> He was also kind enough to have noted on the phonautogram the frequency of the fork’s vibration – 500 simple vibrations per second.

This was used to restore the audio speed to restore frequencies, but...

> It turns out that when people wrote “500 simple vibrations per second” in the nineteenth century, they meant 250 Hz in modern terminology.

I love this: such a misunderstanding in what someone in the 19th century saw as 500 somethings, and we see today as 250 somethings, shows that even the apparently intuitively clear understanding of what a description means can be very wrong. (I wonder what this means for more vague descriptions from further in the past and other societies and dead languages.) Or, that measurement and terminology problems, which every developer will be familiar with, are an age-old human problem!