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98 



MATHEMATICAL and 



voUr us with; and for which we return him our fincere 

 thanks. Providence was found to be 16' in longitude 

 weft from Cambridge. Mr, Winthrop has hitherto found 

 the longitude of Cambridge to be 71^^ weft from the royal 

 obfervatory at Greenwich; fo that the longitude of Provi- 

 dence is about 71^ 16^ from the royal obfervatory, 



" I ihall now give the reader a fliort account of the parallax 

 herein mentioned, and how the planets are affefted thereby. 



" The horizontal parallax of the Sun is that apgle at 

 the Sun's center, which is included between two lines fup- 

 pofed to be drawn, one from the Sun's center to the cen- 

 ter of the earth, the other from the Sun's center to the furface 

 of the earth. Or, in other words, it is the angle, under which 

 thefemidiametcr of the earth would appear to an eye, at the 

 center of the Sun. The way that parallax affeds the Sun 

 and planets is, itiliakes them appear below their true pla--- 

 ces in the heavens, except they be in the zenith of the ob- 

 ferver; in that cafe parallax hath no eff'ed at all; and the 

 reafon is, becaufe the obferver is in that right line which 



joins the centers of the earth and planet. Parallax may af-^ 

 fe6t the planets places feveralways; as if the obferver fhould 

 vicw^ the planet upon a vertical, cutting the ecliptic at right 

 angles; in this cafe, parallax will aff"ed; its place in refpedto 

 latitudeonly ; but if the obferver be fituated in the plane of 

 Ae ecliptic, it will then alter its place, inrcfpe£tto longitude 

 only; and if the planet be viewed in an oblique pofition, with 

 xefpcd; to the ecliptic, parallax will aff'ed: its place both 

 in longitude and latitude. The horizontal parallaxes of the 

 planets are to each other in a reciprocal proportion to their 

 idifiances; that is, the planets which are neareft have the 

 greateft parallax, andthofe which are moH: remote, theleaft. 

 Thence it follows, if two planets are viewed together, that 

 which is neareft will appear juft fo much below the other, 

 as what the difference of their parallaxes is. The nearer a 

 planet is .to the horizon of the obferver, the greater is its 

 -parallax, and in the horizon it is the greateft polTible; and 

 is then called the horizontal parallax. 



. , " Hence 



