eS 
165 
is within, and the stem (bent at right angles) on the outside 
front. 
It is intended to employ this apparatus for three pur- 
poses; namely, to determine, first, the magnetic declina- 
tion; secondly, the periodical and the irregular variations 
of the declination; thirdly, the corresponding variations 
of the horizontal magnetic force. 
In using the instrument in the determination of the declina- 
tion, it is to be combined with the theodolite and the transit in- 
strument. The transit instrument is to be fixed close to the 
southern window of the observatory; there being also an 
aperture in the roof for the purpose of adjusting it to the 
meridian by means of the pole star and 8 urse minoris. The 
centre of the theodolite is placed, as nearly as possible, at the 
point where the line of collimation of the transit instrument 
intersects that of the magnetic bar. When an observation 
is made, the telescope of the theodolite.is directed to the 
lens of the magnetic collimator, and the vertical wire of the 
latter is made to’ bisect the cross in the focus of the telescope. 
When this is done, the line of collimation of the telescope 
is parallel to that of the magnetic bar. But as the latter 
line may not coincide with the magnetic axis of the bar, a 
similar observation is to be made with the bar inverted; and 
the mean of the two readings will obviously give the direc- 
tion of the magnetic meridian, freed from the error of colli- 
mation. To determine the angle between this and the érue 
meridian, the transit telescope is to be turned over, and em- 
ployed as a collimator. The telescope of the theodolité 
being directed to its object glass, the middle wire in the focus 
of the transit is to be observed, in the same manner as the wire 
of the collimator bar was in the former part of the observa- 
tion. The line of collimation of the theodolite telescope is 
then in the true meridian; and the angle read off on the 
limb i is the supplement of the declination. — 
. In observing the diurnal and irregular variations of the 
