Journalistic accounts mention that at this point the witness smiled. “I knew it would be no difficulty,” he said, “despite the appearance of danger, for I had on several occasions read in the press of those incidents which are called railway sway, and I had similarly read of the explanation, offered by engineers, that the forces are caused by the nature of swiftly moving air as shown in the studies of the late Italian, Baroni. Thus, I was assured that these forces would operate to hold my person to the surface of the coach, and I should be utterly safe in my undertaking.”
At this point, the prosecutor asked for further elucidation, which Pierce gave in garbled form. The summary of this portion of the trial, as reported in the Times, was garbled still further. The general idea was that Pierce— by now almost revered in the press as a master criminal— possessed some knowledge of a scientific principle that had aided him.
The truth is that Pierce, rather proud of his erudition, undertook his climb over the cars with a sense of confidence that was completely unfounded. Briefly, the situation was this:
Beginning around 1848, when railway trains began to attain speeds of fifty or even seventy miles an hour, a bizarre and inexplicable new phenomenon was noted. Whenever a fast-moving train passed a train standing at a station, the carriages of both trains had a tendency to be drawn together in what was called “railway sway.” In some cases the carriages heeled over in such a pronounced fashion that passengers were alarmed, and indeed there was sometimes minor damage to coaches.
Railway engineers; after a period of technical chatter, finally admitted their perplexity outright. No one had the slightest idea why “railway sway” occurred, or what to do to correct it. One must remember that trains were then the fastest-moving objects in human history, and the behavior of such swift vehicles was suspected to be governed by some set of physical laws as yet undiscovered. The confusion was precisely that of airplane engineers a century later, when the “buffeting” phenomenon of an aircraft approaching the speed of sound was similarly inexplicable, and the means to overcome it could only be guessed at.
However, by 1851 most engineers had decided correctly that railway sway was an example of Bernoulli’s Law, a formulation of a Swiss mathematician of the previous century which stated, in effect, that the pressure within a moving stream of air is less than the pressure of the air surrounding it.
This meant that two moving trains, if they were close enough, would be sucked together by the partial vacuum of air between them. The solution to the problem was simple, and soon adopted: the parallel tracks were set farther apart, and railway sway disappeared.
In modern times, Bernoulli’s Law explains such diverse phenomena as why a baseball curves, why a sailboat can sail into the wind, and why an airplane wing lifts the aircraft. But then, as now, most people did not really understand these events in terms of physics: most jet-age travelers would probably be surprised to learn that a jet flies because it is literally sucked upward into the air by a partial vacuum over the wings’ upper surface, and the sole purpose of the engines is to propel the wings forward fast enough to create a stream of passing air that produces this necessary vacuum.
Futhermore, a physicist would dispute even this explanation as not really correct, and would insist that a rigorous explanation of events is even further from the public’s “common sense” idea about these phenomena.
In the face of this complexity, one can readily understand Pierces own confusion, and the erroneous conclusion. he drew. Apparently he believed that the airstream around the moving carriage, as described by “Baroni,” would act to suck him down to the carriage roof, and thus help him to maintain his footing as he moved from car to car.
The truth is that Bernoulli’s Law would not operate in any way on his body. He would simply be a man exposed to a fifty-mile-an-hour blast of rushing air, which could blow him off the train at any moment, and it was absurd for him to attempt what he did at all.