MONEY 
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are inseparable from cabs, so our inquiries 
lead us to the cost of production of cabs and 
an equation connecting the supplies of con 
veyance or transport obtainable from cabs 
with different quantities of them. The equa 
tion sought expresses indirectly the rapidity 
of circulation of cabs, so to speak. Now the 
problem of money is very like this problem. 
In the case of money, the thing demanded 
is facility for effecting exchanges, and the 
quantity of this facility depends upon the 
number of things with which it is associated 
(that is the quantity of coins of each sort), and 
their rapidity of circulation. Broadly speak 
ing we may say that rapidity of circulation 
refers to the number of times that a thing 
can repeat in a given period the services 
associated with it. It is true that the demand 
for money is a derived demand reflecting the 
demand for other things, but so is the demand 
for many articles. 
Coming back to the case of the cabs, we 
may say that the price of cabs is settled by the 
demand for them in relation to the marginal 
cost of production of different quantities of 
them ; but the demand for them is derived 
from the consumers’ demand for rides, and the 
supply forces, while expressing directly the cost 
of production of different quantities of cabs,