620 Proceedings of the Royal Society 
Similarly, there are a few buyers who, if pushed to extremity, 
would buy some goods above market price ; some also will just 
buy at market price; some will not buy unless the price is below 
market price. This is represented by the demand curve. 
Now, I contend that when the market price is fixed, those 
traders who are perfectly in different whether they buy or sell at 
that price reap no benefit by the trade; but these will be few in 
number. 
Looking at the demand curve, the ordinate X A from the axis OY 
to A represents the value set on some of the goods by some buyers, 
but these buyers have got the goods for the sum represented by 
the ordinate x = OM; the difference between these two ordinates 
X A — x is the difference in price between what was given and what 
might have been given for a certain small quantity a 7 J °f goods. 
Ay is therefore the benefit reaped by buyers from the purchase 
of the quantity \y ; and integrating the benefits derived from the 
sale of each successive quantity, we find the area MDCBAN 
represents the whole gain to buyers by the purchase of the quantity 
y of goods. Similarly, it is easy to show that the area MDcSaP 
represents the gain to sellers by the same transaction; these areas 
represent the gain in money. Each product &y(x — X A ) being 
the product of a quantity by the gain in money per unit of quantity. 
Thus the whole benefit to the two leading communities is repre¬ 
sented by the sum of the two above named areas, and the reparti¬ 
tion of the benefit between the two communities is perfectly 
definite. 
Professor Jevons has used curves to integrate what he terms the 
utility gained by exchange in a manner analogous to the above; 
but utility, as he defines it, admits of no practical measurement, 
and he bases his curve, not on the varying estimates of value set 
by different individuals each on what he has or what he wants, but 
on the varying utility to each individual of each increment of 
goods. The above estimate of the gain due to trade, deduced from 
the demand and supply curves as originally drawn in my Eecess 
Studies’ article is, I believe, novel, and gives a numerical estimate 
in money of the value of any given trade, which might be approxi¬ 
mately determined by observing the effect of a change of prices on 
the trade; the curves throughout their whole lengths could cer- 
