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

214 INTRODUCTION TO MATHEMATICS
example, the above graph represents the size
at any time of a population with a uniform
birth-rate, where the x corresponds to the
time reckoned from any convenient day, and
the y represents the population to the proper
scale. The scale must be such that OA represents the population at the date which is
taken as the origin. But we have here come
upon the idea of "rates of increase" which
is the topic for the next chapter.
An important function nearly allied to the
exponential function is found by putting
- x2 for x as the argument in the exponential
function. We thus get exp. (-x2). The
graph y =exp. (-x2) is given in fig. 30.
Y
Fig. 30.
The curve, which is something like a cocked
hat, is called the curve of normal error. Its