10'2 Prof. H. A. Rowland on a new Theory 



30. Young's Paraffin Oil, specific gravity *890, a trade 

 sample of one of the heaviest lubricants made ; brownish in 

 colour, fluorescent, transparent, and a very good insulator. 

 In electro-optic experiments the action of this paraffin is 

 similar to that of the preceding, equally regular and pure, 

 but remarkably stronger. The extinction-bands are as fine 

 in this liquid as in cumol, if not finer. Against a weight 

 of 4 pounds on the fixed compensator the bands are easily 

 moved out beyond the balls ; but against a weight of 7 pounds 

 the axal band is barely, if so much as, divided. There is no 

 other liquid that I have examined, except CS 2 , which is clearly 

 superior to this heavy paraffin in strength and range of electro- 

 optic action. I am not sure whether cumol should be placed 

 above or below it. 



Glasgow, July 1, 1879. 



[To be continued.] 



XII. On Professors Ayrton and Perry's new Theory of the 

 Eartlis Magnetism, icith a Note on a new Theory of the 

 Aurora. By H. A. Rowland, Professor of Physics in the 

 Johns Hopkins University* . 



SOME years ago, while in Berlin, I proved by direct expe- 

 riment that electric convection produced magnetic action ; 

 and I then suggested to Professor Helmholtz that a theory of 

 the earth's magnetism might be based upon the experiment. 

 But upon calculating the potential of the earth required to 

 produce the effect, I found that it was entirely too great to 

 exist without producing violent perturbations in the planetary 

 movements, and other violent actions. 



I have lately read Professor Ayrton and Perry's publication 

 of the same theory ; and as they seem to have arrived at a 

 result for the potential much less than I did, I have thought 

 it worth while to publish my reasons for the rejection of the 

 theory. 



The first objection to the theory that struck me was, that 

 not only the relative motion but also the absolute motion 

 through space of the earth around the sun might also produce 

 action. And to this end I instituted an experiment as soon as 

 I came home from Berlin. 



I made a condenser of two parallel plates with a magnetic 

 needle enclosed in a minute metal box between them ; for I 

 reasoned that, when the plates were charged and were moved 

 forward by the motion of the earth around the sun, they would 

 then act in opposite directions on the enclosed needle, and so 

 * Communicated by the Physical Society, having been read June 29th, 



