March 4, 1880] 



NATURE 



419 



two distinct sciences, in appearance so different, were 

 resolved into one, and all the facts which they compre- 

 hended were connected by one identical principle. If 

 the theoretical consequences of CErsted's discovery were 

 considerable, those which had regard to economic and 

 industrial applications were incalculable, and were to 

 cause a veritable revolution in the relations of mankind. 

 But if this grand discovery opened out such vast horizons 

 and disclosed a new world, it required the concourse of 

 genius to effect the conquest. France has still the best 

 part of this honour, thanks to Ampere and Arago. 



Within a week CErsted's experiments had been repeated 

 before the Academy. Already Ampere brought before it 

 his discovery of the reflex action of the currents, and he 



had laid the foundation of that magnificent chapter of 

 electro-dynamics, one of the finest, most profound, and 

 most perfect of which the science of all times can boast. 

 A week later anil it was Arago's turn, he having discovered 

 the attractive action of the current in iron filings, a dis- 

 covery of which he made good use as we shall soon see. 



Arago then, as he himself informed us, showed his 

 experiments to Ampere, and these two great physicists 

 for a brief period united their efforts. The object pursued 

 was the magnetisation of steel by the current. From the 

 first, Ampere guided by his new views on the constitution 

 of the magnetism in the magnet, saw at once the con- 

 ditions of success. He indicated that in order to obtain 

 a steady and powerful magnetisation, it is necessary to 



roll up in screw-form a part of the wire conductor and at 

 that point to place the needle ; and his theory is so sure, 

 so precise, that he assigned the position of the poles in the 

 magnet so as to give in the spiral the force of the current 

 and the direction of the screw. The prediction has been 

 entirely confirmed by experience. 



These leading experiments thus established the prin- 

 ciple of the electro-magnet, on which has been based 

 for the most part the mode of action in electric-tele- 

 graphy. 



But all the world knows that this admirable mechanism 

 by which the properties of the magnet are associated 

 with those of electricity, has since received almost num- 

 berless applications in science and industry. We might 

 cite by the thousand these magneto-electric motors, these 

 clocks which disseminate in every town the time which 

 the electric wire draws up to a central regulator, these 

 checks on the railway which arrest a train so efficiently in 

 the presence of a danger signal, &c. 



And now, gentlemen, a still greater future seems 

 reserved for the electro-magnet. This marvellous facility 

 of developing at a distance by means of a simple electrical- 

 conducting wire a magnetic power capable of raising 

 enormous weights occupies the engineers of the present 

 day, and it seems that the time is not far distant when 

 the telegraphic wire will transport afar mechanical force 

 even as it now transmits human speech. 



Such, gentlemen, were the fruits of the momentary 

 union of these two men, so great and yet so different. 



Perpignan, to its great credit, pays this day to Arago, 

 a portion of the debt of France. I desire to express here 

 the hope that the city of Lyons will equally honour the 

 memory of Ampere, the immortal founder of electro- 

 dynamics, the geometric scholar, the philosopher who has 

 pointed out the principle in galvanometry on which is 

 based to-day the grand system of inter-oceanic telegraphy, 

 the man, finally, whose candour equalled his genius, and 

 whose slightest ideas are almost always marked with the 

 stamp of keenness and profundity. 



In order to conclude the grand series of Arago's dis- 

 coveries, I ought to recall that of magnetism by rotation ; 

 it belongs to the year 1824. Humboldt tells us that Arago 

 made it on the slope of the beautiful hill at Greenwich 

 during some operations bearing on the measurement of 

 magnetic intensity. Arago remarked that a magnetic needle 

 attained repose sooner when it oscillated within its copper 

 frame than when separated. This was but the first link 

 in a chain of fertile truths which led Faraday to the 

 great discovery of induction. 



Gentlemen, it is impossible in sketching" the life of 

 Arago, not to recall the importance of his teaching 

 and of the writings which he devoted to the 

 diffusion of scientific learning. If his discoveries and 

 his labours merit the recognition of posterity, his pen 

 and his speech were the better part of the great influ- 

 ence which he exercised in his time. We know with 

 what avidity his memoirs were read on their appear- 

 ance. The collection of our Anuuaircs du Bureau des 

 Longitudes preserves also the trace of an ircident which 

 shows the impatience of his readers. One year Arago, 

 absorbed by some important work, allowed his Annuaire 

 to appear without a summary. The press rebelled and 

 made itself the echo of the public displeasure. They 

 even went the length of contending that the Bureau had 

 failed in the duties which were imposed by its regulations. 

 There was nothing of the kind ; but Arago understood to 

 what an extent this sentiment was flattering to him, 

 although expressed in an indirect and hardly courteous 

 manner. He executed and published apart a memoir 

 which was given gratuitously to all purchasers of the 

 Annuaire. His biographic memoirs, his academic 

 reports, his sessional lectures, his analysis of correspon- 

 dence as permanent secretary were the object of an 

 interest which is never disappointed. His admirable 



