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forming an inferior opposite current in the Straits. This is sufficient to produce the two currents, and to perpetuate them without interruption.

There is an experiment which confirms the agreement of this hypothesis with the laws of hydrostatics. Take a long box, divide it into two by a board fixed in the middle, let there be a small hole in the board, which you can shut at pleasure. Fill one end of the box with water, and the other with oil to an equal height. On hastily opening the hole in the board that divides them, the water, which is heaviest, will be seen to run into that end of the box where the oil is. On the contrary, the oil will be carried in the same manner, and at the same time into that end where the water is, over which it will spread itself. It may indeed be objected, that as oil cannot mix with water, it must get at top; but the same thing happens to two waters of unequal gravity, when one is coloured and much salter than the other. If the box be made of glass instead of wood, you will have a distinct idea of the two opposite currents.

The air in like circumstances acts exactly like water, and it is easy to make the experiment. Let there be two rooms with a door from one to the other; let one room be warmed that the air in it may dilate itself and become lighter, this will be the Atlantic. The other cold room, the air of which is not so thin and light, will represent the Mediterranean; let the door, which is the Strait between the two seas, be opened, and a lighted candle placed on the threshold, whilst another is held at the top; it will be seen by the flames of these two candles that the cold air passes from the cold room into the hot at bottom towards the threshold; and the warm air into the cold room at top. The warm air soon cools in the cold room, but the heat of the warm room being kept up by a fire, the double current of the air will appear very evident for some time, till the air of the two chambers be equally warm, and consequently, equally heavy.

If there be a warm rooni on each side of a large cold room, the same thing will happen at the two doors, that is to say, the cold air will enter at bottom, and the warm at top. This explains what Count Marsigli says of the currents in the Straits of Constantinople, where the salt water of the Mediterranean enters at bottom into the Black Sea, and is there rendered lighter by the quantity of fresh water that runs into it; after which it flows again, in the same Strait, above the salt water, into the Mediterranean; as is seen in the Strait of Gibraltar, The currents are stronger at Constantinople than

at Gibraltar, because the difference in the degrees of saltness of the water, which comes in, and that which goes out, is greater, namely, according to Marsigli as 73 to 62, whereas it is not so great in the Straits of Spain.

There is one very plausible objection to this theory, namely, that as the Atlantic sea is in the same climate with the Mediterranean, the evaporation must be the same in both; and consequently their water be of the same gravity, especially if we consider the great quantity of fresh water which so many rivers carry into the Mediterranean. To this it is answered, that it is well known that the sea is less salt towards the poles than near the equator; an invariable current brings this fresher water from the poles towards the equator; some large rivers, as the Guardiana and the Guadalquivir, empty themselves at the two sides of it at the same time, and pass by the Strait with their fresh water to run into the Spanish sea; and lastly, a daily flux and reflux incessantly agitate and mix these waters from top to bottom: these different circumstances united, shew that the water of the Atlantic cannot be so salt as the Mediterranean, the evaporation of which continually augments its weight and saltness. What we have said above of a perpetual current running from the poles to the line, is supported by sufficient authorities. Navigators attest that they always go quicker in this, than in the contrary direction, and they every year see large shoals of ice carried from the north to the south. Several causes may contribute to the formation of this current, and it may be proved that the water it carries along doth not contain much salt. When the water freezes it becomes lighter, and the ice swims at top. Though this ice be composed of salt water there is but very little salt in it, as might be shewn by many experiments, and by what happens in salt works. On these shoals of ice from salt water, there fixes a quantity of snow, rain, vapours, &c. the wind drives these shoals upon one another till they form vast mountains of ice. When these mountains come to melt, they produce an immense quantity of fresh water, which does not easily mix with the salt, but remains at top. It cannot flow back towards the Poles, where there is still more ice and fresh water; it is therefore continually carried to the south, where the water is salter, and consequently lower.

In fine, it remains only to inquire, why, on the two sides of the Straits of Gibraltar the current of water is subject to the flux and reflux, and does not run into the Mediterranean, es in the middle. Ships coming from the Mediterranean are

wont to observe this current, and commonly keep on the African side, to wait for and follow it; partly because the coast is less dangerous, and partly because the flux and reflux is much greater than on the Spanish side. These side currents prove the possibility of several currents existing at one time in the same channel, running one below another, and in contrary directions.

When two drops of water touch, and unite according to the laws of attraction and cohesion, if one be considerably larger than the other, and be put in motion, it draws the other to it, and carries it along. A current is nothing else, but a multitude of cohering drops in motion; it must therefore carry with it a part of the water on its sides.

1760, Feb.

XXX. Immense Chesnut Tree at Tamworth.

MR. URBAN,

As your monthly labours will be records to ages to come, I submit the following calculation of the age of a celebrated chesnut tree, which in all probability is the oldest, if not the largest tree in England, being 52 feet round, to be transmitted by your means to posterity.

This eminent tree is the property of the Rt. Hon. Lord Dacre at Tortsworth, alias Tamworth, Gloucestershire.

I may with reason fix its rising from the nut in the reign of King Egbert, anno 800. From this date, to attain to such maturity and magnitude, as to be a signal tree, for a boundary or land-mark, called, by way of distinction, the great chesnut tree at Tamworth, in the reign of King Stephen, I cannot allow less age than 335 years, which brings it down to the first year of King Stephen, anno 1135; from this date, we are certain of its age by record to the present year, 1762; 627 years. In all 962 years.

Mr. Evelyn, in his fifth edition, has this remarkable passage relating to this tree, viz. Boundaries to great parishes, and gentlemen's estates; famous for which, is that great chesnut at Tamworth, in Gloucestershire, which has continued a signal boundary to that manor from King Stephen's time, as it stands on record.

If any regard is to be paid to the three periods given to oak and chesnut, viz. 300 years growing, 300 years standing, and 309 years decaying, it favours my conjecture, that this

stately old chesnut tree is very little less, possibly more, than a thousand years old; and yet such vigour remains, it bare nuts anno 1759; from them young trees are raised.

1762, Feb.

Yours, &c.

P. C.

XXXI. Remarkable Phenomenon of the Bath Waters.

A Letter from Dr. D. W. Linden to Dr. Sutherland, at the Hot Wells, Bristol, concerning a remarkable Phenomenon of the Bath Waters.

DEAR SIR,

IN compliance with your request, I send you a brief account of my last examination of the Bath waters. The phenomenon which most struck me, were certain cakes, of a blackish colour, which at this time of the year are found floating upon the surface of these waters, and which I had never seen before, having been at Bath only in the winter months, when they do not appear. I had, indeed, heard much of them, and was told that they were a vegetable substance, the conferva gelatinosa; but, upon examination, I found this to be a mistake, and that the black cakes were mineral.

That they are not the conferva gelatinosa is manifest, from their appearing so early as the beginning of May; for the conferva does not appear till July, and it does not flower till August. Besides, the conferva is found only on stagnant waters; and it is absurd to suppose that a mineral hot spring should have any communication with a standing pool, whence it should receive this plant, as it could not receive the plant without such a mixture of the water as would render it cold, and annihilate its virtue: that the Bath water cannot originally produce the plant, is certain; for it is continually in a state of agitation, which renders the growth of it impossible.

Upon a close examination of these cakes, some of which have a greenish hue, I found that those which had lain near the wall for some time undisturbed, had caused a natural crystalization of the salts in the Bath water; and these salts, on some of the walls in the Abbey-house spring, were more than half an inch thick; such a crystalization could not be caused by a vegetable substance.

Having now shewn what these cakes are not, I will tell you what they are; for, upon applying the common vitrioline

solvent, I found them to be neither more nor less than the mucilagium ferri, or slimy substance, that is always a concomitant of iron-stone, iron-earth, or iron-ore; if there is any medical virtue in iron, it ought to be sought in this slimy substance: and I shall shew, in a Treatise on the Bath waters, which I am now preparing for the press, that the Bath waters derive great medical efficacy from these cakes, especially in external applications.

Those persons who have supposed these cakes to be vegetable, have been deceived into that opinion, by the solid fibrous parts which they have discovered in them, after having washed them from the mud and other extraneous bodies, that have been found mixed with them. But those who are acquainted with practical mineralogy, a science which is essentially necessary to those who undertake the analysis of mineral waters, know, that the slimy substance in iron-ore, when agitated in waters that contain salt, will form itself into fibres and branches, resembling those of vegetables; and upon this principle it is, that, in curious chymistry, small branches and fibres are formed in liquids by the solution of metals and minerals, and have obtained the name of philosophical trees.

Some experiments, indeed, have been made upon these cakes, by distillation; and it has been presumed, that they are vegetable, because they yield only an insipid water, without any metalline or mineral particles; but this is wholly fallacious and inconclusive; for the mucilagium ferri, or any other metal or mineral, mixed with common or saline water, will, in distillation, yield only an insipid water, with, out mineral particles, because these particles are prevented from rising in the steam, by their own weight. I am, Sir, yours, &c.

1762, May.

D. W. LINDEN,

XXXII. Account of Fires kindled of themselves.*

THE great consumption of sea coal in the port of Brest made it necessary to form a kind of magazine, constructed of timber and planks rudely joined together, where many hundred chaldron were kept piled in a vast mass, and constantly exposed to the weather. No accident was ever

*From the Memoirs of the Royal Academy of Paris.

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