590 
Proceedings of the Royal Society 
In order to explain the unusual images, Yince represents rays pro- 
ceeding from the points b and a, rising in the air and then bending 
down to reach the eye. This reflexure is caused by a rapid change 
in the refractive power of the atmosphere. 
He has not, however, given any measurement by help of which 
we may estimate the extent and manner of the refraction ; he only 
mentions that the proportions of the distances were noted ; yet the 
measurements were at his hand. He was using a telescope 
magnifying 30 or 40 times, and we infer from his narrative that 
the instrument was on a stand. How, with scarcely any trouble, he 
might have ascertained exactly the power, and have measured the 
angular aperture of the field; indeed to an astronomer these are 
matters of course. He could easily have compared the sizes and 
distances with the width of the field-bar. As it is, we cannot tell 
what may be the angle of elevation of the upper image. 
The drawing is that of a fore-and-aft rigged sloop, a kind of 
coasting vessel never built very large; the existence of ratlins 
would indicate it to be a large one of its class, and, for the purpose of a 
rough estimate, we may take the top-mast as 60 feet above the water. 
The eye was 25 feet above the sea-level, wherefore the edge of 
the actual horizon must have been distant 6 J miles. In proportion, 
there must have been about 35 feet of the height concealed, so that 
