Today, when people travel between different countries which use various forms of currency, there is a fairly secure and standard system I for exchanging money. Financial experts maintain arid participate in an intricate global system for determining the relative values of the currencies of countries all over the world. In the Roman world, as in many ancient civilizations, exchanging currencies was a more problematic process. Often individual cities issued their own coinage. A traveler needing the coinage of a new city had to go to someone known as a "money-changer." This individual would compute how much the coin of one city or country was worth in terms of the coin of another city or country. Since the value of coins was determined by the value of the metal used in them, money-changers would weigh the coins and try to compute how...