NAVIGATION CURTISS. 129 



Now there be some that are very inquisitive to have a way to get the 

 longitude, but that is too tedious for seamen, since it requireth the deep 

 knowledge of astronomy, wherefore I would not have any man think that the 

 longitude is to be found at sea by any instrument ; so let no seamen trouble 

 themselves with any such rule, but (according to their accustomed manner) 

 let them keep a perfect account and reckoning of the way of their ship. 



Such a record of the way of a ship appears to have been made 

 with chalk on a wooden board called a log board which folded like 

 a book and from which each day a position for the ship was deduced. 



But while the longitude problem at sea remained unsolved, con- 

 tributions to progress in navigation were being made in other direc- 

 tions. Mercator and Wright developed a correct sailing chart about 

 1600. Gunter's tables in 1620 made possible the application of 

 logarithms to navigation. In 1631 a device, called the vernier, for 

 accurate reading of scales became known. In 1635 Gellibrand pub- 

 lished his discovery of the annual change in the variation of the com- 

 pass needle. In 1637 Norwood helped remove one of the greatest 

 stumbling blocks in the way of correct navigation by determining 

 improved values of the length of a minute of arc on the earth's 

 surface, or the true nautical mile. His value was about six-tenths 

 of 1 per cent too large. In 1699 Halley constructed the first com- 

 pass variation chart. 



In the meantime some progress was being made with the longitude 

 problem. It was recognized that the only accurate method of 

 determining the longitude is by knowing the difference at the same 

 instant between the time at the meridian of Greenwich and that of 

 the observer. The determination of the local time for the observer 

 by astronomical observations of the altitude of suitably situated 

 heavenly bodies was an old, well-known and frequently practiced 

 operation. But the simultaneous determination of Greenwich time 

 presented great difficulties. Obviously if the ship were near enough 

 to a station on the Greenwich meridian a rocket or a loud explosion 

 could be used as a signal at some stated Greenwich time. The ship's 

 time for the same instant could then be observed and the difference 

 between these two times for the same instant would be the longitude 

 of the ship east or west of Greenwich. But for ships out of signal 

 range from Greenwich observations of celestial phenomena had to 

 be employed. At present chronometers are carried on board ship 

 which, after being corrected and rated at departure, keep accurate 

 Greenwich time throughout the voyage and thus render longitude 

 determination relatively easy. But chronometers of satisfactory 

 accuracy were not available till late in the eighteenth century. 



The best method known for determining Greenwich time at sea 

 by observation before the chronometer became available was that 

 depending upon the measurement of the distance of the moon from 



