Now, we cannot measure or determine any quantity, except by considering some other quantity of the same kind as known, and pointing out their mutual relation.
要度量或确定一个量，我们必须先把另一个同类的量看作已知，再说明两者之间的关系。
If it were proposed, for example, to determine the quantity of a sum of money, we should take some known piece of money, as a louis, a crown, a ducat, or some other coin, and shew how many of these pieces are contained in the given sum.
例如，要确定一笔钱的数量，我们就要取某种已知的钱币，如路易、克朗、杜卡特或别的硬币，再说明给定的钱里含有多少个这样的硬币。
In the same manner, if it were proposed to determine the quantity of a weight, we should take a certain known weight; for example, a pound, an ounce, &c. and then shew how many times one of these weights is contained in that which we are endeavouring to ascertain.
同样，要确定一个重量，就要取一个已知重量，比如一磅、一盎司等，再说明这个已知重量在待测重量里含有多少次。
If we wished to measure any length or extension, we should make use of some known length, such as a foot.
如果要量一段长度或延展，我们就要用某个已知长度，比如一英尺。
度量不是孤立地看一个量，而是拿它同一个已知单位比较。
钱数要用某种硬币作单位。
重量要用磅、盎司等作单位。
长度要用英尺等作单位。
同类量才能直接比较：长度不能直接用盎司来量。