MAGNETISM. 



75 



explained ($ 97), as also the general form 

 of the instrument. 



Fig. 67. 



(290.) The simplest construction is 

 that represented mfg. 67. The needle 

 D d is a flat oblong piece of steel, broader 

 at the middle, and tapering to a point at 

 the extremities. A slender cylindrical 

 axis is passed at right-angles through 

 its centre, and moves freely in circular 

 apertures made in the middle of the 

 lateral horizontal bars, H h, fastened to 

 a vertical graduated circle, CC, indicat- 

 ing the angle which the needle makes 

 with the horizon. This circle is fixed 

 to a flat stand, ST, provided with one 

 or more levels ; the horizontally of 

 which is adjusted by means of screws 

 placed at the corners of the stand. The 

 usual mode of observing with such an 

 instrument is first to ascertain the di- 

 rection of the magnetic meridian by a 

 common compass, and then, removing 

 the compass to a sufficient distance, so 

 that it may not affect the position of the 

 dipping-needle, to fix the circle of the 

 latter in the plane''of this meridian, and 

 then to render it perfectly level by means 

 of the screws of the stand. For the ad- 

 justment of the instrument in the meri- 

 dian, in any particular place where the 

 bearing of a distant object is exactly 

 known, the frame containing the needle 

 is occasionally provided with two sight 

 vanes, placed on an index moving hori- 

 zontally on the top of the vane, and 

 which may be directed to that object. 



(291.) Great care should be taken 

 that no iron or steel enters into the con- 

 struction of any part of the frame- work 

 of the apparatus, as such material 

 might produce a sensible action upon 

 the needle : great attention should even 

 be paid to the purity of brass that may be 

 employed in its construction, and to its 

 exemption from all magnetic properties. 



(292.) It was formerly deemed an 



advantage to make the needles of con- 

 siderable dimensions, so as even to ex- 

 ceed a foot in length. But experience 

 has shown that more is lost than gained 

 in point of accuracy by giving to them 

 a length greater than six or eight inches ; 

 and considerable convenience, of course, 

 results from this reduction of size, as 

 the instrument is thus rendered more 

 portable, as well as less expensive. 



(293.) With a view to diminish fric- 

 tion, Mr. Mitchell, in the year 1772, 

 proposed that the two ends of the axis 

 of the dipping-needle should be sup- 

 ported on friction-wheels ; and two in- 

 struments with this improvement were 

 executed for the Board of Longitude 

 by Mr. Nairne. The needles were a 

 foot in length, and the ends of the axes 

 were made of gold alloyed with cop- 

 per, and the friction-wheels on which 

 they rested were four inches in diame- 

 ter, these wheels being themselves ba- 

 lanced with great care. The ends of the 

 axes of the friction-wheels were like- 

 wise made of an alloy of gold and cop- 

 per, and moved in small holes made in 

 bell metal ; and opposite to the ends of 

 the axes of the needles and of the fric- 

 tion wheels, were placed flat agates, 

 finely polished. Each magnetic needle 

 vibrated in a circle of bell-metal, divided 

 into degrees and half degrees ; and a 

 line passing through the middle of the 

 needle to the ends pointed to the divi- 

 sions. The needles were nearly ba- 

 lanced before they were rendered mag- 

 netical ; and by an ingenious contrivance 

 of Mr. Mitchell, of a cross fixed on the 

 axes of the needles, on the arms of which 

 were cut very fine screws, to receive 

 small buttons, admitting of being screwed 

 nearer to or farther from the axis, the 

 needles could be adjusted both ways, to 

 a great nicety, after being magnetised, 

 by reversing the poles, and changing 

 the sides of the needle. The frame of 

 the instruments were provided with 

 levels for the horizontal adjustment, 

 after they had been placed in the plane 

 of the magnetic meridian *. 



(294.) In a subsequent volume of the 

 Philosophical Transactions t, a dipping- 

 needle is described by Dr. Lorimer, 

 calculated for making observations on 

 the dip at sea, where, from the un- 

 steadiness of the supports, the difficulty 

 of attaining any degree of accuracy is 

 very great. The needle was of the 



Philosophical Transactions for 1772, p. 4?6. a 

 t For 17/5, p. 79. 



