94.3 



deduces •0042 as the annual decrease of the horizontal force during 

 the period comprehended by the observations, the value of the force 

 on the 1st of March 184-7, the mean epoch being 3*53043, \^ith a 

 probable error of +'00025. 



For the purpose of deducing the values of the total magnetic force 

 and its secular change from those of the horizontal force, it is neces- 

 sary to know the magnetic inclination corresponding to the epoch 

 and its secular change. From the observations of the inclination, 

 75° 16''09 is deduced as the value of this element on the 1st of March 

 1847? with a secular increase of 0'*89 annually ; and 13'8832 as the 

 value of the total force in absolute measure, at the same epoch. As 

 the annual increase of 0''89 in the inclination will not account for an 

 annual decrease of more than '0033 in the horizontal force, there 

 remains '0009 as indicative of a small annual decrease in the total 

 force during the period of the observations, and the author considers 

 that the probabilities are in favour of such a decrease. 



The general fact of an annual variation of the horizontal force at 

 Toronto, the force being greater in the summer than in the winter 

 months, is shown by three independent methods of experiment, viz. 

 the observations from which the foregoing conclusions have been 

 drawn, the regular observations with the bifilar magnetometer, and 

 observations undertaken expressly with the view of ascertaining the 

 fact. The author also infers the probable existence of an annual 

 variation of the total force, the force being greatest in the winter 

 months, or when the sun is in the southern signs, and least in the 

 summer months, or when the sun is in the northern signs. 



The results obtained from the observations at Hobarton are next 

 briefly stated. The investigation, conducted in the same manner as 

 at Toronto, shows at Hobarton a decrease of south inclination of 

 0'*89 on the average of the months from April to August inclusive, 

 that is, in the southern winter ; and an increase of 0'-85 from October 

 to February inclusive, that is, in the southern summer. 



The series of observations on the horizontal force shows an annual 

 variation of the same character as respects the seasons, and almost 

 identical in amount with that at Toronto. In the months from 

 October to February inclusive, or in the summer months at Ho- 

 barton, the horizontal force is '0017 greater on the average than its 

 mean amount; and from April to August inclusive, or in the winter 

 months at Hobarton, it is on the average '0013 less than its mean 

 amount. 



The inferences drawn from these variations of the inclination and 

 horizontal force, taken jointly as respects the total force at Hobarton, 

 are that this force is subject to an annual variation, being higher 

 than its mean amount from October to February, and lower than its 

 mean amount from April to August. 



It thus appears that in the months from October to February the 

 magnetic needle more nearly approaches the vertical position, both 

 at Toronto in the northern hemisphere, and at Hobarton in the 

 southern ; and that the total force is greatest at both stations from 

 October to February, and least from April to August. 



