334 



azimuth, an arrangement which is necessary in the adjust- 

 ments. The suspending wire passes round a small grooved 

 wheel, on the axis of which the stirrup rests by inverted Ys ; 

 and the instrument is furnished with a series of such wheels, 

 of different diameters, for the purpose of varying the interval 

 of the wires. This interval is altered, at the upper extre- 

 mity, by means of two screws (one right handed and the 

 other left handed,) cut in the same cylinder ; the wires being 

 lodged in the intervals of the threads, and their distance re- 

 gulated by means of a micrometer head. 



The third instrument is that used in determining the 

 changes in the vertical component of the magnetic force. It 

 is a magnet resting on agate planes, by knife edges, and 

 brought to the horizontal position by weights. There is a 

 small cross of wires near each end, and the changes of posi- 

 tion of the magnet are read oiF by a pair of microscopes. 

 From these changes of position, the changes of the ver- 

 tical force are inferred, when we know the incUnatio7i at the 

 place of observation, the azimuth of the plane in which the 

 needle moves, and the angle which the line connecting the 

 centre of gravity and centre of motion makes with the mag- 

 netic axis of the needle. As the determination of this latter 

 constant would require that considerable additions should 

 be made to the apparatus, Mr. Lloyd has preferred to adjust 

 the weights so that the angle in question shall be nothing. 

 The weights are small brass screws moving in fixed nuts, one 

 on each arm; the axis of one of these screws is peipendicalar 

 to the magnetic axis of the bar, and its movement conse- 

 quently produces the desired adjustment ; the other screw 

 (the heavier) is parallel to the magnetic axis, and serves to 

 adjust the magnet to the horizontal position. Each of these 

 adjustments admits of an easy test. 



The supports of the magnet, and of the micrometers, are 

 firmly attached to a massive slab of marble, which is ce- 

 mented to the stone pillar on which it is placed ; and a spirit- 



