CRUSHING INSTRUMENTS.--THE NUT-CRACKERS, ROLLING-MILL, AND
GRINDSTONE.--PRESSURE OF ATMOSPHERE.--SEED DIBBLES AND DRILLS.
Importance of Leverage in Crushing Power.--Nut-crackers a Lever of
the Second Order.--The Chaff-cutting and Tobacconists’
Machines.--Jaws of various Animals.--The Wolf-fish or
Sea-wolf.--The Rolling-mill and its Action.--Gunpowder-mills and
Granulating Machine.--The “Jacob’s Ladder.”--The Mangle and its
various Adaptations.--The Grindstone.--Primitive Grindstones of the
Savage Races.--The Kafirs and the Inhabitants of
Palestine.--Ceasing of the Millstone.--“Facing” of
Millstones.--Tusk of the Elephant and its Structure.--Its Facings
always preserved.--Power of Self-renewal.--Pressure of
Atmosphere.--The Napier Coffee Machine.--The Cupping
Instrument.--The Pneumatic Peg.--The Magdeburg Hemispheres.--Plane
Surfaces of Glass or Metal.--Suckers of the Cuttle-fish.--Foot of
the Water-beetle.--The Limpet.--The Star-fish and its Mode of
Progression.--The Sucking-fish and the Fables connected with
it.--Its real Structure.--Modification of the Dorsal Fin.--The
Gobies and Lump-fish.--The Gecko and Tree-frog.--The Lampern and
the Medicinal Leech.--Seed Dibbles and Drills.--Labourers versus
Machinery.--Natural Dibble of the Grasshopper.--The Daddy
Long-legs.--Drills and Dibbles of the Ichneumon-flies.--A wonderful
Specimen from Bogotá.--The Pelecinus and its Mode of laying Eggs.
CRUSHING INSTRUMENTS.
As we are on the subject of leverage, we will take some examples of
levers in Art and Nature, without, however, even attempting to exhaust
the topic.
On the right hand of the illustration is shown a very familiar example
of a lever, namely, nut-crackers, with a nut between them. This useful
implement is simply an adaptation of levers of the second kind, the
power being represented by the human hand, the weight by the nut, and
the fulcrum being the joint of the instrument.
The common chaff-cutter, which is worked by hand, is another familiar
example of this kind of lever, and so is the knife used by tobacconists
in cutting cake Cavendish into threads, and by druggists for similar
purposes. In these instruments the point of the knife is jointed to some
fixed object, and becomes the fulcrum; the hand of the cutter supplies
the power, and the weight is the object which is being cut. It will be
seen that, by increasing the length of the handle, very great power can
be obtained.
[Illustration: JAWS OF WOLF-FISH.]
[Illustration: NUT-CRACKERS.]
Exchanging the power for weight, we have in the common tongs, whether
used for the coals or for sugar, a leverage of a similar character, the
weight moving over a greater space than the power. A good example of
this is to be found in the deltoid muscle of the human arm. The muscle,
which furnishes the power, contracts about an inch, and, so doing, moves
the hand over some forty inches of space. It has been well stated that
if a man is able to hold in his hand, and with extended arm, a weight of
twenty-five pounds, the muscle must be exerting a power of forty times
as great, _i.e._ about a thousand pounds.
* * * * *
THERE is little doubt that, in such Crushing Instruments as have been
mentioned, the idea has been taken from the jaws of sundry animals. We
know, for example, that with ourselves, if we desire to crack a walnut
or a filbert in our teeth, we always put it as far back as possible, so
as to make the leverage as powerful as possible. No one would ever dream
of cracking a nut with his front teeth, an act which would be very much
like that of trying to break a piece of coal by pinching it with the
tongs.
The left-hand figure of the illustration represents part of the jaws of
the Wolf-fish, or Sea-wolf, as it is sometimes called, and a very
wonderful crushing machine it is. The Sea-wolf (_Anarrhicas lupus_),
sometimes called the Sea-cat, or Swine-fish, is tolerably common on our
coasts, and, as it sometimes attains a length of seven feet, and is
proportionately stout and muscular, the power of its bite may be
estimated. The fish in question feeds chiefly on crustacea and
hard-shelled molluscs, and is therefore furnished with an apparatus
which can crush their shells. Extremes meet. The Sea-anemones, which are
mere films of animal matter, and can be torn in pieces with the finger
and thumb, can seize, swallow, and digest a crab or an oyster in spite
of the thick and strong shells in which they are enclosed. So can the
Sea-wolf, and fishes of a similar character. But nothing intermediate
can touch them, and it is curious to reflect that such opposite means
should produce a similar effect.
On reference to the illustration, the reader will see how exact is the
parallel between the Nut-crackers and the Sea-wolf’s jaws, both being
worked on the same principle, and both being furnished with a series of
projecting points, which are used for the purpose of preventing the
escape of the object which is to be crushed. The terrible grasping power
of the crocodile, the dolphin, and other predacious creatures can be
explained on the same principle.
THE ROLLING-MILL.
We now come to another variation of the Crushing Machine, _i.e._ that in
which the motion is constant, and not intermittent, as is the case with
those machines which have just been mentioned.
Perhaps some of my readers may have visited those great iron-works in
which huge masses of iron are rolled into plates of greater or less
thickness, or are cut up into strips as easily as if they were butter.
The mechanism is in its principle simple enough. The cylindrical rollers
are placed nearly in contact, and forced towards each other by
mechanical means, such as levers, screws, or springs, or all three
combined. These cylinders revolve in opposite directions, and, if any
object be placed between them, they draw it through them, and present it
on the other side in a flattened condition.
[Illustration: JAWS OF SKATE.]
[Illustration: CRUSHING-MILL AND ROLLER.]
Many years ago, one of my schoolfellows, who had been brought up
entirely under the care of some maiden ladies, was visiting a workshop,
and must needs put his finger between two revolving rollers. Of course
the hand was drawn between them, and simply squeezed flat. The machine
was instantly stopped, and the hand extricated; and the strange thing
was, that the crushed and shapeless hand afterwards recovered its full
power, though not its shape, and was able to touch the keys of the
piano.
The whole process of the Rolling-mill is singularly interesting, whether
it be used for large or small objects.
Supposing that the grooved rollers of the illustration were cut across
so as to present a number of points, it is evident that anything which
got between them would be bitten to pieces, each piece being of a
tolerably uniform shape.
This plan is now adopted in the granulation of gunpowder. After the
future powder has emerged from the hydraulic press in the form called
“press-cake,” it was formerly broken to bits with wooden or copper
mallets, and then placed in a very peculiar kind of sieve. This was
shaped like an ordinary sieve, but the bottom was made of cowhide,
pierced with innumerable holes. A round pebble was placed in the sieve,
and, when the latter was violently shaken backwards and forwards, the
powder was driven through the holes by the pressure of the stone, and
was afterwards separated into its various degrees of fineness.
I have only twice seen this process, and confess to have been in a very
nervous state on both occasions. The sieve is whirled about with
enormous velocity, and the pebble flies round as if it were a thing
alive. Let but a broken needle or a fragment of stone get into the
sieve, or even let the stone itself break asunder, and there will be an
instantaneous explosion, which will hurl the house, the machinery, and
the workmen into unknown regions.
Now, however, the mode of granulating powder is radically altered. There
is a series of double cylinders, such as shown in the illustration, and
each of them has the ridges cut into teeth in regular order. Thus the
first set of rollers or cylinders merely bites the press-cake into
convenient pieces, though seldom of the same weight.
The press-cake, thus bitten to pieces, is passed through a series of
cylindrical sieves, each graduated with the utmost accuracy, and being
turned by means of machinery. Being set on a slope, the powder runs by
its own weight down them, and all those particles which cannot pass
through the meshes are poured out untouched at the lower end.
The portions which are too large to pass the openings of the first sieve
are then handed onwards by means of a machine called a “Jacob’s Ladder,”
which consists of a series of little vessels or buckets strung on a
tape, and revolving over a couple of wheels. The first set of buckets
takes the coarsely bitten press-cake to the second set of rollers, the
teeth of which are comparatively small. Thence it is passed over to a
third set, and so forth, until it is delivered in any quality of grain
which may be required.
The modern Mangle, again, affords a good example of this principle. The
old obtrusive, costly, and cumbrous Mangle, which was nothing more than
a heavy box of stones upon rollers, has given place to the modern system
of duplex action in rollers, and one of the old Mangles is not easily to
be seen, unless it be worked as a curiosity. In fact, it is nearly as
obsolete as the spinning-wheel, which yet may be seen in some of our
country villages, where scarcely one per cent, of the population has
ever been in a town, and many of them, the women especially, make it
their boast that they have never been beyond the outskirts of their
village.
This clumsy machine is now replaced by the very simple invention which
has been in vogue for some years, and which can not only release, but
regulate, the pressure at any moment, by means of springs, levers, and
weights. This machine is, in fact, exactly the same as that which is
represented in the illustration, except that the rollers are quite
smooth. They can be adjusted to almost any amount of pressure by levers
and weights which are attached to the upper roller, and, when the linen
has passed through them, it has undergone the double operation of
wringing and mangling. This disposition of the rollers has long been
anticipated in the jaws of the Skate which crush to pieces the shells of
the whelks, periwinkles, &c., on which the creature feeds.
THE GRINDSTONE.
Being on the subject of jaws and teeth as a mode of breaking to pieces
objects which are placed between them, we will take those implements
which grind to powder, or “triturate,” instead of breaking or
flattening.
From the very earliest ages, and as soon as man had begun to discover
the “staff of life,” the art of grinding naturally assumed an
ever-increasing importance.
The first and most primitive mode of grinding corn and converting it
into meal was that which was followed by Sarah, when she welcomed her
husband’s guests, which we know, from internal evidence, was followed by
the uncivilised races who formerly inhabited this island, and by many
semi-savages of the present day.
Nothing could be simpler than the machinery used, and nothing could
cause a greater waste of muscular power. Two stones were employed, a
large one upon which the grain was placed, and a smaller which was held
in the hands, and used for grinding the corn to powder, just as the
painters of the last century used to grind their colours. The Kafirs of
Southern Africa use this simple mill, and so exactly do they keep
unconsciously to the customs of long-perished natives, that if one of
their mills were buried for a few years and dug up again, it might be
mistaken for one of the ancient “querns.” As the stone held in the hand
was rounded, it naturally wore a rounded hollow in the lower stone, and
this made the process of trituration easier. Perhaps some of my readers
may have noticed that when a chemist makes up a prescription, and is
obliged to reduce one of the ingredients to powder, he always does so by
rubbing, and not by pounding, as is generally believed. He works the
pestle round and round the mortar with a kind of twisting motion, and
thus obtains a powder much too fine to have been produced by any amount
of pounding.
[Illustration: TOOTH OF ELEPHANT.]
[Illustration: GRINDSTONE.]
The labour of this operation is necessarily very severe, and therefore
the Kafir of the present day, as did his predecessors of the long-lost
races, declines to do it himself, but hands it over to the women. In
Palestine, as in other parts of the world, a simple mill has been
invented, which takes away much of the labour, and, above all, releases
the grinder from the obligation of leaning with her fall weight upon the
upper stone. In this mill the stones are similar. The upper is moved
backwards and forwards round a pivot, and the grain is passed between
them by means of a conical aperture in the upper stone, which answers
the purpose of our “hopper.”
In order to work this mill, two women are required, sitting opposite
each other, with the mill between them, holding the same handle, and
assisting each other in turning the stone backwards and forwards. No one
who has not seen this operation can fully appreciate the force of the
saying that “two women shall be grinding at the mill; the one shall be
taken, and the other left.”
It is worthy of remark that, even at the present day, the custom of
grinding corn is carried out in Palestine as it was so many centuries
ago, and that it is repeated in Southern Africa among the Kafir tribes.
In both parts of the earth the first sound of early morning is caused by
the millstones of the grinding women, and the amount and duration of the
noise afford a sure test of prosperity. Cessation of the millstones
signifies adversity and a thin population, as has been said by a writer
who lived not very far from three thousand years ago. Speaking of
tribulation, he mentions that “the grinders cease because they be few,
and that the doors shall be shut in the streets when the sound of the
grinding is low.”
After awhile improvements were gradually introduced into the business of
grinding, not the least of which was covering its surface with ridges,
instead of leaving it entirely smooth, as it had been formerly. Millers
of the present time know the value of these ridges, and the additional
grinding power which this “facing” gives to a stone. One of these stones
is represented in the illustration, so as to show the system on which
the ridges and grooves are constructed.
* * * * *
NOW, passing from Art to Nature, we find that the whole system of the
millstone, its movement and its ridged surface, existed in the times
when man had not yet come upon earth.
The reader is probably aware that among the tooth-bearing animals there
are three types of teeth. First come the incisors, or cutting teeth,
which occupy the front of the jaw, and find their fullest development in
the rodent animals, such as the beaver, the squirrel, the rabbit, and
the rat. Next them come the canine or piercing teeth, which are so
highly developed in all the cat tribe. Lastly, there are the molar or
masticating teeth, so called from a Latin word signifying a millstone,
because their office is to grind food.
As it is with these last that we have now to treat, we will say nothing
about the others.
The molar teeth find their greatest development in the Elephant, the
structure of whose molars is exactly like that of our modern millstones.
There is certainly one very great difference. When the surface of a
millstone is rubbed away, the stone must be re-faced, and sooner or
later is worn out altogether, and must be replaced with a new one. This,
however, is not the case with the Elephant’s molar teeth, which not only
keep their facing perfectly sharp, but have the faculty of renewing
themselves as fast as they are worn away.
How these important objects are attained we shall now see.
If the reader will refer to the upper left-hand figure of the
illustration, he will see that its surface is for the most part round,
with irregularly oval figures, close and thick at one end, and almost
disappearing at the other. These are the “facings” of the Elephant’s
tooth, and they are formed as follows:--
The tooth, which is of enormous size, is not solid, but is composed of a
number of plates laid side by side, like a pack of cards when set on
their edge. Each of these plates is composed of a hard external layer of
enamel, and an internal layer of comparatively soft bony matter. A slice
of badly made toast affords a familiar parallel, the half-charred
outside representing the enamel, and the soft, sodden interior being
analogous to the bony matter. In order to show the arrangement of these
plates, a side view of part of the tooth is given on the same
illustration. Sometimes, when the teeth of fossil elephants are
discovered, these plates all fall asunder, the material which connected
them having been dissolved away in the earth.
When, however, we look upon the upper surface of a recent tooth, we see
it present the appearance which is shown in the illustration. The
elongated oval marks are the edges of the hard enamel plates, while the
spaces between them are filled with the soft bony matter. It will be
evident, then, that if two teeth such as these be in opposite jaws, and
perform the task of grinding food, their surface will always be well
“faced.” Owing to the different hardness and density of the enamel and
bony substance, the latter will wear away with comparative rapidity,
leaving the former to project slightly, and thus to preserve the facing
of the natural mill.
This is, indeed, but a modification of the beautiful animal mechanism
which keeps the teeth of a rodent animal always sharp, and always
bevelled off at the proper angle. If we could invent some plan whereby,
in our millstones, we could make the facing of much harder material
than the stone, we should make an advance in the miller’s art that would
render the millstones of the future as far superior to those of the
present as are our present millstones to the hand “quern” of the Kafir
women.
Yet another improvement has to be made. Would it be possible to
construct a millstone which should not only retain its facing, but
possess the power of renewing itself in proportion as it is worn out?
This property is found in the Elephant’s tooth, and the illustration
will give a tolerably good idea of the simple and beautiful mechanism by
which it is brought into operation.
The tooth, instead of being one solid mass, consists, as I have already
stated, of a series of plates set side by side. These plates are so
constructed that they are more worn away in front than behind. In
proportion as they are worn, a new tooth is built up behind the old one,
and gradually pushes off the old one. Now, if we could only construct
millstones with such properties, we should possess an absolutely perfect
instrument.
PRESSURE OF ATMOSPHERE.
There are many useful inventions which depend on the weight of the
atmosphere and the creation of a more or less perfect vacuum. There is,
for example, the common Pump, which raises water simply by the action of
the atmosphere. A pipe passes into the water, and in that pipe an
air-tight piston is inserted. When the piston is drawn upwards a vacuum
is formed, and the water is at once forced into it by the pressure of
the atmosphere.
Then there is the graceful and useful Napier Coffee-making Machine,
consisting of a glass globe, and vase of the same material.
Coffee and boiling water are put into the vase, and some hot water into
the globe. The two are then connected with the tube, and under the globe
is placed a spirit-lamp. Presently the water in the globe boils,
expelling the air and filling the globe with steam. The lamp is then
removed, and the steam in the globe is condensed, leaving a vacuum. The
pressure of the atmosphere then comes to bear upon the coffee in the
vase, which is forced through the tube into the globe, producing
beautifully clear and well-flavoured coffee.
* * * * *
SURGERY employs the weight of the atmosphere in the operation called
“Cupping,” now rarely employed, but formerly in such constant use that
scarcely any man who had attained middle age had not undergone it. The
operation was intended for the purpose of removing the blood from some
definite spot. Persons, for example, who appeared to have a tendency to
apoplexy were regularly cupped between the shoulders twice a year,
_i.e._ in the spring and autumn.
The mode of performing the operation is as follows:--A vase-shaped glass
vessel called a cupping-glass is placed close to the skin. The flame of
a spirit-lamp is then introduced for a moment in the glass so as to
expel the air, and the glass is rapidly placed with its mouth downwards
on the skin. If this be done with sufficient rapidity, the partial
vacuum in the cupping-glass causes it to adhere to the skin, which is
forced into it by atmospheric pressure, as shown in the illustration.
The blood is, of course, drawn towards the surface by the same means.
The glass is then quickly removed, and a little brass instrument
applied, which, at the touching of a spring, sends out a number of small
lancet-blades so formed as to make very slight cuts. The glass is again
applied, and rapidly becomes filled with blood from the cuts, the air
having forced it in exactly as it forces the coffee in Napier’s machine.
* * * * *
IN the upper right-hand corner of the illustration is shown the
Pneumatic Peg, a comparatively recent invention, and useful in cases
where much strength is not required. The base of the peg is fitted with
a sort of cup made of india-rubber. When this base is pressed against a
smooth and flat surface, such as a pane of glass, the air is forced out
of the cup, and a vacuum formed. The pressure of the atmosphere then
causes the cup to adhere to the glass with sufficient force to enable
objects to be suspended from it.
The boy’s well-known toy, the Sucker, is made on exactly the same
principle. A piece of leather, generally circular, though the shape is
not of much consequence, has a hole bored through its centre, so as to
allow a string to be attached. The leather is then soaked in water until
it is quite soft. If it be firmly pressed on any smooth object, such as
a stone, the air is forced from under it, and it becomes capable of
sustaining a weight in proportion to its dimensions. As the air has a
pressure of about fifteen pounds on every square inch, it is easy to
calculate the weight which it will uphold, a margin being left for
imperfection of vacuum.
[Illustration:
SUCKERS OF CUTTLE.
LIMPET.
STAR-FISH.
SUCKING-FISH.
FOOT OF GECKO.
LAMPERN.
CUPPING-GLASS.
PNEUMATIC PEG.
SUCKER.
MAGDEBURG HEMISPHERES.]
* * * * *
THE lower figure represents the instrument called the Magdeburg
Hemispheres, which are made for the purpose of showing the enormous
power of air-pressure. They are two hollowed hemispheres, having their
edges very accurately ground together. When used, a little lard is
rubbed on the edges in order to insure their exact fit, and they are
then pressed tightly together. The air is removed by means of the
common exhausting syringe, and it is found that the two adhere together
with such force that two strong men cannot pull them asunder. But, if
the tap be turned, and air admitted, they come apart without the least
difficulty.
Similarly, if two plates of glass or metal be ground to exactly plane
surfaces, and pressed together, they adhere nearly as strongly as if
they were one solid piece.
* * * * *
WE will now turn from Art to Nature, and examine some natural producers
of vacuum.
One of the most celebrated is that series of suckers which may be found
upon the arms of the various Cuttles. At the upper part of the
illustration a figure is given of part of an arm, on which are four
suckers. When the animal wishes to attach itself to any object, it
presses the disc of the sucker against it, and simultaneously withdraws
the centre, exactly as the boy does with his toy sucker. And, as each
arm contains a great number of suckers, it is evident that the holding
power must be very great. Indeed, on one occasion when a comparatively
small specimen had fastened on a man’s arm, he could not remove it, but
was obliged to have it cut away piecemeal by an assistant.
The common Water-beetle has similar suckers upon its first pair of feet,
and can adhere to smooth surfaces with great tenacity.
* * * * *
ON the left of the cuttle-arm is the common Limpet, shown as it appears
when adhering to the rocks. Every visitor to the seaside who has
attempted to remove the Limpets may remember how difficult it is to stir
them when they have once taken their hold. If they can be taken by
surprise, they come away with a touch; but if they become alarmed, they
press the edges of the foot firmly against the rock, withdraw the
centre, and thus create the necessary vacuum.
* * * * *
NEXT follows a Star-fish, shown as it appears when in the act of
walking, or rather, gliding along.
This movement is obtained by the use of a vast number of long suckers,
exactly resembling the pneumatic peg, except that they are flexible,
and can be curved in any direction. It is really beautiful to see the
manner in which a Star-fish will glide along by means of its suckers,
its arms accommodating themselves to the irregularities of the ground,
and its multitudinous suckers protruded and withdrawn with a
never-ceasing movement.
And, as the Star-fish is apparently blind, not having any organs which
can even be conjectured to serve the purpose of vision, this mode of
directing its course is not easily understood. Yet, blind though it may
be, it guides itself with as much accuracy as if it possessed eyes, and
evidently does so with a definite purpose, using its suckers with as
much decision as a centipede uses its legs.
These suckers can be seen very well by placing a Star-fish in a shallow
vessel of sea-water, and laying it on its back. The suckers immediately
protrude themselves from their little apertures, and the arms slowly
curve themselves so as to find something to which the suckers can
adhere. Presently one or two of the suckers will take hold of the bottom
of the vessel. Others soon follow, and in a very short time the
Star-fish is on its legs, if we may so call them, and is quietly gliding
on its way.
* * * * *
BELOW the Star-fish is seen the celebrated Sucking-fish (_Echeneis
remora_) about which so many strange tales have been told, and which is
possessed of a structure remarkable enough to need no aid from
invention. The dorsal fin of this fish is modified in a most singular
manner. The spines of which it is so largely composed are metamorphosed
into flattened plates very much resembling the laths of a Venetian
blind, and form an instrument of suction identical in principle, though
not in form, with those which have already been described. When the
sucker is pressed against a smooth surface, a vacuum is formed, and the
fish in consequence adheres firmly to the object.
The fact has been known for centuries, though it has only been lately
discovered, that the sucker was not a separate apparatus, but merely one
of the fins modified in a simple though effective manner. Indeed, any
one who has some slight notion of the structure of a fin can easily see,
by looking at the Sucking-fish from above, that the apparatus is
nothing more than the dorsal fin laid flat.
I may mention here that the name of Echeneis is taken from two words
signifying “ship-holder.” It was given to the fish on account of a
curious notion which was fully believed until quite modern times, that
the Sucking-fish had the power of attaching itself to ships, and holding
them so firmly that they could not proceed in spite of sails and oars.
The word Echeneis is used by Aristotle in his “History of Animals.” The
specific name _remora_, or “delay,” is Latin, and is given to the fish
for the same reason.
The little Gobies, which are so plentiful along our coasts, have the
ventral fins formed into a sucker, with which they can cling firmly to
any object, such as a leaf of seaweed or a smooth rock or stone. A
similar modification of the ventral fins is also found in the
beautifully coloured Lump-fish, or Lump-sucker, sometimes called the
Cock-paidle. One of these fishes, when placed in a bucket of water,
adhered so strongly to the bottom, that, when lifted by the tail, it
bore the whole weight of the pail and water.
* * * * *
JUST below the Sucking-fish is drawn a foot of the curious little
lizard, the Gecko, so called from its peculiar cry. It is common in the
West Indies, and haunts houses, traversing their walls just as flies run
up panes of glass. It is enabled to perform this movement by means of
the structure of the feet. As the reader may see by reference to the
illustration, the toes are greatly widened and flattened. If the lower
surface be examined, it will be found to be furnished with a number of
plates very much resembling those of the sucking-fish, and performing
the same office.
So rapid is the operation of these plates, that the animal can even leap
upon a perpendicular flat surface, and stick there. Perhaps the reader
may remember that the beautiful Tree-frogs, which cling so tightly to
leaves, are furnished with suckers on their toes, whereby they can hold
on even to an upright pane of glass. In fact, the smooth surface of the
glass seems to please them, and when they adhere to it they give an
excellent opportunity of examining the structure of the feet with a
magnifying-glass.
Another example of the pressure of the atmosphere has been slightly
mentioned, when treating of the ball-and-socket joint. This is the joint
by which the thigh-bone is attached to the hip. As the rounded head of
the thigh-bone fits exactly into the cavity of the hip, and is,
moreover, well lubricated with the animal oil called synovia, no air can
obtain admission between the two. Consequently, they are held together
so firmly by the pressure of the atmosphere, that they retain their
places even after the whole of the muscular attachments have been
removed. Not without very great force can the thigh-bone be dislodged
from the shallow socket in which it lies; but, if a hole be bored so as
to admit the air, it comes out at once.
Similarly, however firmly a limpet may cling to the rock, if the finest
needle were introduced so as to admit air, the creature could not retain
its hold for a moment.
* * * * *
THE last figure on the illustration represents the common Lampern
(_Lampetra fluviatilis_).
The mouth of this little fish is formed on the principle of the sucker,
and very firmly it can adhere, as I can state from much personal
experience. Indeed, it is rather alarming, to those who are unacquainted
with the character of the fish, to have it turn round and fasten upon
the hand. However, it is quite harmless, and those who are accustomed to
them will have half-a-dozen hanging on their hand at a time, and take no
notice of them.
* * * * *
ALREADY has it been mentioned that Surgery has pressed into its service
the weight of the atmosphere by means of cupping. She also makes use of
Nature in a similar manner by employing the Leech for local and surface
bleeding.
The mouth of the Medicinal Leech forms an exact parallel with the
cupping-glass and lancets, only that it is very far superior in its
powers. To make the analogy perfect, the lancets ought to be within the
cupping-glass, and the latter ought to be able to exhaust the air from
itself, and to be attached to a reservoir into which the blood could be
passed.
I need hardly mention that the action of sucking as practised by the
young of all mammalian beings, from man downwards, is due to the same
principle. By the action of sucking a partial vacuum is formed, and the
pressure of the atmosphere upon the breasts forces the milk into the
mouth of the young.
We might multiply examples _ad infinitum_, and we will therefore pass to
another subject.
SEED-DRILLS.
[Illustration:
ICHNEUMON-FLY.
GRASSHOPPER.
SEED-DRILL.]
Among the modern improvements in agriculture we may reckon the invention
of the Seed-drill as one of the most important. By means of this
invention, seed is greatly economized, the supply can be regulated, and
the sower knows exactly where every grain of seed goes. There is no
scattering, as in the wasteful broadcast plan, by which the seeds are
flung almost at random over the field, and may or may not fall into the
furrows. The Seed-drill, on the contrary, either stamps holes or ploughs
narrow furrows, measures the seed into them, and in some machines
replaces the earth. The former kind of machine rather deserves the name
of a dibble, and was invented for the purpose of superseding the use of
the hand-dibble.
It is really a pitiful thing to see human beings endowed with reason and
aspirations performing such a task as dibbling by hand, one going
backwards with a dibble in each hand, and the other following and
putting seed into the holes. Yet the field labourers have the greatest
objection to the machine dibble, as, indeed, they have to any sort of
labour-saving machine, thinking that it will lessen the demand for
labour, and prevent them from earning a livelihood.
I well remember how a country clergyman, pitying the hard toil of the
hand-dibblers, took occasion when he visited town to purchase a machine
dibble wherewith one man could set eight rows of beans at once. It was a
very simple affair, comprehensible even by the dull brain of a Wiltshire
labourer. His trouble was all in vain, for no one would use it, and
there was such a disturbance about it in the village, that for the sake
of peace its owner laid it up in a loft and abandoned its use. There
might be some semblance of reason in thinking that it would deprive them
of their field labour, but no cottager would even use it in his own
garden, though it was freely offered to any one who wished to borrow it.
* * * * *
THESE machines have their parallels in Nature, two of which are
represented in the illustration.
The lower left-hand figure represents the female Grasshopper depositing
her eggs. She is furnished with a sharply pointed ovipositor, composed
of two blades. When she is about to lay her eggs, she searches for a
suitable piece of ground, where the earth is tolerably soft, and with
the closed ovipositor bores a hole. She then separates the blades
slightly, and an egg glides between them into the ground, precisely as
is done by the machine dibble with its beans. When I first saw and used
the instrument, some twenty-five years ago, the parallel struck me at
once.
* * * * *
THE female of the familiar Daddy Long-legs (_Tipula_) acts in a similar
manner. She is furnished with an ovipositor too short to be used like
that of the grasshopper, and so she attains her object in a rather
different manner. Making use of her long stilt-like legs, she sets
herself nearly upright, with the point of the ovipositor in the ground.
She then twists herself from side to side, just after the principle of
the bradawl, and so proceeds until she has made a hole large enough for
her purpose. The blades of the ovipositor are then separated, and the
egg placed in the hole, as has been described of the grasshopper.
* * * * *
THE upper figure represents one of the large Ichneumon-flies depositing
the egg in the grub of some wood-inhabiting larva. How she bores the
hole has already been described when treating of Boring Tools, and the
process need not again be discussed. The principal point at present is,
that after the hole is bored, an egg can pass between the blades of the
ovipositor, though they are but little thicker than human hairs.
One of the most extraordinary instances of this kind of ovipositor is
found in an Ichneumon-fly brought from Bogotá. The body, from the head
to the end of the tail, is not quite an inch long, while the ovipositor
is six inches and a half in length, and scarcely thicker than that of
the insect whose portrait is given in the illustration. Nothing is as
yet known of its habits, so that the object of this wonderfully long
ovipositor is a mystery. But that it should be used like other
ovipositors is evident enough, and the chief wonder is, what are the
mechanical means whereby an egg can be propelled between blades so long
and slender.
There is a genus of Ichneumon-flies called Pelecinus. They deposit their
eggs in wood-boring larvæ, and we might imagine that the ovipositor
would be a long one. It is, however, extremely short, and the requisite
length is obtained by the form of the abdomen, the joints of which are
so long and narrow that they almost look as if they had passed through a
wire-drawing machine, the length of the head and throat being
three-eighths of an inch, and that of the abdomen an inch and a half.
This long abdomen belongs only to the female, that of the male being
short and club-shaped.
USEFUL ARTS.