A P P E N D I X. 



Account of the Observations made with the MARtNE 

 Dipping Needle, conftrufted for the Board of Longi- 

 tude by Mr. Nairne, from whom I received the following 

 defcription of the inftrument. 



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'TT^HE figure (plate 9.) is a reprefentation of the 

 inllrument, hanging by an univerfal joint on a 

 " triangular fland. It is adjufted fo as to hang in a plane 

 ** perpendicular to the horizon, by means of a plumb line, 

 ** which is to be fufpended on a pin above the divided 

 ** circle, and the dovetail work, which alters the pofition 

 " of the inftrument, by turning the button A. The two 

 " 90° on the divided circle, are adjufted fo as to be per- 

 *' pendicular to the horizon, by the fame plumb line and 

 *' the adjufting fcrew B : and at the loweft 90°, when 

 "' it is adjufted, the pointer C is fixed. The length of the 

 " magnetic needle is twelve inches, and its axis (the ends 

 ** of which were of gold alloyed with copper) refted on 

 " fridion wheels of four inches diameter, each end on two 

 *' fridlion wheels ; which wheels were balanced with great 

 *' care. The ends of the axes of the fri£tlon wheels were 

 " likewife of gold alloyed with copper, and moved in fmall 

 " holes made in bell metal; and oppofite the ends of the 

 " axes of the needle and the fridion wheels, were flat 

 " agates finely polifhed. The magnetic needle vibrated 



S 8 "within 



