On the Cosmical Features of Terrestrial Magnetism. 97 



I believe that what I can claim as new in my equation (II.) is 

 just this, that the magnitude Z which there occurs has acquired, 

 through my developments, a definite physical meaning, whence 

 it follows that it is fully determined by the arrangement of the 

 constituent molecules of the body existing at any given instant. 

 Thus only does it become possible to deduce from this equation 

 the important conclusion which follows. 



[To be continued.] 



XIV. On tlie Cosmical Features of Terrestrial Magnetism. By 

 Edward Sabine, R.A., D.C.L. Oxford, and LL.D. Cam- 

 bridge, President of the Royal Society. 



[The Reade Lecture, delivered in the Senate House of the University of 



Cambridge, in May 1862.J 



In directing our attention to the subject of terrestrial magnetism, 

 we may conveniently separate the phenomena into two distinct 

 classes, — viz. those which originate in and indicate the magnetic 

 state of our own planet, and those which admit of a traceable con- 

 nexion with bodies exterior to the earth, or forces emanating from 

 them. The title of the lecture which I shall have the honour of ad- 

 dressing to you will lead me to dwell chiefly on the second of these 

 two classes of phenomena, namely, on those which with more or 

 less certainty indicate cosmical relations. We are now justified 

 in predicating that such cosmical relations are discernible in all, 

 or almost all, those fluctuations and changes of direction which, 

 as many of my present audience are aware, our magnetic needles 

 are continually undergoing, whether they appear in the form of 

 momentary affections or in regularly recurring periods, such as 

 a day, a month, a year, or a cycle of years. 



The advances which have been made in this department of 

 magnetical science date chiefly from the epoch (in 1840) when, 

 on the joint recommendation of the Royal Society and of the 

 British Association for the Advancement of Science, observatories 

 were established in several parts of the globe devoted to magnetic 

 phenomena as their especial and primary object. Prior to this 

 epoch, the knowledge possessed of the periodical magnetic varia- 

 tions was in the highest degree vague, desultory, and inconclu- 

 sive ; whilst the attempts which had been occasionally made to 

 systematize or explain them were either wholly conjectural, or 

 were based on an assumed connexion between the variations of 

 temperature and of magnetism, in which the consistency of the 

 hypotheses with the few facts then known respecting the mag- 



Phil. Mag. S. 4. Vol. 24. No. 159. Aug. 1862. H 



