MAGNETISM. 



needles are to be suspended, is'well de- 

 serving of attention. In order to pro- 

 vide a concave surface, by which the 

 needle may rest on the pivot which is to 

 support it, such, that the point of sus- 

 pension may be just above the centre of 

 gravity, it is generally necessary, in the 

 (Straight needle, to make a perforation in 

 its centre, and to rivet into the hole a 

 piece of hammered brass, the lower side 

 of which has been hollowed into a coni- 

 cal cavity, while its upper convex sur- 

 face is allowed to project a little above 

 the level of the upper surface of the 

 needle. It is found, however, that brass is 

 not capable of being rendered sufficiently 

 hard to resist the continued action of the 

 point against which it rubs in every mo- 

 tion of the compass. In process of time 

 it is worn into an irregular hole, giving 

 rise to great friction, and loss of mobi- 

 lity in the compass. This defect is 

 usually remedied by inserting in the 

 upper part of the brass, a piece of po- 

 lished agate, ground concave, with a de- 

 cided centre. The best compasses, made 

 for nautical use, are thus furnished with 

 agate caps. 



(236.) Some have considered the per- 

 foration of the needle at the centre, for 

 the purpose of suspension, as prejudicial 

 to the regularity of the magnetic power, 

 and as tending to the creation of an ad- 

 ditional number of poles. But, in re- 

 ality, the derangement occasioned by the 

 perforation of a magnetic bar at the 

 point of neutrality, is not found to be at- 

 tended with any sensible inconveniences 

 in practice*. If the shape given to the 

 needle be that of the pierced rhombus, 

 as recommended by Captain Kater, no 

 such objection will arise, since the cross- 

 bar which connected the obtuse angles 

 of the rhombus has nothing to do with 

 the magnetism of the steel bars forming 

 the sides of the parallelogram. It would, 

 no doubt, be easy to balance a straight 

 bar, without removing any part of its 

 substance, by the addition of a rim of 

 copper, or other non-magnetic substance, 

 to the circumference of the card, so that 

 the centre of gravity of the moveable 

 part of the instrument may be brought 

 sufficiently low to be under the point of 

 suspension. But a little reflection will 

 show, that more would be lost than 

 gained by this expedient ; for every ad- 

 dition that is made to the weight of the 

 parts which have to move along with 

 the needle, lessens the efficacy of the 



* Coulomb, Mcmoires de Mathematique et de 

 Physique presntcs a 1' Academic. Tom. ix., 1780. 



magnetic force which gives them mo- 

 tion, and the friction also, being aug- 

 mented in the same proportion, con- 

 spires to diminish the freedom of the 

 motion of the needle, and to impair its 

 sensibility. 



(237.) The best precaution to be taken 

 for ensuring the steadiness of the move- 

 ments of the compass, under all circum- 

 stances, is to balance the needle accu- 

 rately upon its centre, before the card is 

 applied. Care should be taken that the 

 card is uniform throughout in its thick- 

 ness and texture, and be perforated with 

 a circular hole in its centre, so that 

 when united to the needle, the equili- 

 brium of the whole may be perfectly 

 preserved. In order to fix it to the 

 needle, the latter is tapped with two 

 small screw-holes, at the distance of 

 about half an inch from each end ; and 

 the card being placed so that the meri- 

 dian line marked on it is in the same 

 vertical plane with the axis of the needle, 

 and holes being made in it opposite to 

 those in the needle, small screws are in- 

 troduced, so as firmly to fasten them 

 together. In order to secure the steadi- 

 ness of the compass during the violent 

 and irregular movements to which the 

 ship is liable, the suspension of the box 

 by the gimbals should be made with 

 great care; the several axes of motion 

 being so adjusted as that the point of 

 suspension on which the needle, with its 

 card, is supported, be exactly in the 

 same line with both these axes. 



(238.) Complaints are frequently 

 made by seamen, that, in a rough sea, 

 the ordinary compasses are so unsteady 

 as to prevent their being easily ob- 

 served ; an inconvenience, which they 

 are apt to ascribe to the needle's being 

 too strongly magnetic, and therefore too 

 easily disturbed by the irregularities in 

 the motion of the vessel. This supposed 

 defect they endeavour to remedy by 

 adding a weight to the card : and this is 

 often done, very injudiciously, by load- 

 ing it with sealing-wax. Sometimes 

 they stick a few pieces of paper on the 

 under side of the card, to serve as vanes 

 which, acting upon the air, may create 

 a resistance to the oscillations of the 

 needle. It has even been proposed, 

 with a similar design, tQ make the 

 needle move in oil, or other liquid, 

 keeping it still suspended, as usual, on 

 its pivot the fluid serving to check the 

 vibrations. But all these expedients, 

 calculated to diminish the mobility of 

 the needle, by counteracting the opera- 



