[ i66 ] 
than the Sun’s mean diftance, when her elongation is 
2 5 0 in the inferior part of her orbit, it is plain, that 
the necefiary obfervations may eafily be made, when 
her menftrual parallax will be at a medium, three 
times greater than the Sun’s; and confequently amount- 
ing for the whole difference to 42". 
To avoid embarraffment in defeription, I have hi- 
therto fuppofed, that all the obfervations are made in 
the meridian; in which cafe the right afeenfions will 
be the fame as they would appear from the center of 
the Earth ; and confequently, the planet’s longitudes 
thence deduced, nearly the fame : but Tis eafy to fee, 
that if the quartile obfervations are made when the 
planets are confiderably to the eaft or weft of the me- 
ridian, and fo chofen, that the place of the obferver 
be further diftant from the common center of gravity, 
than the center of the Earth is from that center, that 
the bale of the obfervations will be confiderably en- 
larged. Thus, in our latitude, fuppoling that the 
quartile obfervations are made four hours before and 
four hours after the planet paftes the meridian, this 
will produce an enlargement of the bafts by one of the 
Earth’s femidiameters : and as the whole bafe or dia- 
meter of the epicycle comes out, according to Mr. 
Malkelyne, no more than 1.6 of the Earth’s femi- 
diamers; the bafe will, according to this method, come 
out 2.6 ; and confequently, at the next oppofttion, 
the menftrual parallax of Mars will be thereby en- 
larged to 50", the greateft to 62'' 4» and that of 
Venus at a mean, to 74" 4. 
It muft however be acknowledged, that no kind 
of obfervations of the places of the planets are of equal 
validity with thofe taken with the beft inftruments in 
the 
