218 



LECTURES TO SCIENCE TE AGREES. 



little pith ball as nearly as possible of the same size as the 

 ball at the end of the torsion arm, and also carefully gilded. 

 It is fixed to the extremity of a fine rod or stem of shel- 

 lac, which is of such a length as to bring the centre of the 

 carrier ball, when in position, precisely to the level of the 

 centre of the torsion ball ; and there is a proper geometrical 

 arrangement for placing the carrier ball again and again 



FIG. 1. 



in precisely the same position, with its centre, let us say, at 

 the same distance from the centre of the circle in which 

 the torsion ball moves as is the centre of the torsion ball, 

 and with one side in contact with the torsion ball when 

 the middle of the torsion ball is opposite to the zero of the 

 scale upon the cylindrical glass case. Lastly, as to the 

 torsion head. The cylindrical piece, H, is cemented into the 



