1892.] Changes produced by Magnetisation in Wires. 495 



emission suddenly set up, certainly within two days and possibly 

 within a few hours, was probably much greater than that of our Sun ; 

 yet within some fifty days after it had been discovered, at the end of 

 January, its light fell to about 1 /300th part, and in some three 

 months to nearly the l/10,000th part. As long as its spectrum 

 could be observed the chief lines remained without material alter- 

 ation of relative brightness. Under what conditions could we suppose 

 the Sun to cool down sufficiently for its light to decrease to a similar 

 extent in so short a time, and without the incoming of material 

 changes in its spectrum ? It is scarcely conceivabJe that we 

 can have to do with the conversion of gravitational energy into 

 light and heat. On the theory we have ventured to suggest, the 

 rapid calming down, after some swayings to and fro of the tidal 

 disturbances, and the closing in again of the outer and cooler gases, 

 together with the want of transparency which might come in under 

 such circumstances, as the bodies separated, might account reasonably 

 for the very rapid and at first curiously fluctuating waning of the 

 Nova ; and also for the observed absence of change in its spectrum. 



I may, perhaps, be permitted to remark that the view suggested 

 by Dr. William Allen Miller and myself, in the case of the Nova of 

 1866,* was essentially similar, in so far as we ascribed it to erupted 

 gases. The great suddenness of the outburst of that star, within a 

 few hours probably, and the rapid waning from the 36 magnitude to 

 the 8'1 magnitude in nine days, induced us to throw out the addi- 

 tional suggestion that possibly chemical actions between the erupted 

 gases and the outer atmosphere of the star may have contributed to 

 its sudden and transient splendour ; a view which, though not im- 

 possible, I should not now, with our present knowledge of the light- 

 changes of stars, be disposed to suggest. 



II. " On the Changes produced by Magnetisation in the Length 

 of Iron and other Wires carrying Currents." By Shelford 

 Bidwell, M.A., LL.B., F.R.S. Received April 28, 1892. 



The changes of length attending the magnetisation of rods or 

 wires of iron and other magnetic metals which were first noticed 

 by Joulef in 1841, and have in recent years formed the subject of 

 many experiments by myself, J have been found to be related to 

 several other phenomena of magnetism. Maxwell§ has suggested 



* < Roy. Soc. Proc.,' vol. 15, p. 146. 



f « Joule's Scientific Papers (Phys. Soc.),' pp. 48, 235. 



X ' Phil. Trans.,' vol. 179, A, p. 205 ; ' Roy. Soc. Proc.,' No. 237 (1885), p. 265 ; 

 No. 242 (1886), p. 109; No. 243 (1886), p. 257; vol. 43, p. 406; vol. 47, p. 469. 

 § ' Electricity and Magnetism,' vol. 2, § 448. 



