In solving an inventive problem, it is very important to understand the casual relation of phenomena taking place in a system. Constructing such chains is a very interesting process which sometimes yields completely unexpected results.

The following story is popular in the engineering environment.
Auxiliary engines of one of the first American spaceships had 5 ft in diameter although, according to the designers' calculations, their optimal diameter should have been a little larger. The limitation was caused by that the engines were delivered to the assembly site by railway which went through several tunnels. The distance between the rails was standard – 4 ft and 8.5 and the tunnel diameter was not much larger. There emerges a question why the standard distance between rails was 4 ft and 8.5? How did that figure arise? The American railway was designed after the English pattern. In England, in turn, the distance between rails was selected by analogy with the tram track which was just 4 ft and 8.5.
But why? The thing is that first trams in England were produced at the same factory as horse-drawn vehicles. The distance between the wheels of such a vehicle was 4 ft and 8.5. Horse-drawn vehicles were made so that their wheels matched old ruts on roads and the distance between the ruts was 4 ft and 8.5 all over England.
Why exactly that distance? First roads in Great Britain were broken by Romans, to be more exact, by their chariots. The length of a standard Roman chariot axle was – yes, you are quite right – 4 ft and 8.5. One thing is unclear. That length (4 ft and 8.5) is not an integer in any of the known systems of measures. Why did it occur to Romans to choose that length for chariot axles?
The answer is simple. Two horses were usually harnessed to such a chariot. Four ft and 8.5 was just the average size of two horse croups. There goes the answer to the first question. Even now, when the man goes to outer space, his advanced technical achievements often directly depend on the size of a horse which lived two thousand years ago.

Sent by Sergey Khruschev.
Sculpture fragment: Sergey Bondarenko.