of a cement brass, as shown at _E_, Fig. 36, and while it is slowly
revolving in the lathe touch the flat surface _a_ with a sharpened
pegwood wet with muriatic acid, which dissolves the blue coating of
oxide of iron. (2) The surface of the screwhead is coated with a very
thin coating of shellac dissolved in alcohol and thoroughly dried, or a
thin coating of collodion, which is also dried. The screw is placed in
the ordinary polishing triangle and the flat face at _a_ polished on a
tin lap with diamantine and oil. In polishing such surfaces the thinnest
possible coating of diamantine and oil is smeared on the lap--in fact,
only enough to dim the surface of the tin. It is, of course, understood
that it is necessary to move only next to nothing of the material to
restore the polish of the steel. The polishing of the other steel parts
is done precisely like any other steel work.

[Illustration: Fig. 36]

The regulator is of the Howard pattern. The hairspring stud is set in
the cock like the Elgin three-quarter-plate movement. The richest finish
for such a model is frosted plates and bridges. The frosting should not
be a fine mat, like a watch movement, but coarse-grained--in fact, the
grain of the frosting should be proportionate to the size of the
movement. The edges of the bridges and balance cock can be left smooth.
The best process for frosting is by acid. Details for doing the work
will now be given.

[Illustration: Fig. 37]

[Illustration: Fig. 38]

To do this frosting by acid nicely, make a sieve by tacking and gluing
four pieces of thin wood together, to make a rectangular box without a
bottom. Four pieces of cigar-box wood, 8" long by 11/2" wide, answer
first rate. We show at _A A A A_, Fig. 37, such a box as if seen from
above; with a side view, as if seen in the direction of the arrow _a_,
at Fig. 38. A piece of India muslin is glued across the bottom, as shown
at the dotted lines _b b_. By turning up the edges on the outside of the
box, the muslin bottom can be drawn as tight as a drum head.

HOW TO DO ACID FROSTING.

To do acid frosting, we procure two ounces of gum mastic and place in
the square sieve, shown at Fig. 37. Usually more than half the weight of
gum mastic is in fine dust, and if not, that is, if the gum is in the
shape of small round pellets called "mastic tears," crush these into
dust and place the dust in _A_. Let us next suppose we wish to frost
the cock on the balance, shown at Fig.