l^ • TLi EffeQs and Thiorf af Br^taQiali mar the Surface of the Sea.. 



fpecificaUy lighter than the diier atmofphere; and therefore float or rife, from that principle, 

 as fteam from water ; and in their rifing (the furrounding corpufcles from the fame caufe 

 imbibing a part of the moiflurc) become continually drier as they afcend, yet continue 

 afcending untilthey become equally denfe with the air *. However, tliefc conjecflures I 

 (hall leave, and proceed to the following obfervations upon rcfra£tions : 



In the year 179?, when at Ailonby in Cumberland, I made fomc remarks on the ap- 

 pearance of the Abbey-head in Galloway, which in diftance from Ailonby is about feven 

 leagues; and from my window, at fifty feet above the level of the fca, at that time of 

 tide, 1 obfer\'ed the app eataoce of the land about the Head as reprefented in PI. VIlI. Fig. 1 . 

 There was a dry fand xy, called Robin Rigg, between me and the Head, at the diftance from 

 my houfe of between three and four miles; over which I faw the horizon of the fea H O ; 

 the fand at this time was about three or four feet above the level of the fea. The hummock J 

 is a part of the headland ; but appeared infulated or detached from the reft, and confiderably 

 elevated above the fea, with an open fpace between. I then came down about twenty-five 

 feet, when I had the dry fand of Robin Rigg xy in the apparent horizon, and loft all that 

 floating appearance feen from above ; and the Abbey-head appeared everywhere diftlniV 

 from the furface of the fand. This being in the afternoon, the wet or rnoifture on tb^ fand 

 would in a great meafure be dried up. I have reafon therefore to conclude, that evaporation 

 is the caufe of a lefs refradion near the furface of the fea ; and when fo much fo as to make 

 an obje£t appear elevated wholly above the horizon (as at d in Fig. i .) there will from 

 every point of this obje£l iftue two pencils of rays of light which enter the eye of the 

 obfervtr, and that below the dotted line AB (parallel to the horizon of the fea HO) the ob- 

 jeds on the land will appear inverted. 



To explain this phenomenon, I fhall propofe the following theory, and compare it with 

 the obfervations which I have made. Suppofe HO, Fig. 2. to reprefent the horizontal fur- 

 face of the fea, and the parallel lines above it the lamina or ftrata of corpufcles, which, next 

 »he fluid, are moft expanded, or therareft; and every lamina upwards increafing in denfity, 

 till it arrive at a maximun\ (and which I fiiall in future call the maximum of denfity) at the 

 liae D C, above which it again decreafes in denfity ad wfiintum. 



Though this in reality may be the cafe, 1 do not with to extend the meaning of the 

 word denfity farther than to be taken for the refractive power of the atmofphere ; that is, a 

 ray of light entering obliquely a denfer lamina, to be refrained towards a perpendicular to 

 its fuxface v and in entering a rarer lamina the contrary ; which lamina: being taken at in- 

 finitely fmall diftances, the ray of light will form a curve agreeable to the laws of dioptrics. 



In order to eftablilh this principle in horizontal refraftions, I traced over various parts 

 of this fliore, at different times, when thofc appearances feemed favourable, with a good 

 telefcope, and found objefts fufficient to confirm it ; though it be difficult at that diftat;ce 

 of the land to get terreftrial objefls well defined fo near the horizon, as will afterwards 

 appear. 



• Mr. Hamilton, in his very curious Effay on the Arcent of Vapours, does not allow of this principle even 

 asanafliftant; though by a remark .(page 1 5) he takes notice of thofe appearances in the horiion of the fea, 

 and fays they arife from a ftrong or vinufaal degree of refraction ; the contrary of which I hope to illuftrate in 

 tbc courfc of this paper. 



O&e 



