76 Mr. J. P. Joule on the Effects of Magnetism 



The evidence which I am able to offer upon this subject Is 

 necessarily, from the nature of the subject, only negative ; and 

 the conclusion to which it leads will be destroyed by the 

 establishment of any one fact in opposition to it. But such a 

 fact must be established beyond doubt : and I have no hesi- 

 tation in saying that no form of experiment anterior to my 

 own has been such as to place its conclusions beyond doubt. 

 I insist upon the two points — of juxtaposition of the modified 

 and unmodified spectra, and of exclusion of other light — as 

 indispensably necessary for accuracy. 



The experimenter must also be on his guard against the 

 apparent effects of mere intensification or dilution of light in 

 modifying its quality. Every one knows, for instance, that 

 when the setting sun is beginning to assume a slightly yellow 

 colour, its light received on a white plaster wall appears of a 

 full blush red, although there is not the smallest reason to 

 think that the quality of the light is really altered. 

 I am. Gentlemen, 



Your obedient Servant, 



G. B. Airy. 



XVII. O71 the Effects ()f Magnetism upon the Dimensions of 

 Iron and Steel Bars. By J. P. Joule, Eisq.^ 



A BOUT the close of the year 1811, Mr. F. D. Arstall, an 

 ingenious machinist of this town, suggested to me a new 

 form of electro-magnetic engine. He was of opinion that a 

 bar of iron experienced an increase of bulk by receiving the 

 magnetic condition ; and that, by reversing its polarity rapidly 

 by means of alternating currents of electricity, an available 

 and useful motive power might be realized. At Mr. Arstall's 

 request I undertook some experiments in order to decide how 

 far his opinions were well-founded. 



The results of my inquiries were brought before the public 

 on the occasion of a conversazione held at the Royal Victoria 

 Gallery of Manchester on the 16th of February IS^ii, and are 

 printed in the eighth volume of Sturgeon's Annals of Elec- 

 tricity, p. 219. As many of my readers may not have access 

 to that work. I will, with the permission of the Editors of the 

 Philosophical Magazine, quote a small portion of the paper, 

 which is necessary to complete the history of this interesting 

 branch of investigation. 



" A length of thirty feet of copper wire, one-twentieth of an 

 inch thick, and covered with cotton thread, was formed into a 



* Communicated by the Author. 



