i8 Mr. Vince's Observations upon 
The cliffs at Calais being very visible, I directed my telescope 
towards them, in order to examine whether there was any thing 
unusual in their appearance ; when I observed an image of 
the cliffs, above the cliffs themselves, together with an image 
of the sea separating them, as is represented in fig. 5. ; in which, 
x y represents the horizon of the sea, AB the cliffs, a b their 
image, and v w the image of the sea between them : the depth 
of a b was much less than that of AB. It is probable, however, 
that vw might not be the image of the sea immediately adjoin- 
ing to the cliffs, but a partial elevation of the sea at some dis- 
tance from them ; and that the image v w might intercept 
some part of the image a b, which would otherwise have been 
visible : we must not therefore conclude, that the image a b, so 
far as it appeared, was less than the corresponding part of the 
object. From the memorandums which I made at the time of 
observation, I do not find that I examined the appearance of 
the cliff AB, and its image a b ; which, had there at that time 
been any striking marks in them, would have determined whe- 
ther the object and its image were of the same magnitude. 
The image a b was, however, erect ; the boundaries on the top 
of AB and a b agreeing together. Having examined this for 
some time, and taken a drawing of the appearance, during 
which I could discover no variation, I directed my telescope to 
other objects ; and, upon turning it again to the same cliffs, 
after the space of about six or seven minutes, the images a b 
and vw were vanished; but, examining them again soon after, 
the images were again visible, and in every respect the same 
as they appeared before. A short time after, they disappeared, 
and did not appear any more. 
Soon after tne above appearances, I observed a ship C, with 
