movement of control rod 43 in the direction of arrow 3 will tend to propel dropper bar 93 in the direction of arrow 3 but will also tend to draw the guide bar 91 back in a direction opposite to arrow 3 in other words movement of the control 43 in the direction of arrow 3 will tend to cause guide bar 91 to move in a direction opposite to arrow 3 and in that manner aid the return force of spring 44 lever 48 tends to compensate for the force exerted by dropper bar 93 on guide bar 91 because the lever is pivoted around point c and therefore any motion at either point b or point d will cause an opposite directional effect at the other end point c therefore acts as a fulcrum around which lever 48 pivots if point d moves in the direction of arrow 3 then point b will tend to move in a direction 180 u00b0 opposite to arrow 3 and visa versa in practice it has been found desirable to determine the return forces generated by return springs 44 and 45 which are connected to guide bar 91 and dropper bar 93 at points a and e respectfully in this regard it has been found advantageous to employ a return spring 44 which has a return force which is equal to or greater than the return force provided by spring 45 some modifications of the present inventioon may be obvious to those of ordinary skill in.