of Terrestrial Magnetism. 437 



to which cause each phase must be attributed. A few observa- 

 tions, and those subject to some uncertainty, tend to show that 

 at M. Helena the annual variation of tbe inclination is small, so 



IttZv mus< ; c ° r r der ?* variatioQS ^ the horizontal force as 

 depending entirely on those of the total force 



th,i\ Makerst0 !"V he r lu6S of the hori ?o«tal component have 

 equinoxes*. 1 " "****> aUd *"* mmima a little afte ' the 



In general in this as in tbe declination, tbe months of April 

 f tt Z IT TV by the g-^est diurnal excursions; S 

 Lf^hil\ Mr - ] ? r0Un t0 the ^aordinary perturbations, 

 but it probably depends on some other cause. But for the prin- 



S to the 8 ° U th Y P °t °? thG maXlma and — a > - Tust 

 refei to the original works, this memoir being already too long. 



B. Vertical Component. 

 This is given by the balance magnetometer arranged at rieht 



those of the horizontal force. At St. Helena the curve has a 



Tetirti b / lt ,r th this differeuce ft ™ the curve shown by 

 the bihlar, that the curve of the horizontal force is nearly a 

 curve of cosines (reckoning from noon), and that of the ve tical 

 force a curve of sines. But the rudiments of a secondary ™ od 

 are seen in a slight undulation, which it makes at aboutH^as 

 also m the intervals of the intersection of the curve with the axis 

 being greater from midnight to noon than from noon to mfd! 

 n glit. ine maximum occurs between 5 h and 6 h and the mi 

 annua at 20". Its amount is smaller from October to Jta? 

 during which period it is retarded, and greater from April to Sep' 

 tember, when it is in advance. P 



At the Cape of Good Hope the march of the vertical force is 

 so similar to that of the decimation, that it is susceptible of the 

 same analyses and gives the same results. In these curves the 



in l n'ui Pei S°i 5 T ayS ^ d0 r iates > tb ™£h somewhat modified 

 in par^. See Sabine, Obs. Cape, p. 40, pi. 5. fie; 2 



At Toronto and at Hobarton the usual antagonism displays 

 t f m this component. The double period is developed as 

 much as in the decimation and the hours of minima and maxima 

 are almost identica with those of the decimation. The double 

 period displays itself at Hobarton in a somewhat singular man- 

 ner, producing a maximum towards seven in the evenin- 



But to follow out all these irregularities would be very tedious; 

 we wi therefore abstain, referring the reader to the works already? 

 quoted. We will only add, that the explanation of many ano- 



