The Wealth of Nations by Part 2 Chapter 3 Page 40

the same number of labourers cannot be increased, but in consequence either of some addition and improvement to those machines and instruments which facilitate and abridge labour; or of a more proper division and distribution of employment. In either case an additional capital is almost always required. It is by means of an additional capital only that the undertaker of any work can either provide his workmen with better machinery or make a more proper distribution of employment among them. When the work to be done consists of a number of parts, to keep every man constantly employed in one way requires a much greater capital than where every man is occasionally employed in every different part of the work. When we compare, therefore, the state of a nation at two different periods, and find, that the annual produce of its land and labour is evidently greater