50 GAUSS AND WEBER ON TERRESTRIAL MAGNETISM. 



rations are made at every 2| minutes, as long as it may appear 

 desira])le to do so. An example is added : 



10'' 22' 



23 



875-501 



875-05 I 



876 40 >■ 876-27 for IC' 22' 30 



870-60 I 



876-!)0J 



Observers in general are requested to pursue the course here 

 pointed out whenever occasion may require it ; and, in such case, 

 it cannot be doubted that, whenever changes of such magnitude 

 occur, a body of con'esponding observations in close detail will 

 be collected, and will furnish interesting conclusions respecting 

 these remarkable phaenomena. 



If observers, instead of a clock beating seconds, are furnished 

 Avith time-pieces markmg other diAisions of time, they must ar- 

 range their obsen^ations in an analogous manner, corresponding 

 to the beats of the time-piece. The observations with a chrono- 

 meter are more difficult than with a clock, particidarly if the 

 second hand is not truly centred, as is sometimes the case. 



It may be well to add some general precautions for unprac- 

 tised observers. 



It is of the first importance that the movement of the needle 

 should be perfectly free. Spiders sometimes get into the box, 

 and attach their web to the needle. This may be so fine as pos- 

 sibly to escape obsen'ation with the eye. Previously to each 

 term, therefore, the finger should be passed carefully round the 

 needle on every side. Any impediment which may exist to free 

 motion will diminish the time of vibration of the needle. The 

 most minute spider's thread has a very considei'able effect in 

 this respect, of which a curious example will be related in its 

 place. 



In night observation it is necessary to illuminate the scale, 

 which, at Gottingen, during the term-observations, is done by 

 means of two Argand lamps. There is always an upwai-d cur- 

 rent of heated air above the flame, and, therefore, if one of the 

 lamps is placed near and below the telescope, such a current 

 passing before the object-glass will impair the distinctness of 

 vision, and cause the divisions of the scale to appear tremu- 

 lous and xmdidating. This inconvenience frequently occurred 



