Intelligence and Miscellaneous Articles. 



493 



that of ccy, since the pitch of the reciprocal screw in this case is 

 infinite, and therefore P is zero. 



We hope that the account we have now given of the author's 

 Theory of Screws, so far as it goes, will be intelligible to readers 

 unacquainted with the subject, and that it may induce some of them 

 to peruse the work itself. Our limits will only allow us to add that 

 in an Appendix of sixteen pages Mr. Ball gives an account of the 

 memoirs bearing on the subject of his treatise. 



LVIII. Intelligence and Miscellaneous Articles. 



ON THE POLAR AURORAS. BY G. PLANTE. 

 " IVTOTHINGr should be neglected that can induce complete con- 

 -^ viction, in the theory of the forces of nature, and enable us to 

 pass from the disquiet of investigation to the security of known truth. 

 Should we be quite sure, for example, of the theory of the rainbow, 

 if we had not, by spirting drops of water in the sunshine, repro- 

 duced in all its details that brilliant phenomenon ? The experi- 

 ments of the cabinet are modest, but useful, and therefore valuable." 

 These words of an illustrious Member of the Academy encourage 

 me to pursue the analogies between the effects of electric currents 

 of high tension and the grand electrical manifestations of uature. 

 De la Rive's experiment has already brought out the connexion of 

 the polar auroras with terrestrial magnetism, but does not suffice 

 to explain all the circumstances accompanying them. In the ex- 

 periments which are the subject of the present memoir the electric 

 flow takes place in the presence of aqueous masses, as in the atmo- 

 sphere ; and hence result a series of phenomena altogether similar 

 to the various phases of the polar auroras. 



1. If the positive electrode of the powerful secondary battery 

 which I employ is put in contact with the moist sides of a vessel 

 containing salt water in which the negative electrode is previously 

 immersed, there is seen, according to the greater or less distance 

 of the liquid, either a wreath round the electrode (fig. 1), or an arc 

 bordered with a fringe of bright rays (fig. 2), or a sinuous line 

 Fig. 1. Fig. 2. 



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