98 POPULAR LECTURES AND ADDRESSES. 



was from three to four minutes " sun behind time," 

 you will understand exactly how each line lies 

 as you see it on the chart, being, as I have told you 

 before, always in a direction perpendicular to the 

 line from the ship, to the point on the horizon 

 under the sun. 



Look again at the lines determined by altitudes 

 of Polaris and Arcturus, observed on the night 

 before reaching Cape Finisterre. You see how 

 they intersect exactly in the place where the 

 vessel was at the time, and can understand how 

 important the full information thus given was in 

 the case of approaching land. Porto Santo was 

 sighted at noon on the pth of May, and Madeira 

 two hours later. No more astronomical obser- 

 vations were needed. 



56. LUNARS. I have spoken to you of the 

 marvellous accuracy of the marine chronometer, but 

 till Harrison's invention of the first useful artificial 

 marine chronometer, fulfilling Sir Isaac Newton's 

 anticipation, was given to the world, in 1765, 

 through the well-judged beneficence of the British 

 Government, the only chronometer generally avail- 



