EULOGY ON AMPERE. 139 



eliminate it; bow intercept it "? I see some smile at my question, and 

 bear them exclaim, Do not mariners cover with pieces of canvas or pea- 

 jackets the iron cannon in the neighborhood of the compass, wheu- 

 ever they wish to obtain exactness in their bearings ? Screens, then, 

 ought efiectually to furnish the means of protecting the needle from 

 terrestrial magnetism. As to that, a glass sphere, inclosing the com- 

 pass, would answer. 



A single word will dispel this illusion. Xo substance, thick or 

 thin, has yet been discovered through which magnetic action, like that 

 of gravity, does not exert its full force, without any abatement. The 

 custom of covering cannon, balls, and anchors, with sails, tarred or un- 

 tarred, or with anything else, to prevent their action on the compass, 

 belongs to the thousand and one usages recorded in treatises on naviga- 

 tion before science had diffused its light around it. Even when ex- 

 posed, such usages diffuse and perpetuate themselves, and are the blind 

 powers which govern the world. The researches of Ampere did not abso 

 utely require (which, in fact, would have been an impossibility) that his 

 apparatus should be completely free from the attraction of the earth; it 

 was suflicient that this attraction should not counteract the movement 

 of the needle ; and this simple reflection became the ray of light that 

 guided the illustrious physicist, and gave rise to a kind of compass 

 never before thought of. 



To understand the invention of Ampere by vrhich a magnetic needle 

 could be so arranged as to be free to obey the action of a galvanic cur- 

 rent, and undisturbed by the magnetism of the earth, suppose an ordi- 

 nary dipping-needle apparatus so placed that its graduated circle shall 

 be perpendicular to the magnetic meridian of the place, and then so 

 inclined to the horizon that the graduated circle and the needle within 

 it shall be at right angles to the direction of the magnetic dip of the 

 place where the experiment is made. In this condition the magnetism 

 of the earth will act perpe ndicular to the direction of the needle and be 

 opposed by the pivot on which the lower point of the axis rests. It will 

 therefore be free to take any position in the plane of the divided circle 

 which an extraneous force may give it. Ampere was therefore quite 

 right in calling his new instrument an astatic compass. 



Ampere's astatic needle, placed before a conducting wire, takes a 

 direction exactly perpendicular to that wire, neither one second more nor 

 less ; and, a very remarkable circumstance, a very feeble electricity ijro- 

 duces as much effect as a current of sufficient intensity to reduce metals 

 to a state of incandescence. Here, then, is one of those simple laws that 

 science loves to record, and the mind receives with confidence, and be- 

 fore which false theories will inevitably disappear. 



The discovery of Oersted reached Paris through Switzerland. At our 

 weekly meeting on Monday, September 11, 1820, a member of the 

 academy from Geneva repeated before you the experiment of the Dan- 

 ish savant. Seven days later, on the 18th of September, Ampere pre- 



