232 Instructions for the Scientific Expedition 



netic meridian, by turning the moveable arm of the torsion circle," 

 the collimator is to be turned, by its independent motion, until some 

 point about the middle of the scale coincides with the vertical wire 

 of the fixed telescope. This point of the scale is to be noted in the 

 usual manner. 



3. The magnet is then to be removed, and the brass weight at- 

 tached. Note the new point of the scale which coincides with the 

 wire of the telescope. Then, if the magnet had been placed (in the 

 previous experiment) in its direct position (i. e, north to north) the 

 error of the plane of detorsion is 



^(^+0 



V being the difference of the two readings, converted into angular 

 measure. If, on the other hand, the magnet had been reversed (i. e. 

 north end to south) the error is 



"'(^-0 



The moveable arm of the torsion circle is then to be turned through 

 this angle, in the opposite direction ; and the magnetic axis will be 

 in the magnetic meridian. 



The difference of the two readings, corresponding to a given eiTor, 

 being much greater in the reversed than in the direct position of the 

 magnet, it follows that the former affords a much more delicate me- 

 thod of making the desired adjustment. 



4. The brass weight remaining attached, turn the moveable arm 

 of the torsion circle through 90°. Then turn back the collimator, 

 until some point about the middle of the scale coincides with the 

 vertical wire of the fixed telescope ; and note the reading. 



5. Now remove the brass weight, and replace the magnet. The 

 magnetic force of the earth will bring it back towards the magnetic 

 meridian, and the scale will be thrown out of the field of the tele- 

 scope. Then turn the moveable arm of the torsion circle, until the 

 point of the scale last noted is brought to coincide again with the 

 wire of the telescope ; the magnetic axis is then in the plane per- 

 pendicular to the magnetic meridian, and the adjustment is complete. 



Observations. — The observations to be made with this instrument 

 are those of the absolute value of the horizontal intensity, and its 

 changes. 



From the explanation of the principle of the instrument, given 

 above, it is manifest that it will serve to determine the moment of 

 the force exerted by the earth upon the free magnetism of the sus- 

 pended bar. Let X denote (as before) the horizontal part of the 

 earth's magnetic force ; m the moment of free magnetism of the bar; 

 then 



mX = F. 



F having the same meaning as before (page 230), Hence, substi- 

 tuting the values of F and G, we have 



jw X = M» -— sin V ; 



