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the consideration of one of the first great problems
in flying machines； namely； the supporting
surfaces；not its form； shape or arrangement；
（which will be taken up in their proper places）； but
the area； the dimensions； and the angle necessary
for flight。
AREA NOT THE ESSENTIAL THING。The history
of flying machines； short as it is； furnishes many
examples of one striking fact： That area has
but little to do with sustaining an aeroplane when
once in flight。 The first Wright flyer weighed
741 pounds； had about 400 square feet of plane
surface； and was maintained in the air with a 12
horse power engine。
True； that machine was shot into the air by a
catapult。 Motion having once been imparted to it；
the only thing necessary for the motor was to
maintain the speed。
There are many instances to show that when
once in flight； one horse power will sustain over
100 pounds； and each square foot of supporting
surface will maintain 90 pounds in flight。
THE LAW OF GRAVITY。As the effort to fly
may be considered in the light of a struggle to
avoid the laws of nature with respect to matter；
it may be well to consider this great force as a
fitting prelude to the study of our subject。
Proper understanding； and use of terms is very
desirable； so that we must not confuse them。
Thus； weight and mass are not the same。 Weight
varies with the latitude； and it is different at various
altitudes； but mass is always the same。
If projected through space； a certain mass
would move so as to produce momentum； which
would be equal at all places on the earth's surface；
or at any altitude。
Gravity has been called weight； and weight
gravity。 The real difference is plain if gravity
is considered as the attraction of mass for mass。
Gravity is generally known and considered as a
force which seeks to draw things to the earth。
This is too narrow。
Gravity acts in all directions。 Two balls suspended
from strings and hung in close proximity
to each other will mutually attract each other。
If one has double the mass it will have twice the
attractive power。 If one is doubled and the other
tripled； the attraction would be increased six
times。 But if the distance should be doubled the
attraction would be reduced to one…fourth； and
if the distance should be tripled then the pull
would be only one…ninth。
The foregoing is the substance of the law；
namely； that all bodies attract all other bodies
with a force directly in proportion to their mass；
and inversely as the square of their distance from
one another。
To explain this we cite the following illustration：
Two bodies； each having a mass of 4
pounds； and one inch apart； are attracted toward
each other； so they touch。 If one has twice the
mass of the other； the smaller will draw the larger
only one…quarter of an inch； and the large one
will draw the other three…quarters of an inch；
thus confirming the law that two bodies will attract
each other in proportion to their mass。
Suppose； now； that these balls are placed two
inches apart；that is； twice the distance。 As
each is； we shall say； four pounds in weight； the
square of each would be 16。 This does not mean
that there would be sixteen times the attraction；
but； as the law says； inversely as the square of
the distance； so that at two inches there is only
one…sixteenth the attraction as at one inch。
If the cord of one of the balls should be cut； it
would fall to the earth； for the reason that the
attractive force of the great mass of the earth is
so much greater than the force of attraction in
its companion ball。
INDESTRUCTIBILITY OF GRAVITATION。Gravity
cannot be produced or destroyed。 It acts between
all parts of bodies equally； the force being
proportioned to their mass。 It is not affected by
any intervening substance； and is transmitted
instantaneously； whatever the distance may be。
While； therefore； it is impossible to divest matter
of this property； there are two conditions
which neutralize its effect。 The first of these is
position。 Let us take two balls； one solid and
the other hollow； but of the same mass； or density。
If the cavity of the one is large enough to receive
the other； it is obvious that while gravity is still
present the lines of attraction being equal at
all points； and radially； there can be no pull which
moves them together。
DISTANCE REDUCES GRAVITATIONAL PULL。Or
the balls may be such distance apart that the attractive
force ceases。 At the center of the earth
an object would not weigh anything。 A pound
of iron and an ounce of wood； one sixteen times
the mass of the other； would be the same；absolutely
without weight。
If the object should be far away in space it
would not be influenced by the earth's gravity；
so it will be understood that position plays an
important part in the attraction of mass for mass。
HOW MOTION ANTAGONIZES GRAVITY。The second
way to neutralize gravity； is by motion。 A
ball thrown upwardly； antagonizes the force of
gravity during the period of its ascent。 In like
manner； when an object is projected horizontally；
while its mass is still the same； its weight is less。
Motion is that which is constantly combating
the action of gravity。 A body moving in a circle
must be acted upon by two forces； one which tends
to draw it inwardly； and the other which seeks to
throw it outwardly。
The former is called centripetal； and the latter
centrifugal motion。 Gravity； therefore； represents
centripetal； and motion centrifugal force。
If the rotative speed of the earth should be retarded；
all objects on the earth would be increased
in weight； and if the motion should be accelerated
objects would become lighter； and if sufficient
speed should be attained all matter would fly off
the surface； just as dirt dies off the rim of a
wheel at certain speeds。
A TANGENT。When an object is thrown horizontally
the line of flight is tangential to the earth；
or at right angles to the force of gravity。 Such
a course in a flying machine finds less resistance
than if it should be projected upwardly； or directly
opposite the centripetal pull。
_Fig 1。 Tangential Flight_
TANGENTIAL MOTION REPRESENTS CENTRIFUGAL
PULL。A tangential motion； or a horizontal
movement； seeks to move matter away from the
center of the earth； and any force which imparts
a horizontal motion to an object exerts a centrifugal
pull for that reason。
In Fig。 1； let A represent the surface of the
earth； B the starting point of the flight of an object；
and C the line of flight。 That represents a
tangential line。 For the purpose of explaining
the phenomena of tangential flight； we will assume
that the missile was projected with a sufficient
force to reach the vertical point D； which
is 4000 miles from the starting point B。
In such a case it would now be over 5500 miles
from the cen