GAUSS AND WEBER ON TERRESTRIAL MAGNETISM. 87 



From this rule then we obtain the most advantageous di- 

 mensions of such an apparatus, by knowing the Hmit in re- 

 spect to thickness, which is determined by the nature of the steel. 

 The thickness of the bar must not amount to much more 

 than 12i milUmetres, as otherwise the steel cannot be properly 

 hardened and magnetized throughout. We thence obtain the 

 following dimensions of the deflecting bar, as those which com- 

 bine the greatest advantages, namely, for its breadth and thick- 

 ness 12| millimetres, and for its length 100 millimetres. We 

 have also the length of the magnetic needle 100 millimetres, 

 and the smallest admissible distance between them, 300 milli- 

 metres. 



By following these rules we obtain an apparatus, \\\th. which, 

 in mean latitudes, the smallest deflections to be measured ex- 

 ceed 22°, as in the experiments related. At greater distances 

 from the magnetic poles of the earth, this deflection becomes 

 somewhat smaller; nearer to the magnetic poles it is much 

 larger. Therefox'e, if these deflections can be accurately mea- 

 sured to within a tenth part of a degree, a final result can be 

 obtained to within the 200th part of the force itself; since all other 

 measurements required in the determination of the absolute in- 

 tensity can be made with greater accuracy. This result, it is 

 true, is far inferior to that which can be obtained with the mag- 

 netometer ; but such results may still be of great utihty in the 

 absence of more accurate determinations. 



Weber. 

 V. 



Explanations of the graphical representations, and of the 

 table of results. 



In Plates IV.-IX. are given the graphical representations of 

 the changes of declination during six terms, amounting, in all, 

 to forty-six curves, from fourteen stations, viz. BerHn, Breda, 

 Breslau, Catania, Freiberg, Gottingen, the Hague, Leipzig, 

 Milan, Marburg, Messina, Munich, Palermo, and Upsala. The 

 graphical representations begin with the November term of 1835, 

 when the Association was strengthened by the accession of seve- 

 ral new and zealous cooperators. The representations of two 

 terms of the year 1836 have been omitted, viz. those of March 

 and May, as the changes they present are comparatively less in- 

 teresting than those of the two terms of January and July, be- 



