4 POPULAR LECTURES AND ADDRESSES. 



For the third. A Massey's log (or instead of 

 Massey's log, the old log-ship and glasses), the 

 ordinary mariner's compass, a traverse table, 

 mathematical drawing instruments, and a common 

 clock or watch. 



For the fourth. The lead [with improvements 

 described in 37 below], the instruments used for 

 the third mode, arid a chart. 



I shall first briefly describe the instruments, 

 beginning with the sextant. 



4. THE SEXTANT. The sextant is an admirably 

 devised instrument, invented by Sir Isaac Newton 

 (and first made by Hadley), for measuring, on 

 board ship, the angle between any two visible 

 objects. The general principle of the instrument 

 is this : One object, A, is looked at directly, the 

 other, B, by two reflections first, from a silvered 

 mirror, and next from a piece of unsilvered plate- 

 glass, in the manner illustrated in the drawing 

 before you. The second of these mirrors is fixed 

 on the framework. The first mirror, which is 

 movable round an axis, is turned by the observer 

 until the doubly reflected image of B is seen, like 



