ed in each case by an arrow. Every change of direction means
a loss of forward velocity, the loss increasing with the violence and
extent of the change. The pneumatic tyre also scores because, on account
of its elasticity, it gives a "kick off" against the obstacle, which
compensates for the resistance during compression.

[Illustration: FIG. 168.--Section of the mechanism of an air-gun.]

THE AIR-GUN.

This may be described as a valveless air-pump. Fig. 168 is a section of
a "Gem" air-gun, with the mechanism set ready for firing. In the stock
of the gun is the _cylinder_, in which an accurately fitting and hollow
_piston_ moves. A powerful helical spring, turned out of a solid bar of
steel, is compressed between the inside end of the piston and the upper
end of the butt. To set the gun, the _catch_ is pressed down so that its
hooked end disengages from the stock, and the barrel is bent downwards
on pivot P. This slides the lower end of the _compressing lever_ towards
the butt, and a projection on the guide B, working in a groove, takes
the piston with it. When the spring has been fully compressed, the
triangular tip of the rocking cam R engages with a groove in the
piston's head, and prevents recoil when the barrel is returned to its
original position. On pulling the trigger, the piston is released and
flies up the cylinder with great force, and the air in the cylinder is
compressed and driven through the bore of the barrel, blocked by the
leaden slug, to which the whole energy of the expanding spring is
transmitted through the elastic medium of the air.

There are several other good types of air-gun, all of which employ the
principles described above.

THE SELF-CLOSING DOOR-STOP

is another interesting pneumatic device. It consists of a cylinder with
an air-tight piston, and a piston rod working through a cover at one
end. The other end of the cylinder is pivoted to the door frame. When
the door is opened the piston compresses a spring in the cylinder, and
air is admitted past a cup leather on the piston to the upper part of
the cylinder. This air is confined by the cup leather when the door is
released, and escapes slowly through a leak, allowing the spring to
regain its shape slowly, and by the agency of the piston rod to close
the door.

THE ACTION OF WIND ON OBLIQUE SURFACES.

Why does a kite rise? Why does a boat sail across the wind? We can
supply an answer almost instinctively in both cases, "Bec