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Proceedings of the Royal Society 
into a particular field is not constant, but increases, hence P'D' is 
steeper than PD. I see at present no means of experimentally 
ascertaining the gain reaped by producers represented by the area 
PDDP' ; it can be approximately estimated by considering the pre- 
vailing rate of interest in the producing community and the amount 
of capital required for the production of the unit of the article. 
We see that the gain of a manufacturing capitalist may be 
divided into two parts — the profit as a trader, and the interest as a 
capitalist. 
In safe trades, where there are few fluctuations in price, the 
former gain may perhaps be the most important,; in more specu- 
lative trades the latter. 
There is yet a third source of gain to the manufacturing com- 
munity : the labourer who produces the goods earns his wages by 
the manufacture, and this is an advantage to him. In the diagram, 
the area OP'D'D" represents the wages paid for labour alone. 
The length of the lines between OY and P'D' represent the wages 
of labour per unit of goods, increasing as the quantity of goods 
required increases. This is lost to the community if the manu- 
facture is stopped. Thus the whole sum paid by the consumer is 
the area OMDD"; and this is made up of three parts, one of which 
