[ i 6 + ] 
It is equally evident, that when the Moon is in 
quartile with Mars, and moving towards a conjunction, 
an obferver, at the Earth’s center, will fee Mars 
more backward in the ecliptic, than if feen from 
the common center of gravity, by 15" i. j and that, 
when the Moon is in her oppofite quartile with 
Mars, and moving from her conjunction, that then 
an obferver at the Earth’s center, will fee Mars 
advanced in his orbit more forward by 15" 4., than if 
feen from the common center of gravity j and the 
one obfcrvation checqued with the other, will, accord- 
ing to a mean elliptic motion, differ by the quantity 
of 31" j and fuch will be the fecond and fourth 
obfervations above propounded. 
Now, from the firft, third, and fifth, obfervations, 
three points of Mars’s orbit will be given j which, by the 
help of the theory of Mars’s motion in an elliptic orbit, 
whofe aphelion, eccentricity, and nodes, are known 
fufficiently near for this purpofe ; the intermediate 
places of Mars may be inferred with the requilite 
degree of accuracy : and particularly, as the two 
intermediate obfervations, viz. the fecond and fourth, 
will be nearly at equal intervals of time between the 
three others : from hence it follows, that the difference 
between the inferred, or computed places, at the 
quartiles, and the obferved places at thofe times, will 
be the menftrual parallax required. 
It is to be noted, that the times above fpecified 
are the moft favourable for the obfervation ; and could 
thofe be made uninterruptedly from weather, there 
would be the lefs occafion for any other: but, as much 
as poffible to prevent difappointments of this kind, it 
will be right to begin the obfervations, a month preced- 
