four pieces of thin wood together, to make a rectangular box without a
bottom. Four pieces of cigar-box wood, 8" long by 1½" 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. 39. Before we commence to frost,
the cock should be perfectly finished, with all the holes made, the
regulator cap in position, the screw hole made for the Howard regulator
and the index arc engraved with the letters S and F.

[Illustration: Fig. 39]

It is not necessary the brass should be polished, but every file mark
and scratch should be stoned out with a Scotch stone; in fact, be in the
condition known as "in the gray." It is not necessary to frost any
portion of the cock _C_, except the upper surface. To protect the
portion of the cock not to be frosted, like the edges and the back, we
"stop out" by painting over with shellac dissolved in alcohol, to which
a little lampblack is added. It is not necessary the coating of shellac