le turns round. Pieces of wood may be interposed between
the ring and the body of the piston, to keep the ring nearly in its right
position; but these pieces of wood should be fitted so loosely as to give
some side play, else the disposition would arise to wear the flange of the
piston into a groove.

706. _Q._--What kind of tool is used for finishing surfaces by scraping?

_A._--A flat file bent, and sharpened at the end, makes an eligible scraper
for the first stages; or a flat file sharpened at the end and used like a
chisel for wood. A three-cornered file, sharpened at all the corners, is
the best instrument for finishing the operation. The scraping tool should
be of the best steel, and should be carefully sharpened at short intervals
on a Turkey stone, so as to maintain a fine edge.

707. _Q._--Will you explain the method of fitting together the valve and
cylinder faces?

_A._--Both faces must first be planed, then filed according to the
indications of a metallic straight edge, and subsequently of a thick
metallic face plate, and finally scraped very carefully until the face
plate bears equally all over the surface. In planing any surface, the
catches which retain the surface on the planing machine should be relaxed
previously to the last cut, to obviate distortion from springing. To
ascertain, whether the face plate bears equally, smear it over with a
little red ochre and oil, and move the face plate slightly, which will fix
the color upon the prominent points. This operation is to be repeated
frequently; and as the work advances, the quantity of coloring matter is to
be diminished, until finally it is spread over the face plate in a thin
film, which only dims the brightness of the plate. The surfaces at this
stage must be rubbed firmly together to make the points of contact visible,
and the higher points will become slightly clouded, while the other parts
are left more or less in shade. If too small a quantity of coloring matter
be used at first, it will be difficult to form a just conception of the
general state of the surface, as the prominent points will alone be
indicated, whereas the use of a large quantity of coloring matter in the
latter stages would destroy the delicacy of the test the face plate
affords. The number of bearing points which it is desirable to establish on
the surface of the work, depends on the use to which the surface is to be
applied; but whether it is to be finished with great elabora