An introduction to mathematics, by A. N. Whitehead.

2 INTRODUCTION TO MATHEMATICS
or coal-gas or steam-at a constant temperature. Let v be the number of cubic feet in
its volume and p its pressure in lb. weight
per square inch. Then the law, known as
Boyle's law, expressing the relation between
p and v as both vary, is that the product
pv is constant, always supposing that the
temperature does not alter. Let us suppose,
for example, that the quantity of the gas
and its other circumstances are such that
we can put pv =1 (the exact number on the
right-hand side of the equation makes no
essential difference).
P \C
O v M v
Fig. 2.
Then in Fig. 2 we take two lines, OV and
OP, at right angles and draw OM along OV
to represent v units of volume, and ON along