GAUSS AND WEBER ON TERRESTRIAL MAGNETISM. 85 



measured with the accuracy it deserves only with the magneto- 

 meter. It is therefore unnecessary to state that such extreme 

 accuracy cannot be attained with the small apparatus. And in 

 order to obtain with it a good approximation, it must combine 

 all the advantages of which it is susceptible. 



The difficulty of an accurate measurement of intensity, with 

 other instruments than the magnetometer, is thus stated in the 

 memoir " On Terrestrial Magnetism and the Magnetometer :" 



*' In all cases, if the elimination is to be satisfactory, the ex- 

 periments must not be performed at too small distances ; conse- 

 quently the effects are always comparatively small, and the 

 means j^re^dously in use are inadequate to measure them with 

 the necessary precision. It is this difficulty which has called 

 for, and has given rise to the construction of a new apparatus, 

 which may with propriety receive the name of magnetometer, 

 since it serves to execute, with an accuracy equaling that of the 

 most delicate astronomical determinations, all measurements — 

 both of the force of magnetic needles, and of the intensity of the 

 earth's magnetism (at least its horizontal portion). The (hori- 

 zontal) direction of the earth's magnetic force is determined ac- 

 curately with it to w ithin one or two seconds of arc ; the com- 

 mencement and termination of a vibration is observed with it to 

 within a few hundi-edths of a second of time, and consequently 

 more acciu'ately than the passage of stars behind the wires of a 

 transit." 



There are two cii'cumstances, chiefly, on which the accuracy 

 of an absolute measurement of intensity depends ; first, the 

 magnitude of the deflection produced ; secondly, the delicacy of 

 the instrument in measuring this deflection. In constructing an 

 apparatus for this purpose we may therefore follow two different 

 paths : we may either make the amount of deflection the main 

 object, and pay only as much attention to the means of mea- 

 surement as may be consistent therewith ; — or we may attend 

 chiefly to accuracy in the means of measurement, and let the 

 amount of the deflection be the second object. The latter ^^lan 

 leads to much greater accuracy than the former, for this rea- 

 son : the amount of deflection soon attains a limit, on account 

 of the necessary condition of a considerable distance between 

 the deflecting bar and the needle, so that the deflection produced 



