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the central annular ledge, (the former to be of a size a little 

 less than requisite in the. plate when finished,) are already 

 made in their proper places. The plates cast from this model 

 will require no measurement nor piercing, and only want to 

 be filed, turned and polished. 



The mirror C is to be adjusted by making it, by a fore ob- 

 servation, parallel to the back horizon glass, by turning with 

 the left hand the plate A by its milled edge, till the images 

 of the object viewed are seen to coincide ; and then fastening 

 the plate A to the index by the screws s s s 5 : it follows, that 

 the back horizon glass must itself be for this purpose previ- 

 ously adjusted at right angles to the index-glass, in the way 

 before mentioned, Avhich may always on land be easily per- 

 formed. And I should imagine that even atsea, when the sea 

 is calm, it might perhaps be practicable, by fixing up two 

 papers, with a black line drawn vertically on each, on the adja- 

 cent sides of two masts of the ship ; by which lines, the qua- 

 drant fixed on two points of a moveable stand, placed on deck 

 between the two masts, may have the back horizon glass ad- 

 justed as above, in order by this means to try at any time whe- 

 ther the little mirror C has its adjustment altered i which, 

 however, must be very unlikely to happen, especially if the 

 contiguous sides of the plate A and of the index be not po- 

 lished, lest the plate should slide on the index. At other 

 times the back horizon glass is to be adjusted by the mirror C, 

 supposed to be itself right in position ; and indeed considering 

 that this mirror is not, as the horizon glasses are, rested on the 



