Answer :
At the end of
seventeen days the snail will have
climbed 17
ft., and at
the end of its eighteenth daytime 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 twentyfour 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 twentyfour
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 day
