royal Society, the Baron de Humboldt 

 disclaims any intention of examining which are the magnetic 

 stations that at the present time deserve the preference, and 

 which local circumstances may admit of being established. It 

 is sufficient that he has solicited the co-operation of the Royal 

 Society to give new life to a useful undertaking in which he has 

 for many years been engaged. Should the proposition meet with 

 their concurrence, he begs that the Royal Society will enter into 

 direct communication with the Royal Society of Gottingen, the 

 Royal Institute of France, and the Imperial Academy of Russia, 

 to adopt the most proper measures to combine what is proposed 

 to be established with what already exists ; and adds, that, per- 

 haps, they would also previously concert upon the mode of 

 publication of partial observations and of mean results. 



M. de Humboldt finally refers to the labours and accurate 

 observations of M. Gauss at the Observatory of Gottingen. 

 The method?, however, adopted by M. Gauss being already 



