of Edinhurgh, Session 1882 - 83 . 
135 
two would inevitably mix. If, on the other hand, the superior air 
be conformable to the upper layer its altitude must be greater than 
that of the general atmosphere which would press in to displace it. 
In neither case could there be repose. 
But the question arises, “how is this layer of lighter air pro- 
duced!” It cannot be from the sun’s warmth in still weather, 
because then the heating is at the surface of the earth or of the 
water. The warmed air mixes with the cold above, ascending 
while the other descends, and giving rise to the too-well-to-the- 
astronomer-known boiling of the sun’s edge. 
The only other source of such warm air is in the south, whence 
the south-west wind brings it to us in gusts and squalls. At the 
oncoming of the breeze we may see changes in the appearance of 
distant objects on the horizon ; they are displaced, distorted, hori- 
zontally, vertically, obliquely, according to the passing whims of 
Eolus. Seldom more than telescopic, they are as changeable as the 
squall itself. We can scarcely imagine that the wind should gently 
lay a warm coverlet over the quiet cold air of the north and leave it 
there in repose. 
But, I shall be told, these images have been seen, your patient is 
alive and well. Professor Vince saw him from Eamsgate. 
I have already shown that Vince’s narrative is inconsistent with 
ordinarily known appearances, and I have pointed out that the 
same observer, after eight years’ further experience as Professor of 
Astronomy and of Experimental Philosophy at Cambridge, was able 
to see through a telescope magnifying thirty times, a building four- 
teen miles away in all the beautiful proportions of near perspective. 
This feat could only have been performed by one totally ignorant 
of, or utterly careless of, the simplest laws of geometry and of 
optics. 
Yet I needed not to have gone so far. When a picture is pre- 
sented to us, we, without requiring an explanation, form an idea of 
what it means. It may be the picture of a horse, badly drawn, 
yet still we recognise the limner’s meaning, he meant to have drawn 
a horse. So when I look at Vince’s figure, I perceive that he 
meant to have drawn a sloop ; he has succeeded in drawing a sloop 
such as neither I, nor you, nor any other person ever saw. But I see 
a great deal more, I recognise in the picture the unmistakable well- 
known features of an old acquaintance. 
