quence of the eduction of the electricity. But it appears more probable that the water is condensed into clouds by the eduction of its heat, and that then the surplus of electricity prevents their coalescence into larger drops, which immediately succeeds the departure of the lightning.

5. The immediate cause why the barometer sinks before rain is, first, because a region of warm air, brought to us in the place of the cold air which it had displaced, must weigh lighter, both specifically and absolutely, if the height of the warm atmosphere be supposed to be equal to that of the preceding cold one. And secondly, after the drops of rain begin to fall in any column of air, that column becomes lighter, the falling drops only adding to the pressure of the air in proportion to the resistance which they meet with in passing through that fluid.

If we could suppose water to be dissolved in air without heat, or in very low degrees of heat, I suppose the air would become heavier, as happens in many chemical solutions, but if water dissolved in the matter of heat, or calorique, be mixed with an aerial solution of water, there can be no doubt but an atmosphere consisting of such a mixture must become lighter in proportion to the quantity of calorique. On the same circumstance depends the visible vapour produced from the breath of animals in cold weather, or from a boiling kettle; the particles of cold air, with which it is mixed, steal a part of its heat, and become themselves raised in temperature, whence part of the water is precipitated in visible vapour, which, if in great quantity, sinks to the ground; if in small quantity, and the surrounding air is not previously saturated, it spreads itself till it becomes again dissolved.

NOTE XXVI.-SPRINGS.

Your lucid bands condense, with fingers chill, The blue mist hovering round the gelid hill. CANTO III. 1. 19.

THE surface of the earth consists of strata, many of which were formed originally beneath the sea, the mountains were afterwards forced up by subterraneous fires, as appears from the fissures in the rocks of which they consist, the quantity of volcanic productions all over the world, and the numerous remains of craters of volcanoes in mountainous countries. Hence the strata which compose the sides of mountains lie slanting downwards, and one or two or more of the external strata not reaching to the summit when the mountain was raised up, the second or third stratum or a more inferior one is there exposed to day; this may be well represented by forcibly thrusting a blunt instrument through several sheets of paper, a bur will stand up with the lowermost sheet standing highest in the centre of it. On this uppermost stratum, which is colder as it is more elevated, the dews are condensed in large quantities; and sliding down pass under the first or second or third stratum which compose the sides of the hill; and either form a morass below, or a weeping rock, by oozing out in numerous places, or many of these less currents meeting together burst out in a more copious rill.

The summits of mountains are much colder than the plains in their vicinity, owing to several causes; 1. Their being in a manner insulated or cut off from the common heat of the earth,

which is always of 48 degrees, and perpetually counteracts the effects of external cold beneath that degree. 2. From their surfaces being larger in proportion to their solid contents, and hence their heat more expeditiously carried away by the ever-moving atmosphere. 3. The increasing rarity of the air as the mountain rises. All those bodies which conduct electricity well or ill, conduct the matter of heat likewise well or ill. See note VII. Atmospheric air is a bad conductor of electricity, and thence confines it on the body where it is accumulated, but when it is made very rare, as in the exhausted receiver, the electric aura passes away immediately to any distance. The same circumstance probably happens in respect to heat, which is thus kept by the denser air on the plains from escaping, but is dissipated on the hills where the air is thinner. 4. As the currents of air rise up the sides of mountains they become mechanically rarefied, the pressure of the incumbent column lessening as they ascend. Hence the expanding air absorbs heat from the mountain as it ascends, as explained in note VII. 5. There is another, and perhaps more powerful cause, I suspect, which may occasion the great cold on mountains, and in the higher parts of the atmosphere, and which has not yet been attended to; I mean that the fluid matter of heat may probably gravitate round the earth, and form an atmosphere on its surface, mixed with the aerial atmosphere, which may diminish or become rarer, as it recedes from the earth's surface, in a greater proportion than the air diminishes.

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 scak 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 Philadelphia were said by Dr. Franklin to be 52; whether this variation is to be accounted for by the difference of the sun's heat in 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, Part II.

Other streams of cold water arise from beneath the snow on the Alps and Andes, and other high mountains, which is perpetually 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, Part 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 anchor'd pinna, and his cancer-friend.
CANTO III. 1. 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 weathers, 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 I which often amounts to two thousand. Dict. raisonné, 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 buoyant or the contrary. See note on Ulva, Part 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. Amon. Acad. 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 ancients, 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 rheumatism and gout. Dict. raisonné, 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 re-produce 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 shown 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 beard his toothless lips array, And teach the unwieldy sturgeon to betray. CANTO III. 1. 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 ten drills 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 showing 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 mouth 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 tendrils above mentioned, 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 at 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 Wolga, 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.

Or with fine films, suspended o'er the deep,
Of oil effusive lull the waves to sleep.
CANTO III. 1. 87.

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, Part 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 surfac 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 a quired a greater momentum than the lowest sid (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. 1. 91.

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 prodigious

108

ly. They bore their passage in the direction of the fibres of the wood, which is their nourish ment, 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 year 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 Zealand; but it was happily discovered a few years afterwards that these insects had totally abandoned that island, (Dict. Raisonné, 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.

[Notes XXXI, XXXII. 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 a 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 XXXI.-MAELSTROM. Turn the broad helm, the flattering canvas urge From Maelstrom's fierce innavigable surge. CANTO III. 1. 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 farther northward, which gives rise to the Gulph-stream, as described in note on Cassia in Part 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 completely to fill it and stand many feet above it, less disturbance must appear on the surface. The Maelstrom is described in the Memoirs of the Swedish Academy of Sciences, and Pontopiddon's Hist. of Norway, and in the 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 the 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 one's 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

NOTE XXXII.-GLACIERS. Where round dark crags indignant waters bend Through rifted ice, in ivory veins descend. CANTO III. 1. 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 Part 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 Amænit. 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. 1. 529.

109 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.

4 oz. wt. of lava

gave

20 oz. meas. of air. 104

NOTE XXXIII-WINDS.

While southern gales o'er western oceans roll, And Eurus steals his ice-winds from the pole. CANTO IV. 1. 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 alkali, and probably from fixed alkali, 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 alkali 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 avoirdupois ounces, and as vital air is above 800 times lighter than water, it follows that every

1

elvain

5

3,

3

4

lime stone chalk

white iron ore dark iron ore molybdena

............ steatites

2

............ barytes

26

2

80

2

3

stream tin

black wad

In this account the fixed air was previously extracted from the lime-stones by acids, and the heat applied was much less than was necessary to extract all the air from the bodies employed. Add to this the known quantities of air which are combined with the calciform ores, as the ochres of iron, manganese, calamy, gray ore of lead, and some idea may be formed of the great production of air in volcanic eruptions, as mentioned in note on Chunda, Part II. and of the perpetual absorptions and evolutions of whole oceans of air from every part of the earth.

But there would seem to be an officina aeris, a shop where air is both manufactured and destroyed in the greatest abundance within the polar circles, as will hereafter be spoken of. Can this be effected by some yet unknown law of the congelation of aqueous or saline fluids, which may set at liberty their combined heat, and convert a part both of the acid and alkali of seawater into their component airs; or on the contrary, can the electricity of the northern lights convert inflammable air and oxygene into water, whilst the great degree of cold at the poles unites the azote with some other base? Another officina aeris, or manufacture of air, would seem to exist within the tropics, or at the line, though in a much less quantity than at the poles, owing perhaps to the action of the sun's light on the moisture suspended in the air, as will also be spoken of hereafter; but in all other parts of the earth these absorptions and evolutions of air in a greater or less degree are perpetually going on in inconceivable abundance; increased probably, and diminished at different seasons of the year by the approach or retrocession of the sun's light; future discoveries must elucidate this part of the subject. To this should be added, that as heat and electricity, and perhaps magnetism, are known to displace air, that it is not impossible but that the increased or diminished quantities of these fluids diffused in the atmosphere may increase its weight as well as its bulk; since their specific attractions or affinities to matter are very strong, they probably also possess gen