76 



MAGNETISM. 



usual shape and size," and moved verti- 

 cally on its axis, which had two conical 

 points, slightly supported in two corre- 

 sponding hemispherical sockets, inserted 

 into the opposite sides of a small up- 

 right brass parallelogram, about an 

 inch and a half broad, and six inches 

 high. Into this parallelogram was fixed, 

 at right angles, a slender brass circle, 

 about six inches diameter, silvered and 

 graduated to every half degree, on 

 which the dip is indicated by the needle. 

 This, for the sake of distinction, he 

 called the circle of magnetic inclina- 

 tion. This brass parallelogram and, 

 consequently, the circle of inclination, 

 also turned horizontally on two other 

 pivots, the one above and the other be- 

 low, with corresponding sockets in the 

 parallelogram. These pivots were fixed 

 in a vertical brass circle, of the breadth 

 and thickness of two-tenths of an inch, 

 and of such a diameter as to allow the 

 circle of inclination and the parallelo- 

 gram to move freely round within it. 

 This second circle fhe calls the general 

 meridian. It was not graduated, but 

 had a small brass weight fixed to the 

 lower part of it, to keep it in a vertical 

 position ; and the circle itself was 

 screwed, at right angles, into another 

 circle, of equal internal diameter, of the 

 same thickness, and twice the breadth, 

 which was silvered and graduated on 

 the upper side to every half degree. It 

 represented the horizon: for it swung 

 freely in gimbals, and was, conse- 

 quently, always horizontal. The whole 

 was contained in a mahogany box, of 

 an octagon shape, with a glass plate at 

 the top, and one on each side for, some 

 way down. That part of the frame 

 which contained the glass could be lifted 

 off when requisite. The whole box 

 turned round upon a strong brass 

 centre, fixed in a double plate of maho- 

 gany, glued together cross-wise, to pre- 

 vent its warping or splitting ; and this 

 again was supported by three brass feet, 

 frosted so as to prevent their slipping 

 when the vessel rolled considerably. 

 When not wanted for use, it was en- 

 closed in an outer square box, in order 

 to preserve it effectually. 



The peculiar advantage of this in- 

 strument consists in the freedom which 

 is allowed to the needle of obeying the 

 tendency, impressed upon it by terres- 

 trial magnetism, of placing itself in the 

 line of the dip, in consequence of the 

 power which it has of moving in dif- 

 ferent planes at right angles to one an- 



other. Its position with respect to the 

 respective circles points out also, upon 

 simple inspection, not only the inclina- 

 tion, or dip, but also the magnetic 

 bearings in a horizontal plane. Hence 

 by directing the vertical circle to the 

 sun, or other object in the heavens, the 

 magnetic amplitude of the object is also 

 readily determined. Dr. Lorimer's com- 

 pass, though exceedingly plausible in 

 theory, presents such difficulties in its 

 practical execution, as can scarcely be 

 overcome by the most exquisite work- 

 manship. 



(295.) The dipping-needle formerly 

 used by the Royal Society, and which 

 has been regarded as the model for the 

 construction of instruments of this 

 kind, is described by Mr. Cavendish, in 

 the 66th volume of the * Philosophical 

 Transactions *.' In this instrument, the 

 ends of the axis roll on horizontal agate 

 planes ; and a contrivance is applied, by 

 which the needle may, at pleasure, be 

 lifted off from the planes, and laid down 

 on them again in such a manner as to 

 be supported always by the same points 

 of the axis resting on the same parts of 

 the agate planes, the motion by which it 

 is let down being very gradual and with- 

 out shake. The general form of the in- 

 strument, the size and shape of the 

 needle, and the cross used for balancing 

 it, were the same as in the dipping- 

 needle constructed by Nairneon the plan 

 of Mr. Mitchell, already described, 

 293. The mode of using the instrument 

 was as follows : the dip was observed 

 first with its front to the west, and then 

 with its front to the east ; after^ which 

 the poles of the needle were reversed, 

 and the dip observed both ways as be- 

 fore. Care was taken that the needle 

 was rendered equally magnetical after 

 the poles were reversed, as it has been 

 before ; this equality being ascertained 

 by counting the number of vibrations 

 made by the needle in a given time in 

 both cases. The mean of these four 

 observations was the true dip. 



(296.) In order to estimate the influ- 

 ence of the several causes of error which 

 might singly vitiate the result, but which 

 may be made to compensate one an- 

 other by combining these different modes 

 of observation ; let us suppose fig. 68 

 to present us with a front view of the 

 needle ; and S N to be the direction of 

 the magnetic axis, or line according to 

 which its magnetism is exerted ; and let 



* For the year 1776, p. 375. 



