( ) 
or cylindrical Figure. For this purpofe therefore I 
have found, that in the latter Determination of the 
iirft Propofition, if the Sine of the Angle under CMG 
be denoted by a, the Tangent of the Complement of 
this Angle to a right one be denoted by and the Se- 
cant of this Complement by c ; the Root of this E- 
quation 2;’ — — ^aa){^c — a will exceed 
the Sine of the Angle under F M A, that is the Sine 
of the Angle of Incidence, by the Sine of the Angle 
under CMG; and the Sine of the Angle under FMO, 
which is double the refradled Angle, will be the Root 
of this Equation aa xin ; this Angle 
being acute, when the Tangent of the Angle under 
C M G is lefs than half the Radius^ or when the 
Angle itfelf is lefs than 1.6 degr. 33/. and 
when this Tangent.is more .than half the Radius, the 
Angle'undcr 6 M.F is obtuie. 
■ r 7 
The 
