112 Mr. J. E. Moore on a Continuous and 



a circular opening at its centre. The cover of the instrument 

 rests in a groove in the wooden base, fitting closely about the 

 circular disk at its top, and is provided with a plane glass 

 window for the admission of a beam of light. Placed dia- 

 metrically opposite, on the upper surface of the vulcanite disk, 

 are two coils of a few turns of stout wire, hereafter to be 

 called the H-deflecting coils. From the centre of this same 

 disk rises a glass cylinder, covered at its upper end by a 

 second vulcanite disk, to which is attached the suspension- 

 tube. The suspension -tube carries at its upper end a torsion 

 head, provided with binding-posts and adjusting devices for 

 regulating the bifilar wires. From this torsion-head is sus- 

 pended, by means of the silver bifilar wires, the moveable 

 magnetometer system. The upper or H-magnetometer helix is 

 held by a hard rubber clamp, serving at the same time as a 

 terminal block for the lower ends of the bifilar wires. To the 

 lower part of this clamp is fastened a hard rubber shaft, sup- 

 porting centrally, at its lower end, a horizontal hard rubber 

 bar, to the extremities of which are fastened (by means of 

 suitable adjusting devices) the horizontal unifilar torsion-wires 

 carrying the I-magnetometer helix. 



Each of the magnetometer helices having the same number 

 of turns of wire will, when so connected as to develop opposite 

 polarities in the ends lying in the same direction, form an 

 astatic combination for horizontal deflexions. If it should be 

 necessary to make the vertical deflexions also independent of 

 the earth's magnetic field, one can easily arrange a flat coil 

 of wire with its centre in the vertical axis of suspension of 

 the moveable system, and its plane horizontal, and pass through 

 the coil such an electric current as will just equal and neutra- 

 lize, in its magnetic effect, the vertical component of the 

 earth's magnetic field at the centre of the 1-magnetometer 

 helix. 



§ 5. In using the instrument, it should be set up on some 

 firm support (generally, though not necessarily, so that the 

 planes of the H-deflecting coils are parallel to the earth's 

 magnetic meridian) and properly levelled until the suspended 

 system is perfectly free, and hangs centrally in the instru- 

 ment. The length of the bifilar wires should then be adjusted 

 until the centre of the H-magnetometer helix lies in a hori- 

 zontal line passing through the centre of the H-deflecting 

 coils on either side of the instrument. The stress on the 

 bifilar wires should then be equalized, by means of the adjust- 

 ing devices at either end of the horizontal bar carrying the 

 I-magnetometer helix, the horizontal torsion-wires should be 

 regulated in length, so that the centre of gravity of the 



