Mr. Huddart's Observations 
light was weaker than the upper one ; on coming down upon 
deck, I found it again as strong as before. We proceeded 
on, and soon lost the lower light from the deck; and upon 
drawing the upper light near the horizon, it like the former 
shone exceeding bright. I again went aloft, when it diminished 
in brightness ; but from the mast head I could then see the lower 
light near the horizon as strong as before. This is in conse- 
quence of the double quantity of light entering the eye by the 
two pencils of rays from every point. To illustrate which, we 
compare the vessel, fig. 4, to a lighthouse built upon the shore, 
and A the place of the observer ; and having brought down 
the light so low as to view it in the direction a a , another light 
would appear in the horizon at x from the pencil d d; and had 
the vessel been still enough to have observed it at this time with 
a good glass, I doubt not but the two images might have been 
distinctly seen : as the light dropped, (by increasing the dis- 
tance) the two images would appear continually 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 se- 
parate by refraction the pencils a a and dd to a greater angle 
than double the angle that the lamps and reflectors appear 
under, the two images would be blended, and the strong ap- 
pearance 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. 
