614 ADDITIONS. 



the south, till we arrive at the points of convergence of all theii 

 directions ; for there are only two such poles, one in the Arctic and 

 one in the Antarctic region. But in consequence of the irregularity 

 of the magnetic constitution of the earth, if we follow the inclination 

 of the magnetic force round the earth on any parallel of latitude, we 

 find that it has two maxima and two minima, as if there were four 

 magnetic poles. The isodynamic map is a new presentation of the 

 facts of this subject ; the first having been constructed by Colonel 

 Sabine in 1837. 



I have stated also that the magnetic elements at each place are tc 

 be observed in such a manner as to bring into view both their periodi- 

 cal, their secular, and their irregular or occasional changes. The ob- 

 servations made at Toronto in Canada, and at Hobart Town in Van 

 Diemen's Land, two stations at equal distances from the two poles of 

 the earth, and also at St. Helena, a station within the tropics, have 

 been discussed by General Sabine with great care, and with an amount 

 of labor approaching to that employed upon reductions of astro- 

 nomical observations. And the results have been curious and unex- 

 pected. 



The declination was first examined. 2 This magnetical element is, 

 as we have already seen (p. 232), liable both to a diurnal and to an 

 annual inequality ; and also to irregular perturbations which have been 

 termed magnetic storms. Now it was found that all these inequalities 

 went on increasing gradually and steadily from 1843 to 1848, so as 

 to become, at the end of that time, above twice as large as they were 

 at the beginning of it. A new periodical change in all these elements 

 appeared to be clearly established by this examination. M. Larnont, 

 of Munich, had already remarked indications of a decennial period in 

 the diurnal variation of the declination of the needle. The duration 

 of the period from minimum to maximum being about five years, 

 and therefore the whole period about ten years. The same conclusion 

 was found to follow still more decidedly from the observations of the 

 clip and intensity. 



This period of ten years had no familiar ir.eaning in astronomy ; and 

 if none such had been found for it, its occurrence as a magnetic period 

 must have been regarded, as General Sabine says, 3 in the light of a 

 fragmentary fact. But it happened about this time that the scientific 

 world was made aware of the existence of a like period in a pheno 



5 Phi'. Trans. 1852 and 1856. 3 Phil. Trans. 1S56, p 382. 



