of Terrestrial Magnetism. 439 



According to this distinguished writer, an analogous double 

 period is wanting at Hobarton, and the total force has a simple 

 progression with the minimum at 20 h or 21 h , and the maximum 

 between 5 h and 6 h , the intermediate march being continued 

 without interruption. But on carefully examining the curves 

 themselves, given by him at p. 68, pi. 3, it will be seen that the 

 simple period exists only in appearance, and that in certain 

 months the secondary period is very obvious, and rudiments of 

 it exist in all, though to a very small extent. This difference is 

 certainly owing to the great difference of latitude and magnetic 

 force between the two places. 



There are not yet a sufficiency of published observations at 

 St. Helena to determine this law ; but a copious series of ob- 

 servations at the Cape show a period in the variations of the 

 total force almost complementary to that of the declination. 

 The similarity of the two kinds of curves, which generally differ 

 about three hours in their phases, renders a more complete ana- 

 lysis unnecessary. 



D. Complex Period of the Needle. 



The process which we have described for determining the 

 motions of the needle consists in a series of decompositions of 

 the forces, rendered necessary from the mode in which the mag- 

 netic bars are supported. The laws of the variations of the 

 components being determined, we may deduce from them what 

 would be the motion of a needle, not on an axis, but suspended 

 by a single point, which would be its centre of gravity, and free 

 to obey every magnetic variation in whatever direction it took 

 place. To give an idea of the combined motions which the 

 needle makes in a complete oscillation, we may refer to two 

 figures in particular in Sabine's Plate, Hobarton Observations, 

 vol. i. pi. 3, the first of which belongs to December at Hobarton, 

 the other to June at the same place. The principle upon which these 

 figures are traced is the following : — The point where the two 

 axes intersect represents the mean diurnal position of the needle 

 in declination as well as in inclination. Along the horizontal 

 axis a distance is taken representing the variation of the decli- 

 nation for a given hour, and from the point so obtained, an ordi- 

 nate is erected representing on the same scale the variation of 

 the inclination for that hour. Thus are obtained the figures to 

 which we have referred. 



In Colonel Sabine's Plates, he has given the curves for each 

 month of the year, and they are all extremely instructive, but 

 the two to which we have particularly referred, the march of the 

 curve in the two extreme months of the year, is well shown. 

 It may be seen from these, and still better from the whole scries, 



