method, while it is not necessary that the apparatus be similar to the
electric car or that the external activities in the experiment be
identical with their performance in the service. After several
unsatisfactory efforts, in which I worked with too complicated
instruments, I finally settled on the following arrangement of the
experiment which seems to me to satisfy those two demands.

The street is represented by a card 9 half-inches broad and 26
half-inches long. Two heavy lines half an inch apart go lengthwise
through the centre of the card, and accordingly a space of 4
half-inches remains on either side. The whole card is divided into
small half-inch squares which we consider as the unit. Thus there is
in any cross-section 1 unit between the two central lines and 4 units
on either side. Lengthwise there are 26 units. The 26 squares which
lie between the two heavy central lines are marked with the printed
letters of the alphabet from A to Z. These two heavy central lines are
to represent an electric railway track on a street. On either side the
4 rows of squares are filled in an irregular way with black and red
figures of the three first digits. The digit 1 always represents a
pedestrian who moves just one step, and that means from one unit into
the next; the digit 2 a horse, which moves twice as fast, that is,
which moves 2 units; and the digit 3 an automobile which moves three
times as fast, that is, 3 units. Moreover, the black digits stand for
men, horses, and automobiles which move parallel to the track and
cannot cross the track, and are therefore to be disregarded in looking
out for dangers. The red digits, on the other hand, are the dangerous
ones. They move from either side toward the track. The idea is that
the man to be experimented on is to find as quickly as possible those
points on the track which are threatened by the red figures, that is,
those letters in the 26 track units at which the red figures would