The Wealth of Nations by Part 4 Chapter 3 Page 8

I think, it is now generally acknowledged, are a very uncertain criterion, on account of the inaccuracy of the valuation at which the greater part of goods are rated in them. The course of exchange is, perhaps, almost equally so.

When the exchange between two places, such as London and Paris, is at par, it is said to be a sign that the debts due from London to Paris are compensated by those due from Paris to London. On the contrary, when a premium is paid at London for a bill upon Paris, it is said to be a sign that the debts due from London to Paris are not compensated by those due from Paris to London, but that a balance in money must be sent out from the latter place; for the risk, trouble, and expense of exporting which, the premium is both demanded and given.

But the ordinary state