444 Mr. R. EL M. Bosanquet on the Horizontal 



In considering the mean value of H we must remember 

 that this quantity itself is in a perpetual state of change. If 

 we go to the Greenwich observations, we see that the mean 

 value for the day varies generally from day to day with a 

 certain regularity. The diurnal changes are considerable, 

 and are superposed on the others ; and the whole mean value 

 is increasing regularly at the rate of about '00027 per year. 

 So that all we can say is that, at the beginning of March 

 1884 a mean value of H was found at Oxford which was 

 about -18011. 



The only other information that I have been able to obtain 

 about the values of the magnetic constants at Oxford is 

 derived from the differences of Gauss's tables applied to the 

 known Greenwich values. 



In Gauss's 'Atlas des Erdmagnetismus ' there is a table of 

 the magnetic elements for points differing 10° in longitude 

 and 5° in latitude. Forming the differences appropriate to 

 Oxford, taking its position to be 1 J° west and J° north of 

 Greenwich, and finding the Greenwich quantities from the 

 data in Everett's book of Units, I get, 



Mean Greenwich value of H for") 1010/t 

 1884 j ' 1818e 



Correction deduced from Gauss's *) 

 differences for Oxford . . . j 



•00227 



Value for Oxford thus obtained. '17959 



Eesult of present determination. '18010 



It appears therefore that the observed result is somewhat 

 nearer the Greenwich value than the indications of Gauss's 

 differences would lead us to suppose, We shall see that this 

 is also the case with the dip and vertical force. 



Although the effect of terrestrial induction on the magnets 

 during vibration is eliminated by the process I employ, it is 

 yet of interest to determine its amount ; if for no other reason, 

 because in the deflection-experiments the two magnets are 

 not so far removed from one another but that they are within 

 each other's influence. In this case the approximate field 

 about the needle east and west, due to one of the magnets, is 



ar ^— . And, since the field produced varies inversely as r 3 , 



and the other magnet is at twice the distance of the needle, 

 the field about one of the magnets due to the other is 



TT | ori 



— — — . As the induction due to H itself is small, this is 

 16 



