[chaxor]

The Stanford lecture by Robert Channing for one of the first courses in chemical engineering (ChE 101?) on youtube is fun for this.

He goes through how many of the units are defined and how it has had an effect on various projects throughout the years.

One of the best ones is about how the size of the space shuttles' tanks were constrained. They had to be able to fit on a train, so things had constraints relating to the distance between railroad tracks. Those railroad track sizes could be traced back to car axel sizes, and therefore previous horse-carriage axels. These carriages of course had the Romans before them, but all of these had axels defined by the width of two horses.

So ultimately the size constraints on the space shuttle arose from the distance between two horses asses. Kind of fun.