GAUSS AND WEBER ON TERRESTRIAL MAGNETISM. 53 



obsen^ations ; it is, however, not requisite, when a small change 

 is found, to bring back the scale to its former position ; it is 

 sufficient to note down in the registry the point of division cor- 

 responding to the plumb-line. 



It may probably not be superfluous to draw attention to one 

 or two points of comparatively minor importance. It has been 

 supposed, that the magnetometer and telescope are so arranged 

 that the mean position of the magnetic declination corresponds to 

 about the centre of the scale. However, at times of consider- 

 able vai'iation, this centre frequently gets entirely out of the field 

 of view, and then the above method of verification will no longer 

 answer. If at such a time the verification appears necessary, 

 the quieting bar must be made to perform an exactly op]jo- 

 site office to that which it generally serves ; namely, to give the 

 magnetometer a vibration of sufficient extent to reach, and even 

 to go rather beyond, the spot required, and thus to allow the 

 plumb-hne to appear in the middle of the field, at that part of the 

 vibration where the motion is slow, and where consequently the 

 corresponding division of the scale can be determined with accu- 

 racy. It is obvious that if such cases occur in the course of a 

 periodical series, the magnetometer must be again quieted in 

 time for the next observation, and, consequently, skill in the use 

 of the quieting bar is of great moment. 



When the declination falls very nearly in the centre of the scale, 

 unpractised observers must be on their guard not to confound 

 the plumb-line with the vertical line of the telescope. In our 

 apparatus both resemble one another so much, that with a very 

 quiet state of the needle, a mistake is very possible, and did, 

 indeed, once occur. When there is danger of such a mistake, 

 it may be expedient temporarily to remove the plumb-hne. 



With respect to the form of communication, some persons 

 are accustomed to send in the observations in full, others the 

 partial and final results only, and several merely the latter. The 

 last may be sufficient, if the calculations have been revised, and 

 the communicated numbers collated ; but the observations them- 

 selves should be preserved, in case a reference should be wished ; 

 and when unusually great changes occur, communication, in fuU 

 detail, is most desirable. Besides the results of the observations, 

 it is always proper to notice, in connection, the value of the parts 

 of the scale (or the measurements on which the determination is 

 founded), the time of vibration, the correction and rate of the 



