C 162 3 
From what has been faid. I fuppofe it will appear, 
that the effe&s of the menftrual parallax are worthy 
of confideration ; and that nothing has been yet 
executed, whereby it has received a determination 
fufficiently accurate j for, in regard to obfervations 
upon the Sun, the whole quantity is too fmall to be 
minutely obferved in right afcenfion : and with re- 
fpedt to the application to Mars and Venus, as 
fuggefted by Dr. Gregory, I do not know that any 
thing has been done ; and indeed no wonder, as the 
theory of the motion of Mars and Venus has not been 
as yet fo critically reduced to computation, as to 
render their parallaxes (though in themfelves much 
greater) deducible with equal certainty as that of the 
Sun. 
What I therefore have now to propofe, is a method 
of obferving the menftrual parallaxes of Mars and 
Venus, without laying any undue ftrefs upon the 
theory of their motions. 
The firft opportunity of making an obfervation for 
this purpofe, will be at the next oppofition of Mars -, 
which, according to the Nautical Almanack, will 
happen the 26th of O&ober next, in the morning 5. 
I will therefore endeavour to illuftrate this matter by 
taking that as an example. 
The diftance of Mars from the Earth will then be 
fomewhat lefs than the mean diftance, that is, as 
1 to 2.2 ; and. confequently his double menftrual 
parallax, according to Mr. Mafkelyne, will be near 
31^ in the point of oppofition. Now, as the Moon 
of Venus, a negled of the menftrual parallax may make it T *-_ 
part of the whole. 
Will 
