If the fan of
impulse jewel _i_ is set radial to the axis of the balance, the
engagement of the tooth _A^2_ would be at a disadvantage if it took
place prior to this jewel passing through an arc of five degrees inside
the periphery of the escape wheel. It will be evident on thought that if
an escape-wheel tooth engaged the impulse stone before the five-degrees
angle had passed, the contact would not be on its flat face, but the
tooth would strike the impulse jewel on its outer angle. A continued
inspection will also reveal the fact that in order to have the point of
the tooth engage the flat surface of the impulse pallet the impulse
jewel must coincide with the radial line _c g_. If we seek to remedy
this condition by setting the impulse jewel so the face is not radial,
but inclined backward, we encounter a bad engaging friction, because,
during the first part of the impulse action, the tooth has to slide up
the face of the impulse jewel. All things considered, the best action is
obtained with the impulse jewel set so the acting face is radial to the
balance staff and the engagement takes place between the tooth and the
impulse jewel when both are moving with equal velocities, i.
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