A Special Place in the History of Oceanography

TThehe BosporusBosporus SStraittrait A SpecialSpecial PPlacelace inin thethe HHistoryistory ooff OOceanographyceanography

Th is article has been published in Oceanography, Volume 18, Number 2, a quarterly journal of Th e Oceanography Society. Copyright 2005 by Th e Oceanography Society. All rights reserved. Reproduction of any portion of this article by photo- 16 Oceanography Vol.18, No.2, June 2005 copy machine, reposting, or other means without prior authorization of Th e Oceanography Society is strictly prohibited. Send all correspondence to: [email protected] or Th e Oceanography Society, PO Box 1931, Rockville, MD 20849-1931, USA. BY BRUNO SOFFIENTINO AND MICHAEL E.Q. PILSON

The Black Sea and the Bosporus Strait hold an important place in the history

of oceanography. In 1680, the impressive geographical features and remark-

able current patterns of the Bosporus sparked the imagination of young Luigi

Ferdinando Marsigli to solve the long-standing puzzle of two-layer ﬂ ow. The

observations and experiments were published in the 1681 book by Marsigli,

originally published in Italian, Observations around the Bosporus Strait or True

Canal of Constantinople, Presented in a letter to Her Sacred Royal Majesty Queen The Bosporus Strait Christina of Sweden by Luigi Ferdinando Marsigli. (This book was recently translated into English by the authors of this article.) Marsigli understood that

the Bosporus currents were a simple consequence of the different water densi-

ties in the Black and Mediterranean Seas. He demonstrated this density differ-

ence by building a physical model that captured the salient features of the phe-

nomenon (Figure 1). Marsigli made a two-compartment box with the divider

connected by two openings at top and bottom, and showed that waters of dif-

ferent densities in the two compartments would ﬂ ow to the opposite side in a

manner consistent with his observations. British scientists had long suspected

that two-layer ﬂ ow was occurring in places like the Strait of Gibraltar and the

Skagerack, but at the time, no one had managed to convincingly demonstrate

the existence of an undercurrent, nor had anyone put forth a convincing ex-

planation for it.

The story of Marsigli in Istanbul, the breadth of his observations, and the

premises of his famous experiment are very interesting but relatively unknown

to oceanographers. This article therefore has two objectives: the ﬁ rst is to give

an account of Marsigli’s work that is more focused on his measurements and

techniques than has ever been presented; the second is to understand how a

21-year-old man from a town uninvolved in matters of the sea managed to ex-

plain a phenomenon that had occupied the minds of many prominent British

Oceanography Vol.18, No.2, June 2005 17 scientists and engineers for the previous Turkish and Venetian diplomats. This hill battle from the start and prematurely 30 years. Most of Marsigli’s measure- was the fi rst trip abroad for this 21-year- left Istanbul less than a year later, carry- ments, when translated in modern units, old. The connection with the Venetian ing sanctions against Venice that were turn out to be consistent with current embassy was a wonderful opportunity to to be enforced for more than a century knowledge and show that he was a re- spend time with scholarly people, Italian (Stoye, 1994). Despite all of this, Marsi- markably keen and thorough experimen- and foreign, and to learn about other cul- gli was surprisingly immune from this talist. We conclude that Marsigli was the tures, including Muslims, that western- hostile situation, most likely as a conse- right man in the right place: his unique ers found fascinating and disquieting in quence of his young age and lower rank, educational background and experience equal measure (Stoye, 1994). In the mid- and his propensity to learn the local lan- in hydrostatics, ostensibly inappropri- to late 1600s, Ottoman relations with the guage and customs. This was fortunate, ate for the study of sea currents, came to rest of Europe signifi cantly chilled, and as he certainly could not have carried out fruition thanks to a locality—the Bospo- the work of Western diplomats became the range of observation that we know rus—that was exceptionally well suited proportionately more diffi cult (Abbot, about without the ability to freely move for observation and measurement. 1920). Harassment of foreign envoys and restriction of their movements in and Bruno Soffi entino ([email protected]) is TRAVELING TO ISTANBUL around Istanbul was a frequent occur- NASA Astrobiology Postdoctoral Researcher, Luigi Ferdinando Marsigli’s eventful rence. Within the overall dismal spec- University of Rhode Island Graduate School journey began on July 22, 1679 when he trum of European-Ottoman relation- of Oceanography, Narragansett, RI, USA. set sail from Venice for Istanbul. He was a ships, the Ottoman’s relationship with Michael E.Q. Pilson is Emeritus Profes- “junior member” of the Venetian embas- Venice was among of the worst. Accused sor of Oceanography, University of Rhode sy to the Ottoman court, where he would of espionage immediately upon arrival, Island Graduate School of Oceanography, be working as a messenger between the Marsigli’s diplomatic party faced an up- Narragansett, RI, USA.

Figure 1. Luigi Ferdinando Marsigli’s physical model of gravity-driven, two-layer fl ow, published in 1681. Th e model consists of a box divided by a partition (A) with openings at top and bottom, containing water of diff erent density in the two compartments. Dense water (dark-colored side) fl ows to the opposite compartment through the bottom opening, causing lower-density water (light-colored side) to move in the opposite direction through the top opening. Th e conceptual synthesis of this model arose from the combina- tion of Marsigli’s atypical background in hydraulics and hydrostatics, his careful measurement of water densities in the Mediterranean and Black Seas, and the unique geographical and hydrologi- cal features of the Bosporus Strait.

18 Oceanography Vol.18, No.2, June 2005 about, and he could not have learned so Montanari. With this instrument he mea- Marsigli was intrigued by local reports much about the region without access to sured the weight of water of the Adriatic on the current patterns of the Bosporus. local people. and Aegean Seas, at several locations in Fishermen told him that fi shing nets Marsigli made a remarkable range of the Sea of Marmara and the Black Sea, cast from the stern of boats anchored in observations during his voyage to Istan- and of water retrieved from the under- the canal would resurface in front of the bul. He measured the density of waters at current. We have found no evidence that boat if allowed to sink to a certain depth. various locations in the Bosporus Strait Montanari encouraged Marsigli to make He also found out from an English mer- and the Mediterranean and Black Seas. those measurements, and as far as we can chant, Sir Dudley North, that an English He measured the speed of the Bosporus tell, Marsigli was at the time of departure ship captain had lowered a weight in the surface current with a new type of cur- unaware of the two-layer fl ow problem. water and had seen it change direction of rent meter, and he determined the depth The hydrometer is not pictured in Obser- drift when it reached a certain depth. at which the fl ow reversed direction. He vations, but from its sketchy description, Encouraged by John Finch, the Eng- drew a map of the Bosporus region, re- it seems to have been similar to a later lish ambassador in Istanbul, Marsigli set portedly using only a compass. He used design found in Marsigli’s 1725 book out to investigate the currents. He sur- a barometer to compare the height of Histoire Physique de la Mer. It consisted veyed the Bosporus and sketched a chart various locations to sea level and sur- of a small, long-necked bottle weighted showing the essential features of the area mised that the Black Sea and the Sea of with enough lead shot that it could fl oat (Figure 2). Compared to a modern map, Marmara were at the same altitude. He upright, on which metal rings of known the scale is only approximate, but con- made repeated observations of the wa- weight could be placed until it sank to a sidering the fact that it was drawn using ter level and meteorological conditions reference mark. From Marsigli’s reported only a compass and no surveying instru- in Istanbul and correctly concluded that weight of rainwater (42 grains), of water ments, it is remarkably good for such tides were imperceptible in the area, and off the coast of Izmir (82.5 grains), the a large geographical area. The compass that wind and weather were instead most average value of salinity in that region orientation is fairly accurate, and the important in determining water height. (~38 ppt) (Miller et al., 1970), and a met- localities are in the right relative posi- He illustrated an oil seep in the isle of ric conversion of 0.055g/grain (Zupko, tion. The surface currents also appear Zakinthos. He gave an account of the 1981) we calculated that the bottle had correct. The small nearshore currents seasonal migrations of fi sh to and from a volume of about 79.1 ml, a reasonable running opposite to the main direction the Black Sea. He dissected and illustrat- size for that type of instrument. of fl ow are obviously eddies caused by ed the anatomy of a mollusk, probably Marsigli found that Black Sea water local topography. Marsigli correctly in- a mussel (Marsigli, 1681). For the pur- at the northern mouth of the Bosporus terpreted currents C and D as such, but poses of this article, we will only concern weighted 56.25 Venetian mint grains. he incorrectly interpreted current B as ourselves with the measurements and This corresponds to a salinity of about a freshwater plume from the river that observations that are pertinent to his 12.5 ppt. Such a value is not inconsistent discharged in Istanbul harbor. Current model of two-layer fl ow. with what is found today at that location D is a curious case: it is located in what (16.5 ppt; Ozsoy et al., 1993). He also is today the harbor of Haydarpasa, north MARSIGLI’S OBSERVATIONS found that water from the undercur- of Cadi Koi; however, there is no report OF TWOLAYER FLOW rent was 10 grains heavier than surface of this eddy in modern sources. Consult- The most important measurement that water, which corresponds to a salinity ing a navigation chart, it is apparent why. led Marsigli to understand the causes difference of about 8.7 ppt. This is less Today the harbor is protected by two of two-layer fl ow was water density, or than the difference between the two end- large breakwater jetties that prevent any “weight,” as he describes it. Marsigli member waters, so it is probable that his signifi cant circulation. brought from home a “hydrostatic am- sample did not come from the bottom of To measure the intensity of the sur- poule,” or hydrometer, that he acquired the channel, where the salinity would be face fl ow, Marsigli had someone build from one of his teachers, Geminiano close to that of Mediterranean water. for him a paddle-wheel current meter:

Oceanography Vol.18, No.2, June 2005 19 Figure 2. Marsigli enjoyed many opportunities to explore the Bosporus during his 11-month stay in Istanbul; this map summarizes many of the observations that he made. Th e map is remarkably accurate considering it was drawn without surveying instruments, using only a compass for orientation. Th e currents of the Bosporus were by no means young Marsigli’s only interest in Istanbul: he observed and noted details of the fauna, fl ora, geography, and meteorology, as well as of the politics, military, and social customs of the place. Th e latter information would eventually become very useful to him in a few years’ time, when fi ghting the Turks as a Habsburg army offi cer.

“...a machine [made] out of wood, con- right angle in its submerged portion. In 42.9 cm long, and the axle about 87.5 to sisting of a wheel with six paddles, each such position, the index made thirty-eight 156.4 cm long1. The current was mea- two palms, four inches, and two quarts of revolutions in the time of one-hundred sured by counting the revolutions of the Roman measure in length; in the wheel I swings of a pendulum eight inches and six index during 100 swings of the 16.8 cm- placed an axle seven palms long, on which eighths long” (Marsigli, 1681, p. 27). long pendulum. Marsigli does not report I mounted an index one palm and two According to Frazier (1974), this is the in Observations whether or not someone quarts long at one end, for the purpose fi rst recorded use of this current meter assisted him with the measurements, of showing the revolutions that the wheel design. Translation of the dimensions but we fi nd it diffi cult to imagine that he made as it was held horizontally and was into modern units (Zupko, 1981) reveal could have done it alone. A pendulum of propelled by the water, that struck it at that the paddles were between 33.1 and those dimensions has a relatively short

1 Th e range of metric values has to do with the fact that “palms” and “quarts” units varied from city to city, and from trade to trade. Because Marsigli did not specify which ones he used, we translated all the ones that seemed likely candidates.

20 Oceanography Vol.18, No.2, June 2005 period, so counting swings and revolu- the measured depths would correspond slope would not account for the fl ow. tions at the same time would seem quite to 14, 17, and 20 meters. These fi gures are Marsigli built his famous “box” (Fig- a challenging exercise. In either case, consistent with modern knowledge: the ure 1) in Italy, upon his return from Is- at the southern end of the Bosporus, depth of the interface is known to vary tanbul, and tested it in the presence of the current was 38 revolutions per 100 between 10 and 20 meters (Ritchie, 1969) Luc’Antonio Porzio, a physician from swings, which translates to approximately at the southern mouth where Marsigli Padua who later made his name for writ- 2 to 2.5 knots accounting for the uncer- was stationed. Marsigli also managed to ing the fi rst treatise on water purifi cation tainty in the dimensions and assuming get a sample of water from the undercur- using sand fi lters. Upon encouragement a one-to-one correspondence between rent: “...with the use of a vessel, closed from (ex) Queen Christina of Sweden, current velocity and tangential velocity with a valve that I could open with a rope whom he visited in Rome, Marsigli wrote of the wheel. This fl ow velocity is lower while submerged, I collected waters that up his Istanbul experiences and results than the modern value of 3 to 4 knots I found weighed ten grains more than on in the compact little book that was pub- known in that locality. The underestima- the surface of the Canal” (Marsigli, 1681, lished in Rome in 1681 (i.e., Observa- tion might have been due to the fact that p. 74). Unfortunately, no further descriptions). The book reached the Royal Soci- one of the paddles broke shortly after tion of the vessel is given. It would be ety of London in 1684, and although the deploying the instrument (!!), as Marsigli interesting to know whether it was some two-layer fl ow problem had interested reported; otherwise it could have been ad hoc design, or whether it was similar British scientists for some time, the sig- that the instrument was deployed near to the design of Robert Hooke, who had, nifi cance of this work went largely unno- shore, where the current is weaker. Com- a couple of decades earlier, constructed ticed (Deacon, 1997). paring this measurement with others a vessel for sampling subsurface water in different parts of the Strait, Marsigli (Deacon, 1997). MARSIGLI’S BACKGROUND correctly concluded that the surface cur- Finally, Marsigli checked whether the What special attributes of Marsigli’s rent was higher in the narrowest part of Black Sea was at a higher elevation than background put him in the position to the canal, at the Rumeli Castle, and that the Sea of Marmara. He did this by tak- understand the causes behind two-layer at that location it was one-third greater ing a barometer to various locations and fl ow? The evidence points to an unusu- than at the southern mouth. comparing the height of the mercury ally diverse education. Marsigli was born Marsigli did not have the means to column, and found that, if anything, the in an affl uent, noble family of Bologna, measure the undercurrent, but he inves- Black Sea was lower than the Sea of Mar- and had access to good education in the tigated it by lowering into the water a mara. Of course, this seems like a use- form of private tutoring by local univer- weighted piece of wood painted white, or less exercise to us, as a barometer would sity professors. His teachers were some of a simple lead weight, and noticing the di- never be able to detect such small differ- the best Italian scientists of the time: the rection and strength with which the rope ences in elevation above sea level amidst astronomer, mathematician, and hydrol- pulled his hand as it changed depth (Fig- the meteorological variations of atmo- ogist Geminiano Montanari; the physi- ure 3). At the southern end of the Bospo- spheric pressure. In Observations, Marsi- cian Marcello Malpighi; and the bota- rus, Marsigli found the current switching gli made the remarkable statement that nist Lelio Trionfetti. Though Marsigli direction “...with great haste and strength “...although I don’t consider inclination showed considerable inclination toward at a depth of 8, 10, or 12 Turkish Paces, important in this Canal, I report to Her mathematics and the natural sciences, each Pace being approximately equiva- Majesty the relative level [of Black Sea he never enrolled in the university and lent to the distance between the open and Sea of Marmara] that can be read never obtained a formal degree. There arms of a man of average size” (Marsigli, with the Mercury, or Torricellian Tube...” is no evidence that he ever considered 1681, p. 56). Given that an arm span is (Marsigli, 1681, p. 33). It is hard to tell an academic career. Instead, he was in- approximately equal to height (Schott, whether this statement is made with the terested in politics and government, and 1992), a reasonable metric guess for the benefi t of hindsight, or whether Marsi- ultimately he became an army offi cer at Turkish Pace would be around 1.7 m, and gli truly thought all along that channel the service of the Habsburg Empire and

Oceanography Vol.18, No.2, June 2005 21 Figure 3. Th is drawing depicts the be- havior of a tethered weight lowered through the water column from a boat anchored in the middle of the Bosporus channel. Marsigli carried out this experiment to demonstrate the presence of an undercurrent running opposite to the surface current. As the weight crossed the two oppositely running water layers, the currents dragged the weight in the opposite direction. He correctly estimated that the depth of this boundary varied between 10 and 20 m on diﬀ erent occasions.

remained a professional soldier, with with his student Guglielmini (a contem- those days. In the spirit of this casual or- varying degrees of fortune, for the rest of porary of Marsigli), he became a central ganization, detailed records of the min- his life. The most peculiar feature of this fi gure in the late 1600 Italian school of utes and participants were never kept, eccentric character is that throughout his hydraulics (Maffi oli, 1994). Curiously, but from Montanari’s correspondence, it life he remained a scientist at heart, mak- Montanari never formally taught this is known that Marsigli was a frequent at- ing observations, drawing, publishing, subject at the University, despite the fact tendee. The conceptual ancestry of Mar- and patronizing science under the most that Bologna had long been confronted sigli’s physical model of the Bosporus is extraordinary and often unfavorable cir- with water issues because of the recur- suggested by the descriptions and draw- cumstances (Stoye, 1994). rent fl oods of the river Reno. Montanari ings of some of Montanari’s experiments Geminiano Montanari appears to performed experiments on the equilib- (Maffi oli, 1994), and by the fact that, in have had the most signifi cant infl uence rium level and fl ow of fl uids of differ- Observations, Marsigli often refers to the on Marsigli’s thinking on water move- ent density (e.g., water and mercury) in teachings of his mentor Montanari. ment. An astronomer and mathemati- various systems of connected vessels, and The young man from Bologna was, cian by training, Montanari was hired by discussed the results at gatherings of his therefore, well equipped to tackle the the University of Bologna in 1664, and Accademia, which were held in an infor- two-layer fl ow problem: he had learned soon after arriving he founded the Ac- mal and convivial atmosphere mostly in principles of hydraulics and hydrostatics cademia della Traccia, a scientifi c society his own home. These meetings could be from a pioneer of the fi eld, and he had that he lead from 1665 until 1677. Mon- attended by anybody with an interest in participated in experiments and demon- tanari became interested in problems of the discussions without distinction of strations of fl uid processes akin to those hydrostatics and water movement, and social status, an unusual arrangement in he would fi nd in nature.

22 Oceanography Vol.18, No.2, June 2005 SERENDIPITY of miles from Gibraltar, and even then ACKNOWLEDGEMENTS As we thought about the details of Mar- they would have been only slightly dif- We owe special thanks to Ms. Roberta sigli’s methods, experiments, and back- ferent (by hydrometer measurement) Doran, Head Librarian of Pell Library, ground, we realized that in and of them- from the Atlantic. Graduate School of Oceanography, selves they were not suffi cient to have led This reasoning also suggests why the URI, for her efforts in fi nding and ob- Marsigli to his conclusions about two- British did not came up with an expla- taining copies of Marsigli’s book and layer fl ow. The Bosporus was the other nation for two-layer fl ow despite their other references. necessary ingredient. We could not think long-lasting concern with the currents of another place but the Bosporus where in the Strait of Gibraltar. Although Brit- REFERENCES Marsigli could have fruitfully applied ish scientists had anecdotal evidence for Abbott, G.F. 1920. Under the Turk in Constantino- ple. MacMillan and Company, London, UK. his knowledge. In no other place where an undercurrent in Gibraltar similar to Deacon, M. 1997. Scientists and the Sea, 1650-1900: two-layer fl ow is known to occur are two that available to Marsigli in the Bosporus, A Study of Marine Science. 2nd ed. Ashgate Pub- basins with so strikingly different water they could never take the next step and lishing Company, Burlington, VT. properties as the Black Sea and the Medi- produce irrefutable, tangible evidence Frazier, A.H. 1974. Water Current Meters in the Smithsonian Collections of the National Museum terranean Sea connected by such a small by systematic investigation. They were of History and Technology [now the National channel over such a small geographical simply working in a setting that was inac- Museum of American History]. Smithsonian area. As keen an observer as he was, it is cessible to human measurements. They Institution Press, Washington, D.C. Maffi oli, C. 1994. Out of Galileo: The Science of Wa- unlikely that Marsigli would have under- could not go to the Strait of Gibraltar day ters 1628-1718. Erasmus, Amsterdam. stood the causes of two-layer fl ow had in and day out and observe the undercur- Marsigli, L.F. 1681. Osservazioni Intorno al Bosforo he not been able to observe so closely rent and convince themselves that it ex- Tracio overo Canale di Constantinopoli Rappre- sentate in Lettera alla Sacra Real Maestá Cristina the conspicuous current pattern, and to isted. When Edmond Halley satisfactorily Regina di Svezia da Luigi Ferdinando Marsigli. make measurements of density in water explained the surface current in Gibraltar Nicoló Angelo Tinassi, Roma. Translated into from the two communicating basins and as the result of net evaporation in the English by B. Soffi entino and M.E.Q. Pilson. Marsigli, L.F. 1725. Histoire Physique de la Mer. from the oppositely fl owing layers. The Mediterranean (Deacon, 1997), he and Amsterdam, NL. causal association of the currents to the other British scientists had no urgency to Miller, A.R., P. Tchernia, and H. Charnock. 1970. different density of the water was pos- search for a new model, in the absence of Mediterranean Sea Atlas of Temperature, Salin- ity and Oxygen From Cruises of R.V. Atlantis and sible because the Bosporus represented pressing evidence for an undercurrent. R.V. Chain. Woods Hole Oceanographic Institu- a natural analog to the experimental set- In conclusion, Marsigli owes his place tion, Woods Hole, MA USA. tings that Marsigli had seen. It was close in oceanography to two equally impor- Özsoy, E., Ü. Ünlüata, and Z. Top. 1993. The evo- lution of Mediterranean water in the Black enough in spatial scale and properties to tant elements: his conceptual prepara- Sea: interior mixing and material transport by allow the conceptual extrapolation. tion toward explaining what he would double diffusive intrusions. Progress in Ocean- Imagining Marsigli carrying out his eventually see, and the unique and eas- ography 31:275-320. Ritchie, G.S. 1969. The Black Sea Coast Pilot, Com- observations in the Strait of Gibraltar ily accessible features of the Bosporus prising the Dardanelles, Marmara Denizi, the makes this apparent. He could not have region. Upon close inspection, there is Bosporus, Black Sea, and Sea of Azov. 11th ed. anchored a boat in the middle of the a third ingredient in Marsigli’s success Royal Hydrographer of the Navy, London, UK. channel, and he could not have unmis- story that cannot be disregarded: a lucky Schott, G.D. 1992. The extent of man from Vitru- vius to Marfan. Lancet 340(8834-8835):1518- takably detected a change in the fl ow di- twist of fate. Who knows if he would 1520. rection with depth. Water samples clear- have had a place in the history of science, Stoye, J. 1994. Marsigli’s Europe, 1680-1730. The ly coming from the bottom layer would had the departure of the Venetian am- Life and Times of Luigi Ferdinando Marsigli, Soldier and Virtuoso. Yale University Press, New have been very diffi cult to retrieve. bassador not been delayed by personal Haven, CT. Furthermore, to realize that Mediterra- reasons, allowing young Luigi Ferdinan- Zupko, R.E. 1981. Italian Weights and Measures nean and Atlantic waters have different do Marsigli to get back from his travels from the Middle Ages to the Nineteenth Cen- tury. Memoirs of the American Philosophical densities, samples of the Mediterranean and join the party leaving for Istanbul Society (Memoir 145). American Philosophical would have had to be collected hundreds (Stoye, 1994)? Society, Philadelphia, PA.

Oceanography Vol.18, No.2, June 2005 23