TERRESTRIAL REFRACTION AND MIRAGE. 279 
us a description of it presently (hear, hear). There was a very curious 
effect of mirage also observed at Lydd in the summer by Captain Osborn, 
R.A. He had been firing with the Maxim machine gun at a target, 
and during the firing, he observed a wedge-shaped gap in the target, 
as if a portion of the target had been removed by the hail of bullets, 
but in a very short time after the firing ceased the target appeared 
quite whole and undamaged. He communicated this to me, and I 
think there can be no doubt of the reason of it—it was in fact an arti- 
ficially produced mirage. The heated gases rise from the muzzle, and 
form a medium in which the density increases upwards ; he was ob- 
serving the upper portion of the target through this medium and I have 
no doubt that that caused negative refraction ; it caused the bending 
of the rays so that the sky above the target was seen in the shape of a 
wedge, something like the shape of the vertical section of the rising 
gases (see Fig. 8). That is a very interesting case of an artificially 
Fre. 8. 
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produced mirage, and I think it says a great deal for Captain Osborn’s 
powers of observation. 
I have to say a few words about another part of the subject which 
is called “looming.” Distant objects are said to loom when they appear 
considerably above their true positions, that is to say, when images of 
them appear considerably raised above their true positions; and the 
phenomenon is usually accompanied by a great apparent proximity of 
the objects. The effects of looming are really quite extraordinary, and 
I have had some diagrams constructed in order to show some of the 
effects that may be observed. A diligent search has convinced me that 
the best examples are those recorded by Commander William Scoresby 
who went on his third voyage in his ship the “ Baffin ” to the Green- 
land Whale Fishery in the spring of 1822. He made many obser- 
vations, and took many sketches of these extraordinary looming 
appearances. Before describing, much in his own words, these 
appearances, I will give you his views of the climatic conditions which 
are favourable to these wonderful phenomena. These are :— 
(1) A rapid evaporation which takes place in a hot sun from the 
surface of the sea. 
(2) The unequal density occasioned by partial condensations, 
when the moist air becomes chilled by passing over consider- 
able surfaces of ice. The question is very much complicated 
by the presence of large irregularly shaped icebergs such as 
