228 



Report on Terrestrial Magnetism. 



[July 



sphere, to abundant rains, to the hour of the day or night, solar time; 

 that is, to the influence of the sun, and what is isochronous under dif- 

 ferent meridians : we ought, in addition to these observations of the 

 horary variation, to observe the annual course of the absolute variation 

 of the inclination of the needle, and of the intensity of the magnetic 

 forces, of which the increase from the magnetic equator to the poles is 

 unequal in the American or Western, and in the Asiatic or Eastern hemi- 

 spheres. All these data, the indispensable basis of a future theory, can 

 acquire certainty and importance only by means of fixed establishments, 

 which are permanent for a great number of years, observatories in which 

 are repeated at settled intervals and with similar instruments, observa- 

 tions 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 preparati- 

 ons 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 isodynamie lines ; and the influence which, unquestion- 

 ably, the configuration and articulation of the continents exert upon the 

 slow or rapid march of these curves. He will, he considers, be fortu- 

 nate 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 instructi- 

 ons, but varying them according to the progressive state of physical 

 knowledge and the improvements which may have been made in 

 instruments and the methods 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 magnt-tic 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 un- 

 dertaking in which he has for many years been engaged. Should the 

 proposition meet with their concurrence, he begs that the Royal Socie- 

 ty 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, perhaps, 

 they would also previously concert upon the mode of publication of 

 partial observations and of mean results. 



