363 
iron arismgfi'om its rotation. 
of the instrument, I will here briefly notice their effects. The 
angle which the axis SN, fig. i. makes with the horizon 
H O, being 70° 30', if S O N is in the magnetic meridian, and 
the compass is adjusted, so as to have its centre in the centre 
of the instrument, SN will be the magnetic axis, and the 
centre of the plate, as there represented, would, by the rota- 
tion of the limb Si^^N, describe a parallel of latitude, its 
longitude being indicated on the circle R r : and if the in- 
dexes at 0, o' be brought to coincide with the centre of 
the plate would then describe the equator. If the limb SQN 
be slided through G G' until the index there coincide with 
19° 30', from O towards S, and the indexes 0, o' be made to 
coincide with R as represented in Fig. 2, the centre of the 
plate, by the revolution of the limb, would describe a secon- 
dary both to the meridian and equator, and its latitude would 
be indicated on the circle R r. If the point Q, Fig. 1 , or zero 
on the limb SQN, be brought to coincide with the index at 
g, and the instrument make a quarter of a revolution about 
GI , so that the index at K may point to 90°, the centre of 
the plate would describe the meridian when the indexes at 
0, o' coincide with ce ; and the latitude would be deter- 
mined from the degrees indicated on Rr. This is repre- 
sented Fig. 3, where the contrary side of the instrument to 
that seen in Fig. 1, 2, is placed in front, in order to show 
the situations of the screws, which clamp the arm A B and 
the limb S Q N in their respective situations. Thus, by a 
proper adjustment of the indexes at K,^, 0,0', the centre of 
the plate may be made to describe any circle of the sphere. 
After making a very few sets of experiments with this 
instrument, I found that it was necessary to attend very par- 
