necessary to employ some concrete object, the subdivisions of which were familiar, to express the fractional parts whether it be of an estate or anything else. The most common unit in use was the rod of copper divided into 12 thumbs. Accordingly, if a Roman wished to say that Balbus was heir to one-twelfth of an estate he expressed this by the homely formula that Balbus had come in for one inch, the denominator 12 being mentally supplied, as everyone knew that there were 12 inches in the copper bar. The same principle of taking some familiar object, the ordinary method of dividing which was known to all men, is seen in the method of expressing one-tenth. The Roman denarius was divided into 10 libellae; accordingly, when Cicero wishes to say that a certain person had come in for a tenth part of an estate he says that he has come in for a libella (heres ex libella). From this the reader will at once see that we might just as well declare that the word denarius is an abstract word meaning unity as make the same assertion about the as. Again, when the Roman land surveyors elaborated their system of mensuration, they found that the simplest method of expressing the fractional parts of the jugerum was to employ the old duodecimal method of the as. Nor is this without a parallel elsewhere. As the yard was the common English unit of linear measure, it was applied to the most common unit of land, the quarter of the hide, which was accordingly termed a yard of land, or a virgate (virga terrae). The English analogy is even still more complete, for as the as or foot-rod became the unit of weight, so in Cambridge the yard of butter is identical with the pound of butter64.
Our next step will be to trace the process by which the as or rod became the general weight-unit, the pound (libra). The term libra is not the oldest Latin name for weight, for pondus or its cognate verb pendeo, which literally means to hang, is the true claimant for that position. Libra seems properly to mean the balance, as is seen from the legal formula (employed in Mancipatio) per aes et libram, by means of copper and the balance. From the fact that its chief use was to weigh asses of copper, the mass of an as came