REPORT ON THE MAGNETISM OF THE EARTH. 117 



the variation, furnished to him by the Admiralty, the East India 

 Company, and from other sources. If to the hues of equal .va- 

 riation were added the magnetic meridians and their normals, 

 the isodynamic lines, with those of equal dip, such a globe would 

 form the most complete representation of facts connected with 

 terrestrial magnetism that has ever been exhibited, and might 

 indicate relations which have hitherto been overlooked. 



Having discovered that a peculiar polarity is imparted to iron 

 by the simple act of rotation, I was led to consider whether the 

 principal phsenomenon of terrestrial magnetism is not, in a great 

 measvu-e, due to its rotation. The subsequent discovery by 

 Arago, that analogous effects take place during the rotation of 

 all metals, and Faraday's more recent discovery, that electrical 

 currents are not only excited during the motion of metals, but 

 that such currents are transmitted by them, render such an 

 opinion not improbable. It is, however, to be remarked, that, 

 in all these cases, motion alone is not the cause of the effects 

 produced ; but that these effects are due to electricity induced 

 in the body by its motion in the neighbourhood of a magnetized 

 body. If, then, electrical currents aie excited in the earth in 

 consequence of its rotation, we must look to some body exterior 

 to the earth for the inducing cause. The magnetic influence 

 attributed by Morichini and Mrs. Somerville to the violet ray, 

 and the effect M'hich I found to be produced on a magnetized 

 needle when vibrated in sunshine, and which appeared not to 

 admit of explanation without attributing such influence to the 

 sun's rays, might appear to point to the sun as the inducing 

 body. The experiments, however, of Morichini and Mrs. So- 

 merville, have not succeeded on repetition ; and in a recent re- 

 petition of my own experiments, in a vacuum, by Mr. Snow 

 Harris, the effects which I observed were not detected. I had 

 found that the effects produced on an unmagnetized steel needle 

 differed from those produced on a similar needle when magnet- 

 ized, and therefore considered that the idea of these effects 

 being independent of magnetism was precluded ; but Mr. Har- 

 ris's results may possibly be considered to indicate that they 

 were due solely to currents of air excited by the sun's rays. 

 These circumstances render it doubtful whether the sun's rays 

 possess any magnetic influence independent of their heating 

 power ; but besides this, supposing such influence to exist, if 

 electric currents were induced in the earth during its rotation, 

 they would be nearly at right angles to the equator, and would 

 therefore cause a magnetized needle to place itself nearly per- 

 pendicular to the meridians, or parallel to the equator. 



Altiiough it Avould therefore appear that the rotation of the 



