NATURE 



[Nov. 2 1, 1889 



(6) When I mounted higher, a mirage, if seen at all, was 

 further off than when I stood lower. 



If, when looking at the watery layer of a mirage, I mounted 

 higher, the " water " narrowed, and the strip of land beyond it 

 widened, until at a certain height of my head the "water " had 

 narrowed into a wavy line of fragments. Further mounting 

 caused the "water " to disappear. If, on the contrary, I stooped, 

 the "water" appeared to widen, the strip of land above it to 

 narrow, until at last the mirage joined the sky. 



On one occasion, when the mirage was about a mile and a 

 half distant, and on another occasion when about 250 yards 

 distant, I caused the "water" to appear and disappear by 

 a vertical movement of my head not exceeding i foot. 



(7) Objects situated in the watery layer but rising out of it, 

 or on the strip of land beyond it, were reflected in the " water" 

 much as in true water ; but all was ill-defined, and the inverted 

 reflections often broken and lengthened. 



(8) It appeared to me that objects on the strip of land beyond 

 the watery layer were also to be seen faintly reflected in the 

 land that lay between them and the ' water." And when, as 

 in (6), I had raised my head until the "water" had just 

 dwindled away, objects near the horizon were reflected inverted 

 in the region from which " water " had vanished. 



(9) By the aid of my glasses I came to the conclusion that 

 objects were not really, as they appeared to the naked eye, 

 "drawn up" by the mirage. But it seemed rather that, an 



Fig. 



object being seen above its (often elongated) reflected image, and 

 both being ill-defined, to the naked eye the whole appeared 

 like the object "drawn up." In this way clumps of grass 

 appeared as trees. 



(10) In (i) I have mentioned the usual form of the mirage. 

 But with various slopes, &c., of the ground, the form of the 

 mirage varied. Sometimes the "water" opened out into the 

 sky both ways ; and several times I saw an isolated patch of 

 " water" over an isolated patch of bare hot earth. 



Conclusions as to Summer Mirage. — It seemed, then — 



( I ) That this mirage was due to a layer of relatively warm air 

 close to the earth. 

 ^■(2) That this mirage-giving layer was not more than about 



leet in depth, and that it may have been less. 



(3) That there were not, to any noticeable extent, vertical 

 elongations of objects nor extensions of normal horizon. 



{4) That in this mirage there were no images, erect or 

 inverted, seen above the real object. 



In fact, it seemed that the sky and terrestrial objects were 

 simply reflected in a sheet of warmer air lying close to the 

 ground. (Of course the paths of the rays would be curved. ) 



II. The Winter Mirage. 



[I was told that this mirage is seen in winter, and best on fine 

 mornings after hard frost. What I saw were, it seemed, but 

 poor specimens.] 



(i) I saw this mirage several times, always about sunrise and 

 after a frost. Before sunrise, as soon as there was any light, I 



Fig. 2. 



looked out into the plains with my binoculars. It appeared as 

 if the horizon were higher than usual, and that one could see 

 tracts of land, with houses and other objects, that were usually 

 concealed below the horizon. 



Further, it seemed that this extension of horizon was not 

 really continuous, as it at first appeared, but that it was divided 

 into layers. As far as I could judge, the line (a) was beyond 

 the normal limits of the horizon, the tract from (o) to the limit 

 (j8) was more or less a repetition of the tract below (a), and 

 from ((8) to (7) was again more or less a repetition of the same 

 tract. As to what one could see above the line (7), I could 

 make no trustworthy observations. 



Before sunrise, this extension of the horizon was seen all 



round ; and, though the layers referred to could be distinguished 

 fairly well, there were as yet no " watery layers " to be seen. 



The land seen just above the lines (o) and (.8) was paler than 

 that seen just below these lines. 



(2) Thanks to a most convenient distribution of cattle of 

 various colours, and of other objects, I was able, with the aid 

 of my glasses, to make out a good deal. 



But the images changed as the cows moved, the appearances 

 varied as time went on, and were so different in different parts 

 of the horizon, that I could only arrive at some general con- 

 clusions. 



There would be, for example, just below, or on the edge of, 

 the line (a), a cow. This I will call the ^^ first co'c," or the 



