212 Description of the Instruments 
at any instant of time by a telescope placed at a distance. 
Each division of the scale is in actual space the ^^th 
part of an inch, corresponding in angular quantity to 
0'‘7085,or 42"‘51; and the visual angle under which it is 
seen in the telescope is so considerable, that each divi¬ 
sion can be subdivided mentally into tenths, and thus 
the changes of position noted within every 4" of arc 
nearly. 
The reading telescope may be placed upon a stone, or 
wooden pedestal, at a distance of 8 or 10 feet from the 
centre of the magnetic bar, thus rendering it unnecessary 
to approach the instrument for the purpose of observ¬ 
ation. It is furnished with two wires, one horizontal 
and the other vertical, which ought to be correctly fixed 
in its line of collimation, or optical axis; and, on first 
adjusting the instrument, it is necessary to observe the 
points of the scale coinciding with the vertical wire when 
the magnetic bar is in the direct and inverted positions.* 
Half the sum of these readings is the magnetic axis, or 
zero point of the bar; and half their difference is the 
deviation of the line of collimation of the telescope from 
the magnetic meridian: the telescope should be then 
moved, until its vertical wire coincides with the point of 
the scale, corresponding to the magnetic axis of the bar. 
The angle which the vertical wire of the telescope forms 
with the true meridian becomes constant, and can be 
readily determined by any instrument for measuring 
horizontal angles; and when the telescope is thus per¬ 
manently fixed, it is evident that as the different parts of 
the scale which is carried on the bar are successively 
brought on to the vertical wire, each change must be a 
change in the declination. Every change in the position 
of the scale becomes, in fact, a determination of the 
* This is effected by merely turning the magnetic bar over: in one 
case the scale is above the bar; in the other, below. 
