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LV. A Dtfcourfe on the Parallax of the 
Sun. . By the Rev. Thomas Hornfby, M. A. 
Savilian Profeffor of AJlronomy in the Uni- 
verfty of Oxford, and F. R. S. 
— — - ***Yv/ A vuiiw i-'vjiii ivJ LliC 
theory and P rac ^ ica ^ P art °f aftronomy, that every 
method of determining it hath been employed by the 
aftronomers of every age. Mr. Flamfleed informs 
us, in the 9 2d and 96th numbers of the Philofophi- 
cal Tranfadhons, that from fome obfervations made 
upon the planet Mars, he had found the Sun’s paral- 
lax not to exceed 10 feconds ; and Dr. Halley, in a 
memoir written exprefsly with a view to afcertain the 
exadt quantity of it, , fuppofes it not to be greater than . 
12 a"? 
When we confider the imperfedt date of'aftrono- 
my at the Ume when Mr. Horrox lived, we cannot 
iumciently admire the wonderful genius of that youn? 
gentleman, who at the age of 24 could colledt from 
his own obfervations, that the parallax of the Sun 
did not exceed 14 feconds; while many celebrated 
aftronomers, whole tables were then in the greateft 
repute, had afhgned a parallax of more than two 
minutes to the Sun, which Kepler had fuppofed could 
not be lefs than 59 feconds, and which Hevelius, who ’ 
pubhfhed the admirable treatife of Mr. Horrox, in— 
titled, Venus in Sole vifa, fixed at 4 1 feconds. 
HE quantity of the Sun’s parallax 
is of fuch importance both to the 
