[ r SS ] 
with the San ; the fame may alfo be determined 
with advantage from the fum of the menftrnal paral- 
laxes of thefe two planets, when they happen to be 
in the required politions at the fame time, which will 
indeed happen but feldom ; or even from the fum of 
the menftrual parallaxes of Mars and the Sun, which 
may be obferved together at every oppolition of Mars 
to the Sun ; the fum of the menftrual parallaxes of 
Mars and Venus in thefe circumftances, according to 
the numbers ufed by Mr. Smeaton, will fometimes 
amount to 87", and the fum of thofe of Mars and 
the Sun to 52".. 
Nevil Mafkelyne, 
1 
X 2 
XXIV. A 
