62 GAUSS AND WEBER OX TERRESTRIAL MAGNETISM. 



between the magnetic declinations of 8 a.m. and 1 p.m. does not 

 contain (apart from the irregular anomalies and the secular de- 

 crease) any veiy important fluctuations dependent on season ; at 

 least there is no certain indication of any such difference between 

 the summer and winter months. 



The mean declination itself, deduced from the observations of 

 the three years, is 18° 37' 56"'9 for the 1st of October, 1835 ; 

 meaning, of course, thereby, only the mean value of the hours 

 chosen for our observations, from which the mean value of all 

 hours of the day may perhaps differ a little, though probably 

 but httle. But all our pre\dous researches abundantly show that 

 without very long and wearisome labour nothing can be fixed 

 with certainty respecting this. 



Hitherto we have spoken only of monthly mean numbers. 

 The complete publication of the separate obsenations would for 

 the present be regarded as superfluous, since, being confined to 

 one place, they present no interest but that arising from the ir- 

 regular fluctuations which they display. This end may however 

 be attained in a better manner than by the mere view of the num- 

 bers, by a methodical combination, in which the amount of the 

 fluctuations is reduced to a definite measure, and the general 

 character of the periods, in respect to the magnitude of the fluctu- 

 ations during them, may be accurately compared. For the sake 

 of precision, I understand here by fluctuation of the magnetic 

 declination, the difference from that of the preceding day at the 

 same hour ; and, (according to analogy with what are called mean 

 eiTors of obsen'ation,) I understand by mean fluctuation, during 

 any given interval of time, the square root of the mean of the 

 squares of the several fluctuations. It must here be remarked, 

 that when several equal inten-als, or inteiwals considered as 

 equal, are united in one, the arithmetical mean of the partial 

 mean fluctuations must not be taken as the general mean, but 

 we must revert to the squares, and take the square root of their 

 arithmetical mean. The results of the three years' observations, 

 calculated in this manner, and expressed in seconds, are con- 

 tained in the following table : 



