“VARIATION or tur COMPASS. 253 
‘The beft inftrument for obferving the flar’s magnetic 
azimuth is a theodolite, furnifhed with fpirit-levels, and a 
{mall telefcope with a perpendicular wire. A common 
circumferentor may, however, anfwer the purpofe. When 
this inftrument is ufed, a fine thread or hair muft be 
firetched along from the top of one fight to that of the 
other, direQtly over the center of the compafs; and the 
-obferver muft be very careful to place the fights perpen- 
dicular to the horizon when he makes the obfervation; 
for this purpofe a fmall pocket fpirit-level, in the form of 
a carpenter’s f{quare, would be very convenient. 
By the common circumferentor we cannot, indeed, take 
the bearing of an obje& with very minute accuracy ; for 
though the eye can very well judge of the coincidence of 
two lines, or of the point of the needle with any whole 
degree on the compafs, yet the parts of a degree cannot 
readily be obferved to greater exactnefs than one third or 
one fourth of the whole. This inconvenience may, how- 
ever, be eafily remedied, and at a very trifling expence, 
in the following manner. 
Let one of the fights, by means of a fcrew, be made 
movable at right angles to the index ; and on the end of 
the index, clofe to the movable fight, fet off, on each fide 
of the central line, the tangent of three degrees to a radius 
equal to the whole length of the index, or diftance between 
the two fights. Let each of thefe degrees be divided into 
fix equal parts; then will a nontus divilion on the fight, 
where ten equal parts muft correfpond with eleven on the 
index, fubdivide thefe parts into minutes of a degree. 
It will be unneceflary to make the fight move in the 
arch of a circle, the difference between this and the tan- 
gent, in fo {mall an arch, being quite imperceptible. With 
this fimple improvement the common circumferentor will 
take the bearing of an object true to a minute, thus: Let 
the end of the needle be made exaétly to coincide with the 
Kk neareft 
