PROFESSOR TAIT ON MIRAGE. _ 575 
venons de le dire, quelquefois deux seulement, l’inférieure constamment droite, la supérieure 
renversée, d’autrefois enfin on n’ en apercevoit qu'une seule directe et reposant sur l’horizon. 
Les cétes de Calais qui présentoient aussi des phénoménes analogues, offroient aussi les mémes 
variétés, quelquefois on les voyoit doubles un instant aprés elles étoient invisibles. Toutes ces 
apparences sont contraires 4 lidée d’ une sphéricité parfaite des couches dair qui produisoient 
ces phénoménes, et l’on concoit en effet qu’étant le résultat d'une équilibre non stable, ils 
peuvent difficilement s’ accorder avec une forme constante. 
On this I would remark, generally, that I think VincE is here rather hardly 
treated. It seems to me, on comparing the two explanations, that the 
reproach of ‘‘ autant des lois différentes qwil y a @images visibles” is not 
merited by ViNcE, and would perhaps more justly apply to his censor. It is 
certainly most unfortunate that Vince did not note the level of the apparent 
horizon ; though, unless he had done so from a great many different heights 
above the sea, I fail to see how the observation would have helped to decide 
- between the various possible explanations. Bror evidently expected a depres- 
sion, for he states as much in reference to the elevated patches of sea and the 
“heavy fog” which VincE observed ; yet this is inconsistent with his own 
figure! But the following passage from VINCE’s paper (in which I have itali- 
cised some words) seems to have escaped the notice of Bror. 
“The usual refraction at the same time was uncommonly great; for the tide was high, 
and at the very edge of the water I could see the cliffs of Calais a very considerable height 
above the horizon; whereas they are frequently not to be seen in clear weather from the high 
lands about the place. The Vrench coast also appeared both ways, to a much greater distance 
than I ever observed it at any other time: s 
Now, one of the most striking of ViNce’s observations was that of a ship 
(hull down) with an inverted image above it, both projected on the confused 
image of the French cliffs as a background. If Buiot’s explanation were 
correct, this background must have been visible by rays of a truly schlangen- 
Jormg character (as GILBERT calls them), for they must have been at least twice 
(more probably thrice) concave downwards; with a convexity downwards, 
somewhere between the spectator and the ship (and probably another between 
the ship and the French coast). It seems much more likely that the ship’s hull 
was really beyond the ordinary horizon, and that the French cliffs were visible 
by rays originally concave upwards so as to rise up, as it were, behind the ship ; 
and then concave downwards, according to the theory I have propounded, from 
the ship to the spectator. 
Biot’s memoir shows, throughout, the pervading influence of his almost 
daily observations of rays which were concave upwards, because passing very 
close to the ground over extensive surfaces of hot sand. If his explanation 
of VINcE’s observation were correct, there would have been an inverted image 
VOL XXX. PART II. a6 
