GAUSS AND WEBER OX TERRESTRIAL, MAGNETISM. 59 



That the comparison of the forenoon means should give in 

 general a greater decrease than the comparison of those of the 

 afternoon, is only a consequence of what has been stated above, 

 viz. that the diurnal movements in the first year are smaller than 

 in the second, and those of the second smaller than in the third. 

 That difference must, therefore, not be considered as a real one, 

 but merely as accidental ; and we may expect, by a longer con- 

 tinuation of the observations, a difference in the opposite direc- 

 tion. As then there is no sufficient reason for preferring one 

 of the results to the other, we can only take the mean of the two. 

 The mean is, in the first year, 2' 36"'5 ; in the second, 4' 55"-9. 

 AVe might regard this as a proof of an increasing rate of dimi- 

 nution in the declination : but this would be nothing more than 

 a bad reason for a thing which in itself is true. It is known that 

 the declination, which increased through all Europe during the 

 last century, attained its maximum in the present century, and 

 is now decreasing afi'esh. This transition must necessarily pro- 

 duce at first an imperceptible, and gradually a greater decrease. 

 But although, from want of earlier observations, the precise year 

 cannot be fixed in which the transition took place at Gottingen, 

 yet it must be inferred firom the observations made at other places, 

 that it must have been at an earlier period than would follow 

 from those two numbers, if we were to consider them as pure 

 effects of the slow movement which we term secular. All other 

 experience shows that 2' 19"*4 is too great to be looked upon as 

 a regular'increase for a year. .We, therefore, regard this differ- 

 ence as being for the most part accidental ; so that, for the pre- 

 sent, and until we have further experience to guide us, we must 

 consider the mean value, 3' 46"'2, as the annual decrease of the 

 decUnation from 1834 to 1837. 



As the difference between the decUnation in the forenoon.and 

 afternoon is subject to an inequality, evidently varying with the 

 season of the year, the question arises whether the change de- 

 pending on the period of the year affects one of these declina- 

 tions only, or one more than the other, or both equally, and 

 what are the existing laws in this respect. A longer series of 

 years, it is true, will be required to find out this law, than is 

 needed to determine the mere difference between the declinations ; 

 nevertheless, it will be desirable to see Avhat light the present 

 observations throw on the subject. 



