472 



Additional Notes, 



the central point. At the end B, of the piece A B, another 

 speculum of glass is to be in the same manner erected, wliicli 

 may be somewhat less than the other, with a square or oblong 

 spot in it unsilvered, that a star, by a direct ray, may be seen 

 through it; and the back of this speculum should be guarded 

 ■with a thin brass plate, with an aperture in it equal to the 

 unsilvered part of the glass ; the tilgQ of the aperture toward 

 H to be exactly straight, dividing between the silvered and 

 unsilvered part of the speculum, and standing in the line of 

 the axis of the telescope. This speculum is to be set at an 

 angle of about 20 deg. with the square of the piece A B, or 

 at 1 1 deg. with the sides of it. Upon the piece A B, the 

 telescope P Q is fixed, of a good aperture and field, with 

 the axis placed as above. ^,.. 

 The limb is to be graduated 



by diagonals, or parallel cir- f i }\ 



cles, to half degrees and half 

 minutes, beginning from C, 

 "which are to be numbered 

 as whole ones. And if it 

 be practicable to face wood 

 -with brass without warping, 

 the whole face should be so Qs^^ _ 

 covered ; if not, then along p 

 the outward edge of the 

 limb a narrow strip of brass 

 plate may be let into the 

 face of it, finely and equally 

 indented on the edge, to 

 take a screw fitted to that 

 toothing to be fixed on the 

 moving index at L, as your 

 instruments are made that 

 count by revolutions; and 

 then, before this is used, it 

 would be proper to take the 

 distance of the two objects 

 first nearly by a fore-staff, 

 andfi-om thence accordingly 

 to set the index. This screw, 

 at land, would be highly use-.r^ 

 ful, but at sea it cannot be "^ 

 wrought, while the instru- 

 ment is directed by the same 



H 



