of the Angle fubtended by Two Obje£fs t See. 3^9 
The rays GB, BC, and the fpeculum B, being fixed in refpc£t 
of each other, and of the plane GAP, the plane CBG will 
alfo be given in pofition ; that is, its inclination to the plane of 
motion, or to any other fixed plane, will conftantly be the 
fame : whereas the inclination of the plane BCT to the plane 
of motion, or other fixed plane, will be continually changing 
while the reflector C revolves with the radius CP. The pofition 
of the plane GBC conftitutes the fourth and laft of the data* 
and it will be immaterial to what fixed plane it is referred. In 
the enfuing folutiort the fituation of this plane will be defined 
by its inclination to the fixed fecondary of the plane of motion 
which paffes through the point O. 
5. The enumeration of thefe data leads to the conftruo 
tlon of the problem, a few obfervations being previoufly 
inferted to prevent repetitions and unneceffary references. iff:, 
The obje&s obferved are underftood to be lucid or illumined 
points, and fo diftant, that the rays which flow from either of 
them may be efteemed parallel without error as far regards 
thefe obfervations : fuch objects are the fixed ftars, any given 
points in the dhks of the fun or planets, &c. 2dly, As in mea- 
furing the angular pofitions of objects which lie in the fame 
plane, thefe objects are referred to the circumference of a circle, 
the centre of which is coincident with the fpeftator’s eye ; fo 
in eftimating the pofitions of objects which lie in different planes, 
and of the inclinations of thefe planes to each other, the ob- 
je£ts, &c* are referred to the circumference of a fphere, of 
which the centre coincides with the centre of the fpe&ator’s 
view: applying this to the prefent cafe, fince the lines 
CT, CB, SG (fig. 1 .) are lituated in different planes ; in order 
to eftimate their pofitions, any point may be afl'umed as 
the centre of a fphere, and through that point lines are to be 
drawn parallel to the given lines CT, CB, SG, the points in 
G g g 2 which 
