DIFFERENCES IN THE MAGNETIC NEEDLE. 1 ] J 



fome of Ihe loofe ftones that lay upon the ground, and 

 applied (hem to the needle, but they produced no efFeft ; 

 and I iherel'ore concluded that there was iron ore in the hills, 

 of which I had remarked other indications, both here and in 

 tlie neighbouring parts, " 



On landing at Pier Head 1 found the ilones lying on the The author's 

 furface to be porphyry, of a dark bluifli colour; but although ^^^^^^^ '^"^ 

 I undcrftand this fpecies is ufually found to poflefs fome 

 magnetic po^^'er, a piece did not produce any fenfible effe&. 

 upon the needle of the tlieodolite when applied to it. In the 

 following obfervations the theodolite always Hood about four 

 feet from the ground, that being nearly the length of its legs. 

 I firfl; took an extenfive fet of healings from the top of the 

 hill, amongfl: which were two flalions whence Pier Head had 

 been before fet. The firfl, called Extentive Mount, diftant 34 

 miles, differed from its back bearing 4* S5^ to the right, and 

 the fecond, illand a, diflant 29| miles, differed 4** 45'' the fame 

 way. I now moved the theodolite three yards to the wetlward, 

 and the fame two obje£ts bore 2° 10' to the right of their back 

 bearing ; on moving it three yards lo the fouth-eaftward from 

 the firfl place, they differed 2* to the left ; and on moving the 

 theodolite four yards to (he northward, the fame two objefls 

 bore I*' 10' lo ihc right of their back bearings. On the fol- 

 lowing morning I determined to try the magnetifm more par- 

 ticularly. Taking the thedolite and dipping-needle, I landed 

 upon the fliore of the Head, whence the top of the hill bore 

 N 50° W, about one-third of a mile. The variation of the 



degrees from its direftion, but it then fwung back to its error of* 

 bO° where it was ftationary. In Arnhem Ibiith bay a fmall piece 

 of llmilar ftone diew the needle of the theodolite entirely round, 

 yet the bearings taken in this place did not fhow any difagreement 

 from the variation and bearings taken in the neighbouring places, 

 where the ftone did not produce any fuch efFecSt. In moft place* 

 on fl)ore, where I had occafion to take angles, it was my ptaftice 

 to try the efFeft of a piece of the ftune upon the theodolite, in order 

 to detc6t the prefence of iron ore, as well as on account of my 

 furvey. It commonly happened that no efFL<5t was apparent, but 

 yet I could not truft implicitly to the angles, (particularly on the 

 main land,) unlefs obfervations f.»rthe vaii-.ttion were taken before 

 the inftrumentwas moved, or I had a back bearing of fome ftaticn 

 where uich oblervationi had been mad',', 



K 2 theodolite 



