838 



winter months, the principal or western loop is formed by the mo- 

 tion from 8'' A.M. till 5^ p.m. ; in the months from April to August, 

 three-fourths of the whole diurnal motion occur between 6^ a.m. and 

 6''p.M., the remaining fourth forming a slightly inflected side to each 

 of the figures : it is this side which is gradually twisted up to form the 

 eastern loop of the winter months. 



It is evident that no proper comparison can be made of the areas 

 of these figures on account of the involved forms in the winter 

 months; the areas, however, of the figures from April to August, 

 differ very little. 



Perimeters of the Figures. — The twisting of the perimeters, which 

 renders a comparison of the areas of little value, does not appear to 

 affect the length of the motion, and this therefore seems a fair sub- 

 ject for examination. The following are the values of the angular 

 motion, or length of the perimeter, for each month, as obtained ap- 

 proximately from Plate VII. 



December and January shew the least perimeters, April, May, and 

 June, the greatest, though the perimeters for the months from April 

 to August are nearly constant. 



Hourly Angular Motions, — Having obtained the approximate 

 motion from hour to hour for each of the monthly figures, we find 

 that, on the whole, they follow nearly the same law, that indicated 

 in the following numbers, which are the means for each two hours 

 of the hourly motions from the 12 separate months. 



\2^ 14h 16h Igh 20'> 22l> Oh 2^ i^ 6l> 8h lOh 



0'-43 0'-18 0'-46 0'-62 1'19 l'-60 l'-34 1'08 0'-99 0'-60 0'-57 0'-29 



These numbers give the following curious result ; — That the velo- 

 city of motion of the north end of a magnet freely suspended in the 

 direction of the magnetic force is a maximum when the sun makes its 

 superior transit of the magnetic meridian (between lO** and ll*" a.m.), 

 and a minimum when it makes its inferior transit of the same meridian 

 (between 10** and 11*'p.m.). This result is the more curious that 

 the epoch of the minimum velocity of the diurnal motion is an epoch 

 of maximum disturbance ; and, in as far as the declination is con« 



VOL. II. 2 F 



