298 Royal Society. 



may be, the force of torsion must always, and may without any diffi- 

 culty, be taken into account with the greatest exactness. 



The two apparatus may likewise be made use of for another pur- 

 pose, which, though not immediately connected with the principal 

 subject of the memoir, may still be adverted to in this place. They 

 are the most sensible and convenient galvanometers both for the 

 strongest and weakest energies of the galvanic current. To measure 

 the strongest, it is only required to bring the conducting wire single, 

 and at a considerable distance (at least several feet), into the magne- 

 tic meridian below or above the needle ; for very weak energies a 

 multiplier is wound round the box containing the needle. Some of 

 the experiments were made with a multiplier of 68 circumvolutions, 

 producing a length of wire equal to 300 feet. No pair of large plates 

 is requisite; a pair of small buttons, or even simply the ends of two 

 different metallic wires dipped in acidulated water, produce a cur- 

 rent indicated by the movement of the image along many hundred 

 parts of the scale ; but on using a pair of plates of very moderate di- 

 mensions, the image of the whule scale, as soon as the circuit is com- 

 pleted, is seen rapidly to dart through the field of vision of the tele- 

 scope. It is obvious that by this method the measurement of galvanic 

 forces may be conducted with a degree of ease and precision unattain- 

 able by the hitherto employed laborious modes by means of observed 

 times of vibration ; and it is literally true that by it we are enabled to 

 follow from second to second the gradual decrease of the intensity of 

 a galvanic current, which, it is well known, is more rapid in the be- 

 ginning. If, in addition, instead of the single, a double (astatic) 

 needle is used, no degree of electro-magnetic energy will be found 

 too small to admit of being still measured with the utmost precision. 

 Here, therefore, a wide field is opened to the naturalist for most in- 

 teresting investigation. 



Not a small portion of this unpublished memoir of Prof. Gauss is 

 taken up by the developement of the mathematical theory j and also by 

 various methods peculiar to the author, such as the determination of 

 the momentum of inertia of the vibrating needle, independently of the 

 assumption of a regular figure j by his experiments with a view to esta- 

 blish the above-mentioned fundamental law for the magnetic effects j 

 and, finally, by the details of the experiments to determine the value 

 of the intensity of terrestrial magnetism, of which last the following 

 may be given as the results, as far as they relate to the intensity of 

 the horizontal part of that force. 



1. May 21 17820 



II. May 24 1*7694 



III. June 4 17713 



IV. June 24— 28 .. 17625 

 V. July 23, 24 .. 17826 



VI. Julv25, 26 .. 17845 



VII. Sept. 9 17764 



VIII. Sept. 18 17821 



IX. Sept. 27 17965 



X. Oct. 15 17860 -yd 



