■ [ '■■■■"■"""■ g* 



JOURNAL 



OF 



NATURAL PHILOSOPHY, CHEMISTRY, 



AND 



THE ARTS. 



yuLr i7Qg. 



ARTICLE I. ^ 



Objirvations upon an unufiial Horhontal Refra^ion of the Air, ivith Hemarksonthe Fariatlons 

 to which the lower Parts of the Atmofphere are fomttimes fubjtSt. By the Rev. S. FiNCE, 

 A.M. F.R.S. and Plumian Pnfejfor of Aflronomy and Experimental Philofophy in the 

 XJniverfity of Cambridge "*", 



X. HE uncertainty of the refra<Slion of the air, near the horizon, has long been known to 

 aflronomers, the mean refraftion varying by quantities which cannot be accounted for, from 

 the variations of the barometer and thermometer ; on which account, altitudes of the heavenly 

 bodies, which are not more than 5° or 6°, ought never to be made ufe of, when any confe- 

 quences are to be deduced from them. The caufe of this uncertainty is probably the great 

 Quantities of grc^s vapours and exhalations of various kinds which arc fufpcnded in the air, 

 near to the earth's furfacc, and the variations to which they are fubje£l: ; caufes of which we 

 have no inftruments to meafure the effe<Sls which they produce in refrafling the rays oi light. 

 In general, the courfe of a ray paffing through the atmofphere, is that of a curve, which is 

 concave towards the earth : the effe£t of which is, to give an apparent elevation to the objecl, 

 and thu«the heavenly bodies appear above the horizon, when they are adualiy belg-kV it; but if 



* Philof. Tranf. 1799, p. 13. 

 V-GL. HI. — JutY 1799 U wilt 



