64 Instrument for finding the true Lunar Distance. 
is moveable along AE, around the centre O; it is the line of inter- 
section of the plane of BC with that of AE. ‘The inner curvature 
of BC, moves in the plane of AE, and along its middle curvature 
PQ, as the vernier c, travels along AE, and the plane of the are 
BC, always passes through the common centre O. 
With the observed altitudes and distance of the two bodies, taken 
in the usual manner, for finding longitude from a lunar observation, 
the true distance is determined, by first marking off on the two ares 
