part going to the other, while still another part is directed
upward and goes over the top of the obstruction.
This makes the handling of a glider on an obstructed
field difficult and uncertain. To handle a glider successfully
the place of operation should be clear and the wind
moderate and steady. If it is gusty postpone your flight.
In this connection it will be well to understand the velocity
of the wind, and what it means as shown in the
following table:

Miles per hour Feet per second Pressure per sq. foot
10 14.7 .492
25 36.7 3.075
50 73.3 12.300
100 146.6 49.200

Pressure of wind increases in proportion to the square
of the velocity. Thus wind at 10 miles an hour has four
times the pressure of wind at 5 miles an hour. The
greater this pressure the large and heavier the object
which can be raised. Any boy who has had experience
in flying kites can testify to this, High winds, however,
are almost invariably gusty and uncertain as to direction,
and this makes them dangerous for aviators. It
is also a self-evident fact that, beyond a certain stage,
the harder the wind blows the more difficult it is to
make headway against it.

Launching Device for Gliders.