PROFESSOR TAIT ON MIRAGE. 577 
in the case of air irregularly heated, and thus we have a very probable explana- 
tion of the series of inverted images figured by Scorressy. The strata which 
produced these, in all likelihood produced direct images also, but (except on 
very rare occasions) so small in vertical dimensions as to have escaped 
observation. In the absence of wind such strata, once formed, would last for 
a long time, in consequence of the very small thermal conductivity of air. I 
might also refer to an interesting case of inverted images seen from a balloon by 
TISSANDIER.* The height at which the balloon was situated is not stated 
expressly, but from the context it must have been somewhere about 6000 
feet. This, of course, proves the existence, at a great elevation, of a stratum 
in which there was a comparatively rapid diminution of refractive index 
with increasing height. 
I will quote, in conclusion, ScorEsBy’s account of his remarkable observa- 
tion of an isolated inverted image of a ship, which was situated far beyond the 
horizon. His drawing is reproduced as the second of the series in fig. 1. The 
obvious and simple explanation of this is what has already been mentioned for 
TISSANDIER’S observation, though, of course, it could also be accounted for by 
an infinite number of different laws of refractive index, all of more or less 
ingenious complexity. 
“The atmosphere, in consequence of the warmth, being in a highly refractive state, a 
great many curious appearances were presented by the land and icebergs. The most extra- 
ordinary effect of this state of the atmosphere, however, was the distinct inverted image of a 
ship in the clearsky, . . . . the ship itself being entirely beyond the horizon. 
It was so extremely well defined, that when examined with a telescope by DoLtonp, I pal 
distinguish every sail, the general ‘rig of the ship, and its particular character; insomuch 
that I confidently pronounced it to be my Father’s ship, the ‘ Fame,’ which it afterwards proved 
to be; though, on comparing notes with my Father, I found that our relative position at the 
time gave our distance from one another very nearly thirty miles, and some leagues beyond 
the limit of direct vision.” + 
It seems hard to reconcile the clearness of definition in this case with any 
other than a stable state of equilibrium of a transition stratum. The mirage 
of the desert, where the equilibrium is essentially unstable, is always exceed- 
ingly unsteady. 
Brot makes a point, to which I have not yet alluded, from Vuincr’s state- 
ment that the inverted image appeared to rise as the object moved farther 
away. His mode of explaining this, however, savours of the “ autant des lois 
différentes,” &c. ; and, besides, the result follows quite as directly from my ex- 
* GLAISHER’S Tvavels in the Air, p. 297 (1871). 
+ Scorzspy’s Journal of a Voyage to the Northern Whale Fishery (1823), p. 189. 
