1898-1902. No. 6.] TERRESTRIAL MAGNETISM. 



THE OBSERVATIONS. 



When the apparatus had been duly levelled, a sight at the mark 

 was first made, with the .subsequent reading of both verniers of the 

 horizontal circle. The double needle was then placed upon the pin, and 

 the alhidade so turned as to bring the wire of the telescope into coinci- 

 dence with its reflected image in the mirror of the double needle, while 

 the ivory disc was kept rubbing on the projecting peg fixed for that 

 purpose upon the lid of the magnet-box. The moment of coincidence 

 was noted by the observation watch, and then both the verniers of the 

 horizontal circle were read. The setting of the needle was invariably 

 attended with great difficulty: the needle always oscillated backwards 

 and forwards, more or less regularly, often in jerks. After a varying 

 number of readings with the needle in both the positions, P, and P^, 

 the observation concluded with a fresh sight at the mark, and reading. 



If the reading of the position of the needle in the magnetic meri- 

 dian be called M, the mean of the readings of the sights at the mark 

 before and after, m, and the azimuth of the mark, reckoned from the 

 south point through west, a, we obtain, as the value for the easterly 

 declination, D: 



D = M—m-^a. 



The following fist contains all the determinations of decHnation made 

 during the Expedition, in chronological order, with the addition of the 

 hour in local time to the nearest minute for each setting of the needle. 



In some cases, in deflection observations, when the double needle 

 was used for tiie determination of the horizontal intensity as a deflected 

 magnet in both positions, P^ and P^, I have given the mean of the 

 readings belonging to the four positions of the deflector as the meridian 

 reading. Every such case is indicated by a record of the deflector used, 

 Vor VI, under the heading "Needle", beside the position of the double 

 needle, P, or P^. 



