17 
an unusual horizontal Refraction of the Air. 
and the ship separated ; but I observed no second image, as in 
the first case ; a second image, however, might probably have 
appeared, if the ship had continued to descend. 
Upon moving my telescope along the horizon, in order to 
examine any other ships which might be in sight, I observed, 
just at the horizon x y, (in fig. 4.) the top a of the mast of a 
ship; and here an effect was observed, which had not been 
before discovered ; for there was an inverted image B, vertical 
to a , an erect image C, both of them very perfect and well de- 
fined, and an image v w of the sea between them, the water 
appearing very distinctly. As the ship was coming up towards 
the horizon, I continued to observe it, in order to discover the 
variations which might follow, and found, that as the ship 
approached the horizon, the image C gradually disappeared, 
and at last it vanished ; after that, -the image v w of the sea 
disappeared ; and during this time the image B descended ; 
but the ship did not rise so near to the horizon as to bring the 
main-masts together. Had I directed my telescope to the same 
point of the horizon a little sooner, I should have seen the two 
images, before the ship itself was visible. In fact, the images 
were visible, when the whole ship was actually below the ho- 
rizon; for, from the very small part of the mast which was at 
first visible, that part must then have been below the horizon, 
and appeared above it by the usual refraction ; the altitude of a , 
above the horizon, having then been much less than the in- 
crease of altitude which arises from the common horizontal 
refraction. The discovery of ships in this manner might, in 
some cases, be of great importance ; and, on such occasions, it 
might be worth while to appoint proper persons to make ob- 
servations for that purpose. 
MDCCXCIX. 
D 
