From the intersection of the radial line A b with the arc a lay off three and a half degrees to the left of said intersection on the arc a, and through the point so established draw the radial line A e.  From the intersection of the radial line A b with the arc a erect the perpendicular line f, and at the crossing or intersection of said line with the vertical line A k establish the center of the pallet staff, as indicated by the small circle B.  From B as a center sweep the short arc l with a 5” radius; and from the intersection of the radial line A b with the arc a continue the line f until it crosses the short arc l, as shown at f’.  Lay off one and a half degrees on the arc l below its intersection with the line f’, and from B as a center draw the line B i through said intersection.  From B as a center, through the intersection of the radial line A b and the arc a, sweep the arc g.

The space between the lines B f’ and B i on the arc g defines the extent of the locking face of the entrance pallet C.  The intersection of the line B f’ with the arc g we denominate the point o, and from this point as a center sweep the short arc p with a 5” radius; and on this arc, from its intersection with the radial line A b, lay off twelve degrees, and through the point so established, from o as a center, draw the radial line o m, said line defining the locking face of the entrance pallet C.

[Illustration:  Fig. 22]

It will be seen that this gives a positive “draw” of twelve degrees to the entrance pallet; that is, counting to the line B f’.  In this escapement as delineated there is perfect tangential locking.  If the locking face of the entrance-pallet stone at C was made to conform to the radial line A b, the lock of the tooth D at o would be “dead”; that is, absolutely neutral.  The tooth D would press the pallet C in the direction of the arrow x, toward the center of the pallet staff B, with no tendency on the part of the pallet to turn on its axis B.  Theoretically, the pallet with the locking face cut to coincide with the line A b would resist movement on the center B in either direction indicated by the double-headed arrow y.

A pallet at C with a circular locking face made to conform to the arc g, would permit movement in the direction of the double-headed arrow y with only mechanical effort enough to overcome friction.  But it is evident on inspection that a locking face on the line A b would cause a retrograde motion of the escape wheel, and consequent resistance, if said pallet was moved in either direction indicated by the double-headed arrow y.  Precisely the same conditions obtain at the point u, which holds the same relations to the exit pallet as the point o does to the entrance pallet C.