Viewing page 358 of 504

This transcription has been completed. Contact us with corrections.

165)

measured by an Angle which is equal to twice the horizontal Refraction. And so, in our Atmosphere, that Angle amounts to about 1° 0' or 170° 52'.

[[left margin]] Effects of an horizontal Refrn. in our Atmosphere, considered. [[/left margin]] 

10. In order then that we may the better argue concerning the Refraction of Light in the Atmosphere of the Moon, let us consider, in this Discourse, the Effects of the horizontal Refraction of Light in our Atmosphere. For these Effects, which depend partly ^[[insert]] up [[/insert]] on the [[underline]] Height [[/underline]] at which our Atmosphere ceases to refract the Rays of Light, would appear very singular and curious, if the Eclipses of the Sun, or of fixed Stars, by the Interposition of the Earth, were observed, for Instance, from the Globe of the Moon. And the like may be said of the Refraction of Light in other Planets also.

[[left margin]] Height of the Atmosphere. Sir I. Newton neglected the great cold in the upper reigons in considering Refraction, for which no Tables can serve universally. [[/left margin]] 

11. [[underline]] That Height [[/underline]] is by Sir [[underline]] Isaac Newton, [[/underline]] p. 463. made of 35 or 40 Miles. For he calculated with great Pains, upon a physical Hypothesis, a Table of the Refractions of Light, from the Zenith to the Horizon. In the making of which I suspect he took no sufficient Notice of the Condensation of our Air, by the the great Coldness which reigns in its upper Regions: whose Effect is so great, that no such Table can serve universally.

[[left margin]] An increase in the Height of our Atmosphere, increases the apparent semidiameter of the Earth; but this does not remove the Objectn. in No. 2. [[/left margin]]
[[strikethrough]] Plate. [[/strikethrough]] Fig. 43.

12. It is true that the higher we suppose our refracting Atmosphere to be, the greater is the Number of Feet [[strikethrough]] to be [[/strikethrough]] which must be added to the real Semidiameter of the Earth, as seen from any Distance whatsoever. But that Addition to make up the apparent Semidiameter of the Earth is very inconsiderable. For I find that if the Height (FX or FX) of our refracting Atmosphere, be of 35 or 40 Miles (as Sir [[underline]] Isaac Newton [[/underline]] does suppose) the Addition (QP or QP) to the Semidiameter of the Earth, to make up her