Page:Philosophical Transactions - Volume 029.djvu/438

 different in every differing Horizon; exactly after the ſame manner as the Rainbow ſeen in the ſame Cloud is not the ſame Bow, but different to every ſeveral Eye.

Nor is it to be doubted, but the Pyramidical Figure of theſe Aſcending Beams is Opticall: ſince according to all likelyhood they are parallel-ſided, or rather taporing the otherway. But by the Rules of Perſpective, their Sides ought to converge to a Point, as we ſee in Pictures the Parallcl Borders of ſtreight Walks, and all other Lines parallel to the Axis of Viſion, meet as in a Center. Wherefore thoſe Rays which aroſe higheſt above the Earth and were neareſt the Eye, ſeemed to terminate in Cuſps ſufficiently acute, and have been for that reaſon ſuppoſed by the Vulgar to repreſent Spears. Others ſeen from afar, and perhaps not riſing fo high as the former, would terminate as if cut off with Plains parallel to the Horizon, like truncate Cones or Cylinders: theſe have been taken to look like the Battlements and Towers on the Walls of Cities fortified after the ancient manner. Whilſt others yet further off, by reaſon of their great Diſtance, good part of them being intercepted by the Interpofition of the Convexity of the Earth, would only ſhew their pointed Tops, and becauſe of their Shortneſs have gotten the Name of Swords.

Next the Motion of theſe Beams, furniſhes us with a new and, as it ſeems to me, moſt evident Argument to prove the diurnal Rotation of the earth: (though that be a matter which, at preſent, is generally taken by the Learned to be paſt diſpute) For thoſe Beams which rise up to a Point, and did not preſently diſappear, bat continued for ſome time, had moſt of them a ſenſible Motion, from Eaſt to Weſt, contrary to that of the Heavens; the biggeſt and talleſt of them, as being neareſt, ſwifteſt; and the more remote and ſhorter, ſlower. By which means, the one overtaking the other, they would