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an unusual horizontal Refraction of the Air. 
an E, bv E pass, but should operate in the tract through which 
am E, b w E pass, an erect image would be visible, but there 
would be no inverted image; and, should it operate in the latter 
case, but not in the former, there would be only an inverted 
image. 
As the phaenomena are very curious, and extraordinary in 
their nature, and have not, that I know of, been before ob- 
served, I have thought proper to lay a description of them, with 
all the attending circumstances, before the Royal Society. 
They appear to be of considerable importance ; as they lead us 
to a knowledge of those changes to which the lower parts of 
the atmosphere are sometimes subject. If, when these pheno- 
mena appear, a vessel, furnished with a barometer, thermo- 
meter, and hygrometer^ below, and also at the top of the mast, 
were sent out to pass below the horizon and return again, and 
an observer at land, having like instruments, were to note, at 
certain intervals, the situation and figure of the images, it 
might throw further light upon this subject, and lead to useful 
discoveries respecting the state of the atmosphere, from a con- 
junction of the causes which affect these instruments. 
