422 Mr. J. C. McConnel on the 



§8. 



Several physicists * have recently undertaken the defence of 

 the old theory on the nature of unipolar induction against the 

 objections which I have brought against it. I shall therefore 

 conclude this memoir by calling the attention of the reader 

 to the following results of the above-described researches. 



The assertion of the old theory that a magnet is incapable of 

 producing induction in a conductor with which it is fixedly 

 united is in opposition to the requirements of the mechanical 

 theory of heat, and cannot therefore be sustained. 



The old theory supposes that a magnet in rotation about 

 its axis produces in a conductor at rest placed at a distance, 

 an induction of the same magnitude as if the conductor were 

 in rotation with the same angular velocity in the opposite 

 direction round the magnet at rest. 



This method of regarding it is in opposition to the me- 

 chanical theory of heat, and must therefore be erroneous; the 

 induction produced in the conductor at rest when the magnet 

 is in rotation amounts generally only to an insignificant frac- 

 tion of the induction due to the rotation of the conductor with 

 the same velocity round the magnet at rest. 



If a magnet and a conductor at some distance are in rota- 

 tion in the same direction, with the same angular velocity 

 round the axis of the former, according to the old theory no 

 induction should be produced in the conductor. But this is 

 also in opposition to the mechanical theory of heat, which 

 shows us that the induction in the conductor is of nearly equal 

 amount, whether the magnet is in rotation or whether it is 

 not. The rotation of the magnet is in fact without sensible 

 effect upon the magnitude of the induction. 



LIY. On the Cause of Iridescence in Clouds. 

 By James C. McConnel, M.A.\ 



IN a recent number of this Magazine!, Dr. Johnstone Stoney 

 has suggested an explanation of the iridescent hues 

 sometimes seen tinging the edges and thinner portions of the 

 clouds. During my residence last winter in the Engadine, I 



* Hoppe, Wied. Ann. t. xxviii. p. 478 ; t. xxix. p. 544. F. Exner, 

 Sitzuiif/sber. der Kais. Akad. der Wiss. in Wien, 8 Juli 1886. For the 

 refutation of these articles see Edlund, Wiedemann's Annalen, t. xxix. 

 p. 420, t. xxx. p. 655 ; Sitzungsber. der K. Akad. der Wissensch. in Wien, 

 13 Januar 1887. 



t Communicated by the Author. 



% Phil. Mag. July 1887, p. 87 j Trans. Roy. Dublin. Soc. Feb. 16 and 

 March 23, 1887. 



