probable that the utmost Power of its Spring cannot exert it self, to so great an Extension, and that no part of the Atmosphere reaches above 45 Miles from the Surface of the Earth.
This seems confirm'd from the Observations of the Crepusculum , which is observ'd commonly to begin and end when the Sun is about 18 Degrees below the Horizon; for supposing the Air to reflect light from its most rarified Parts, and that as long as the Sun illuminates any of its Atoms , they are visible to an Eye not intercepted by the Curvity of the Earth, it will follow from Fig. 5. Plate 1. that the proportion of