FLYING OF KITES.
It was said of Benjamin Franklin that when he wished to fly a kite in
order to ascertain if lightning could be drawn down from the clouds, he
managed to have a boy with him in order to avoid ridicule. It was
considered too frivolous in those days for grown-up men to amuse
themselves with kites, and a good many besides Benjamin Franklin have
feared to face the ridicule that was inevitable if they took up or even
discussed the question of artificial flight. Nineteen years ago, when I
commenced my own experiments, I was told that my reputation would be
greatly injured, that mankind looked upon artificial flight as an
_ignis-fatuus_, and that anyone who experimented in that direction was
placed in the same category as those who sought to make perpetual-motion
machines or to find the philosopher’s stone. Although I had little fear
of ridicule, still I kept things as quiet as I could for a considerable
time, and I had been working fully six months before anyone ascertained
what I was doing. When, however, it became known that I was
experimenting with a view of building a flying machine, the public
seemed to think that I was making honest and praiseworthy scientific
investigations; true, I might not succeed, still it was said that I
would accomplish something, and find out some of the laws relating to
the subject. No one ridiculed my work except two individuals, and both
of these were men whom I had greatly benefited. As is often the case,
those whom you find in difficulties and place on their feet seek to do
you some injury as compensation for the benefits they have received.
At the present time it is not necessary for any man to take a small boy
with him as a species of lightning-rod to ward off ridicule when he
flies a kite. I have been one of a committee on kite-flying at which
some of the most learned and serious men in England were my colleagues
in investigating the subject. The behaviour of kites is certainly very
puzzling to those who do not thoroughly understand the subject. A kite
may be made with the greatest degree of perfection, and placed in the
hands of one of considerable experience; nevertheless, it may behave
very badly, diving suddenly to the ground without any apparent cause.
Then, again, this same kite will sometimes steadily mount in the air
until it reaches a height difficult to account for. If the surface of
the earth should be perfectly smooth, and the wind should always blow in
a horizontal direction, kites would not show these eccentric
peculiarities, but, as a matter of fact, the air seldom moves in a
horizontal direction; it is always influenced by the heat of the surface
of the earth. Heated air is continually ascending in some places only to
be cooled and to descend in other places. If one is attempting to fly a
kite where the air is moving downwards, he will find it an extremely
difficult matter, whereas, if he is fortunate enough to strike a current
of air which is rising, the kite will mount much higher in the air than
can be accounted for, except we admit of the existence of these upward
draughts of air. On one occasion many years ago, I was present when a
bonded warehouse in New York containing 10,000 barrels of alcohol was
burnt. It was nine o’clock at night, and I walked completely around the
fire, and found things just as I had expected. The wind was blowing a
perfect hurricane through every street in the direction of the fire,
although it was a dead calm everywhere else; the flames mounted straight
in the air to an enormous height, and took with them a large amount of
burning wood. When I was fully 500 feet from the fire, a piece of partly
burnt 1-inch board, about 8 inches wide and 4 feet long, fell through
the air and landed very near me, sending sparks in every direction. This
board had evidently been taken up to a great height by the tremendous
uprush of air caused by the burning alcohol. It is very evident that a
kite made of boiler iron could have been successfully flown under these
conditions providing that it could have been brought into the right
position.
[Illustration: Fig. 7.--The circulation of air produced by a difference
in temperature.]
The sketch (Fig. 7) shows a device consisting of a spirit lamp and a box
of ice. The lamp heats the metallic plate, expands the air which rises
and is cooled by convection on coming in contact with the top plate, and
descends as shown. However, a fire is not necessary to accomplish this
result; it is taking place all over the earth, all the time. A great
number of plants depend upon a rising current of air to transport their
seeds to distant places. Seeds of the thistle and dandelion variety are
sometimes able to travel hundreds of miles, to the great vexation of
farmers; and there is a certain class of small spider known as “Balloon
Spiders” which also depend upon a rising current of air to carry them
from the place of their birth to some distant part where they, of
course, hope to start a colony. When I was a boy of eight, I noticed
small spiders webbing down from the sky. I was greatly puzzled; it
appeared to me that they had attached their web to some stationary
object high in the air and were spinning a web in order to lower
themselves to the earth. What could that stationary object be? As the
sky was clear, I was quite unable to understand this phenomenon, but
afterwards I learned from scientific books that there was a class of
spiders that managed to rise high in the air by the aid of the wind. It
appears that they climb a high tree until they have reached the
uppermost extremity and then, from a leaf or twig that projects into the
air, they wait for an ascending current of air. Although the spider is
exceedingly small--the size of a pin’s head--it has about 200
spinnerets, its ordinary web being formed of no less than that number of
extremely fine threads. These are spun out singly into the air until an
almost invisible mass of fine webs interlacing each other in all
directions and forming an approximately cylindrical network about half
an inch in diameter and 18 inches long is produced. Whenever an upward
draft of air approximately vertical occurs, it takes this weightless
tangle of fine webs with it, and so soon as the spider finds there is
sufficient pull to lift its weight, it lets go and ascends with the air.
When the Nulli Secundus ascended at Farnborough and landed at the
Crystal Palace, Mr. Cody, who was on board, reported what he supposed to
be a very curious and unaccountable phenomenon. The balloon was covered
with many thousands of minute spiders that it had picked up in the air
on the voyage. Certainly this of itself is very strong evidence of the
existence of these ascending currents of air.
[Illustration: Fig. 8.--_a_ represents a kite in a horizontal wind, _e_,
_e_, _e_; _b_, the same kite in a rising column of air, the wind blowing
in the direction shown at _f_, _f_. If the kite is a good one, it may
pass over to the point _c_.]
When in Boston about fifteen years ago, I went to Blue Hill to witness
the remarkable kite flying which was taking place at that time. The
kites experimented with were of the Hargrave type, and of enormous
dimensions. A steel wire and windlass worked by a steam engine was
employed. I was told that on certain occasions the kites mounted
extremely high, much higher than they were able to account for; but on
this particular occasion, although they let out a great amount of wire,
the kite did not mount very high. I have heard much discussion first and
last regarding the flight of kites, and I think it is generally admitted
that they do sometimes rise upwards and continue moving to the windward
until they pass directly over the spot where they are attached to the
earth. It was not, however, till about three years ago that I witnessed
this phenomenon myself. Mr. Cody, who is the inventor of a very good
kite, had been flying kites at the Crystal Palace for some months, and
on one occasion I saw his kite rise, pass to the windward and directly
over our heads. I took hold of the cord with both hands, and was
somewhat surprised to find what the lifting effect was. The kite was,
however, of large dimensions, but by no means so large as Mr. Cody’s
“man-lifting kites.” In the drawing (Fig. 8) I have shown, at _a_, the
action of a kite in a horizontal wind, lines _e_, _e_, showing the
direction of the wind. A good kite will easily mount 45°, the angle
shown, but on the occasion just mentioned, the sun had been shining
brightly into the valley where the experiments took place, and an
upward current of air had been determined. The cooler air was, of
course, rushing in from each side and mounting in about the centre of
the valley, and Mr. Cody’s kite, instead of flying in a horizontal wind,
soon reached a point where the wind was ascending at an angle, as shown
at _f_, _f_. The kite would therefore mount until at _b_, where it
presented the same angle to the wind as with the horizontal wind at _a_,
and if it should be made to fly at a higher angle, it might pass over to
the position shown at _c_. But it must not be imagined that this
phenomenon can be witnessed every day in the year. It is only on rare
occasions that one is fortunate enough to find a wind which is blowing
at a sufficiently sharp upward trend to cause a kite to pass to the
windward over the point of support. Neither must it be supposed that
this favourable condition of things is of long duration. As the centre
of the upward current is constantly moving, it is certain that very soon
it will move away from the point from which the kite is being flown.
What is true of kites is also true of flying machines. It is very
difficult indeed to make a kite mount providing that it is in a
descending current of air, and one is just as likely to find a
descending current as any other. Flying machines will, therefore, have
to be made with a considerable amount of reserve energy, so as to be
able to put on a spurt when they encounter an adverse current. If a
machine is made that is able to maintain itself in the air for any
considerable length of time, it will not be a very difficult task to
know when a current of air of this kind is encountered, because, if the
engine is working up to speed, and everything is in perfect order, and
still the machine is falling, it is very certain that an unfavourable
current has been encountered, and efforts should be made to get out of
it as soon as possible. Then, again, if the machine has an abnormal
tendency to rise without any increase in the number of rotations made by
the screws, the aeronaut may be certain that he has encountered an
upward and favourable current of air which, unfortunately, will not
last. It should, however, be borne in mind that, while the width of the
upward current is not very great, nevertheless, it may extend in a
practically straight line for many miles.