278 W. WEBER ON THE ARRANGEMENT AND 



passes ; the upper end of the pivot carries the mirror, which 

 must be above the box. The two wires having the mirror be- 

 tween them pass through the same aperture. This circular aper- 

 ture is usually closed by two semicircular flaps, in which there 

 are small slits for the pivot and the wires. 



8. Before the magnet bar is laid in the stirrup, a weight of the 

 same size, but unmagnetic, is placed therein, and the wires are 

 suffered to arrange themselves in their natural position, in which 

 both are in one vertical plane throughout their whole length. 

 The alidade of the stirrup is then brought as exactly as possible 

 into the mean magnetic meridian from which the changes of 

 variation are to be measured. The other alidade on the pivot 

 of the mirror should be so fixed as to form a right angle with 

 the alidade of the stirrup, in order that the noniuses may be far 

 apart. The weight in the stirrup is moved until the mirror is 

 situated exactly between the two wires, when the axis of the 

 mirror should be very nearly horizontal. Employ the first ro- 

 tation to direct the mirror towards the scale, Mdthout disturbing 

 the alidade. If the scale does not appear in the telescope, it 

 will be seen by the naked eye a little above or beneath, and may 

 be brought into the field by the help of a light running weight 

 placed on the stirrup. The first observation is then performed, 

 and the position of the scale determined. 



9. The time of vibration for determining the directive force of 

 the wires may be observed before the magnet-bar is inserted, 

 and again Avith a known increase of the moment of inertia. 

 It is better, however, to perform this experiment somewhat later, 

 when the distance of the Avires from each other has been accu- 

 rately adjusted, in case this distance has not been previously 

 determined by calculation, and regulated accordingly. 



10. The magnet-bar is then placed in the reverse position, 

 (north towards the south) and the position of the scale again ob- 

 served : this ought to agree with the observation (8.) If the 

 two readings do not coincide, agreement must be attained by 

 merely turning the stirrup with its alidade. The coincidence of 

 the two readings proves that the magnetic axis of the bar is 

 situated in the magnetic meridian. The less the directive force 

 arising from the mode of suspension exceeds the magnetic di- 

 rective force, (see 8.), the more delicate is this test, so that it 

 may be impossible to obtain a perfect coincidence of the two 

 readings ; a difference of a few divisions of the scale may then 

 be considered as unimportant. The influence of the hourly va- 



