Speculation, Trade Imbalances, and Black Markets
While trade among two merchants can be facilitated by transferring ownership of specie from one to the other without moving the actual silver, the actual practice of trade involves many producers and consumers in multiple states, occurring over a longer period of time, but the aggregate effect is the same, and the efficiency of banking in transferring the ownership of specie is all the more efficient in a large and complex system of exchanges.
This ability to transfer ownership rather than silver is further extended by the use of credit, such that a bank in England may exchange letters of credit that are owed by French citizens with a bank in France that has letters of credit in equal amount that is owed by English citizens. The banks may exchange these letters and collect from debtors on their own shores.
Doing so requires bankers to engage in speculation - to extend credit to a French merchant on the expectation that a letter of credit to an English merchant will later come in to be exchanged for the French debt, enabling the banker to hold the silver (and perhaps profit by lending it) in the interim.
The banker thus undertakes the risk of having to move specie at his own risk and expense when the balance of trade cannot be worked out by exchanging letters of credit. For example, if England gains in trade with Portugal, but always loses in trade with Holland, the balance between gain and loss must be transported from Portugal from Holland. To the banker, this may constitute an additional measure of gain or loss depending on the amount at which letters of credit are discounted in each nation.
Cantillon also considers, somewhat superficially, the fluctuation between the value of metals in a system where more than one is used as currency. Given that copper is used for small transaction (customer to merchant), silver is used for moderate ones (merchant to producer), and gold is used for large amounts (among merchants, producers, and banks), there is considerable profit or loss to be hand when the exchange values of the metals themselves fluctuates over time.
He also concedes, but refrains from exploring, the exchange of money between states for reasons other than trade: princes may borrow or lend to one another, governments may buy services or political favors, private citizens may invest in businesses in other nations, and the like. These may also be handled through the banking system rather than by the transport of specie, but as there is often no reciprocal trade for them, transportation is inevitable.
A brief mention is made of the ability of banking to facilitate trade when a government has prohibited the exportation of its specie, or has taxed it. This is a considerable risk to the bankers who must count on a balance of trade so that letters of credit may be exchanged without an imbalance.
However, the entire notion of prohibiting the export of specie is inhibitive to trade and the benefits it could bring to a ruler's own subjects. That is, if the king of Portugal strictly prohibit the export of specie, even to threaten the property and life of transporters, then the English merchants see no method of receiving profit from their goods and will not send their manufactures there. The Portuguese subjects, in need of foreign goods that cannot be produced domestically, will offer considerably higher prices for them - and the profit goes to those who disregard the law to smuggle goods into the country and gold out of it.
No advantage is gained by such a law, and it is arrogance for a ruler to think that those who want things will not find a way to obtain them, as someone will facilitate the trade if the profit is there. The only way that a king who is covetous of gold to keep it in his own country is to encourage production among his citizens such that imported goods can be repaid with the profit of exports.