The Mint Act of 1792
After determining an average silver content in the Spanish Milled Dollar via the merchants, Congress drafted the Mint Act of 1792, which stated:
“The U.S. Dollar or Unit shall be of the value of the Spanish Milled Dollar as the same is now current.
To wit, 371 1/4 grains of fine silver.”
That is what an attorney would call “a finding of historical fact”. Congress was not creating this “thing” (ie, “dollar”), it had no power to create this “thing” because the “thing” was mentioned in the Constitution. It (Congress) was finding out what this “thing” was at that moment. And that was a decision of Constitutional Law. Now we know what the word “dollar” means in the Constitution as a historical fact.
We can’t change the 371 1/4 grains of silver written into The Mint Act of 1792 because we don’t have the box of samples used. Even though our chemists today may be more accurate than the Framers, we can’t perform the experiment. More importantly, we can’t go out into the market as it existed then and determine if the coins they picked were really representative. We would have to assume they acted in good faith to the best extent of their knowledge. Ultimately it doesn’t matter because it was an arbitrary number anyway. They (Framers) needed to find a number (371 1/4 grains of fine silver) and they found it.