VOL. LXXXVII.] PHILOSOPHICAL TRANSACTIONS. 89 



me and the head, at the distance from my house of between 3 and 4 miles, over 

 which I saw the horizon of the sea, ho ; the sand at this time being about 3 or 4 

 feet above the level of the sea. The hummock d is a part of the head land, but 

 appeared insulated or detached from the rest, and considerably elevated above the 

 sea, with an open space between. I then came down about 25 feet, when I had 

 the dry sand of Robin Rigg, xy, in the apparent horizon, and lost all that float- 

 ing appearance seen from above, and the Abbey Head appeared every where distinct 

 to the surface of the sand; this being in the afternoon, the wet or moisture on the 

 sand was probably in a great measure dried up. I have reason therefore to con- 

 clude, that evaporation is the cause of a less refraction near the surface of the sea; 

 and when so much so as to make an object appear elevated wholly above the horizon, 

 as at d, there will from every point of this object issue 2 pencils of rays of light, 

 which enter the eye of the observer; and that below the dotted line ab, parallel to 

 the horizon of the sea ho, the objects on the land will appear inverted. 



To explain this phenomenon, I shall propose the following theory, and compare 

 it with the observations which I have made. Suppose ho, fig. 2, to represent the 

 horizontal surface of the sea, and the parallel lines above it, the lamina or strata 

 of corpuscles 3 which next the fluid are most expanded, or the rarest; and every 

 lamina upwards increasing in density till it arrive at a maximum, and which I shall 

 in future call the maximum of density, at the line dc, above which it again de- 

 creases in density ad infinitum. Though this in reality may be the case, I do not 

 wish to extend the meaning of the word density farther, than to be taken for the 

 refractive power of the atmosphere; that is, a ray of light entering obliquely a 

 denser lamina to be refracted towards a perpendicular to its surface; and in entering 

 a rarer lamina, the contrary; which laminae being taken at infinitely small distances, 

 the ray of light will form a curve, agreeable to the laws of dioptrics. In order to 

 establish this principle in horizontal refractions, I traced over various parts of this 

 shore at different times, when those appearances seemed favourable, with a good 

 telescope, and found objects sufficient to confirm it; though it be difficult at that 

 distance of the land to get terrestrial objects well defined so near the horizon, as will 

 afterwards appear. 



One day observing the land elevated, and seeing a small vessel at about 8 miles 

 distance, from my window I directed a telescope to her, and thought her a fitter 

 object than any other I had seen for the purpose of explaining the phenomena of 

 these refractions. The telescope was 40 feet above the level of the sea. The 

 boat's mast about 35 feet. The barometer at 29.7 inches, and Fahrenheit's ther- 

 mometer at 54°. The appearance of the vessel, as magnified in the telescope, 

 was as represented in fig. 3, and from the mast head to the boom was well defined. 

 I pretty distinctly saw the head and shoulders of the man at the helm; but the hull 

 of the vessel was contracted, confused, and ill defined: the inverted image began 

 to be well defined at the boom, for I could not clearly perceive the man at the helm 

 inverted, and from the boom to the horizon of the sea the sails were well defined, 



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