216 



I now proceed to the construction of the apparatus em- 

 ployed in these measurements. 



The magnet is cyUndrical ; its length is three inches, 

 and diameter one-fourth of an inch. A mirror is attached to 

 the stirrup by which it is suspended, by means of which the 

 varying position of the magnet may be observed with a 

 telescope at a distance, after the method of Gauss. This 

 mirror is of course vertical ; and it has a motion round a ver- 

 tical axis, by means of which it may be adjusted to any de- 

 sired position of the observing telescope. The mirror is 

 circular, and is three-fourths of an inch in diameter. The 

 moveable part of the stirrup to which it is attached has the 

 form of a cross ; and it is rendered vertical by means of three 

 screws, near the extremities of three of the arms of the 

 cross, the heads of which project and hold it. The mirror 

 is maintained in contact with these heads by springs at the 

 back. 



The box is octagonal ; the interval between the opposite 

 sides is four inches, and that between the top and bottom 

 two inches. The 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 encloses the suspen- 

 sion 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 



