Luigi Ferdinando Marsigli is a name that kept cropping up reading around cartography. I kept sidelining him as biographers seemed to place his interests mostly elsewhere. Born in 1658 to a wealthy and noble family, he studied – among other things – maths, anatomy and natural history. After a colourful back story he washes up in the South of France around 1706, principally studying marine species of flora (erroneously ‘including’ coral), it wasn’t until I found a research paper by a Hungarian that I understood the importance of Marsigli to the invention of the isobath.
Gercsák is a man I trust, unlike many historians of cartography he spent time actually looking at the map he’s writing about and – better still – takes to task other writers for repeating mistakes by authors who have not looked at the actual map they are referring to:
“Several authors speak of the map as the first printed one that represents the depth of the sea by using isobaths or do not make it clear in their wording if there is only one or more isobaths in the map (e.g., Konvitz 1987, Klinghammer and Papp-V´ary 1983, M´arton 1985, Wallis and Robinson 1987, Klinghammer et al. 1995, Imhof 2007”.
There is only one depth contour drawn on his chart which marks the edge of an underwater abyss in the Gulf de Lion. He wished to demonstrate how geographical features found on land could also appear underwater and ‘connect’ to each other. To this end, at the eastern edge, he has hachured (and the printers have coloured in) this line thus emphasizing the steep nature of the drop in a manner that would be used for geographical features found on land (and confuse some into assuming they see two lines!)Marsigli’s original map is missing, but extant are copies of it printed in Amsterdam nearly two decades later as part of his excellent Histoire physique de la mer.
The last section of Gercsák’s paper demonstrates the discrepancies between Marsigli’s isobath and the actual position of the abyss on a modern map – Marsigli’s line doesn’t come off well. Gercsak gives a number of reasons why the line could be so wrong but to me this is immaterial. He knew there was an underwater chasm and demonstrated this; he showed it on a map so others could see that natural geographic features found on land were ‘echoed’ under the sea, a line that is not there (and is wrong in any case), used to ‘depict’ a feature he could not see, but clearly demonstrating an advancement in knowledge of the structure of our world.