JOURNAL 



OF ^ 



NATURAL PHILOSOPHY, CHEMISTRY, 



THE ARTS. 



JULY 1 797. 



ARTICLE I. 



'Oi/ervathns on Horizontal RefraBiom which affeB the Appearance of Terrejirial OhjeBs, and the 

 Dip or Deprejjion of the Horizon of the Sea. By JOSEPH HVDDART, Efq. F. R, S.* 



J. HE variation and uncertainty of the dip in different dates of the air, taken at the fame 

 altitude above the level of the fea, vi'as the rcafon of my turning my thoughts to this fubje£V, 

 as it renders the latitude obferved incorred, by giving an erroneous zenith diftance of a 

 celeftial objeft. 



I have often obferved, that lowlands, and the extremity of headlands, or points forming an 

 acute angle with the horizon of the fea, and viewed from a diftance beyond it, appear ele- 

 vated above it, with an open fpace between the land and the fea. The moft remarkable in- 

 ftance of this appearance of the land, I obferved at Macao, for feveral days previous to a 

 typhon, in which the Locko loft her topmafts in Macao roads ; the points of the illands 

 and lowlands appearing the higheft, and the fpaces between them and the fea the largeft, I 

 ever faw. I believe it arifes and is proportional to the evaporation going on from the fea. 

 And in reflefting upon this phenomenon, I am convinced that thofe appearances muft arife 

 from refraflion, and that, inftead of the denfity of the atmofphere increafing to the furface 

 cf the fea, it muft decreafe from fome fpace above it; and that evaporation is the -principal 

 caufe which prevents the uniformity of denfity and refraftion being continued by the ge- 

 neral law down to the furface of the earth. And I am inclined to believe, though I men- 

 tion it here as a conjecture, that the difference of fpecific gravity in the particles of the at- 

 mofphere may be a principal agent in evaporation ; for the corpufcles of air, from their 

 aifmity with water, being combined at the furface of the fluid from expanfion, form air 



'* Pliilofophical Tranfnflions for 1797. 



Vet. L— July 1797. IJ fpecifiMlIy 



