AËROSTATICS.--WEIGHT OF AIR.--EXPANSION BY HEAT.
Ascent and Descent.--The Balloon and the Parachute.--Description of
the Balloon.--The Montgolfier Balloon.--Causes of its
Abandonment.--The Gas Balloon.--Hydrogen Gas and its
Manufacture.--The Gossamer Spider.--Reasons of its Ascent and
Descent.--Many Species of Gossamers.--Description of the
Parachute.--Its Mode of Action.--A Balloon converted into a
Parachute.--Toy Parachutes.--Natural Parachutes.--The Dandelion
Seed and its Structure.--The Flying Squirrel.--The Flying
Monkey.--Flying Mice and Flying Opossums.--The Flying Dragon and
its Pseudo-wings.--The Flying Frog.--Weight of Air.--Pressure per
Square Inch.--The Air Ocean and its Storms.--Principle of
Air-currents.--The Sun, the Earth, and the Air.--Ventilation of
Mines.--Choke-damp and Fire-damp.--The Air-shafts.--Chimneys of
Factories.--The Steam-blast.--The Barometer, and Mode of its
Construction.--Water and Mercury.--Sucking Eggs and
Sugar-cane.--Expansion of Water and Metals by Heat.--The
Thermometer.--Wheel-making.
AËROSTATICS.
We will begin this chapter with the only two modes at present known by
which man can ascend from the earth or descend to it with safety,
namely, the Balloon and the Parachute, the latter being generally
attached to the former, and detachable at pleasure.
The Balloon is, in fact, as its name imports, a large, hollow, air-tight
ball, filled with some substance lighter than ordinary air. The original
Balloons by Montgolfier were filled with heated air exactly like our toy
fire-balloons. Just as the supply of hot air is kept up in them by a
sponge dipped in lighted spirits of wine, so in Montgolfier’s balloons
the same object was attained by straw which was kept continually burning
in a grate.
There were, however, two disadvantages about this plan. The first was
the great danger of fire, which on one occasion did ignite a balloon
when at a great height. The second was the perpetual labour required in
keeping the fire alight. Straw burns very rapidly, and so the aëronaut
had no opportunity of making those meteorologic observations in which
consist almost the entire value of the balloon.
[Illustration: GOSSAMER SPIDER. BALLOON.]
Then it was thought that hydrogen gas, being about fourteen times
lighter than ordinary air, would answer the purpose, and such has proved
to be the case. Formerly the gas was made at great expense from
sulphuric acid and zinc, but it is now found that the common coal-gas is
quite as efficient, very much cheaper, and fills the balloon much more
rapidly.
* * * * *
THE same principle, though not the same form, is found in Nature.
There are certain tiny spiders called Gossamers, which have a curious
power of floating in the air. They have been seen on the tops of lofty
spires, and they are sometimes so numerous that the air is full of their
floating webs, and the ground is white with those that have descended.
Their mode of ascent is this. They climb to the top of some elevated
object, if it be only a grass-blade. They then pour out a tuft of long,
slender threads, which shortly begin to tend upwards. As soon as the
Spider feels the pull, it crawls upon the web, and sails away into the
air. The duration and height of the ascent depend much on the wind and
character of the atmosphere.
The web ascends because it is for the time lighter than the atmosphere.
But, as it gradually becomes laden with the moisture that more or less
fills the air, it becomes heavier than the atmosphere, and gently sinks
to the ground.
What may be the object of these aërial voyages no one knows. They may be
for the purpose of capturing minute insects, or they may be for mere
amusement. But in either case they are highly instructive, as showing
the principle on which the balloon was framed.
The little Gossamer Spider is shown on the left hand of the
illustration, clinging to its floating web. I believe that the Gossamer
is not a single species of Spider, but that there are many species which
deserve the name, being able to float in the air when they are small,
but losing that capacity as they increase in size and weight.
* * * * *
NOW we come to another branch of the same subject, namely, the safe
descent from a great height by means of the Parachute.
On the right hand of the illustration is the ordinary Parachute as it
appears when open and closed, in either case having somewhat the
appearance of a large umbrella. It is hung to the balloon in its closed
state, and when detached it falls rapidly for a yard or two with
startling rapidity. The pressure of the air thus forces the ribs open,
and gives sufficient assistance to the atmosphere to insure a gentle
fall.
On one memorable occasion, when the late Albert Smith was in the car of
a balloon upwards of a mile from the ground, the balloon burst.
Fortunately it burst so completely, that the silk was driven into the
closely meshed netting, and formed an extemporised parachute, which took
the voyagers to the earth with safety, except some rather severe
bruises.
Children often amuse themselves with miniature parachutes. They take a
square piece of thin paper, tie threads to the four corners, and then
bring the ends together, a cork taking the place of the car. They then
launch it from a high window, and should there be a favourable breeze,
it is wonderful how far it will be carried before it comes to the
ground.
Once, when a boy of eleven, and consequently thoughtless, I set a
chimney on fire by one of these Parachutes. I wished to see whether it
would go up the chimney, and come out at the top. Unfortunately it was
caught by a flame as it was launched, flew up in full blaze, and, as the
chimney needed sweeping, the result was inevitable.
* * * * *
[Illustration:
FLYING SQUIRREL. DANDELION SEED PARACHUTE
FLYING DRAGON. (OPEN AND CLOSED). (OPEN AND CLOSED).
FLYING FROG.]
In the centre of the illustrations, and at the top, are two examples of
a well-known natural Parachute called the Dandelion seed. The
resemblance to the real Parachute is wonderful, the actual seed
occupying the place of the car, and fulfilling the same office, _i.e._
keeping the seed upright until it reaches the ground.
When the tuft is closed, as is the case before the pretty ball of seeds
bursts from the green envelope in which they had been confined during
the process of development, its form bears the same startling
resemblance to the Parachute.
* * * * *
PASSING from the vegetable world, there will be seen three examples of
Natural Parachutes. Several others will be mentioned, but we have no
space for description or figure. It will be seen, however, that the one
principle which characterizes them all is the exposure to the air of a
flattened and large surface, in proportion to the size of the object.
Before beginning the description, however, I must mention that nearly
all animal parachutes can to a certain extent guide their course, while
neither the balloon, the gossamer, the parachute, nor the various winged
seeds have the least power of guidance, but must follow every current of
air in which they may happen to float.
* * * * *
THE upper figure represents a Flying Squirrel.
There are many species of Flying Squirrel, but they all agree in one
point. The skin of their sides is modified into a very thin fold, which
extends as far as the feet.
It is very elastic, so that when it is not in use it falls into folds or
wrinkles, and is hardly perceptible. But should the Squirrel wish to
pass from one tree to another, without coming to the ground, it spreads
its legs as widely as possible, so as to stretch the membrane into a
wide, flat surface. It then boldly springs into the air, and sweeps upon
its mark with a sort of skimming movement. Except that it does not
revolve, it passes through the air much after the fashion of an
oyster-shell when thrown horizontally.
Many mammalia are constructed after a similar fashion, such as the
Colugo, or Flying Monkey, the Flying Mice, and the Flying Phalangists,
or “Opossums,” as they are popularly called.
* * * * *
IN the centre is the Flying Dragon, or small lizard, which very probably
gave rise to the fabled Dragons in which our ancestors so devoutly
believed. Indeed, on looking back at the old illustrated works on
Natural History, there can be but little doubt on the subject.
In this creature, the ribs, instead of the legs, carry the flat and
elastic membranes. When simply crawling on the branches, after the
manner of tree-lizards, the ribs lie flat against the sides, and the
membranes collapse, so that the shape of the body is little different
from that of any crawling lizard.
But the ribs are movable at will, and, when the creature wishes to pass
from one tree to another, it extends the ribs, stretches the membranes,
and launches itself into the air, exactly as has been narrated of the
Flying Squirrel.
* * * * *
THE lowest figure represents a most extraordinary animal, called the
Flying Frog. Only one specimen is believed to be known, and that was
discovered in Borneo by Mr. Wallace.
Here we have an analogy with the bats of the present day and the
pterodactyles of the past, namely, the elongation of the toes, and the
stretching of a web between them. In the two latter animals, however,
only the toes of the two fore-legs are elongated, whereas, with the
Flying Frog, the elongation is found in both pairs of limbs. The ends of
the toes are furnished with adhesive pads, like those of the tree-frogs,
to which it is probably related.
By means of the four membranes, the creature is able to sweep through
the air for some distance, and, indeed, this power was the reason why it
was caught. It was seen to skim from one tree to another, and was
immediately secured. Had it remained sticking on the tree, it would
probably have escaped observation.
WEIGHT OF AIR.
We have already noticed that hydrogen gas is fourteen times lighter than
air, and infer necessarily that the weight of the atmosphere must be
very considerable if so heavy an object as a balloon, with its car,
instruments, sand-bags, and passengers, can rise and float in it.
We are not conscious of its weight, because it permeates us, and the
pressure is neutralised. But, in fact, we live at the bottom of a vast
ocean which we call the atmosphere; and as, on an average, there is a
pressure of fifteen pounds on every square inch of surface, we have to
sustain an almost incredible weight. Let, for example, any one measure
the surface of his own hand, reduce it to square inches, add together
fifteen pounds for every square inch, and he will then appreciate the
weight of the atmospheric ocean in which we live. On an average, every
human being endures a pressure of some ninety thousand pounds.
This ocean is in perpetual movement, sometimes violently, which we call
storm; sometimes gently, which we call breeze; and sometimes very
gently, which we call calm. There are air-spouts as well as
water-spouts; and, in fact, the water-spout is nothing but a continuance
of the air-spout, as is shown by the moving sand-columns of the desert.
Whatever may be the character of the winds, as we call this movement,
the air is never for a moment still; and, indeed, were it to be still
for any time, the whole human race would perish.
How winds are caused we shall see by the aid of the diagram on the
left-hand side of the illustration.
[Illustration: AIR-CURRENTS. VENTILATION OF MINES.]
The original cause is the sun. His rays fall upon the earth, heating it,
and so by radiation heating the air. Now, as has been remarked, heated
air will cause a heavy balloon to float through ordinary air, and to
carry up a considerable amount of dead weight besides; consequently the
heated air must ascend, while cool and heavier air rushes in to take its
place, and thus the currents are produced. Were the earth set straight
upright, the currents would invariably run in one direction; but, as it
is tilted on one side, the needful variety is obtained, and we find the
winds blowing from all parts of the compass.
The principle, therefore, of all winds is, that heat expands, and
therefore becomes lighter than air at an ordinary temperature.
* * * * *
WERE it not that man has taken advantage of this principle, there could
not be a deep mine in England. In any deep excavation, even though it be
a well, foul air, mostly composed of carbonic acid gas, always collects,
and, being much heavier than atmospheric air, lies at the bottom of the
pit as surely as hydrogen would rise out of it. To breathe this air is
as certain and as sudden death as to take prussic acid, and no mine can
be worked as long as “choke-damp” is in it.
In coal mines there is an additional source of danger, namely, the coal
gas, which is nearly identical with our coal gas of the streets, and
takes fire when brought into contact with flame. To rid the mines of
these gases, a simple, ingenious, and effectual remedy is used. A
ventilating shaft is made, which reaches from the bottom to the mouth of
the pit. At the bottom, diagonal shafts are made, entering the main
shaft, as shown on the right hand of the illustration. One of these is
connected with a furnace, and the other, or others, open into the mine.
The heat of the furnace rarefies the air in the shaft, causing it to
rush upwards with great violence, and so, by creating a partial vacuum,
to force the air in the shaft to follow it. The loss of air thus caused
is supplied by fresh air from above, which, by the law already
described, is obliged to take the place of that which was driven out.
Thus a complete circulation of air is kept up, and a well-managed mine
has a fresher atmosphere than many houses in which the windows are
mostly kept shut, and the only ventilation is accomplished by
occasionally open doors.
The “draught” of our domestic chimneys is owing to this principle, and
the reason why factory chimneys are built of such enormous height is,
that the column of heated air may be increased, and consequently that
the draught may be stronger, and the heat of the furnace made fiercer.
The “Steam-blast,” by which the escape steam of engines is sent into the
chimney, is another example of this principle, the steam taking the
place of the hot air.
Further examples of the weight of the atmosphere are given in the
illustration. That on the right represents the common Wheel Barometer,
which marks the weight of the air by a hand moving in front of a dial.
If the hand moves towards the right, the weight of the air is
increasing; if to the left, it is decreasing.
There are certain words, such as Wet, Change, Fair, Dry, &c., on the
face of the dial, but they are only conventional, the real test of the
weather being the direction in which the hand moves. For example, if
with a west wind the hand moves from Dry towards Fair, rain may be
expected; whereas, if it should move from Wet to Change with an east
wind, we may reasonably think that fine weather is coming.
The whole cause of this revolution of the hand may be found in the
weight of the atmosphere.
It is found that a column of water thirty feet high, or a column of
mercury thirty inches high, is exactly equal in weight to a column of
air of the same diameter, but some forty odd miles high, so that the two
columns precisely balance each other.
[Illustration: SUCKING SUGAR-CANE. SUCKING AN EGG. BAROMETERS.]
Suppose, then, the water or mercury to be placed in tubes closed at the
top and open at the bottom, the water or mercury will exactly balance
the air, and will not escape from the tubes. It necessarily follows that
if the air be heavier than usual, it will force the liquid higher into
the tubes, and, if it be lighter than usual, will allow them to fall
lower. This is the principle of the Barometer.
The mechanism of the hand and dial is shown in the diagram which
occupies the centre of the illustration. For convenience, sake the
mercury column is mostly employed, but several Water Barometers, some
thirty feet in length, have been constructed.
* * * * *
ON the left hand is seen a boy engaged in sucking an egg. The plan
employed is simple enough. A tolerably large hole is made at one end,
and a very small one at the other. The yolk having been broken up by a
long needle, or similar implement, the larger hole is placed to the
lips, and, suction being used, the contents pass into the mouth.
Were it not for the hole at the end opposite the mouth, it would be
impossible to extract the contents, but the air rushes through the
aperture, and so forces out the contents of the egg.
Above is a representation of the way in which Sugar-cane is sucked. The
reader probably knows that the Sugar-cane, like the wheat-stem, has
knots at certain intervals, which divide the cane into a number of
separate parts.
There is quite an art in sucking the Sugar-cane. If a joint be cut off,
and the lips applied to the end, not a drop of the sweet juice would be
extracted. But if a notch be cut close to the joint, as shown in the
illustration, the air can gain access, and then the juice flows easily
enough.
* * * * *
[Illustration: BOILING WATER.]
[Illustration: THERMOMETER.]
It has already been mentioned that air expands when heated. The same
rule holds good when applied to other objects, such as the various
liquids, metals, &c. A very familiar example of this fact is the
“boiling over” of water, when the vessel has been filled too much to
allow for the expansion of the heated liquid.
Advantage has been taken of this principle in the formation of the
Thermometer, a word which signifies “heat-measurer.” Liquid of some kind
is placed in an hermetically sealed tube, generally terminating with a
bulb, and in proportion to the heat the liquid expands, and is forced up
the tube.
Any liquid will answer to a certain extent, but, as water freezes at
32°, it would be useless for measuring degrees of cold below the
freezing point. Coloured spirits of wine are used; but the very best
liquid is mercury, which is a metal in a state of fusion.
This expansion by heat is so powerful in iron, that it is utilised in
several ways.
Take, for example, wheel-making. The iron tire is made rather smaller
than the wheel, and is then placed in a fire until it is red-hot. It
then expands so much that it can be easily slipped over the wheel as it
lies on the ground. Cold water is then dashed on it, and the tire
contracts with tremendous force, binding the parts of the wheel firmly
together.
In all buildings where iron is much used, such as iron bridges, iron
beams, &c., it is necessary to make allowance at both ends, so as to
permit the iron to expand on a hot day and contract on a cool one.
Buildings formed of stone and iron were once thought to be safe in case
of fire. They are now known to be just the contrary, the stone flying
with the heat, and the iron expanding.
USEFUL ARTS.