At the beginning, I found, the Coinage Act of 1792 authorizes "Eagles — each to be of the value of ten dollars or units [i.e., of ten silver dollars], and to contain two hundred and forty-seven grains, and four eighths of a grain of pure … gold." Since the dollar contained 371.25 grains of silver, this brought into legal equivalence 3712.5 grains of silver and 247.5 grains of gold. The ratio was 1:15.

These values were adjusted through the years.