AYRTOX AND PERRY'S THEORY OF THE EARTH'S MAGNETISM 183 



In connection with the theory of the earth's magnetism, I had also 

 framed a theory of the Aurora which may still hold. It is that the 

 earth is electrified, and naturally that the electricity resides for the 

 most part on the exterior of the atmosphere and that the air-currents 

 thus carry the electricity toward the poles, where the air descending 

 leaves it and that the condensation so produced is finally relieved 

 by discharge. 



The total effect would thus be to cause a difference of potential be- 

 tween the earth and the upper regions of the air both at the poles and 

 the equator. At the poles the discharge of the aurora takes place in 

 the dry atmosphere. At the equator the electrostatic attraction of the 

 earth for the upper atmospheric layers causes the atmosphere to be in 

 unstable equilibrium. At some spot of least resistance the upper atmos- 

 phere rushes toward the earth, moisture is condensed, and a conductor 

 thus formed on which electricity can collect; and so the whole forms a 

 conducting system by which the electric potential of the upper air and 

 the earth become more nearly equal. This is the phenomenon known 

 as the thunderstorm. 



Hence, were the earth electrified, the electricity would be carried to 

 the higher latitudes by convection, would there discharge to the earth 

 as an aurora, and passing back to the equator would get to the upper 

 regions as a lightning discharge, once more to go on its unending cycle. 

 I leave the details of this theory to the future. 



Baltimore, May 30, 1879. 



Appendix. Since writing the above, Professors Ayrton and Perry's 

 paper has appeared in full ; and I am thus able to point out their error 

 more exactly. Their formula at the foot of page 40G is almost the 

 same as mine; but on page 407, in the fourth equation, the exponent of 

 n should be -f- instead of \, which increases their result by about 

 600,000,000, and makes it practically the same as my own. 



Rotterdam, July 13. 



