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_.
The facts are, the problems relating to the club-tooth lever escapement are very intricate and require very careful analysis, and without such care the horological student can very readily be misled. Faulty drawings, when studying such problems, lead to no end of errors, and practical men who make imperfect drawings lead to the popular phrase, "Oh, such a matter may be all right in theory, but will not work in practice." We should always bear in mind that _theory, if right, must lead practice_.
If we delineate our entrance pallet to have a draw of twelve degrees when in actual contact with the tooth, and then construct in exact conformity with such drawings, we will find our lever to "hug the banks" in every instance. It is inattention to such details which produces the errors of makers complained of by Saunier in section 696 of his "Modern Horology," and which he attempts to correct by drawing the locking face at fifteen degrees draw.
We shall show that neither _C_ nor _C'_, Fig. 85, is the theoretically correct position for the pallet center for a tangential locking.
We will now take up the consideration of a club-tooth lever escapement with circular pallets and tangential lockings; but previous to making the drawings we must decide several points, among which are the thickness of the pallet arms, which establishes the angular motion of the escape wheel utilized by such pallet arms, and also the angular motion imparted to the pallets by the impulse faces of the teeth. We will, for the present, accept the thickness of the arms as being equivalent to five degrees of angular extent of the pitch circle of the escape wheel.