x = number of hours after 9 am (eg: x = 1 means 1 hr after 9 am, so 10 am)
f(x) = population count x hours after 9 am
f(1) = population count at 10 am (1 hour later)
f(2) = population count at 11 am (2 hrs after 9 am)
f(2) - f(1) represents the difference in population counts from 10 am to 11 am, or put another way, how much the population increased during that time interval.
