The Wealth of Nations by Part 4 Chapter 1 Page 20

Lisbon, or from wherever else it was to be had, fifty tons of gold, which could be coined into more than five millions of guineas. But if there were an effectual demand for grain to the same value, to import it would require, at five guineas a ton, a million of tons of shipping, or a thousand ships of a thousand tons each.

The navy of England would not be sufficient.

When the quantity of gold and silver imported into any country exceeds the effectual demand, no vigilance of government can prevent their exportation. All the sanguinary laws of Spain and Portugal are not able to keep their gold and silver at home. The continual importations from Peru and Brazil exceed the effectual demand of those countries, and sink the price of those metals there below that in the neighbouring countries.