1 4 
Mr. Vince’s Observations upon 
that is, the image of the highest part of the object will be up- 
permost, and the image of the lowest part will be undermost. 
The figures, however, of the sun and moon, when near the 
horizon, will suffer a change, in consequence of the refraction 
of the under limb being greater than that of the upper ; from 
which they assume an elliptical form, the minor axis of which 
is perpendicular to the horizon, and the major axis parallel to 
it. But a perpendicular object, situated upon the surface of the 
earth, will not have its length altered by refraction, the refrac- 
tion of the bottom being the same as that of the top.* These 
are the effects which are produced upon bodies at or near the 
horizon, in the common state of the atmosphere, by what I 
shall call the usual refraction. 
But, besides the ugual refraction which affects the rays of 
light, the atmosphere over the sea is sometimes found to be in 
a state which refracts the rays in such a manner as to produce 
other images of the object, which we will call an effect from 
an unusual refraction. In the Phil. Trans, for 1797, Mr. Hud- 
dart has described some effects of this kind, which he has ac- 
counted for by supposing that, from the evaporation of the 
water, the refractive power of the air is not greatest at the sur- 
face of the sea, but at some distance above it ; and this will 
solve, in a very satisfactory manner, all the phaenomena which 
he has observed. But effects very different from those which 
have been described by Mr. Huddart are sometimes found to 
take place. These I had an opportunity of observing at Rams- 
gate, last summer, on August the first, from about half an hour 
after four o’clock in the afternoon till between seven and eight. 
The day had been extremely hot, and the evening was very 
* See my Complete Systetn of Astronomy, Art. 194. 
