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But if we farther add the great rivers, such as the Danube, the Don, the Dueiper, the Dnister, and several others, which fall into the Black Sea, and flow through the Straits of Constantinople into the Mediterranean, as also that muluitude of rivers, great and small, which run on all sides into the Mediterranean, it will be evident that the heigist which this sea receives annually by those means cannot be less than 30 feet. That evaporation should carry off all this water, seems impossible; for in that case it would be twenty-five times stronger than at Paris, which is not situated in a cold climate. A lake of between 40 and 50 feet in depth, without any issue, would not dry up probably in a year, even under the line. M. de Buffon has nevertheless asserted, that evaporation is sufficient to carry off the surplus water which the Mediterranean receives annually. It was the authority of this celebrated naturalist that engaged M. Waiz to examine the subject with more exactness.
For this end, he copsiders the manner in which salt is made in the Mediterranean by natural evaporation, by receiving the water on a sinooth surface to the height of an inch and half oply. This water evaporates in 24 hours, in the hottest season in the year, provided no rain falls. Dr. Hoffman tells us, that a pound of the Mediterranean water contains two lots (a lot is the 320 part of a pound] of salt: butaccording to the Swedish academician's own experiments, salt water doth not deposit its salt till the evaporation is carried so far that there remain only five lots of salt to thirteen lots of fresh water. According to this calculation, evaporation on the coasts of the Mediterranean, in the hottest days, should carry off from each pound of water in the 24 bours, 24€ lots of water, which makes two thirds of the inch and half which the water had in depth at the beginning. In deep cavities the evaporation must be more slow. Iu this manner the evaporation would, in 24 hours, be one inch and a half. But if we grant that this inch and half of water is entirely evaporated in 24 hours, the salt remaining quite dry, and making the 32d part of the whole mass; the daily evaporation will tlien amount to 1. of an inch, and the aunual evaporation to 44 41 feet, if it be equally hot all the year, and no rain falls. But as the hot weaiher lasts for some months only, and there are few days without rain, and as there are even some whole seasons in which it rains constantly in the Mediterranean, and the evaporation is less, we cannot make the evaporation amount annually to 44 feet, especially as Lemery assures us, in his Course of Chymistry, that at Rochelle, in 15 days in the most proper season, from
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 iti 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; so that the surface and the bottom must both be carried the same way. Building on these hydrostatic truths, M. de Buffon has not 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 salt 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 have been wholly changed into salt, if the salt water continually emptied into it by the neighbouring seas, had 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 to 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 Tariil and Tangier, the wreck of this vessel, with some casks and other light things, appeared after some days on the surface of ihe 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 Strait 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 tlre weight, already the greatest, of the water of the latter, which cannot get away, but by opening itself a passage underneath, and forining an inferior opposite current in the Straits. This is sufficient to produce ihe 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, Jet 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 the 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 warn 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 rooin, 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 salt. ness of the water, which comes in, and that which goes out, is greater, naniely, 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 Guadiana 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 evapora. tion 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 suíficient 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 fux and reflux, and does not run into the Mediterranean, as in the middle, Ships coming from the Mediterranean are