the drawing is in the condition shown at Fig. 93 we
measure the distance on the line _b_ between the points (centers) _A B_,
and we thus by graphic means obtain a measure of the distance between _A
B_. Now, by the use of trigonometry, we have the length of the line _A
f_ (radius of the arc _a_) and all the angles given, to find the length
of _f B_, or _A B_, or both _f B_ and _A B_. By adopting this policy we
can verify the measurements taken from our drawings. Suppose we find by
the graphic method that the distance between the points _A B_ is 5.78",
and by trigonometrical computation find the distance to be 5.7762". We
know from this that there is .0038" to be accounted for somewhere; but
for all practical purposes either measurement should be satisfactory,
because our drawing is about thirty-eight times the actual size of the
escape wheel of an eighteen-size movement.

HOW THE BASIS FOR CLOSE MEASUREMENTS IS OBTAINED.

Let us further suppose the diameter of our actual escape wheel to be
.26", and we were constructing a watch after the lines of our drawing.
By "lines," in this case, we mean in the same general form and ratio of
parts; as, for illustration, if the distance from the intersection of
the arc _a_ with the line _b_ to the point _B_ was one-fifteenth of the
diameter of the escape wheel, this ratio would hold good in the actual
watch, that is, it would be the one-fifteenth part of .26". Again,
suppose the diameter of the escape wheel in the large drawing is 10" and
the distance between the centers _A B_ is 5.78"; to obtain the actual
distance for the watch with the escape wheel .26" diameter, we make a
statement in proportion, thus: 10 : 5.78 :: .26 to the actual distance
between the pivot holes of the watch. By computation we find the
distance to be .15". These proportions will hold good in every part of
actual construction.

All parts--thickness of the pallet stones, length of pallet arms,
etc.--bear the same ratio of proportion. We measure the thickness of the
entrance pallet stone on the large drawing and find it to be .47"; we
make a similar statement to the one above, thus: 10 : .47 :: .26 to the
actual thickness of the real pallet stone. By computation we find it to
be .0122". All angular relations are alike, whether in the large drawing
or the small pallets to match the actual escape wheel .26" in diameter.
Thus, in the pallet _D_, Fig. 93, the impulse face, as reckoned from _B_
as a center, would