GAUSS AND WEBER ON TERRESTRIAL MAGNETISM. 93 



Of course, however, these observations could not furnish an 

 accurate determination of the amount of en-or introduced, as the 

 declination may have altered during the interval, which amount- 

 ed to more than an hour. 



In the graphic representation the second half of the Marburg 

 curve has been drawn on a reduced scale, the reduced divisions 

 representing 28 on the Marburg scale. 



I may here mention a second case of a similar kind. The 

 time of vibration of the magnet bar in Breslau, which, in March 

 1836, amounted to nearly 3-2 seconds, had from that period to 

 November gradually increased, making altogether an increase of 

 about 0"'4. This is no unusual circumstance, as all magnetic 

 bars in the course of time lose some part of their force, though 

 in very various degrees dependent on the unequal tempering of the 

 steel and other circumstances. But, from November 1836, to 

 January 1837*, a decrease in the time of vibration of l"-27 took 

 place. Prof, von Boguslawski, who informed me of this remark- 

 able circumstance, seemed inclined to attribute it in part to an 

 increased intensity of the ten-estrial magnetism. I did not doubt, 

 howevei', that the cause must be sought in the immediate neigh- 

 boiu-hood of the magnet bar, probably in some imjaediment to 

 its free motion, and this supposition was verified by the follow- 

 ing letter of M. Boguslawski : — • 



" You were right in your supposition as to the cause of the al- 

 teration in the time of vibration. By a slight accidental dis- 

 placement of the box, the edge of the small aperture through 

 which the suspension thread passes, had been brought near the 

 thread, though by no means into contact with it. However, some 

 of the finer fibres of the silk must have been touched thereby, for 

 when it was again made to pass quite through the centre of the 

 aperture, the time of ^dbration was found almost identical with 

 that formerly observed." 



This is perhaps the place for some remarks on the movements 

 themselves, which are here represented during six terms. 



In the thi-ee summer terms, (Plates VI. VII. and VIII.) not- 

 withstanding all the great anomalies, the regular diurnal move- 

 ment is clearly seen in the curves, ascending during the hours 



• Probably during the interval no determinations of the time of vibration 

 had been made. 



