MESSRS. EDITORS:--In one of the late numbers of your journal, you
publish a paper, read by Mr. Porter before some learned society in New
York, on something about the possibility or practicability of running
a steam engine at a high rate of speed, and claiming to give a
scientific explanation of the why and wherefore. Now, scientifically,
I know nothing about a steam engine; practically, I know how to stop
and start one. Therefore, you will understand that what I say is not
as coming from one who claims to be wise above what is written, but as
simply being a statement of the case, as it appears to one who wants
to learn, and takes this way to draw out the truth. A scientific
theory, invested with all its sines, coefficients, and other
paraphernalia, is a very pretty thing to look at, no doubt, for those
who understand it, and, when properly applied, is invaluable; but
when, as in this case, a practical question is to be decided, by the
aid of a scientific demonstration, it will not do to throw aside the
main elements of the problem, or any, in fact, of the minor points, no
matter how trivial they may appear.
Mr. Porter's labors were strictly of a scientific nature. He starts
out with the proposition that what he is about to explain is very
simple, and very likely it is; but, for one, I can't see it, and I
want more light. He says that it takes a certain number of pounds to
overcome the inertia of the reciprocating parts of a certain weight,
to give it a certain speed. What is inertia? He says, "we will not
take into account the friction of parts." Now, my understanding of
this point is, that friction is practically one of the main elements
in the problem. How can we hope to obtain a correct solution when he
rubs out one of the terms of the equation? What is friction doing all
the time, while he is theoretically having his reciprocating parts
storing up power and then giving it out again, just at the right time,
and in the right quantity?
What an immense amount of iron has been wasted by being cast into fly
wheels, when a fraction of the amount, if only put into cross heads,
would render fly wheels unnecessary!
Mr. Porter stops short in his discussion. He should have added a table
giving the proportionate length of stroke, weight of parts, and number
of revolutions required to produce the effect of an engine running at
a high speed, without the least fraction of inequality in the strain
on the crank, and then the sun would have fairly risen in the "dawn of
a new era for the steam engine." But, as it is so very simple, we can
all figure it out for ourselves.
In the diagram Mr. Porter gives, to illustrate the travel of the
piston, he wets his finger and draws it over another term in the
equation (a method of elimination not taught by Hutton, Davies, and
other mathematicians). It is a quick way, but is it correct? He says,
"the distance traveled by the piston is the versed sine of an angle
formed by a line from the center of the crank pin, in any part of its
stroke to the center of the circle described by the crank pin, leaving
out of the calculation the angular vibration of the connecting rod."
What he means by the "angular vibration," I do not know. He is wrong
in the statement. If he will think of it he will see it. If he meant
to say that the piston's travel was measured by the versed sine of the
angle formed by the connecting rod and the line of horizontal centers,
he is wrong again, yet nearer the truth than before, just as the
proportion between the length of the connecting rod and the half
diameter of the circle described by the crank pin. This can quickly
be seen by supposing the connecting rod to be detached, and allowed
to fall down on the center line, at any part of the stroke. If he
understood this (as no doubt he did), he should not ignore the facts.
What I am aiming at is this. When a man attempts to demonstrate a
thing mathematically, he must take into his calculation everything
essentially connected with the problem, just exactly as it is, and not
as he would have it; otherwise, he cannot, by any possibility, attain
a correct result. When he claims, as now, the practicability of
running engines at a high speed, I think he is claiming too much.
Build an engine of proper materials, make it strong, and fit
everything as it should be, balance crank and fly wheel to a nicety,
keep everything snugly in its place, and the terrors of a quick stroke
vanish.