39 
on Horizontal Refractions. 
mg any certain part of the contained arc; for the points zCB, 
fig. 7, will have various refractions, though they are at nearly 
the same distance from the observer. And if the observations 
are made wholly over land, if the ground rises to within a small 
distance of the rays of light in their passage from the object to 
the eye, as well as at the situation of the object and observer, 
the refractions will be subject to be influenced by the evapo- 
ration of rains, dews, &c. which is sufficiently proved by the 
observations of Colonel Williams, Captain Mudge, and Mr. 
Dalby, Phil. Trans. 1795, p. 583. 
The appearances mentioned by Colonel Williams, Captain 
Mudge, and Mr. Dalby, (Phil. Trans. 1795, p. 58b, 587,) 
cannot be demonstrated upon general principles, as they arise 
from evaporation producing partial refractions. In those gene- 
ral principles, it is supposed that the same lamina of density is 
every where at an equal distance from the surface of the sea, at 
least as far as the eye can reach a terrestrial object; but in the 
partial refractions, the lamina of the expanded or rarefied me- 
dium may be of various figures according to circumstances, 
which will refract according to the incidence of the rays, and 
affect the appearance of the land accordingly, which I have 
often seen to a surprising degree. But my principal view is 
to shew the uncertainty of the dip of the sea, and that the ef- 
fect of evaporation tends to depress the apparent horizon at x, 
when the eye is not above the maximum of density; and from 
hence the difficulty of laying down any correct formula for 
these refractions, whilst the law of evaporation is so little un- 
derstood, which indeed seems a task not easy to surmount. 
The effect indicated by the barometer and thermometer is in- 
sufficient: and should the hygrometer be improved to fix a 
