Magnitude of the Diurnal Motion of the Magnetic Needle. 223 



P.M. The first falls somewhat before the lowest stand (the great- 

 est mmimum)j the other a couple of hours before the evening 

 maximum; the difference of both therefore does not give the 

 entire magnitude of the diurnal change in the horizontal intensity. 

 The deviations, expressed in decimal parts of the absolute 

 horizontal intensity, are as follows : — 



Alteration of the Horizontal Intensity from 8 o'clock a.m. to 

 8 o'clock P.M. Gottingen time. Difference ■=8'^v.M. — S'^ a.m. 



A minimum of the daily alteration of the horizontal intensity 

 from 8^ A.M. to 8^ p.m. is here plainly shown to fall between 

 1843 and 1844, and a maximum between 1848 and 1849. Hence 

 the same decennial period occurs in the alteration of the hori- 

 zontal intensity as has been proved to exist in the diurnal change 

 of declination. 



In respect to the course of these alterations during the single 

 months of the year, they appear quite to follow the course of the 

 temperature. ObsciTations made during several years will eli- 

 minate the little anomalies which now exist in the warm months, 

 and the law to which all is subject will be thus rendered distinctly 

 visible. 



