WEBER ON A TRANSPORTABLE MAGNETOMETER. 583 



verification telescope is directed on the first needle A, in the 

 copper case as in the open air. The needle C is furnished 

 either with a collimator or a mirror, and is capable of reversal. 

 The direction of its magnetic axis is next to be found, i. e. the 

 angle jx, to which the theodolite must be adjusted, in order that 

 the optical axis of its principal telescope may be parallel with 

 the direction of the magnetic axis of the needle C, whence the 

 angle § (= t — (% — cfi) + ju.) is obtained, if the two needles A 

 and C are sufficiently distant apart to exert no sensible influ- 

 ence on each other, so that their magnetic axes may be regarded 

 as parallel. But if this be not the case, it is easy to determine 

 the angle v formed by the magnetic axes of the two needles*, 

 and to add it as a correction to the value, as above, of g ; i. e. 



? = T — (x — 4>) + f<' + ''- 

 The suspension of the needle in the wooden case is so con- 

 trived, that it may be used either as an unifilar or as a bifilar 

 magnetometer. This contrivance is represented in fig. 9. The 

 variations of the declination and of the horizontal intensity can 

 thus be observed at the same time ; the former with the mag- 

 netometer in the copper case, and the latter with the magneto- 

 meter in the wooden case. In preparing for the latter observa- 

 tions, the telescope of the theodolite is to be directed perpendi- 

 cularly to the magnetic meridian, and the magnetometer in the 

 wooden case is to be placed in the same direction. The time of 

 vibration t of the needle, with the unifilar suspension, must be 

 determined, if not already known, which it will generally be, 

 from the experiments of vibration belonging to the measure- 

 ment of the absolute intensity. The unifilar suspension must 

 then be changed for the bifilar without altering the direction of 

 the magnetic axis, and the time of \'ibration must be observed 

 afresh, the distance apart of the suspension threads being in- 



• From the propositions contained in the Resultate for 1837, page 22 etseq., 

 it follows that if A B C = 90°, A C B = «, and A C = j/, and if m and irJ 

 denote respectively tlie magnetism of the needles A and C, 



3 . „ m — m' 

 «= -77 sm 2 le . — -7p-. 

 2 r3 1 



The value of m and m' must be determined by the deflections S and 3' of a 

 compass needle placed successively east and west at the distance d, namely 



T ~ ^' T ~ 2 * 



