The curator was cataloguing a box of papers left to the museum by a family who had run a mill, and near the bottom she found a passbook. It had been opened for a child, with a hundred units of the old currency, and the account had earned five percent a year ever since. Nobody had touched it.
She was curious, so she worked out what it had done, decade by decade.
The first ten years were disappointing. The hundred had become a hundred and sixty-two — not yet two hundred, after a whole decade. Anyone watching would have concluded that this was a slow and rather pointless arrangement.
By the fiftieth year it had passed a thousand. That is ten times the original, and it had taken half a century to get there.
By the hundredth year it was over thirteen thousand. The curator checked her arithmetic twice, because the number looked like a mistake beside the modest figure written on the first page.
The second half of the century had added more than a hundred times what the first decade added, and the rate had never changed. What changed was the amount the five percent was taken from. Five percent of a hundred is five. Five percent of twelve thousand is six hundred — the same rule, a hundred and twenty times the result.
The curator put a small label beside the passbook in the display case. It reads: this account was ignored for a hundred years, and that is exactly why it is here.