of Terrestrial Magnetism. 397 



that the appearance of irregularity only arises from our being 

 ignorant of the fixed periods, as well as of the accidental causes 

 which influence the needle. Desiring to treat this subject with 

 systematic order, it is necessary to restrict ourselves to the regu- 

 lar variations only. In order to give an idea of the annual 

 variations of the declination without too much multiplying 

 words, we will refer to Sabine^s work (Toronto, vol. ii. p. 20) for 

 the curves traced by him, representing the position of the needle 

 in the two six-monthly periods when the sun is on either side of 

 the equator, at the four observatories of Toronto, St. Helena, 

 Cape of Good Hope, and Hobarton. 



In these figures the red line indicates the excursions of the 

 needle in the months when the sun is in the northern signs, or 

 in the tropic of Cancer ; and the blue line the same in the months 

 when the sun is in the southern signs, or in the tropic of Capri- 

 corn. These curves include only the hours of the day, as being 

 the most marked. The north pole of the needle deviates to the 

 east when the curve is above the axis of the abscissae, and to the 

 west when it is below it. From a simple inspection of these 

 curves, we may draw the following conclusions : — 



At Toronto, the needle at 8 in the morning is throughout the 

 year to the east of its mean position; and in the afternoon, 

 towards 2 p.m., it is always to the west ; 2nd, the excursion is 

 greater in summer than in winter, and the annual difference in 

 this respect is represented by the distance between the two 

 curves ; 3rd, in the intermediate months the needle is between 

 the two limiting curves. 



For Hobarton we get the same laws, but with contrary deno- 

 minations, as we have already said (under Law II. Corollary II.). 



For St. Helena there is the nbtable circumstance, that the 

 curves are seen to bend alternately south and north of the equa- 

 tor, moving with the sun ; yet it is not to be overlooked, that 

 the curve of the months of the June solstice wants the second 

 inflexion, which it would require in order to be symmetrical with 

 the curve of the opposite six months. 



At the Cape of Good Hope the phases are transitional between 

 those of St. Helena and Hobarton. These curves are the gra- 

 phical result of the observations, and we have now only to see 

 whether it is possible that they may have originated from more 

 simple periods, which, being separated from each other, may throw 

 light on the physical cause of the phsenomenon. 



These curves are traced by taking the mean of the six months, 

 and hence they appear more regular than if taken from the dif- 

 ferent months singly ; for if we examine each of the constituent 

 monthly curves separately, we shall find some peculiarities and 

 notable differences, which tend further to confirm the belief that 



