on the Planet Mars: 251 
the apparent brightnefs and magnitude of that {pot, that they 
may be compared together. (See fig. 28.) The agreement of 
the meafures, and the phenomena attending the motion of the 
f{pot, are fufficient to point out the meridian of the circle; for 
which, from a due confideration of thefe circumftances, I have 
fixed on the place where the fpot was O&. 10. 6h. 46’. 
Of the angles colleéted in fig. 28. we find 65° o’ the largeft, 
and-49° 45’ the {malleft; but, on account of the different 
fituation of the earth and Mars, the angle meafured 7’ lefs 
O&. 16. than it would have done had the planets remained in 
the places they were in O&. 5. when the other meafure was 
taken. This being added, we have 65° 7’. The difference be- 
tween the two pofitions is 15° 22’. Now, the conftruction of 
fig. 28. being admitted, we fee that the angles were nearly 
taken at the oppofite extremes of the circle in which the fpot 
moved. However, by the 5th column of Tab. III. O&. 5. we 
have the fituation of the fpot in the circle with’ refpe&t to the 
meridian 281° 44’, and Oct. 16. 114° 6’: therefore the fouth 
polar diftance of the center of the {pot is found, by taking half 
the fum of the fines of thefe angles to radius, as 7° 41’ (half 
of 15° 22’) toa fourth number, which 1s 8° 8’; and the lati-. 
tude of the circle, in which the fpot moved about the pole, 
therefore is 81° 52’ fouth. ‘This being determined, we have 
the following correction for the angles of pofition: radius is 
to fine of the angular diftance of the fpot from the meridian 
as $° 8’ to the required quantity. This muft be added or fub- 
tracted, according as the cafe requires; and thereby we fhall 
have the pofition of the true pole from any one of the 
meatures. . : 
Tfhall now apply the above to determine the fituation of the 
exis of Mars. To this end, we fee that, in the firtt place, the 
Liz moeafures 
