28 



lower point of application, and which point is in 

 effect the " upper end of the cross-line." 



So far as the rotary traction from the upper 

 point continues its course, by drawing on the upper 

 end of the " cross-line " (here situated on the right 

 side) it will produce a continuation of its winding 

 line from that end, by the rear, haK way around 

 the lower part of the rod, to a point at the lower 

 plane section, perpendicularly under and of course 

 collateral with its (the upper left point of applica- 

 tion) own position. We shall thus have a hehcal 

 line completely around the rod from a point on the 

 border of the upper plane section to a point per- 

 pendicularly under it on the border of the lower 

 plane section. This line, however, is interrupted 

 in its continuity of force by the "cross-Hne," 

 which (kept in place by the passive resistance of 

 the lower pohit of application) continues the wind- 

 ing hne in just such proportion as it is moved from 

 its place against the resistance of its other end. 



§ 23. The drawing on the substance of the rod 

 along the winding line will, by the reacting pressure, 

 produce concavities under the varying positions 

 of the rotating point, and the hne of these con- 

 caves, as also of their corresponding convexes, will^ 

 of course, likewise be hehcal. 



