Request your friend to make up his mind as to the time he intends to
rise on the following morning, and then to mention an entirely different
hour to you. To the latter you mentally add twelve, and giving him the
number of the total, request him to look at his watch, and starting at
the hour preceding the one he has selected for rising, to count
backwards until he reaches the number you have given him, beginning with
the number which he previously gave you. Ask him to state the hour at
which he stops, which he will find is the one he selected for rising.
For instance; supposing your friend intends to rise at nine and gives
you four. To four you mentally add twelve and request him to start at
the hour before his getting-up time (which would be eight) and count
sixteen backwards on the face of the watch, starting with the number he
gave you--four--and when he reaches sixteen his finger or pencil will
rest upon nine, the hour he selected for getting up.