horizontal Refra&kn cf the Air. ^^ 



which account, allitiides of the heavenly bodies which are 

 not more than 5° or 6° ought neve-i: to be made ufe of when 

 any confequences are to be deduced from them. The caufe 

 of this uncertainty is probably the crreat quantities of grofs 

 vapours, and exhalations of varioub kinds, which are fuf- 

 pended in the air near to the earth's furface, and the varia- 

 tions to which they are fubjeft ; caufes, of which we have 

 no inftruments to rneafure tlie effeAs which they produce ia 

 refrafting the rays of light. In genial, the courfe of a ray 

 paffing through the atmofphere, is that of a curve which is 

 concave towards the earth, the effetl of which is to give an 

 apparent elevation to the object ; and thus the heavenly bo- 

 dies appear above the horizon, when they are a6luallv below 

 it; but it will not alter the pofition of their parts in rcfpeft 

 to the horizon; that is, the image of the highefi; part -of the 

 objeft will be uppermoft, and the image of the loweft part 

 will be undermoll. The figures, however, of the fun and 

 moon, when near the horizon, will fuffer a change, in con- 

 fequence of the refraftion of the under limb being greater 

 than that of the upper; from wliich they affume an elliptical 

 form, the minor axis of which is perpendicular to the hori- 

 zon, and the major axis, parallel to it. But a perpendicular 

 objedl, fituated upon the furface of the earth, will not have 

 its length altered by refraftion, the refratSlion of the bottom 

 being the fame as that of the top *. Thefe are the effects 

 which are produced upon bodies at or near the horizon, in 

 the common Rate of the atmofphere, by what I fhall call 

 the 7/Jual refraftion. 



But, befides the ufual refraftion which aflefts the rays of 

 light, the atmofphere over the fea is fometimes found to be 

 in a (late which refrafts the rays in fuch a manner as to pro- 

 duce other images of the objeft, which we will call an eifeft 

 from an umifual refraction. In the Philofophical Tranfac- 

 tions for 1797, Mr. Huddart has defcribcd fome effe6ls of 

 this kind, which he has accounted for by fuppofmo- that, 

 from the evaporation of the water, the refractive power of 

 the air is not greateft at the furface of the fea, but at fome 

 ililtance above it; and this will folve, in a very fatisfa6lory 

 • See my Complete Syjletn of AJlronomj, art. 194. 



manner. 



