2. By art. 1, s. 8, the Constitution of the United States authorizes congress "to coin money, and to regulate the value thereof." Changes in the currency ought not to be made but for the most urgent reason, as they unsettle commerce, both at home and abroad. Suppose Peter contracts to pay Paul one thousand dollars in six months - the dollar of a certain fineness of silver, weighing one hundred and twelve and a half grains - and afterwards, before the money becomes due, the value of the dollar is changed, and it weighs now but fifty-six and a quarter grains; will one thousand of the new dollars pay the old debt? Different opinion may be entertained, but it seems that such payment would be complete; because, 1. The creditor is bound to receive the public currency; and, 2. He is bound to receive it at its legal value. 6 Duverg. n. 174.