304 - Royal Society :— 
have their epochs and amounts dependent apparently on a solar 
period of not yet perfectly ascertained duration, manifesting itself 
also by periodical changes in the frequency and amount of the solar 
spots. With the exception of the last-named class, all these varia- 
tions require, for their generalization, that the phenomena should be 
investigated at several points of the earth’s surface widely distant 
from each other; and we have now the knowledge, grounded on 
experience, that a very few years are sufficient for the observations 
at each station, with the instrumental means and methods recom- 
mended by the Royal Society, and when the investigation is made a 
primary object by those who engage in it. 
Absolute Values and Secular Changes.—But interesting and valu- 
able as is the acquisition of a fuller and more precise knowledge of 
the comparatively small magnetic variations produced at the surface 
of the earth by the action or influence of external bodies, even 
greater importance seems to attach,—when ¢errestrial magnetism is 
in question,—to the purposes of that distinct branch of the duties of 
a magnetic observatory, which consists in the determination of the 
absolute values and secular changes of the three magnetic elements. 
By the adsolute values we seek to acquire a knowledge of the 
actual present order and distribution of the terrestrial magnetic 
influence at the surface of the earth, and to provide the materials by 
which the constancy, or otherwise, of the earth’s magnetic charge 
may hereafter be examined; and by determinations of the present 
direction and amount of the secular changes, we seek to become 
acquainted with the laws, and ultimately with the causes, of that 
most mysterious change, by which the magnetic condition of the 
globe at one epoch passes progressively and systematically into that 
of another. It is specially by determimations of this class, obtained 
with the necessary precision in different parts of the globe, that, in 
the words of the Committee’s Report, “the patient inductive inquirer 
must seek to ascend to the general laws of the earth’s magnetism.”’ 
At the time when the Report of the Committee of Physics was 
written, doubts were reasonably entertained, whether the limited 
time, during which the Colonial Observatories were likely to be 
maintained in action, would be sufficient for the determination of 
the secular changes; and it was therefore very properly argued, that 
‘these changes cannot be concluded from comparatively short series 
of observations without giving to the observations extreme nicety, so 
as to determine with perfect precision the mean state of the elements 
at the two extremes of the period embraced.” It is with much 
satisfaction, and with a well-deserved recognition of the pains which 
have been bestowed by the successive Directors of the Toronto Ob- 
servatory, and their Assistants, on this branch of their duties, that I 
am able to refer to the determinations of the absolute values and 
secular changes of the three elements contained in the third volume 
of the Toronto Observations, in evidence that the instrumental means 
which were devised, and the methods which have been adopted, have 
proved, under all the disadvantages of a first essay, sufficient to de- 
