At the end of seventeen days the snail will have climbed 17 ft., and at
the end of its eighteenth day-time task it will be at the top. It
instantly begins slipping while sleeping, and will be 2 ft. down the
other side at the end of the eighteenth day of twenty-four hours. How
long will it take over the remaining 18 ft.? If it slips 2 ft. at night
it clearly overcomes the tendency to slip 2 ft. during the daytime, in
climbing up. In rowing up a river we have the stream against us, but in
coming down it is with us and helps us. If the snail can climb 3 ft. and
overcome the tendency to slip 2 ft. in twelve hours' ascent, it could
with the same exertion crawl 5 ft. a day on the level. Therefore, in
going down, the same exertion carries it 7 ft. in twelve hours--that is,
5 ft. by personal exertion and 2 ft. by slip. This, with the night slip,
gives it a descending progress of 9 ft. in the twenty-four hours. It can,
therefore, do the remaining 18 ft. in exactly two days, and the whole
journey, up and down, will take it exactly twenty days.