474 Proceedings of the Royal Society 
But the most remarkable transformation was effected in a sloop 
which was sailing obliquely across my line of vision from south- 
west to north-east, apparently at the G-ranton Ferry, about four 
miles off. From the set of its sails, and the darkness of a great 
extent of the Firth around, the vessel was evidently carrying with 
it a brisk favouring breeze from the south-west ; and during the 
fifteen or twenty minutes that my observation lasted it must have 
changed its place by two miles. But during all that time it pre- 
sented the appearance of a narrow gap in its entire hull immediately 
before its single mast, and a very much wider one behind the mast. 
The sea was visible through both gaps ; and with the telescope it 
was easy to observe the helmsman at the tiller on a poop completely 
and widely separated from the forward part of the vessel. The 
ferry steamer appearing from Grranton on her passage to Burnt- 
island, I was anxious to see what might happen to her when she 
arrived in the same water with the sloop. But when she came 
into a line with the sloop, it was at once apparent that the sloop 
was about two miles farther off ; and she had previously appeared 
to be nearer, in consequence of the sheet of dark broken water in 
which she had been sailing being apparently raised above the level 
of the calm white water surrounding it on all sides over the rest of 
the Firth. 
During a period of about twenty minutes, spent in watching 
these alterations of form in North Berwick Law, Inchkeith, and 
the sailing vessel, I could not detect the slightest change in the 
character of the appearances. 
I am unwilling to attempt an explanation of these singular 
