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Read the following passage on Mapping of Australia by Flinders. You have approximately 20 minutes to read and answer questions. At the end of the passage, questions are displayed.
Flinders’s mapping of the coast of the Australian continent during the voyages of 1801–03 comprises a ‘running survey’: a series of observations that an assiduous commander would undertake when traversing unknown or little frequented waters in order to fix the position of new lands or dangers he encountered. This was a discipline in which his old captain, William Bligh, was a master. In turn, Bligh had carried out such surveys under the eagle eye of Captain James Cook.2 The accuracy of a running survey depended on the precision with which the track of the ship could be laid down by dead reckoning between positions established by astronomical observations.
The latitude of the ship was deduced from measurements of the altitude of heavenly bodies as they crossed the observer’s meridian. He states that, whenever circumstances and the altitude of the sun (70° or greater) permitted, he would observe a double measurement, observing the altitude of the lower limb and then turning to measure the supplement of the altitude of the upper limb from the opposite horizon. He describes how;
When the sun was so nearly vertical that the rapidity of his motion over the meridian would not allow time for taking two observations, my general custom was then to take one side whilst lieutenant Flinders took the other, and the mean result of both was considered to be the true latitude.3 Some of the larger angles recorded by Flinders took the observer to the limits of the graduated scale of the sextant, and he remarks in a footnote that the reflecting circles employed by the French were better suited for this measurement.
As Flinders’s work would show in areas which had been traversed by Cook during the latter’s first voyage, the precision of deductions of longitude had been vastly improved with the availability of chronometers, provided that a careful check was maintained on their performance, i.e. the rate at which they might be running fast or slow. Flinders carried four chronometers and two pocket watches. The two chronometers manufactured by Arnold (Nos. 82 and 176), which had previously been allocated to Vancouver for his surveys, quickly became defective. Flinders was thus dependent on the two new chronometers made by Earnshaw
To enhance the accuracy of his survey Flinders made regular landings at prominent points on the coast to make prolonged astronomical observations with the more precise instruments that had been supplied for use by John Crosley, the astronomer who had been appointed for the voyage but who had remained at the Cape of Good Hope because of ill health. A Ramsden Universal Theodolite with vernier scales that allowed the horizontal and vertical plates to be read to the nearest minute was used for the observation of altitudes and the more accurate determination of latitude. Three large eight-inch sextants with stands had been specially manufactured for the expedition by the great instrument maker Troughton. Flinders and his brother, to whom he delegated the bulk of the astronomical work, employed these to observe lunar distances. Substantial sets were obtained when time permitted. For example, fifty sets were obtained in Broad Sound, and whilst Samuel Flinders was marooned on Wreck Reef he observed no less than sixty. The sextants were also employed to measure equal altitudes of a heavenly body to derive the local time of the observed body’s ‘meridian passage’ and thus, with observations spaced about seven days apart, to check the rates of the chronometers.
Whilst in Port Lincoln, Flinders employed the large telescope supplied for Crosley and observed an eclipse of the sun, but he considered that the intricate calculations would need subsequent re-computation by an astronomer.
The Memoir of Flinders shows how these ‘absolute’ stations where the precise observations for latitude and longitude were made were the ‘points of departure’ for runs along the coast, with the chronometers being used to fix the longitude of ‘relative’ positions in between.
The chapter gives a detailed account of how, after each run was closed at an absolute station, he calculated and applied proportionally what he terms an ‘approximating correction’ to the relative positions before plotting them on his survey sheets. All these observations that provided the framework of Flinders’s survey were recomputed by Crosley after the voyage.7 As the ship passed along the intervening coast a sequence of compass bearings would be taken to edges of land or conspicuous features such as peaks in an effort to intersect prominent points and to establish the line of the coast in between. The precision of these observations was enhanced in discovery vessels by the issue of azimuth compasses fitted with a sight through which the feature could be observed and its bearing read on a scale, the zero of which had to be aligned with the north point of the compass. Flinders may have supplemented his bearings by measuring the horizontal angle between distinguishable features with a sextant as recommended by Alexander Dalrymple.8 The plotting of the intersection of three bearings, corrected for longitude
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