so Baron Humboldt on advancing the Knoisoledge 



the foiuner having the command of the sloop Uranie from 

 1S17 to 1820, and the latter, who has six times crossed the 

 magnetic equator, commanding the sloop Cocjuille from 1822 

 to 1825. To promote the rapid advancement of the theory 

 of terrestrial magnetism, or at least to establish with more 

 precision empirical laws, it is necessary at the same time to 

 prolong and to vary the lines of con-espondhig observations', 

 also to distinguish in observations of horary variations, what 

 arises from the influence of the seasons, of serene and cloudy 

 weather and of abundant rains, of the hours of day and night, 

 and of the true time at each place, that is, from the influence 

 of the sun, and of all isochronous influences at the different 

 meridians. To these observations of horary declination must 

 be united those of the annual movement of the absolute decliyia- 

 tion, of the inclination of the needle^ and of the intensity of 

 the magnetic forces^ the increase of which from the magnetic 

 equator to the poles is unequal in the Western American and 

 the Eastern Asiatic hemispheres. All these data, indispensa- 

 ble bases for the future theory, can only acquire certainty and 

 importance by the means of establishments which shall remain 

 permanent for a great number of years, of Physical Obser- 

 vatories in which the investigation of numerical elements may 

 be repeated at settled intervals of time, and with similar in- 

 struments. Travellers who cross a country in but one direc- 

 tion and at one epoch merely prepare the way for an under- 

 taking which should embrace the complete delineation of the 

 lines without declination at intervals equally distant ; the pro- 

 gressive removal of the points of intex-section of the terrestrial 

 and magnetic equators; the changes of form in the isogonal 

 and isodynamic lines ; and the influence upon the slow or ac- 

 celerated movement of the curves, which indubitably arises 

 from the configuration and articulation of the continents. It 

 must be considered fortunate if the isolated labours of tra- 

 vellers, whose cause it is my office to plead, have contributed 

 to give animation to a species of investigation which is the 

 work of centuries, and which requiries the concurrence of 

 numerous observers, distributed according to a plan arranged 

 after mature consideration, under the direction of several of 

 the great scientific centres of Europe. The directors should 

 not always confine themselves to tlie narrow limits of the 

 same instructions, but they should vary them freely in adapta- 

 tion to the progressive state of physical science, and the im- 

 provement of instruments and methods of observation. 



When soliciting Your Royal Highness to condescend to 

 communicate this letter to the illustrious Society over which 

 you preside, it is not in any degree my office to inquire, vi^hich 



