540 TRANSACTIONS OF SECTION F, 
the same results were put into arithmetical form, we should have quite a column 
of figures. 
But this diagram shows us the height of the barometer at any time, and 
all its fluctuations. Its life-history for the week and the law of its variations 
are obvious at a glance, in a way which no words could convey to us. So great 
are the advantages of this method that barographs are printed in many of the 
newspapers. 
But the use of such curves is not confined to the registration of atmospheric 
pressure or temperature. They may be used for all purposes. Thus, for 
AWGN 
1870 1910 
Hira. 2: 
example, we might have a curve indicating the variation in successive years of 
the number of marriages per head of the population. 
Line 1 (fig. 2) shows the proportion of marriages to population from 1870 to 
1910. The advantages of this synoptic view are obvious. But they become 
more obvious still when we add other curves. For instance, line 4 shows the 
price of wheat in various years, line 3 the price of coal, line 2 the average of 
money wages, and line 1 the number of marriages per head of the population. A 
simple inspection shows that these curves rise and fall sympathetically, and 
proves beyond doubt that the facts they represent are causally connected. 
How eloquently this diagram represents, on a space that in a printed book 
may be three inches square, a series of relations which would take three or four 
pages to describe even imperfectly in words. And would any description in 
words enable us to follow the changes like this diagram? The diagram, in fact, 
My Money 
S upply 
Oo M x 
Fic. 3. 
plays the part that maps play in geography and when duly appreciated becomes 
as valuable as maps of countries. 
We may use similar diagrams in the exposition of economic facts. It was, 
however, reserved for Cournot to show that the use of curves might go further 
still. Not only might they be used to display statistical facts, but they might 
also be used to solve problems. I will endeavour to illustrate this very in- 
genious and interesting development. __ 
It is a well-known fact that in certain departments of industry the cost of 
making an article increases in proportion to the number produced. The growth 
of corn is a familiar example of this principle. The principle depends on two 
