ON A NEW MAGNETICAL INSTRUMENT. 225 



top and bottom, and the connecting pillars, are formed of gun- 

 metal ; the eight sides are closed by moveable pieces, three of 

 which are of glass, and the rest of ebony. To the top of the box 

 is attached an upright tube of glass, eight inches in length, which 

 incloses the suspension thread. The suspension apparatus at the 

 top of the tube is of the usual construction ; the circular piece to 

 which it is attached has a movement of rotation, and its outer sur- 

 face is graduated to 5, for the purpose of determining the effect of 

 torsion of the suspension thread. 



The base of the instrument is a circle of gun-metal, six inches 

 in diameter, graduated on the edge. The box is connected with 

 this circle by a short conical stem, forming the axis of a second 

 plate, which revolves upon the fixed one. This moveable plate 

 carries two verniers, by which the angle of rotation may be read 

 off to minutes. Two tubular arms, slightly inclined to one an- 

 other, are attached to this plate ; and their other extremities are 

 connected by a cross-piece, which carries a short scale at a distance 

 of eighteen inches from the mirror. This part of the apparatus is 

 employed in determining the total angles of deflection. 



The soft iron bar is a cylinder, twelve inches long, and three- 

 fourths of an inch in diameter. One of its extremities is inclosed 

 in a hollow cylinder of brass, connected with a horizontal pivot 

 which revolves in a fixed socket. The axis of this pivot being in 

 the line passing through the centre of the suspended magnet, and 

 perpendicular to the magnetic meridian, it is obvious that the bar 

 has a movement of rotation in the plane of the magnetic meridian 

 itself. The distance of the axis of the bar from the centre of the 

 magnet is about five inches ; and it is so placed that the induced 

 pole is in the direction of the axis of the pivot, and thus remains 

 fixed during the movement of the bar. 



The changes of position of the suspended magnet are observed 

 at a distance by means of a fixed telescope and scale. The scale, 

 whose divisions are reflected by the mirror, is attached above the 

 telescope to a support near the eye-end. 



Having explained the principle of this instrument, and given 

 the details of its construction, it remains only that I should describe 

 the observations made for the purpose of testing its performance. 

 I shall pass over for the present those which relate to the absolute 

 inclination, because they have yielded results which can be regarded 

 only as approximations to the truth, and I have not succeeded as 



Q 



