6 POPULAR LECTURES AND ADDRESSES. 



mirror is turned in the use of the sextant are 

 measured by very fine divisions on an arc forming 

 a sixth of the circumference of a circle, whence 

 the instrument derives its name of sextant. 



5. Imagine now that I am standing on the 

 ship and looking directly at the horizon. What 

 do you mean by the horizon at sea ? It is the 

 bounding line of sea and sky. It is a real 

 line on the sea. When you look from the deck 

 of a ship at the sea, you are looking down. 

 Look as far as you can along the sea, and you 

 are still looking somewhat downwards. The angle 

 at which you must look down from the true level 

 to see the line of the horizon is called the dip 

 of the horizon. Looking then at the horizon, and 

 turning the silvered mirror about with my left 

 hand, I bring the ghostly image of the sun down 

 till its lower edge touches the sea-horizon. I then 

 look at the divided circle of the instrument, read 

 off the number, and I have the angle through 

 which I have had to turn the silvered mirror to 

 bring down the image of the sun from the direction 

 in which I had to look to see the sun directly, to 



