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attempted to deduce this quantity from the theory of 
gravity, it fhould feem that the earth performs its 
annual revolution round the fun at a greater 
diftance than is generally imagined : fince Mr. Pro- 
feffor Stewart has determined the fun’s parallax to 
be only 6"', 9, and Mr. Mayer, the late celebrated 
Profelfor at Gottingen, who hath brought the lunar 
tables to a degree of perfedion almoft unexpeded, 
is of opinion that it cannot exceed 8" 
In this uncertainty, the aftronomers of the prefent 
age are peculiarly fortunate in being able fo foon to 
have recourfe to another tranfit of Venus in 1769, 
when, on account of that planet’s north latitude, a 
difference in the total duration may conveniently be 
obferved, greater than could poffibly be obtained, or 
was even expeded by Dr. Halley, from the laft 
tranfit. 
The experience which we gained in the year 
1761, the knowledge of the errors, from whatever 
caufe they may arife, which mufl unavoidably be 
committed in obfervations of this kind, will enable 
us to put in pradice every method of folving this 
problem, and to determine with what degree of ac- 
curacy, and within what limits, the true quantity of 
the fun’s parallax may be obtained, and confequently 
the dimenfions of the whole folar fyflem. 
But, before I proceed to give a computation of the 
clfed of parallax at the feveral places where this 
tranfit ought to be obferved, it will be neceflary to pre- 
mife the principles uponwhich the general calculus was 
* Mr. Machin, ProfelTor of Aftronomy at Grcfliam College, 
deduced the fame quantity many years ago. See a tract entitled. 
The Laws of the. Moon's Motion accoreiing to Gravilyy p. 24. 
formed. 
