184 REMARKS on the 



to fome communication between the authors of them, that com- 

 munication is more likely to have gone from India to Greece, 

 than in the oppofite diredlion. It may perhaps be thought to 

 favour this laft opinion, that Ptolemy has no where demon- 

 ftrated the neceflity of afligning a double eccentricity to the 

 orbits of the planets, and has left room to fufpe(fl, that autho- 

 rity, more than argument, has influenced this part of his. 

 fyftem. 



58. In the tables of the planets, we remarked another equation, 

 {fchigram) anfwering to the parallax of the earth's orbit, or the 

 difference between the heliocentric and the geocentric place of 

 the planet. This parallax, if we conceive a triangle to be 

 formed by lines drawn from the fun to the earth and to the 

 planet, and alfo from the planet to the earth, is the angle of 

 that triangle, fubtended by the line drawn from the fun to the 

 earth. And fo, accordingly, it is computed in thefe tables ; for 

 if we refolve fuck a triangle as is here defcribed, we will find 

 the angle, fubtended by the earth's diftance from the fun, coin- 

 cide very nearly with the fchigram. 



The argument of this eqviation is the difference between the 

 mean longitude of the fun and of the planet. The orbits are 

 fuppofed circular ; but whether the inequality in queftion was 

 underftood to arife from the motion of the earth, or from the 

 motion of the planet in an epicycle, the centre of which re- 

 volves in a circle, i« left undetermined, as both hypothefes may 

 be fo adjufted as to give the fame refult with refpe(ft to this in- 

 equality. The proportional diftances of the planets from the 

 earth or the fun, may be deduced from the tables of thefe equar 

 tions, and are not far from the truth, 



59. The preceding calculations mufl: have required the af- 

 fiftance of many fubfidiary tables, of which no trace has yet 

 been found in India. Befides many other geometrical propofi- 

 tions, fome of them alfo involve the ratio, which the diameter 

 of a circle was fuppofed to bear to its circumference, but which. 



we 



