92 PHILOSOPHICAL TRANSACTIONS. [ANNO 1797. 



to approach each other, till blended with double light in one, and disappear at the 

 altitude i, above the apparent horizon of the sea. But, as explained before, if 

 the strength of evaporation did not separate by refraction the pencils aa and dd to 

 a greater angle than double the angle that the lamps and reflectors appear under, 

 the 2 images would be blended, and the strong appearance of light would be of 

 shorter duration. The distance run from the lights, during the time each of the 

 lights shone bright, would have been useful, but this did not occur at the time, 

 nor have I had the like opportunity since. However, I recommend to the mariner 

 to station people at different heights in looking out for a light, in order to get 

 sight of it near the horizon, when it is always strongest. 



Respecting the appearance of the Abbey Head before-mentioned, fig. l, the 

 dotted line ab represents the limit, or the lowest points of the land that can be 

 seen over the sea; for, as above stated, all the objects appearing below this line, 

 are the land above it inverted; and where the land is low, as at d and m, it must 

 appear elevated above the horizon of the sea. 



In fig. 5, let ho represent the curve of the ocean, and d the extreme top of the 

 mount visible at a by the help of refraction; the dotted pencil of rays cc passing 

 from d to the eye in some part a little below the maximum of density, where in- 

 version begins; therefore no land lower than this can be seen; for any pencil from 

 a point in the land lower than this, must in the refraction have a contrary flexure 

 in the curve, and therefore pass above the observer. Let ad be a tangent to the 

 curve at a; then the object d will appear to be elevated by refraction to d; also let 

 av be a tangent to the pencil ax at a; then the angle dax will appear to be an 

 open space, or between d and the horizon of the sea. Suppose a star should ap- 

 pear very near or over the mount d, as at *, 2 pencils would issue from every point 

 of it, and form a star below as well as above the hummock d. There are always 

 confused or ill defined images of the objects at the height of the dotted line, fig. l, 

 above the level of the sea, as before-mentioned; and instead of the points d 

 ending sharp in that line, they appear blunted, and the Abbey Head is frequently 

 insulated at the neck m. 



I have viewed, from an elevated situation, a point or head land at a distance be- 

 yond the horizon of the sea, forming, as in fig. 6, a straight line ab, making an 

 acute angle bao with the horizon of the sea. Seeing the extreme point blunted 

 and elevated, I descended ; and though in descending the horizon cut the land 

 higher, as at ho, ho, yet the point had always the same appearance as a, a, a, fig. 6, 

 though the land is known to continue in the direction of the straight line ab to 

 beneath the horizon, or nearly so, as viewed from the height above. If then from 

 a low situation we view this head land through a telescope, the inclination of the 

 surface ab to the horizon being known to be a straight line, it will appear as in 

 fig. 7, the dotted line (at the height of the point where a perpendicular xy would 

 touch the extreme of the land) being at the limit or lowest point of erect vision. 

 And if a tangent to the curved appearance of the land ab, be drawn parallej to the 



