Halley. Progress of Navigation. Cross-Staff. Finding Longitude. Variation of the Compass.
The last expedition of the 17th century was purely scientific. In 1699 Edmund Halley, the astronomer-royal, in command of the "Paramour Pink," undertook a voyage to improve the knowledge of longitude, and of the variation of the compass. The result of his voyage were the construction of a variation chart, and proposals for finding the longitude by occultations of fixed stars.
During the 17th century very considerable progress was made in the art of navigation, and in systematizing and delineating the vast mass of material that was accumulated by the ceaseless activity of explorers. The Dutch took the lead as map-makers. Mercator invented the useful projection which bears his name; and Ortelius, Hondius, and Hulsius compiled a series of valuable maps.
In finding the latitude at sea, the astrolabe very generally gave place to the cross-staff, because the graduation of the latter was larger and more easily read off. The cross-staff was a very simple instrument, consisting of a graduated pole with cross pieces, called transversaries )of which there were four used according to the altitude), also graduated, which were fitted to work on it. The bearings of the sun were taken by compass, to ascertain when it was near the meridian; then the end of the long staff was placed close to the observers eye, and the transversary moved until one end exactly touched the horizon, and the other the suns centre. This was continued until the sun dipped, when the meridian altitude was obtained. The back-staff was an improvement on the cross-staff, invested by the great Arctic navigator John Davis. It was fitted with a reflector, and it was thus the first rough idea of the principle of the quadrant and sextant. The cross-staff was used for low altitude, because both ends of the transversary could easily be seen at the same time, and the astrolabe for high altitudes.
With the invention of these instruments came instructions for their use , and for working out observations. In England the first of these was The Old Rutter of the Sea, printed in 1490. Then followed the Seamans Secrets of John Davis, and A Regiment of the Sea, containing very necessary Matters, with a perfect Sea Card, by Thomas Hood, published in 1596. Hood also sold compasses constructed on Mr Normans principle, near the Minories. These manuals contained definitions, treatises on the use of the sea card and compass, tables of declination and rules for applying it, rules for dead reckonings and longitude, and instructions in the use of instruments.
Latitude was obtained by observation, but longitude had usually to be reckoned on the chart form the meridian of Grand Canary, which in those days was used by all civilized countries. The differences of time between the eclipse of the moon at the place of the observer and the place for which it was calculated in the ephemeredes for that day was another method in use of finding the difference of longitude. Mariners were also provided with tables giving the number of miles in a degree of longitude for every degree of latitude. Much attention was bestowed upon the phenomena of the variation and dip of the magnetic needle. Robert Norman, the hydrographer, discovered the dip or inclination of the needle in 1576, and in 1581 be observed the variation of the compass at London, and found it to be 11º15´ E. In the same year his Discourse of the Magnet or Loadstone was published by Ballard. In 1580 Mr Borough, comptroller of the navy, found the variation of the compass at Limehouse to be 11º19´ E. It may be observed here that in 1657 there was no variation at London, and that it moved westerly until 1815, when it was 24º27´ W. It is now returning eastwards.
By means of these rough instruments and calculations our Elizabethan navigators and their contemporaries succeeded in delineating the vast regions that were discovered. Thus the sum of human knowledge was augmented, while mens minds were enlarged, and the wealth and prosperity of nations were increased, through the provision of safe guides by which lands and seas could be traversed, and distant countries visited.
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