100 COSMOS. 



tions of inclination with those of the horizontal force, it has 

 been established that in Canada, during the winter months, 

 when the sun is in the southern signs of the zodiac, the total 

 terrestrial force has a greater intensity than in the summer 

 months, while in Van Diemen's Land the intensity is great- 

 er than the mean annual value that is to say, the total ter- 

 restrial force from October to February, which constitutes 

 the summer of the southern hemisphere, while it is less from 

 April to August. According to Sabine,* this intensity of 

 the terrestrial magnetic force is not dependent on differences 

 of temperature, but on the lesser distance of the magnetic 

 solar body from the earth. At Hobarton the intensity dur- 

 ing the summer is 13-574: in the absolute scale, while during 

 the winter it is 13 '543. The secular variation of intensity 

 has hitherto been deduced from only a small number of ob- 

 servations. At Toronto it appears to have suffered some de- 

 crease between 1845 and 1849, and the comparison of my 

 own observations with those of Rudberg, in the years 1806 

 and 1832, give a similar result for Berlin.f 



Inclination. 



The knowledge of the isoclinal curves, or lines of equal in- 

 clination^ as well as the more rapid or slower increase of the 

 inclination by which they are determined (reckoning from 

 the magnetic equator, where the inclination =rO, to the 

 northern and southern magnetic pole, where the horizontal 

 force vanishes), has acquired additional importance in mod- 

 ern times, since the element of the total magnetic force can 

 not be deduced from the horizontal intensity, which requires 

 to be measured with excessive accuracy, unless we are pre- 

 viously well acquainted with the inclination. The knowl- 

 edge of the geographical position of both magnetic poles is 



single one at Hobarton." The time of the maximum of intensity falls 

 at Hobarton between 8 and 9 A.M. ; while the secondary or lesser 

 minimum falls at Toronto about 10 A.M., and consequently the in- 

 crease and diminution of the intensity fall within the same hours in 

 accordance with the time of the place, and not at opposite hours, as 

 is the case with respect to the inclination and the declination. See, 

 regarding the causes of this phenomenon, p. Ixix. (compare also Far- 

 aday, Atmospheric Magnetism, 3027-3034). 



* Phil Transact, for 1850, pt. i., p. 215-217; Magnet. Observ. at 

 Hobarton, vol. ii., 1852, p. xlvi. See also p. 26 of the present volume. 

 At the Cape of Good Hope the intensity presents less difference at 

 opposite periods of the year than the inclination (Magnet. Observ. 

 made at the Cape of Good Hope, vol. i., 1851, p. lv.). 



f See the magnetic part of my work on Asie Centrale, t. iii., p. 442. 



