•MATHEMATICS AND NATURAL PHILOSOPHY. 461 



is perceived to exist in regard to the hours which constitute the epochs of maxima 

 and minima at the three stations, as 'well as in regard to the amounts of the re- 

 spective variations ; these differences are no doubt intimately connected "with the 

 causes of the phenomena, and are likely to lead to their elucidation. 



The domain of periodical variations has thus been considerably enlarged since 

 the Report of the Committee of Physics -was drawn up ; and must henceforth be 

 understood to comprise, in addition to the variations "■whose amount is a function 

 of the hour-angle of the sun, aDd of his longitude 1 ' (or of his declination) (Report, 

 p. 10), — lstly, those variations of the three elements whose amount is a function of 

 the hour-angle of the moon; 2ndly, those variations which were classed in the 

 Committee's Report as " irregular," or " apparently observing no law," but which 

 are now known lo be governed by laws depending on the sun's declination, and 

 hour-angle ; and 3rdly, those variations, both "irregular" and "occasional," which 

 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. 



Absolute Values and Seczdar Changes. — But interesting and valuable 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 terrestrial 

 magnetism is in question, — to the purposes of that distinct branch of the duties 

 of a magnetic observatory, whieh consists in the determination of the absolute values 

 and secular changes of the three magnetic elements. By the absolute 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 determinations of this class, obtaiued 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 main- 

 tained 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 satisfac- 

 tion, and with a well deserved recognition of the pains which have been bestowed 

 by the successive directors of the Toronto Observatory, 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 determine these data with a preci- 

 sion, which is greatly in advance of preceding experience, and, as far as may be 



