a short
span, say an inch or so, and step off the length of one piece of metal,
and punch the last mark. Then do likewise with the other piece of metal,
and see how nearly alike the two measurements are by comparing them.

You will find a variation in the lengths of the two measurements at the
first trials, and very likely will not be able to make the two pieces
register accurately after many trials, even when using the utmost care.

Sooner or later you will learn that you have not stepped paths along the
two bars which were exactly straight, and this will account for the
variations. In order to be accurate a line should be drawn along each
piece of metal, and the dividers should step off the marks on that line.

FINDING CENTERS.--By way of further experiment, it might be well to find
the exact center of the ends of a square bar, putting in the punch marks
and then mounting it in the lathe centers to see how accurately this has
been done.

If either end is out of true the punch marks can be corrected by
inclining the punch, so that when it is struck it will move over the
point in the direction of its true center. This may be followed up by
centering the end of a round bar so as to make it true. This will be
found to be a more difficult job, unless you have a center head, a tool
made for that purpose.

It is good practice, however, to make trials of all this work, as it
will enable you to judge of measurements. It can be done with the
dividers by using care in scribing the centers.

HACK-SAW PRACTICE.--Practice with the hack-saw should be indulged in
frequently. Learn to make a straight cut through a bar. Try to do this
without using a square to guide you. One of the tests of a good mechanic
is ability to judge a straight cut.

The following plan is suggested as a test for the eye. Use a bar of iron
or steel one inch square, and make a cut an eighth of an inch deep
across it; then turn it around a quarter, so as to expose the nest face,
and continue the cut along the side, the same depth, and follow this up
with the remaining two sides, and see how near the end of the first cut
and the finish cut come together. The test will surprise you.

CUTTING METALS TRUE.--When you saw off the end of such a bar for trial
purposes, use a square, after the cut is made, and note how much it is
out of true in both directions. It is a curious fact that most mechanics
are disposed to saw or cut crooked in one direction, either to