C »5* ] 
the Telefcope, in one and the fame Place, both by 
the dired Rays and by the reflex’d ones j but if the 
Index be turned, the Star (hall appear in two Places, 
whofc Diftance is {hewed, on the Brafs Limb, by the 
Index. 
By this Inftrument, the Diftance of the Moon from 
any Fixt Star is thus obferved : View the Star through 
the Perfpicil by the dired Light, and the Moon by 
the Reflcxt (or on the contrary) ; and turn the Index 
till the Star touch the Limb of the Moon, and the 
Index (hall fhew upon the Brafs Limb of the Inftru- 
ment, the Diftance of the Star from the Limb of the 
Moon } and though the Inftrument fhake, by the Mo- 
tion of your Ship at Sea, yet the Moon and Star will 
move together, as if they did really touch one another 
in the Heavens j fo that an Obfervation may be made 
as exadly at Sea as at Land. 
And by the fame Inftrument, may be obferved, 
exadly, the Altitudes of the Moon and Stars, by 
bringing them to the Horizon ; and thereby the Lati- 
tude, and Times of Obfervations, may be determined 
more exadly than by the Ways now in Ufe. 
In the Time of the Obfervation, if the Inftrument 
move angularly about the Axis of the Telefcope, the 
Star will move in a Tangent of the Moon’s Limb, or 
of the Horizon ; but the Obfervation may notwith- 
ftanding be made exadly, by noting when the Line, 
deferibed by the Star, is a Tangent to the Moon’s 
Limb, or to the Horizon. 
To make the Inftrument ufeful, the Telefcope ought 
to take in a large Angle : And to make the Obferva- 
tion true, let the Star touch the Moon’s Limb, not on 
the Outfide of the Limb, but on the Infide. 
II. The 
