Here is an error; the lines defining the center of the pallet staff should have been drawn tangent to the circle s, which represents the locking angle of the teeth.  This would have placed the center of the pallet staff farther in, or closer to the wheel.  Any person can see at a glance that the pallets as delineated are not tangential in a true sense.

[Illustration:  Fig. 85]

We have previously considered engaging friction and also repeatedly have spoken of tangential lockings, but will repeat the idea of tangential lockings at Fig. 85.  A tangential locking is neutral, or nearly so, as regards engaging friction.  For illustration we refer to Fig. 85, where A represents the center of an escape wheel.  We draw the radial lines A y and A z so that they embrace sixty degrees of the arcs s or t, which correspond to similar circles in Fig. 84, and represent the extreme extent of the teeth and likewise the locking angle of such teeth.  In fact, with the club-tooth escapement all that part of a tooth which extends beyond the line s should be considered the same as the addendum in gear wheels.  Consequently, a tangential locking made to coincide with the center of the impulse plane, as recommended by Saunier, would require the pallet staff to be located at C’ instead of C, as he draws it.  If the angle k’ of the tooth k in Fig. 84 was extended outward from the center A so it would engage or rest on the locking face of the entrance pallet as shown at Fig. 84, then the draw of the locking angle would not be quite fifteen degrees; but it is evident no lock can take place until the angle a of the entrance pallet has passed inside the circle s.  We would say here that we have added the letters s and t to the original drawings, as we have frequently to refer to these circles, and without letters had no means of designation.  Before the locking angle k’ of the tooth can engage the pallet, as shown in Fig. 84, the pallet must turn on the center C through an angular movement of at least four degrees.  We show the situation in the diagram at Fig. 86, using the same letters of reference for similar parts as in Fig. 84.

[Illustration:  Fig. 86]

As drawn in Fig. 84 the angle of draft G a I is equal to fifteen degrees, but when brought in a position to act as shown at G a’ I’, Fig. 86, the draw is less even than twelve degrees.  The angle C a I remains constant, as shown at C a’ I’, but the relation to the radial A G changes when the pallet moves through the angle w C w’, as it must when locked.  A tangential locking in the true sense of the meaning of the phrase is a locking set so that a pallet with its face coinciding with a radial line like A G would be neutral, and the thrust of the tooth would be tangent to the circle described by the locking angle of the tooth.  Thus the center C, Fig. 86, is placed on the line w’ which is tangent to the circle s; said line w’ also being at right angles to the radial line A G.