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water of the depth of 6 inches, there did not evaporate enough to make the salt precipitate.

This lays us under a necessity of seeking other discharges for the Mediterranean. Some have imagined that they found one in the contrary direction of the water at the surface, and that at the bottom; by virtue of which the Mediterranean should regularly furnish as much water to the Atlantic as it receives from it. This hypothesis appears at first sight repugnant to the laws of hydrostatics, especially if we suppose

the water of the two seas to be equally salt, and consequently equally heavy; for water never runs but from a higher to a lower place; su that the surface and the bottom must both be carried the same way. Building on these hydrostatic truths, M. de Buffon has pot scrupled positively to deny the fact, and taxes the experiments on which it is founded with falsity.

It cannot be denied, that the principles of hydrostatics furnish an argument against the existence of this double current that seems unanswerable; and our Academician would have adopted the hypothesis of evaporation, if it could have been supported. But all who know any thing of salt works, know that it is only the fresh water that evaporates, and that the salt remains. The same process is observed in making sait from the water of the Mediterranean. If then this sea had lost annually, since it first existed, this quantity of water by evaporation, it would long before now have been reduced to a vast mass of indurated salt. The sixteenth part of its water is pure salt; and by calculation, it will appear that the salt separated from the water would form in 500 years a mass of salt 250 feet high. Now according to the inquiries of Count Marsigli, many places of the Mediterranean are not of this depth: so that in the aforesaid space of time, this sea would liave been wholly changed into salt, if the salt water continually emptied into it by the neighbouring seas, and no issue: but in the many thousand years since this sea has been known, not only this metamorphosis hath not taken place, but even its waters, as far as we know, are not become more salt. We are obliged therefore to give up evaporation, and seek some other expedient get

rid of its redundant water: for this end we must not wholly neglect the double current, but ascertain the fact with all possible exactness, and afterwards endeavour to reconcile it to the laws of hydrostatics.

Besides the testimonies related above, a Dutch transport vessel having been beat to pieces by a French man of war in the middle of the Straits of Gibraltar, between Tariff and

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Tangier, the wreck of this vessel, with some casks and other light things, appeared after some days on the surface of the water, four English miles to the west, towards the Spanish sea. If the direction of the current were the same at bottom as on the surface, from west to east, these wrecks could not have raised themselves against the current so as to swim at top, but would have followed the declivity, which would have carried them towards the Mediterranean.

The impossibility of reaching the bottom of the Strait with the longest line, does not prove that it is without a bottom; but it is highly probable that this difficulty arises from the contrariety of the currents, which bends the line of the lead, and hinders it from getting to the bottom. Count Marsigli made the same observation in the Straits of Constantinople, where the Black Sea has its outlet; and the Turkish fishermen told him that it was always so. There are other authentic examples of opposite currents; it would be in vain therefore to deny the fact; but the natural causes of it remain to be inquired into.

In order to discover them, M. Waiz recapitulates what he had said before, namely, that the water of the Mediterranean contains much salt; secondly, that this sea being in a very warm climate, suffers a great evaporation; thirdly, that the salt is not carried off by this evaporation, but remains behind; fourthly, that salt is about three times specifically heavier than water; fifthly, that salt water is so much diminished by evaporation, that 18 lots of water contain 5 lots of salt, and the water is then much heavier. The author found by his own experiments, that the weight of salt water becomes five times greater before the salt begins to crystalize.

As then there is a continual and copious discharge of salt water into the Mediterranean, and that a great part of this water deposits its salt by evaporation, what is left always remains more salt, and consequently more weighty. Supposing then the surface of the two seas, the Atlantic and the Mediterranean, to be equal, their gravity would not be equal; but the water of the Mediterranean, as the more weighty, would press on that of the Atlantic, and the two seas would run together through the Straits till their waters became of equal weight; so that the Mediterranean would necessarily be lowest. When this happens, the water of the Atlantic, which is highest, cannot take its course through the Strait but by a higher current, by means of which it spreads itself in the Mediterranean; but this would augment the weight, already the greatest, of the water of the latter, which cannot get away, but by opening itself a passage underneath, and 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 room 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 Me. diterranean 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 Hows 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 thau

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at Gibraltar, because the difference in the degrees of salt, ness 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 avd 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 sait, 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 Aux and reflux, and does not run into the Mediterranean, as in the middle. Sbips coming from the Mediterranean are

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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 fux 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 there. fore 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 bak and chesnut, viz. 300 years growing, 300 years standing, and 300 years decaying, it favours my conjecture, that this

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