VOL. LXV.] PHILOSOPHICAL TRANSACTIONS. SQS 



FII. Description of a New Dipping- Needle. By Mr. J. Larimer, of Pensacola. 



P-79- 

 Whenever any one meets with a terrella, or spherical loadstone, the first 

 thing he does is to find out its poles ; and having once discovered them, he 

 knows immediately how any small bit of needle will be affected, when 

 placed on any part of its surface. The poles are most readily discovered by 

 trying where the filings of iron, or a small bit of needle, will stand erect on the 

 terrella ; and this is generally found to be on 2 points diametrically opposite to 

 each other. But the magnetic poles of the earth seem to be situated obliquely 

 to one another (see the Berlin Memoirs, 1757) ; but where they are actually 

 situated, is hitherto unknown ; whether they are on land or water. Yet be 

 these things as they may, it appears evident, that accurate observations, made 

 as near to these magnetic poles as possible, with a good dipping-needle, are the 

 surest way to complete the magnetic theory of this globe, analogous to the 

 method we pursue in examining the terrella. But as all the dipping-needles 

 appeared to be very ill calculated, for the sea service at least, Mr. L. contrived 

 one on a different plan in 1764, and had it executed by Mr. Sisson. He called 

 it a universal magnetic needle, or observation compass ; because he could by it 

 take the dip and amplitude, and even the azimuth, with only one assistant, to 

 take the altitude. The needle was of the same shape and size nearly as those 

 used for the compasses of the royal navy, and played vertically on its own axis, 

 which had 1 conical points, slightly supported in 1 corresponding hemispherical 

 sockets, inserted into the opposite sides of a small upright brass parallelogram, 

 about 1-i- inch broad, and 6 inches high. Into this parallelogram is fixed, at 

 right angles, a slender brass circle, about 6 inches diameter, silvered and 

 graduated to every half degree, on which the needle shows the dip ; and this, 

 for the sake of distinction, he calls the circle of magnetic inclination. This 

 brass parallelogram, and consequently the circle of inclination, also turns hori- 

 zontally on 1 other pivots, the one above and the other below, with correspond- 

 ing sockets in the parallelogram. These pivots are fixed in a vertical brass circle, 

 of the breadth and thickness of -Jj- of an inch, and of such a diameter, as to 

 allow the circle of inclination and the parallelogram to move freely round within 

 it. This 2d he calls the general meridian. It is not graduated, but has a small 

 brass weight fixed to the lower part of it, to keep it upright ; and the circle itself 

 is screwed, at right angles, into another circle, of equal internal diameter, of 

 the same thickness, and twice the breadth, which is silvered and graduated on 

 the upper side to every half degree. It represents the horizon, as it swings 

 freely on gimbols, and is always nearly parallel to it. The whole is contained 

 in a neat mahogany box, of an octagon figure, with a glass plate at top and one 

 on each side, for about \ down. That part of the frame which contains the 



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