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direction of the wind； the vertical rudder a being
almost at right angles to the course of the wind。
_Fig。 72。 Into the Wind at an Angle。_
In turning a circle the same thing takes place
as shown in Fig。 73； with the tail at a different
angle； so as to give a turning movement to the
plane。 It will be seen that in the circling movement
the tendency of the aeroplane is to fly out
at a tangent； shown by the line D； so that the
planes of the machine are not radially…disposed
with reference to the center of the circle； the line
E showing the true radial line。
Referring now to Fig。 71； it will be seen that
this skidding motion of the machine swings the
wings E F inwardly； so that they offer no resistance
to the oblique movement； but the wings E
E； at the other end of the planes are swung outwardly；
to provide an angle； which tends to raise
up the inner end of the planes； and thereby seek
to keep the planes horizontal。
_Fig。 73。 Turning a Circle。_
BAROMETERS。These instruments are used for
registering heights。 A barometer is a device for
measuring the weight or pressure of the air。
The air is supposed to extend to a height of 40
miles from the surface of the sea。 A column of
air one inch square； and forty miles high； weighs
the same as a column of mercury one inch square
and 30 inches high。
Such a column of air； or of mercury； weighs
14 3/4 pounds。 If the air column should be
weighed at the top of the mountain； that part
above would weigh less than if measured at the
sea level； hence； as we ascend or descend the pressure
becomes less or more； dependent on the altitude。
Mercury is also used to indicate temperature；
but this is brought about by the expansive quality
of the mercury； and not by its weight。
_Fig。 74。 Aneroid Barometer。_
ANEROID BAROMETER。The term Aneroid barometer
is frequently used in connection with air…
ship experiments。 The word aneroid means not
wet； or not a fluid； like mercury； so that； while
aneroid barometers are being made which do use
mercury； they are generally made without。
One such form is illustrated in Fig。 74； which
represents a cylindrical shell A； which has at each
end a head of concentrically formed corrugations。
These heads are securely fixed to the ends of the
shell A。 Within； one of the disk heads has a
short stem C； which is attached to the short end
of a lever D； this lever being pivoted at E。 The
outer end of this lever is hinged to the short end
of another lever F； and so by compounding the
levers； it will be seen that a very slight movement
of the head B will cause a considerable movement
in the long end of the lever F。
This end of the lever F connects with one limb
of a bell…crank lever G； and its other limb has a
toothed rack connection with a gear H； which
turns the shaft to which the pointer I is attached。
Air is withdrawn from the interior of the shell；
so that any change in the pressure； or weight of
the atmosphere； is at once felt by the disk heads；
and the finger turns to indicate the amount of
pressure。
HYDROPLANES。Hydro means water； hence the
term hydroplane has been given to machines
which have suitable pontoons or boats； so they
may alight or initiate flight from water。
There is no particular form which has been
adopted to attach to aeroplanes； the object generally
being to so make them that they will sustain
the greatest amount of weight with the least
submergence； and also offer the least resistance
while the motor is drawing the machine along the
surface of the water； preparatory to launching it。
SUSTAINING WEIGHT OF PONTOONS。A pontoon
having within nothing but air； is merely a measuring
device which determines the difference between
the weight of water and the amount placed
on the pontoon。 Water weighs 62 1/2 pounds per
cubic foot。 Ordinary wood； an average of 32
pounds； and steel 500 pounds。
It is； therefore； an easy matter to determine
how much of solid matter will be sustained by a
pontoon of a given size； or what the dimensions
of a pontoon should be to hold up an aeroplane
which weighs； with the pilot； say； 1100 pounds。
As we must calculate for a sufficient excess to
prevent the pontoons from being too much immersed；
and also allow a sufficient difference in
weight so that they will keep on the surface when
the aeroplane strikes the surface in alighting； we
will take the figure of 1500 pounds to make the
calculations from。
If this figure is divided by 62 1/2 we shall find
the cubical contents of the pontoons； not considering；
of course； the weight of the material of which
they are composed。 This calculation shows that
we must have 24 cubic feet in the pontoons。
As there should be two main pontoons； and a
smaller one for the rear； each of the main ones
might have ten cubic feet； and the smaller one
four cubic feet。
SHAPES OF THE PONTOONS。We are now ready
to design the shapes。 Fig。 75 shows three general
types； A being made rectangular in form；
with a tapering forward end； so constructed as to
ride up on the water。
The type B has a rounded under body； the forward
end being also skiff…shaped to decrease as
much as possible the resistance of the water impact。
_Fig。 75。 Hydroplane Floats。_
The third type C is made in the form of a
closed boat； with both ends pointed； and the bottom
rounded； or provided with a keel。 Or； as in
some cases the body may be made triangular in
cross section so that as it is submerged its sustaining
weight will increase at a greater degree
as it is pressed down than its vertical measurement
indicates。
All this； however； is a matter left to the judgment
of the designer； and is； in a great degree；
dependent on the character of the craft to which
it is to be applied。
CHAPTER XII
EXPERIMENTAL WORK IN FLYING
THE novice about to take his first trial trip in
an automobile will soon learn that the great task
in his mind is to properly start the machine。 He
is conscious of one thing； that it will be an easy
matter to stop it by cutting off the fuel supply
and applying the brakes。
CERTAIN CONDITIONS IN FLYING。In an aeroplane
conditions are reversed。 Shutting off the
fuel supply and applying the brakes only bring
on the main difficulty。 He must learn to stop the
machine after all this is done； and this is the
great test of flying。 It is not the launching；
the ability to get into the air； but the landing； that
gives the pupil his first shock。
Man is so accustomed to the little swirls of air
all about him； that he does not appreciate what
they mean to a machine which is once free to
glide along in the little currents which are so unnoticeable
to him as a pedestrian。
The contour of the earth； the fences； trees； little
elevations and other natural surroundings； all
have their effect on a slight movin