The kind of accuracy which we must here consider is that which concerns the acquisition of the exact equivalent for a commodity sold or the giving of an exact equivalent of a good bought. Many other kinds of accuracy are of the first importance in business, but this only is significant in the present connection. As compared with former times, commerce is nowadays conducted on a large scale and with small margins. The difference between the buying and the selling price is often so small that a few cents added or subtracted in each transaction mean riches or ruin to a large concern. Hence it is absolutely necessary that very close calculations be made, and that these calculations be realized in the commercial transactions which follow. Inasmuch as practically every trade involves the use of the medium of exchange, this being the universal equivalent given for goods bought and received for goods sold, it is evident that this medium must be of such a character as to render possible the transfer from buyer to seller of the exact amount agreed upon. Otherwise the closest and most careful calculations might be of no avail.

The importance of a convenient currency may be appreciated by considering the disadvantages which the use of a medium consisting of a single element such as gold or silver would involve. For large payments the weight of the metal would render transportation difficult, expensive, and slow; and for very small payments, say of a few cents, the amount of metal would be so small that it could not be put into the form of a coin which would be usable, and, if the scales were necessary in every exchange, the rapidity absolutely essential in numberless transactions of everyday life would be impossible. Imagine, for example, the difficulties of travel and of conducting a ticket-office at a busy railway station, if in making small change gold had to be weighed and the infinitesimal amounts preserved and carried about by the passengers. Imagine also the expense and inconvenience which would be involved in travelling abroad or in taking long trips at home, if one were compelled to carry the amount needed for the entire journey in the form of gold or silver, as would be the case if one of these metals should constitute our only form of currency. Indeed a convenient currency is quite as essential to commerce as are suitable machines for manufacturing, locomotives for railways, or improved facilities for any other business purpose.

By a safe medium is meant one the use of which does not involve loss. It is desirable, therefore, that the value which it is the purpose of the exchangers to transfer should be always and under all circumstances obtainable through the currency by which the exchange is accomplished, and that the various elements of the medium should be so selected and manipulated as to protect merchants against loss under all circumstances. For example, it is highly desirable that a form of currency should exist which can be sent through the mails or by express without subjecting the parties concerned to loss in case of accident to the train or the ship. So far as possible, also, protection against robbers and against the ordinary accidents of commercial intercourse should be secured.