ASTRONOMY OF EIGHTEENTH CENTURY. 255 



distances from the Nautical Almanack/' says Sir John Herschel, " is 

 worth all the eclipses that have ever happened for inspiring confidence 

 in the conclusions of science. That a man by merely measuring the 

 moon's apparent distance from a star with a little portable instrument 

 held in his hand and applied to his eye, even with so unstable a footing 

 as the deck of a ship, shall say positively, within five rniles, where he 

 is on a boundless ocean, cannot but appear to persons ignorant of 

 physical astronomy an approach to the miraculous. Yet the alterna- 

 tive of wealth and ruin, of life and death, are constantly staked upon 

 the accuracy of these computations, which might almost seem to have 

 been devised on purpose to show how closely the extremes of specu- 

 lative refinement and practical utility can be brought to approximate." 

 He then illustrates these remarks by some particulars relating to a 

 voyage made by Captain Basil Hall, R.N. This officer had sailed from 

 San Bias, on the west coast of Mexico, and, after a voyage of 8,000 

 miles, occupying eighty-nine days, arrived off Rio de Janeiro, having in 

 this interval passed through the Pacific Ocean, rounded Cape Horn, 

 and crossed the South Atlantic without making any land, or even seeing 

 a single ship, except an American whaler off Cape Horn. Arrived 

 within a week's sail of Rio, he set about determining by lunar obser- 

 vations the precise line of the ship's course and its situation at the 

 moment ; and having done this, he continued his voyage by the ready 

 methods which are employed in short trips, but which cannot be trusted 

 in long voyages, when the moon is the only sure guide. Captain Hall 

 thus relates the rest : " We steered towards Rio de Janeiro for some 

 days after taking the lunars, and having arrived within fifteen or twenty 

 miles of the coast, I hove-to at four in the morning till the day should 

 break, and then bore up ; for,, although it was very hazy, we could see 

 before us a couple of miles or so. About eight o'clock it became so 

 foggy that I did not like to stand in farther, and was just bringing the 

 ship to the wind again, before sending the people to breakfast, when 

 it suddenly cleared off, and I had the satisfaction of seeing the great 

 Sugar-Loaf Rock, which stands on one side of the harbour's mouth, 

 so nearly right ahead that we had not to alter our course above a point 

 in order to hit the entrance of Rio. This was the first land we had 

 seen for three months, after crossing so many seas, and being set back- 

 wards and forwards by innumerable currents and foul winds." 



The method of determining the longitude by the transport of time 

 is one which depends entirely upon the excellence of the timekeepers 

 used. Huyghens had no sooner applied the pendulum to clocks than 

 he endeavoured to make these useful to navigators. The difficulties 

 presented by the violent and irregular movements of ships presented 

 insurmountable difficulties ; but his discovery of the isochronism of a 

 steel spring applied to a balance-wheel was, as already mentioned, the 

 first step towards the chronometer. 



In the year 1714 the British Parliament passed an Act offering a 



