the requirement.

So very slight are the points of difference between a good and a bad
engine, that they often escape the eye of those whose business it is
to deal with such works. It is not the brass and steel and bright
metal and elaborate painting that make the really good and
serviceable engine,--but the length, breadth, and depth of its
furnace, the knowledge of proportion shown in its design, and the
mechanical skill exhibited in the fitting of its parts. The
apparently complex portions are really very simple in action, while
the apparently simple parts are those where the greatest knowledge
is required. Any man of ordinary mechanical acquirements can design
and arrange the general form,--the whole mass of cranks, pistons,
connecting-rods, pumps, and the various levers for working the engine;
but to find the correct dimensions of the inner parts of the boiler,
and of the valve-gearing, by which the movements of the steam are
governed, requires a very considerable knowledge of the chemistry of
combustion, of practical geometry, and of the physical properties of
steam. So nice, indeed, is the valve-adjustment of the locomotive,
as depending upon the work it has to do, whether fast or slow, light
or heavy, that a single eighth of an inch too much or too little
will so affect its power as to entirely unfit it for doing its duty
with any degree of economy.

When a single man takes the general charge of five hundred miles of
railroad, upon which the annual pay-roll is a million of dollars,
and which employs over two hundred locomotives and three thousand
cars, earning five million dollars a year,--a road which cost
thirty-three million, has five miles in length of bridges, and over
four hundred buildings,--it is plain that the system of operation