Deák Antal András: A Duna fölfedezése
Tartalom - III.A DANUBIUS PANNONICO-MYSICUS, 1726
T II E DISCOVERY OF THE DANUBE shadows were entered, which largely depended on the alertness of the observer, and if parallel observations were made at several points. The error limits of the method were relatively large. Only a series of careful measurements could reduce the number of errors to an acceptable level. It was this aspect and also the complexity of the process that led to the abandonment of the method. 273 Perhaps this was the reason Marsigli wanted to sink and destroy his measurements. 274 Regrettably, something else also fell through the cracks. Marsigli was the first in the world to order methodical measurements of terrestrial magnetic declination in Hungary. These measurements were necessary because the northern pole of a compass does not ípoint exactly to the astronomical north pole of the Earth but it diverges from it in varying degrees at different points on the earth's surface: towards the west in Europe, Africa and in the Atlantic Ocean. A knowledge of the degree of the declination was essential, especially for sailors. Halley, the English astronomer, produced a map of magnetic declinations valid in the ocean. He connected the points with identical magnetic declinations with an arched line. These lines were first called Halley's lines, later isogones. Lajos Bartha has called my attention to the astonishing fact that Halley's map, which solely depicted marine isogonal lines contains a single terrestrial element, composed of data relevant to Eger-Szolnok-Szeged-Titel. 27 5 He suggested that Halley may have come across them in the 1700 edition of Marsigli's Prodromus , in which Marsigli published the Hungarian magnetic measurement data. However, the question of how Marsigli procured the measurement data was not answered. The answer can be found again in Müller's register of observations. Between the leaves describing the position of the stationary stars and the results of the lunar observations I found a note occupying half a page. The chart, written in pencil, contains the measurement values for magnetic declination. Accordingly Müller made measurements with four compasses arranged according to the cardinal directions in Vienna, Buda, Baja and at the mouth of the Drava and with two compasses in Szeged, in the military camp at Tital, in Szolnok and in Eger. The date and places of the measurements were identical to the relevant data in his astronomical positions. 27 6 Hydrographical section The hydrographic section starts with the first thematic hydrographic map of Hungary the Mappa Potamographica, which depicts the hydrological system of the Danube from Vienna to the Black Sea with many details and river names. This map provides an overall view of nearly every important river and stream flowing into the Danube, and lakes and swamps in the territory of the Hungarian Monarchy, Marsigli wrote in the legend on the map. In the upper left-hand corner of the map he thought it important to call attention to the fact that the places where he had measured the width and depth of the rivers were indicated by small lines crossing the Danube, the Tisza, the Sava and the Maros. These lines, however, cannot be seen on the map. Probably the copperplate engraver forgot them. Nevertheless, the places where the measurements were made were listed separately in a chart in volume VI, so, in the end, the points of measurements can be identified. The four straight lines spanning the chain of mountains rising above the Carpathian Basin are interesting elements on the sheet. The author intended to show the altitude of the beds of some larger rivers compared to the Danube. On a separate page, he drew the cross-section of the Carpathian Basin along these straight lines, so he could illustrate the 27 3 Bartha Lajos: Egy elfelejtett holdtérkép (A forgotten lunar map). Meteor Bd. 23. No. 4. 1996. 27 4 DBM Vol. I. Pars II. Introduction. 27 5 Bartha Lajos: A mágneses deklináció korai adatai a Kárpát-medencében (Early Data on Magnetic Variation in the Carpathian Basin). Technikatörténeti Szemle, Vol. XVI. 1986-87. Bp. 1988. pp. 97, 101, 103. 27 6 BUB Mss di Marsigli Vol. 100. Pars I. p. 12. 143
