28 Mr, Christie On the Magnetism of Iron [JuLY> 



shell 18 inches in diameter, and observed their effects on the 

 needle in various positions, as referred to certain planes passing 

 through its centre. The shell and the needle were placed in the 

 relative positions which he wished to give them, by determining 

 a radius and an angle on a horizontal plane, and a vertical ordi- 

 nate. The requisite computations becoming, from their number, 

 laborious, Mr. Christie resolved to supersede the necessity of 

 them, if possible, by the construction ofaninstrument, by which 

 he could adjust the iron and the needle in their proper relative 

 positions, without any previous computation. In this he suc- 

 ceeded, by means of the instrument he proceeds to describe in 

 the following manner, but in which it became necessary to make 

 use of a plate of iron instead of the heavy iron shell. 



"The instrument is represented in Plate XL, fig. 1. The 

 principal part consists of two strong limbs of brass : one, SQ N, 

 a semicircle, 18 inches in diameter, 2*15 inches broad and '3 

 inch thick : the other consist of two semicircles joined together; 

 SiEM, 1*2 inch broad and '22 thick, and its outer diameter 18 

 inches; s cc n '9 inch broad, -22 thick, and its inner diameter 

 9*2 inches. S iE N n(cs and SQN are attached to each other 

 by strong brass pins passing from S to s and N to w ; so that 

 sMn will revolve about the axis S s n N, while S Q N is fixed. 

 S jE N and SQN are graduated from jE and Q towards S and 

 N, as is likewise s <k n from ai towards s and n. The semicircle 

 SQN passes freely through an opening in the support G I, but 

 may be clamped firmly in any position by means of two strong 

 screws, working into the parts G, G', from the back of the 

 instrument. On the chamfered edge of the opening g, in the 

 face of G G', is an index showing the inclination of the axis S N 

 to the horizon ; and on the part K A: at the foot of the pillar, and 

 attached to it, is an index pointing out on the graduated circle 

 L /, fixed on the table T /, the situation of the fixed limb SQN 

 with respect to the magnetic meridian. R r is another graduated 

 circle, fixed to the moveable limb S iE N ; which, by the index 

 at .r on the fixed limb SQN, shows the angle described by 

 S iEN from the plane of SQN. A very strong brass pin, sol- 

 dered to the foot of the pillar, passes through the table T t and ' 

 a thick, circle of wood, to uhich the legs are attached, and has 

 below a clamping screw, to fix the whole firmly together in any 

 position. The conijjass box N^ S' is fitted on to a stand fixed to 

 the support Vf\ which consists of two parts ; ^" fitted to G, and 

 F slidmg on a tube attached to^'; so that the compass may be 

 elevated or depressed. An arm A B, to carry the cncular plate 

 of iron C c, is connected with the moveable limb S /c N. The 

 part A a consists of two flat pieces, having the limb of the 

 instrument between them, so that the arm may be moved into 

 any position on it, and be fixed in that situation by means of a 

 strong screw working into the face A a. On the cylindrical par 



