284, TERRESTRIAL REFRACTION AND MIRAGE. 
stantly changing, as he says, like the beams shooting out of the Aurora 
Borealis; the images sometimes ran up, and a number of sails appeared, 
and then vanished, and only one or two would be seen; and he says 
that it was very entertaining watching this very curious image above 
the ship through the telescope, it was never the same one second after 
another. Another case mentioned by Dr. Vince is very extraordinary. 
Dover Castle is only visible from Ramsgate in respect of the top of 
its turrets; you cannot see the whole of it. Between Ramsgate and 
Dover there is about 6 miles of sea, and then 6 miles of land rising up 
to a hill which usually obscures the Keep of Dover Castle with the 
exception of the four turrets. On one occasion he saw the castle on 
the near side of the hill; the castle seemed to have been brought from — 
the remote side of the hill, and was seen presented on its near side as 
in Fig. 17; and he explains this by Fig. 18 which however is not very 
Fig. 17. 
convincing tome. Hesupposes 4 B to be the castle which is concealed 
from the point H# ordinarily by the hill, and he supposes the rays from 
the top and bottom of the castle to take the curvilinear paths shewn, 
and the top of the hill to be seen by means of a ray which passes so as 
to be between the other two; and he says that you should see really an 
image of the top of the hill in front of the image of the castle, but that 
as a matter of fact the image at the top of the hill was so faint that you 
could only see the image of the castle. 
There is only one instance more which I will mention of the abnor- 
mal effects of refraction, and that is the most curious case of lateral 
refraction that I have been able to discover. It was observed in 
Geneva in 1818. A barque near Bellerive at a distance of 4000 toises 
(a toise is about the same length as a pace) was seen approaching 
Geneva on the left bank of the lake, and at the same time an image of 
the sails was observed above the water which, instead of following the 
direction of the barque, separated from it, and appeared to approach 
Geneva by the right bank of the lake, the image moving from east to 
west while the barque moved from north to south. At first the image 
was of the same size as the barque but it gradually diminished in size 
to about one-half when the phenomenon ceased. I think that is very 
