Mr. J. A. Broun on Teirestrial Magnetism. 91 



means for a week : thus all the oscillations of the declination- 

 needle in the week March 20 to 25, 1854, were comprised 

 within limits of 2''5 ; and all the oscillations during the three 

 days, March 23, 24, 25, within the limits of 1''3. The ten- 

 dency to an extinction of the diurnal movement is therefore cer- 

 tain, since these limits include all the oscillations due to irregular 

 causes : the effect of disturbance is shown when we take the 

 mean hourly positions for a few days, as in the week March 20 

 to 25, for which the range of the mean variation was only 0''49. 



It is probable that the line of niinimum movement for the sun 

 near the equator is the magnetic line of no-dip, where the free 

 needle rests nearly parallel to the axis of the earth ; and, as 

 already suggested, that the points of minimum movement on 

 this line are those where the terrestrial and magnetic latitude 

 are zero. It seems probable, from other considerations, that 

 the minimum movement does not occur simultaneously over the 

 M'hole line of no-dip, but that the time of minimum varies some- 

 what with the latitude. 



Before I leave this subject I wish to draw your attention to a 

 curious, and I think important fact, deducible from the ranges 

 I have given. You will observe that the range in the month of 

 March increases from 1854 till 1857; so that if March 1858 

 also shows a slight increase, the range for that month will be 

 nearly doubled within four years. January also shows some 

 increase in 1857 : not so for the months of October and August ; 

 the range for these months is either constant, or appears to di- 

 minish slightly. The relation of the increase in March* to the 

 decennial period is evident ; but what shall we say of October 

 and August ? The decennial period only affects certain months 

 at the equator in the period 1854-58. When the longitude of 

 the sun was 0° and 270°, the increase was marked, and not at 

 all when it was 180° and 90° in the semi-period 1854—58. I 

 believe that other months show the increase in other periods. 



I shall not at present enter into the laws of the variations of 

 magnetic dip, nor of the force. I may say, generally, that the 

 diurnal variation of the magnetic dij) near tiie magnetic equator 

 is greatest near the equinoxes when the variation of declination 

 is least, and that it is as great in June and greater in December 

 than in Europe. On the whole, therefore, the variations of dip 

 are greater on the magnetic equator than in Europe. 



The mean diurnal motion of the magnet freely suspended in 

 the direction of the magnetic dip near the equator is as great as, 

 or greater than, that in high latitudes in every month of the 

 year. 



* An increase in March might be produced by a variation of the epoch 

 of niinimum movement. 



