A′
b
intersects it, have found one-half the opening for the crescent, the remaining half being constructed on the opposite side of the line
A′ A2
.
The Horn on the fork belongs to the safety action: more horn is required with the double than with the single roller, on account of the greater angle of opening for the crescent.
The horn should be of such a length that when the crescent has passed the guard point, the end of the horn should point to at least the center of the ruby pin.
Fig. 25.