OF THE MAGNETIC NEEDLE. 



7 



To understand whether this correction be + or — ■ we are to consider 

 1st. That the line joining 0° and 100° on the limb, (north end) passed to the 

 east of the line described by the cross wires ; 2nd. That the line of north 

 and south in the compass-box (north end) also passed to east, but by a 

 greater quantity. Then the north and south line in the compass-box passed 

 (north end) to east of the line joining 0° and 180° on the outer limb. The 

 point to which the needle should have been adjusted is, consequently, 14' 24" 

 west of the point to which it is actually adjusted. Now, let us suppose the 

 magnetic bearing of the meridian taken, it will be 2° 26' 52''' north-east ; but 

 if the point from which the divisions are reckoned, and with it each of the 

 divisions were moved 14' 24" west, then the above bearing would be 14' 24" 

 more. Adding this quantity to 2° 26' 52''' we get 2° 41' 16" as the correct de- 

 clination in Calcutta, in February 1828. It is worthy of remark, that M. De 

 Blossville's determination (see Table I.) is 2° 38', being only 3' 16" less. 

 His instrument did not require this correction, as the needle and teles- 

 cope were referred to the same set of divisions. Whether the circum- 

 stance of its having no nonius for reading the sub-divisions will account 

 for the above small ditference of 3' 16", I cannot pretend to say. It is to be 

 observed, that even with the same observer and instrument, the latter 

 furnished with three nonii, two different needles may vary 2' 24". 



To this cause I am disposed to attribute the discrepancies observ- 

 able in taking magnetic bearings with different theodolites, which I have 

 found sometimes amounted to 1° 30'. When the magnetic declination is 

 observed with the same theodolite with which the bearings were observ- 

 ed, this becomes a matter of no moment, but it must always be an obstacle 

 in determining the absolute amount of the magnetic declination. For 

 this reason I think no great stress can be laid on the contents of Table III. 

 compiled by Lieutenant Colonel Hodgson, which I, nevertheless, give, as 

 it may attract the attention of the several observers to the subject, and 

 induce them, perhaps, to verify their results in the manner indicated. 



