405 



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, and of his longitude " (or of his declination) (Report, p. 10), 

 1 st, those variations of the three elements whose amount is a func- 

 tion of the hour-angle of the moon ; 2ndly, those variations which 

 were classed in the Committee's Report as "irregular," or "appa- 

 rently observing no law," but which are now known to be governed 

 by laws depending on the sun's declination, and hour-angle ; and 

 Srdly, those variations, both " regular " 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. 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 terrestrial 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 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 



