1911-12.] 
The Fata Morgana 
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horizon or beyond it. appear to be at the bottom of the illusory 
valley of water well within the apparent circle of the horizon. 
4. The visibility of the lower parts of the 
opposite coast, which normally should 
have been hidden by the rotundity 
of the water surface. For example, 
the Chateau de Chillon at 34 kilo- 
metres distance appeared to the spec- 
tator to be resting on the quay of 
Morses. 
5. The reduction in relative height or 
dwarfing of the lower parts of the 
opposite coast. 
These details are seen only when the 
phenomenon becomes established. They may 
be preceded by what I have called the mirage 
over warm water (Le Leman, p. 532, etc.). 
I do not dwell on it here, because it is not 
necessary for the purpose of this lecture. 
I propose to confine my attention to the 
principal normal accompaniments of these 
refractions, namely, the depression or elevation 
of the apparent horizon according as the water 
surface is warmer or colder than the air above. 
When investigating the variations of the 
apparent horizon,"' I have carefully determined 
the position of the true horizon by telescopic 
measurement of the positions of a mountain 
top and its reflection in an artificial horizon. 
Midway between these positions, after correc- 
tion for the dip of the horizon, lies the true 
horizon. The hill chosen was the Dent d’Oche, 
which is distant 24*4 kilometres from Morges, 
and rises beyond the opposite coast to about 
2221 metres above sea-level. 
Referred to this true horizon, the apparent 
horizon has been found to vary between the 
* Bull. Soc. Vaud. Sc. nat.,xxx v. 25, Lausanne, 1899 ; see also Arch. d. Sc. phys. et nat., 
viii. 373, Geneva, 1899. 
