JI2 DIFFERENCES IN THE MAGNETIC NEEDLE. 



The author's theodolite in this place I obferved to be S'' 2' E, and the 

 obi. at Pier inclination of the foiith end of the dipping needle 50" 50', the 

 needle flood vertical when the face of the inftrument was 

 S 2° E. I then took the following bearings : Extenfive Mount 

 108** 30', the fame exa6tly as by back bearing. Double Peak 

 11-3° 30'; from hence I rowed round the Head, and landed 

 on a rock, whence the top of the hill bore SSW one-fixth of 

 a mile; Extenfive Mount bore 110° W, the inclination of the 

 dipping-needle 50° 29', and the needle flood vertical when - 

 the inftrument faced S 3° E. Thus the difference was 1|° in 

 the horizontal, and -f" in the vertical direftion of the needle, 

 Afcending the hill, I. made the following obfervations on the 

 top: Extenfive Mount 113° 50', a ifland 133^52', Double 

 Peak 148° 32'; the inclination of the needle was 53'' 20', and 

 is flood vertical at S 3° E. The differences here are 5** 10' in 

 the horizontal, and 2° 30' in the vertical direftion, from what 

 the needle flood at in the firft morning's place. On moving 

 ten yards SSE, the bearings were, Extenfive Mount 108° 44-', 

 Double Peak 143** 25'; the inclination was 52° IS', and the 

 needle was vertical when the inftrument faced S 5° W. In this 

 4th fet of obfervations, the horizontal diredion of the needle 

 is only a few minutes different from the firfl place, but the 

 vertical direction is 1° 28'. From the top of the hill I now 

 moved twenty yards (o the north-eaflward, when Extenfive 

 Mount bore llO**, Double Peak 144** 42'; the inclination of 

 the dipping needle was now 50** 35\ and it flood vertical at 

 S 3® W. Thus it appears that the polarity of the magnetic 

 needle is moft interrupted at the top of the hill, both ac- 

 cording to ihc theodolite and dipping-needle. Whether this 

 may arife from fome particular magnetic fubftance lodged in 

 the heart of the hill, or from the attradive powers of all the 

 fabftances which compofe Pier Head being centered in a 

 fir.iilar point to what I have fuppofed to take place vvilh all the 

 ferruginous bodies lodged within a ftiip, I fiiall not attempt 

 to decide. The greater differences in the horizontal direction, 

 of the needle obferved by Captaia Cook, might have ariieu 

 from his ufing a ommon azimuth c;;T)pa("s, which was pro- 

 bably not further elevated from the ground than to be placed 

 (Jn a ftone. 



'■ ■• - MATTHEW FLINDERS. 



fjlc of France, 



' Manii5l\\, 1804. 



Letter 



