the principles to be understood. The aptitude to design, which must be
what is meant by mechanical intuition, requires very considerable
practical experience, which you will readily learn if you do not keep
yourself above it. If you have used your leisure hours to study why a
certain piece of mechanism was made in a certain way rather than in
another; if you have wondered why one part is thick in one place
rather than in another, apparently in defiance of all rules of the
strength of material; if you have endeavored to ascertain why a
particular device is used rather than another more evident one; if you
have thought and studied why a boss is thrown in here and there in
designs to receive bolts or to lengthen a journal, and if you have in
your mind, by repeated observation, a fair idea of how work is
designed by other people, the so-called _mechanical intuition_ will be
learned and found to be the _combination of common sense and good
practice_.

You will observe that some details have been copied for years and
years, although thoughtful men would say they are not the best, simply
because they are adapted to a large amount of work already done. This
is particularly true of the rolling stock on railroads. The cost of a
change in starting in a new country might be warranted, but it
practically cannot be done when the parts must interchange with so
much work done in other parts of the country. You will find in other
cases that the direct strain to which a piece of mechanism is
subjected is only one of the strains which occur in practice. A piece
of metal may have been thickened where it customarily broke, and you
may possibly surmise that certain jars took place that caused such
breakages, or that particular point was where the abuse of the
attendant was customarily applied.