. The rounding is carefully done, first with a file and
finished with emery paper. The outer edge of the loop _A''_ is a little
rounded, also the inner edge next the cap _C_. This will be understood
by inspecting Fig. 46, where we show a magnified vertical section of the
regulator on line _l_, Fig. 40. The curvature should embrace that
portion of _A''_ between the radial lines _o o'_, and should, on the
model, not measure more than 1/40". It will be seen that the curved
surface of the regulator is sunk so it meets only the vertical edge of
the loop _A''_. For the average workman, polishing the flat parts _m_ is
the most difficult to do, and for this reason we will give entire
details. It is to be expected that the regulator bar will spring a
little in hardening, but if only a little we need pay no attention to
it.

[Illustration: Fig. 44]

[Illustration: Fig. 45]

[Illustration: Fig. 46]

HOW FLAT STEEL POLISHING IS DONE.

Polishing a regulator bar for a large model, such as we are building, is
only a heavy job of flat steel work, a little larger but no more
difficult than to polish a regulator for a sixteen-size watch. We would
ask permission here to say that really nice flat steel work is something
which only a comparatively few workmen can do, and, still, the process
is quite simple and the accessories few and inexpensive. First,
ground-glass slab 6" by 6" by 1/4"; second, flat zinc piece 31/4" by
31/4" by 1/4"; third, a piece of thick sheet brass 3" by 2" by 1/8";
and a bottle of Vienna lime. The glass slab is only a piece of plate
glass cut to the size given above. The zinc slab is pure zinc planed
dead flat, and the glass ground to a dead surface with another piece of
plate glass and some medium fine emery and water, the whole surface
being gone over with emery and water until completely depolished. The
regulator bar, after careful filing and dressing up on the edges with an
oilstone slip or a narrow emery buff, is finished as previously
described. We would add to the details already given a few words on
polishing the edges.

[Illustration: Fig. 47]

It is not necessary that the edges of steelwork, like the regulator bar
_B_, Fig. 47, should be polished to a flat surface; indeed, they look
better to be nicely rounded. Perhaps we can convey the idea better by
referring to certain parts: say, spring to the regulator, shown at _D_,
Fig. 40, and also the hairspring stud _E_. The edges of these parts look
b