Force of the Earth's Magnetism. 15 



during the interval, but that some had undergone slight changes 

 in that respect. A mean between the first and last times of 

 vibration was in such cases taken as the rate in London, cor- 

 responding to that of the several foreign stations ; (the purpose 

 being to compare the force at each of the stations to the force 

 in London). A more strict comparison would obviously have 

 required an interpolation of the times of vibration corresponding 

 to each station in particular ; by computing the proportional 

 part of the whole gain or loss due to the date at, the several 

 stations. And more especially it would have required that 

 reductions should have been applied for the arcs and for the 

 differences of temperature. 



Since the first publication of those results, the subject of the 

 magnetic intensity has increased in public interest, greatly 

 owing to the writings and observations of M. Hansteen ; and 

 an importance has, consequently, attached to my observations 

 greater than I ventured to anticipate at the time. 



Shortly after their publication, M. de Humboldt, whose 

 writings first excited my interest in the subject, kindly wrote 

 me his opinion, that the minute corrections alluded to would 

 not be, as I was disposed to consider, a refinement beyond 

 the occasion : and M. Hansteen has since expressed the same 

 opinion, in a review of the magnetic portion of my volume of 

 experiments printed in PoggendorfF's Annalen der Physik. I 

 have, therefore, for some time past, viewed the correction of 

 the results as a duty to be performed whenever leisure and 

 convenience should enable me to execute it ; and I had 

 commenced the preliminary experiments for determining the 

 amount of the correction for temperature for each needle, in 

 the summer of last year. A short residence during the present 

 summer, in the neighbourhood of the garden of the Horticul- 

 tural Society at Chiswick, a spot exceedingly well calculated 

 for such observations, has enabled me to complete them ; and 

 I cannot take a better opportunity of making the corrected 

 results public, than at the moment when M. Hansteen's journey 

 to Siberia is adding so greatly to our knowledge of the facts in 

 regard to the Magnetic Intensity, and, consequently, is drawing 

 the public attention to the subject. 



1 confine myself to the results obtained with the needles 



