421 



Cape of Good Hope, the Island of St. Helena, some point on the 

 Eastern Coast of South America, and Quebec. In order, he ob- 

 serves, to advance rapidly the theory of the phenomena of ter- 

 restrial magnetism, or at least to establish with more precision em- 

 pirical laws, we ought to e;;tend and, at the same time, to vary the 

 lines of corresponding observations ; to distinguish, in the obser- 

 vations of the horary variations, what is due to the influence of the 

 seasons, to a clear or a cloudy atmosphere, 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 different 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 hemisphere. 

 All these data, the indispensable basis of a future theory, can ac- 

 quire 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, 

 observations 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 pre- 

 parations 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 isodynamic lines 3 and the influence which, 

 unquestionably, the configuration and articulation 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 cooperation 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 accord- 

 ing to the progressive state of physical knowledge and the improve- 

 ments 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 in- 

 tention of examining which are the magnetic stations that at the pre- 

 sent time deserve the preference, and which local circumstances may 

 admit of being established. It is sufficient that he has solicited the 

 cooperation 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 

 Society will enter into direct communication with the Royal So- 

 ciety of Gottingen, the Royal Institute of France, and the Imperial 

 Academy of Russia, to adopt the most proper measures to com- 

 bine what is proposed to be established with what already exists ; 



