146 On the Daily Motion of the Magnetic Needle. 



plainly indicated. As the series embraces too few years to en- 

 able us to pronounce with certainty upon the duration and point 

 of turning of the period, I will assume these to be the same as 

 were found in the case of the declination. The magnitude of 

 the motion will then be expressed by the fonnula 

 9-83 + 3-06 sin (72°-58 + 34°-847i), 



where n expresses the number of years reckoned from 1848. 



From a comparison of the formula with the results of obser- 

 vation given above, the following differences are obtained: — 



DiflTerence. 

 Year. Calculation — Observation. 



1843 -1-0 



1844 -01 



1845 -Hl-6 



1846 -1-4 



1847 -0-4 



1848 —1-6 



1849 +0-7 



1850 +0-9 



1851 -I-0-8 



The differences here are greater than in the case of the decli- 

 nation ; but I must remark in connexion with this, that I have 

 not excluded the days of disturbance. These days exercise, how- 

 e%'er, an important influence, inasmuch as the causes of distm'b- 

 ance always operate in the same sense, and hence do not annul 

 each other when the mean values are taken. 



For the further establishment of the period of intensity, we 

 shall look in vain to the observations of earlier times, some of 

 which, in the case of the declination, we have found applicable, 

 and nothing remains but to await the results of future observa- 

 tions. 



For the present, the simple fact that the magnitude of the 

 magnetic motions is subjected to a regular and very consider- 

 able increase and decrease, appears to me to imply consequences 

 worthy of consideration ; for if, in the effect, a period be shown 

 to exist, it must be the result of a corresponding period in the 

 influencing cause. It is, however, quite certain, that in the tem- 

 perature of the atmosphere — to which at present it is customary 

 to refer the magnetic variations — no such period exists ; and for 

 this reason I hold it to be absolutely necessary, either to give up 

 totally the assumed influence of atmospheric temperature, or to 

 modify it essentially by the introduction of a second coordinate 

 cause. 



