The Botanic Garden

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6. The great condensation of moisture on the summits of hills has
another cause, which is the dashing of moving clouds against them, in
misty days this is often seen to have great effect on plains, where an
eminent tree by obstructing the mist as it moves along shall have a much
greater quantity of moisture drop from its leaves than falls at the same
time on the ground in its vicinity. Mr. White, in his History of
Selborne gives an account of a large tree so situated, from which a
stream flowed during a moving mist so as to fill the cart-ruts in a lane
otherwise not very moist, and ingeniously adds, that trees planted about
ponds of stagnant water contribute much by these means to supply the
reservoir. The spherules which constitute a mist or cloud are kept from
uniting by so small a power that a little agitation against the leaves
of a tree, or the greater attraction of a flat moist surface, condenses
or precipitates them.

If a leaf has its surface moistened and particles of water separate from
each other as in a mist be brought near the moistened surface of a leaf,
each particle will be attracted more by that plain surface of water on
the leaf than it can be by the surrounding particles of the mist,
because globules only attract each other in one point, whereas a plain
attracts a globule by a greater extent of its surface.

The common cold springs are thus formed on elevated grounds by the
condensed vapours, and hence are stronger when the nights are cold after
hot days in spring, than even in the wet days of winter. For the warm
atmosphere during the day has dissolved much more water than it can
support in solution during the cold of the night, which is thus
deposited in large quantities on the hills, and yet so gradually as to
soak in between the strata of them, rather than to slide off over their
surfaces like showers of rain. The common heat of the internal parts of
the earth is ascertained by springs which arise from strata of earth too
deep to be affected by the heat of summer or the frosts of winter. Those
in this country are of 48 degrees of heat, those about Philidelphia were
said by Dr. Franklin to be 52; whether this variation is to be accounted
for by the difference of the sun's heat on that country, according to
the ingenious theory of Mr. Kirwan, or to the vicinity of subterranean
fires is not yet, I think, decided. There are however subterraneous
streams of water not exactly produced in this manner, as streams issuing
from fissures in the earth, communicating with the craters of old
volcanoes; in the Peak of Derbyshire are many hollows, called swallows,
where the land floods sink into the earth, and come out at some miles
distant, as at Ilam near Ashborne. See note on Fica, Vol. II.

Other streams of cold water arise from beneath the snow on the Alps and
Andes, and other high mountains, which is perpetualy thawing at its
under surface by the common heat of the earth, and gives rise to large
rivers. For the origin of warm springs see note on Fucus, Vol. II.

NOTE XXVII.--SHELL FISH.

_You round Echinus ray his arrowy mail,
Give the keel'd Nautilus his oar and sail.
Firm to his rock with silver cords suspend
The anchored Pinna, and his Cancer-friend_.

CANTO III. l. 67.

The armour of the Echinus, or Sea-hedge Hog, consists generally of
moveable spines; (_Linnei System. Nat._ Vol. I. p. 1102.) and in that
respect resembles the armour of the land animal of the same name. The
irregular protuberances on other sea-shells, as on some species of the
Purpura, and Murex, serve them as a fortification against the attacks of
their enemies.

It is said that this animal foresees tempestuous weather, and sinking to
the bottom of the sea adheres firmly to sea-plants, or other bodies by
means of a substance which resembles the horns of snails. Above twelve
hundred of these fillets have been counted by which this animal fixes
itself; and when afloat, it contracts these fillets between the bases of
its points, the number of which often amounts to two thousand. Dict
raisonne. art. Oursin. de mer.

There is a kind of Nautilus, called by Linneus, Argonauta, whose shell
has but one cell; of this animal Pliny affirms, that having exonerated
its shell by throwing out the water, it swims upon the surface,
extending a web of wonderful tenuity, and bending back two of its arms
and rowing with the rest, makes a sail, and at length receiving the
water dives again. Plin. IX. 29. Linneus adds to his description of this
animal, that like the Crab Diogenes or Bernhard, it occupies a house
not its own, as it is not connected to its shell, and is therefore
foreign to it; who could have given credit to this if it had not been
attested by so many who have with their own eyes seen this argonaut in
the act of sailing? Syst. Nat p. 1161.

The Nautilus, properly so named by Linneus, has a shell consisting of
many chambers, of which cups are made in the East with beautiful
painting and carving on the mother-pearl. The animal is said to inhabit
only the uppermost or open chamber, which is larger than the rest; and
that the rest remain empty except that the pipe, or siphunculus, which
communicates from one to the other of them is filled with an appendage
of the animal like a gut or string. Mr. Hook in his Philos. Exper. p.
306, imagines this to be a dilatable or compressible tube, like the air-
bladders of fish, and that by contracting or permitting it to expand, it
renders its shell boyant or the contrary. See Note on Ulva, Vol. II.

The Pinna, or Sea-wing, is contained in a two-valve shell, weighing
sometimes fifteen pounds, and emits a beard of fine long glossy silk-
like fibres, by which it is suspended to the rocks twenty or thirty feet
beneath the surface of the sea. In this situation it is so successfully
attacked by the eight-footed Polypus, that the species perhaps could not
exist but for the exertions of the Cancer Pinnotheris, who lives in the
same shell as a guard and companion. Amoen. Academ. Vol. II. p. 48. Lin.
Syst. Nat. Vol. I. p. 1159, and p. 1040.

The Pinnotheris, or Pinnophylax, is a small crab naked like Bernard the
Hermit, but is furnished with good eyes, and lives in the same shell
with the Pinna; when they want food the Pinna opens its shell, and sends
its faithful ally to forage; but if the Cancer sees the Polypus, he
returns suddenly to the arms of his blind hostess, who by closing the
shell avoids the fury of her enemy; otherwise, when it has procured a
booty, it brings it to the opening of the shell, where it is admitted,
and they divide the prey. This was observed by Haslequist in his voyage
to Palestine.

The Byssus of the antients, according to Aristotle, was the beard of the
Pinna above mentioned, but seems to have been used by other writers
indiscriminately for any spun material, which was esteemed finer or more
valuable than wool. Reaumur says the threads of this Byssus are not less
fine or less beautiful than the silk, as it is spun by the silk-worm;
the Pinna on the coasts of Italy and Provence (where it is fished up by
iron-hooks fixed on long poles) is called the silk-worm of the sea. The
stockings and gloves manufactured from it, are of exquisite fineness,
but too warm for common wear, and are thence esteemed useful in
rhumatism and gout. Dict. raisonne art. Pinne-marine. The warmth of the
Byssus, like that of silk, is probably owing to their being bad
conductors of heat, as well as of electricity. When these fibres are
broken by violence, this animal as well as the muscle has the power to
reproduce them like the common spiders, as was observed by M. Adanson.
As raw silk, and raw cobwebs, when swallowed, are liable to produce
great sickness (as I am informed) it is probable the part of muscles,
which sometimes disagrees with the people who eat them, may be this
silky web, by which they attach themselves to stones. The large kind of
Pinna contains some mother-pearl of a reddish tinge, according to M.
d'Argenville. The substance sold under the name of Indian weed, and used
at the bottom of fish-lines, is probably a production of this kind;
which however is scarcely to be distinguished by the eye from the
tendons of a rat's tail, after they have been separated by putrefaction
in water, and well cleaned and rubbed; a production, which I was once
shewn as a great curiosity; it had the uppermost bone of the tail
adhering to it, and was said to have been used as an ornament in a
lady's hair.

NOTE XXVIII.--STURGEON.

_With worm-like hard his toothless lips array,
And teach the unweildy Sturgeon to betray._

CANTO III. l. 71.

The Sturgeon, _Acipenser, Strurio._ Lin. Syst. Nat. Vol. I. p. 403. is a
fish of great curiosity as well as of great importance; his mouth is
placed under the head, without teeth, like the opening of a purse, which
he has the power to push suddenly out or retract. Before this mouth
under the beak or nose hang four tendrils some inches long, and which so
resemble earth-worms that at first sight they may be mistaken for them.
This clumsy toothless fish is supposed by this contrivance to keep
himself in good condition, the solidity of his flesh evidently shewing
him to be a fish of prey. He is said to hide his large body amongst the
weeds near the sea-coast, or at the mouths of large rivers, only
exposing his cirrhi or tendrils, which small fish or sea-insects
mistaking for real worms approach for plunder, and are sucked into the
jaws of their enemy. He has been supposed by some to root into the soil
at the bottom of the sea or rivers; but the cirrhi, or tendrills
abovementioned, which hang from his snout over his mouth, must
themselves be very inconvenient for this purpose, and as it has no jaws
it evidently lives by suction, and during its residence in the sea a
quantity of sea-insects are found in its stomach.

The flesh was so valued in the time of the Emperor Severus, that it was
brought to table by servants with coronets on their heads, and preceded
by music, which might give rise to its being in our country presented by
the Lord Mayor to the King. At present it is caught in the Danube, and
the Walga, the Don, and other large rivers for various purposes. The
skin makes the best covering for carriages; isinglass is prepared from
parts of the skin; cavear from the spawn; and the flesh is pickled or
salted, and sent all over Europe.

NOTE XXIX.--OIL ON WATER.

_Who with fine films, suspended o'er the deep,
Of Oil effusive lull the waves to sleep._

CANTO III. l. 87.

There is reason to believe that when oil is poured upon water, the two
surfaces do not touch each other, but that the oil is suspended over the
water by their mutual repulsion. This seems to be rendered probable by
the following experiment: if one drop of oil be droped on a bason of
water, it will immediately diffuse itself over the whole, for there
being no friction between the two surfaces, there is nothing to prevent
its spreading itself by the gravity of the upper part of it, except its
own tenacity, into a pellicle of the greatest tenuity. But if a second
drop of oil be put upon the former, it does not spread itself, but
remains in the form of a drop, as the other already occupied the whole
surface of the bason, and there is friction in oil passing over oil,
though none in oil passing over water.

Hence when oil is diffused on the surface of water gentle breezes have
no influence in raising waves upon it; for a small quantity of oil will
cover a very great surface of water, (I suppose a spoonful will diffuse
itself over some acres) and the wind blowing upon this carries it
gradually forwards; and there being no friction between the two surfaces
the water is not affected. On which account oil has no effect in
stilling the agitation of the water after the wind ceases, as was found
by the experiments of Dr. Franklin.

This circumstance lately brought into notice by Dr. Franklin had been
mentioned by Pliny, and is said to be in use by the divers for pearls,
who in windy weather take down with them a little oil in their mouths,
which they occasionally give out when the inequality of the supernatant
waves prevents them from seeing sufficiently distinctly for their
purpose.

The wonderful tenuity with which oil can be spread upon water is evinced
by a few drops projected from a bridge, where the eye is properly placed
over it, passing through all the prismatic colours as it diffuses
itself. And also from another curious experiment of Dr. Franklin's: he
cut a piece of cork to about the size of a letter-wafer, leaving a point
standing off like a tangent at one edge of the circle. This piece of
cork was then dipped in oil and thrown into a large pond of water, and
as the oil flowed off at the point, the cork-wafer continued to revolve
in a contrary direction for several minutes. The oil flowing off all
that time at the pointed tangent in coloured streams. In a small pond of
water this experiment does not so well succeed, as the circulation of
the cork stops as soon as the water becomes covered with the pellicle of
oil. See Additional Note, No. XIII. and Note on Fucus, Vol. II.

The ease with which oil and water slide over each other is agreeably
seen if a phial be about half filled with equal parts of oil and water,
and made to oscillate suspended by a string, the upper surface of the
oil and the lower one of the water will always keep smooth; but the
agitation of the surfaces where the oil and water meet, is curious; for
their specific gravities being not very different, and their friction on
each other nothing, the highest side of the water, as the phial descends
in its oscillation, having acquired a greater momentum than the lowest
side (from its having descended further) would rise the highest on the
ascending side of the oscillation, and thence pushes the then uppermost
part of the water amongst the oil.

NOTE XXX.--SHIP-WORM.

_Meet fell Teredo, as he mines the keel
With beaked head, and break his lips of steel._

CANTO III. l. 91.

The Teredo, or ship-worm, has two calcareous jaws, hemispherical, flat
before, and angular behind. The shell is taper, winding, penetrating
ships and submarine wood, and was brought from India into Europe, Linnei
System. Nat. p. 1267. The Tarieres, or sea-worms, attack and erode ships
with such fury, and in such numbers, as often greatly to endanger them.
It is said that our vessels have not known this new enemy above fifty
years, that they were brought from the sea about the Antilles to our
parts of the ocean, where they have increased prodigiously. They bore
their passage in the direction of the fibres of the wood, which is their
nourishment, and cannot return or pass obliquely, and thence when they
come to a knot in the wood, or when two of them meet together with their
stony mouths, they perish for want of food.

In the years 1731 and 1732 the United Provinces were under a dreadful
alarm concerning these insects, which had made great depredation on the
piles which support the banks of Zeland, but it was happily discovered a
few years afterwards that these insects had totally abandoned that
island, (Dict Raisonne, art, Vers Rongeurs,) which might have been
occasioned by their not being able to live in that latitude when the
winter was rather severer than usual.

NOTE XXXI.--MAELSTROM.

_Turn the broad helm, the fluttering canvas urge
From Maelstrom's fierce innavigable surge._

CANTO III. l. 93.

On the coast of Norway there is an extensive vortex, or eddy, which lies
between the islands of Moskoe and Moskenas, and is called Moskoestrom,
or Maelstrom; it occupies some leagues in circumference, and is said to
be very dangerous and often destructive to vessels navigating these
seas. It is not easy to understand the existence of a constant
descending stream without supposing it must pass through a subterranean
cavity to some other part of the earth or ocean which may lie beneath
its level; as the Mediterranean seems to lie beneath the level of the
Atlantic ocean, which therefore constantly flows into it through the
Straits; and the waters of the Gulph of Mexico lie much above the level
of the sea about the Floridas and further northward, which gives rise to
the Gulph-stream, as described in note on Cassia in Vol. II.

The Maelstrom is said to be still twice in about twenty-four hours when
the tide is up, and most violent at the opposite times of the day. This
is not difficult to account for, since when so much water is brought
over the subterraneous passage, if such exists, as compleatly to fill it
and stand many feet above it, less disturbance must appear on the
surface. The Maelstrom is described in the Memoires of the Swedish
Academy of Sciences, and Pontoppiden's Hist. of Norway, and in Universal
Museum for 1763, p. 131.

The reason why eddies of water become hollow in the middle is because
the water immediately over the centre of the well, or cavity, falls
faster, having less friction to oppose its descent, than the water over
the circumference or edges of the well. The circular motion or gyration
of eddies depends on the obliquity of the course of the stream, or to
the friction or opposition to it being greater on one side of the well
than the other; I have observed in water passing through a hole in the
bottom of a trough, which was always kept full, the gyration of the
stream might be turned either way by increasing the opposition of one
side of the eddy with ones finger, or by turning the spout, through
which the water was introduced, a little more obliquely to the hole on
one side or on the other. Lighter bodies are liable to be retained long
in eddies of water, while those rather heavier than water are soon
thrown out beyond the circumference by their acquired momentum becoming
greater than that of the water. Thus if equal portions of oil and water
be put into a phial, and by means of a string be whirled in a circle
round the hand, the water will always keep at the greater distance from
the centre, whence in the eddies formed in rivers during a flood a
person who endeavours to keep above water or to swim is liable to be
detained in them, but on suffering himself to sink or dive he is said
readily to escape. This circulation of water in descending through a
hole in a vessel Dr. Franklin has ingeniously applied to the explanation
of hurricanes or eddies of air.

NOTE XXXII.--GLACIERS.

_While round dark crags imprison'd waters bend
Through rifted ice, in ivory veins descend._

CANTO III. l. 113.

The common heat of the interior parts of the earth being always 48
degrees, both in winter and summer, the snow which lies in contact with
it is always in a thawing state; Hence in ice-houses the external parts
of the collection of ice is perpetually thawing and thus preserves the
internal part of it; so that it is necessary to lay up many tons for the
preservation of one ton. Hence in Italy considerable rivers have their
source from beneath the eternal glaciers, or mountains of snow and ice.

In our country when the air in the course of a frost continues a day or
two at very near 32 degrees, the common heat of the earth thaws the ice
on its surface, while the thermometer remains at the freezing point.
This circumstance is often observable in the rimy mornings of spring;
the thermometer shall continue at the freezing point, yet all the rime
will vanish, except that which happens to lie on a bridge, a board, or
on a cake of cow-dung, which being thus as it were insulated or cut off
from so free a communication with the common heat of the earth by means
of the air under the bridge, or wood, or dung, which are bad conductors
of heat, continues some time longer unthawed. Hence when the ground is
covered thick with snow, though the frost continues, and the sun does
not shine, yet the snow is observed to decrease very sensibly. For the
common heat of the earth melts the under surface of it, and the upper
one evaporates by its solution in the air. The great evaporation of ice
was observed by Mr. Boyle, which experiment I repeated some time ago.
Having suspended a piece of ice by a wire and weighed it with care
without touching it with my hand, I hung it out the whole of a clear
frosty night, and found in the morning it had lost nearly a fifth of its
weight. Mr. N. Wallerius has since observed that ice at the time of its
congelation evaporates faster than water in its fluid form; which may be
accounted for from the heat given out at the instant of freezing;
(Saussure's Essais sur Hygromet. p. 249.) but this effect is only
momentary.

Thus the vegetables that are covered with snow are seldom injured;
since, as they lie between the thawing snow, which has 32 degrees of
heat, and the covered earth which has 48, they are preserved in a degree
of heat between these; viz. in 40 degrees of heat. Whence the moss on
which the rein-deer feed in the northern latitudes vegetates beneath the
snow; (See note on Muschus, Vol. II.) and hence many Lapland and Alpine
plants perished through cold in the botanic garden at Upsal, for in
their native situations, though the cold is much more intense, yet at
its very commencement they are covered deep with snow, which remains
till late in the spring. For this fact see Amaenit. Academ. Vol. I. No.
48. In our climate such plants do well covered with dried fern, under
which they will grow, and even flower, till the severe vernal frosts
cease. For the increase of glaciers see Note on Canto I. l. 529.

NOTE XXXIII.--WINDS.

_While southern gales o'er western oceans roll,
And Eurus steals his ice-winds from the pole._

CANTO IV. l. 15.

The theory of the winds is yet very imperfect, in part perhaps owing to
the want of observations sufficiently numerous of the exact times and
places where they begin and cease to blow, but chiefly to our yet
imperfect knowledge of the means by which great regions of air are
either suddenly produced or suddenly destroyed.

The air is perpetually subject to increase or diminution from its
combination with other bodies, or its evolution from them. The vital
part of the air, called oxygene, is continually produced in this climate
from the perspiration of vegetables in the sunshine, and probably from
the action of light on clouds or on water in the tropical climates,
where the sun has greater power, and may exert some yet unknown laws of
luminous combination. Another part of the atmosphere, which is called
azote, is perpetually set at liberty from animal and vegetable bodies by
putrefaction or combustion, from many springs of water, from volatile
alcali, and probably from fixed alcali, of which there is an exhaustless
source in the water of the ocean. Both these component parts of the air
are perpetually again diminished by their contact with the soil, which
covers the surface of the earth, producing nitre. The oxygene is
diminished in the production of all acids, of which the carbonic and
muriatic exist in great abundance. The azote is diminished in the growth
of animal bodies, of which it constitutes an important part, and in its
combinations with many other natural productions.

They are both probably diminished in immense quantities by uniting with
the inflammable air, which arises from the mud of rivers and lakes at
some seasons, when the atmosphere is light: the oxygene of the air
producing water, and the azote producing volatile alcali by their
combinations with this inflammable air. At other seasons of the year
these principles may again change their combinations, and the
atmospheric air be reproduced.

Mr. Lavoisier found that one pound of charcoal in burning consumed two
pounds nine ounces of vital air, or oxygene. The consumption of vital
air in the process of making red lead may readily be reduced to
calculation; a small barrel contains about twelve hundred weight of this
commodity, 1200 pounds of lead by calcination absorb about 144 pounds of
vital air; now as a cubic foot of water weighs 1000 averdupois ounces,
and as vital air is above 800 times lighter than water, it follows that
every barrel of red lead contains nearly 2000 cubic feet of vital air.
If this can be performed in miniature in a small oven, what may not be
done in the immense elaboratories of nature!

These great elaboratories of nature include almost all her fossil as
well as her animal and vegetable productions. Dr. Priestley obtained air
of greater or less purity, both vital and azotic, from almost all the
fossil substances he subjected to experiment. Four ounce-weight of lava
from Iceland heated in an earthen retort yielded twenty ounce-measures
of air.

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