532 [April 23, 



state of vertical force, as referred to the mean on each day. The 

 force is here represented by a simple ordinate. The grouping is 

 made by years and by months in the same manner as for the curves 

 already mentioned. The month-curves of the two periods (1841- 

 1847 and 1848-1857) differ, in the magnitude and change of mag- 

 nitude of the ordinates, and in the place and change of place of node. 

 The year-curves of the two periods have some very remarkable dif- 

 ferences. From 1847 to 1849 the magnitude of the ordinates in- 

 creases sensibly; from 1849 to 1850 still more; it then remains 

 nearly stationary. In 1846 the descending node is at llf b nearly ; 

 in 1847 it is at 9 h nearly ; in 1849 at 7 h nearly ; in 1850 at 5 h ; in 

 1851 at 4 h ; and there it continues with little alteration. It is im- 

 portant to observe that, though the instrument was changed in 1848, 

 the change in the place of the node did not then occur suddenly ; it 

 had begun with the old instrument, and continued to advance gra- 

 dually for several years with the new instrument. The author states 

 that he had verified the correctness of the node in the first period 

 from other observations, but he had not succeeded in finding obser- 

 vations corresponding in date with those of the latter period. 

 The paper is followed by eight sheets of curves, as follows : 



I. Diurnal Curves of combination of Declination and Horizontal 



Force. 



(1) Mean of all the days in each year (separately), 1841-1847. 



(2) Mean of all the days in each year (separately), 1848-1857. 



(3) Mean of all the days in the aggregate of the same nominal 

 months (separately) through the period 1841-1847. 



(4) Mean of all the days in the aggregate of the same nominal 

 months (separately) through the period 1848-1857. 



II. Diurnal Curves of Vertical Force. 



(5) Mean of all the days in each year (separately), 1841-1847. 



(6) Mean of all the days in each year (separately), 1849-1857. 



(7) Mean of all the days in the aggregate of the same nominal 

 months (separately) through the period 1841-1847. 



(8) Mean of all the days in the aggregate of the same nominal 

 months (separately) through the period 1849-1857. 



