Take six playing cards and turn up half an inch of the ends of each, the
same side. With them form a hollow cube or box by the arrangement here
depicted:
[Illustration: Fig. 43.]
In the centre of one side cut out a small circle.
After filling this box with tobacco smoke, you can make a ring of it
issue from the aperture by giving a tap to the opposite side, just as
the pressure of the flexible bottom of an oil-can makes the fluid spurt
out.