364 



On the 4th September, 1820, he gave an account to the Academy of 

 the recent investigations of the Danish philosopher, and was in con- 

 sequence charged to repeat them before that body. In preparing 

 these experiments he ascertained some hitherto unobserved pheno- 

 mena. M. QErsted had discovered the action which the voltaic 

 current exerts on a steel needle previously magnetized', M. Arago 

 found that the same current developes the magnetic power in bars 

 of iron or steel, which are at first deprived of this property, and that 

 the magnetism thus communicated to soft iron is temporary, ceasing 

 with the removal of the current, while that imparted to steel is per- 

 manent. He also found, adopting a suggestion of Ampere's, that 

 steel needles were more strongly magnetized by placing them within 

 a helix forming part of the circuit. These discoveries subsequently 

 led, in the hands of others, to the invention of the electro-magnet 

 and its valuable applications ; also to a ready means of making per- 

 manent magnets of great power. 



On the 22nd of November, 1824, Arago communicated to the 

 Academy the results of a series of experiments, showing the influence 

 which metals, not magnetic, and other substances exert on the mag- 

 netic needle ; this effect being to diminish rapidly the amplitude of 

 the oscillations without sensibly changing their duration : and on 

 the following 7 th of March he announced a still more remarkable 

 discovery, the reverse, as it were, of the preceding. Since a needle 

 in motion is stopped by a plate at rest, I\L Arago thought that a 

 needle at rest ought to be carried along by a plate in motion; and he 

 accordingly found that on rotating a copper plate, for instance, beneath 

 a magnetic needle, the needle was moved out of the magnetic meridian 

 and stopped in a position more distant from it in proportion as the 

 rotation of the plate was more rapid ; and if this motion were suf- 

 ficiently quick, the needle rotated itself continuously round the 

 thread from which it was suspended. He added some further facts 

 relating to this subject on July 23rd, 1826. These beautiful ex- 

 periments proved the important fact, that bodies, which in a state of 

 rest exert no action on a magnet, become capable, when they are in 

 motion, of acting upon it as if they are magnetic. Mechanical 

 motion was thus, for the first time, shown to play an important part 

 in the manifestations of electric phenomena. IS^o satisfactory ex- 

 planation was, however, given of these experiments until Faraday, 

 in 1832, referred them to the more general laws of electro-magnetic 

 induction which he had then discovered. 



In 1839, Arago laid before the Academy a proposed system of experi- 

 ments by means of which the theory of emission and that of undula- 

 tions might be submitted to decisive proofs. The object of this commu- 

 nication was to show that the method invented and employed by Mr. 

 Wheatstone to measure the velocity of electricity in metallic conduct- 

 ors, was equally applicable to measure the comparative velocities of 

 light in air and in a liquid. By following the indications of this mt;moir 

 and employing Mr. Wheatstone's revolving mirror, aided by many 

 original and ingenious arrangements of his own, M. Foucault has re- 

 cently completely succeeded in realizing these anticipations of Arago. 



A passing allusion is all that can be made to other important in- 



