576 PROFESSOR TAIT ON MIRAGE. 
(of a part of the top-mast) wrder the lowest of the three images, and objects 
comparatively near hand would have been affected as well as those at a con- 
siderable distance. 
But there is much more to urge against Brot’s view of the phenomena in 
question. VINCE expressly states that ‘the evening was very sultry.” As his 
observations were made at heights above the sea, varying from twenty-five to 
eighty feet, it is pretty clear that this sultriness was not due to the exceptionally 
high temperature of the surface of the sea. Buror, in fact, allows that the 
effects of this were only sensible “jusqu’ & une petite hauteur.” But then he 
assumes (contrary to VINCE’s statement) a rapid descent of temperature at. 
higher levels. This he looks on as developed, how he does not tell us, by the 
cold produced by vapour in condensing! Besides, if this were true, it would 
make the diminution of density upwards /ess, instead of greater than usual, and 
the optical results of such an arrangement would be in contradiction to his 
explanation. 
It is much to be regretted that Vince's description, like his drawings, is of 
the very roughest character. It is quite otherwise with those of ScorEsBy. 
There can be no doubt whatever that Bior’s mode of explanation is alto- 
gether inapplicable to the majority of ScorEsBy’s observations. 
I quote a single passage,* which is apparently decisive. 
“A dense appearance in the atmosphere arose to the southward of us . . . . When 
it came to the S.W. of us, I first noticed that the horizon, under this apparent density, was 
considerably elevated. . . . . . Two ships lying beset about fourteen miles off, the hulls 
of which, before the density came on, could not be wholly seen, seemed now from the mast-head 
not to be above half the distance, as the horizon was visible considerably beyond them.” 
Had the arrangement of strata here been as Bror supposes in VINCE’s case, 
only the top-masts would have remained visible, the apparent horizon would 
have come in front of the hulls, and there would have been inverted images 
of nearer objects visible wnder the objects themselves. 
It will be observed that these observations were taken over a surface of ice 
in which the vessels were “beset.” The sun is said to have been “ powerful,” 
but the lowest strata of air, in contact with ice or ice-cold water, must have 
been colder than those above them. The haze, or “ density” as ScoREsBy 
calls it, probably consisted of minute drops of water, and would thus be much 
raised in temperature by the sun. In connection with this I may mention that 
when a trough, in which brine has been diffusing for some time into water, is 
suddenly and roughly stirred for a short period, it settles in a few minutes into 
a large number of strata of different densities. Something similar must hold 
* Scornssy’s Arctic Regions, i, 387 (1820). 
