( 35 * ) 
muft neceffarily fink and raife the vifible Horizon by 
Turns near one Minute. The Computations of the 
Sun’s Altitudes are all made for the Latitude of 
yT 28', whereas a good Part of them were taken 
under Sail, and upon different Tacks, the Veffel fome- 
times (landing North Eaft or North, and at other 
times South Eaft, for near a quarter of an Hour 
at a time. 
Several of thefe Circumftances may probably have 
contributed to increafe the Inconfiftency of the Ob- 
servations ; but as no particular Notice was taken of 
them at the Time, I content my felf with barely 
mentioning them. 
POSTSCRIPT. 
The Principle on which the Contrivance of this In- 
ftrument depends, was laid down in the before-men- 
tioned Philof. T-ranf. Numb. 420. in one Propofition, 
and feveral Corollaries , the fifth of which contains 
the Grounds of an Approximation for corre&ing 
fome fin all Errors which will arife if the Plane of 
the Inftrument be fuffered to vary too much from 
the great Circle palling through the two Obje&s, 
when the Obfervation is taken. There appears rea- 
fon to think, that there will be very little Occafion in 
Practice for that Correction but it was necefiary to 
mention it, in order to explain the Nature of the In- 
ftrument ^ and as the manner of deducing that Co- 
rollary from the Propofition may not appear obvious 
to every Reader, I have here annexed the Demonftra- 
tion of it. 
Let 
