Queried mid Answers. 



485 



thirsty land j " though, perhaps, rather puzzled as to his way through such 

 a waste of waters, and sometimes wondering at the length of time he is in 

 reaching any of the long-seen lakes ; at last, probably, climbing some mound 

 to take a better view, great is his astonishment to find the delightful scene 

 vanished j and, in place of the refreshing sheets of water, the parched and 

 dusty plain. These appearances are most commonly seen where there is no 

 vegetation, where the soil is dry and black, and the day hot and calm, or 

 with but very little wind. Much, however, depends on the height of the 

 observer's eye above the surface of the ground : the imaginary water dimi- 

 nishing in extent, and withdrawing to a distance, and, perhaps, at last dis- 

 appearing, as he raises himself; and extending and approaching as he sinks 

 down. The phenomenon is seen to most advantage when the water appears 

 not less than a mile distant, with a background of lofty trees or distant hills : 

 the illusion then is perfect, which it is not when it seems to approach close ; 

 for the watery appearance, in such a case, dances and trembles in the air, 

 near the eye, losing its likeness to^eality. As 1 am not satisfied with Y.'s 

 proposed explanation, and have never seen any other, I beg to offer the 

 following, which, I think, accounts for the whoje of the appearances abQV& 

 noticed. -r « ^ ? , - . , 



Let the observer {fig. 122. a) be placed in a situation where the air, near the' 

 surface of the ground, is heated and rarefied as far as the line b c; the influence 

 of the heated earth not extending further, or the rarefied air being swept 

 away or cooled by a light air of wind as it rises above it : let rays of light 

 proceed from points in the sky, as t/, in a direction towards the eye of the 

 observer (d e a), and towards the earth (df) ; the former will reach him in, 

 almost a direct line, but the latter, passing into a rarer medium at the line 

 b Cy will be bent or refracted *, and enter his eye in the direction d «, pro- 

 ducing an indistinct image of the sky, similar to what would be formed by 

 still water placed at d. In like manner, let rays proceed from the distant 

 hill (g), towards the observer (g^ a), and towards the ground (gi) : the 

 former will suffer but little refraction ; but the latter, being bent on entering 

 the rarefied air at b c, will reach him in the direction i «, producing an indis- 

 tinct shade or image of the hill at i. The consequence is, that the observer 

 (c) will see the appearance of a lake at d, with an indistinct image of the 

 hill (g) reflected in at i ; for it is obvious that he cannot see the ground at i 

 or D, the rays {It) proceeding from which being either bent upward too much 

 to reach the observer at c, or arriving in the direction k c, and producing, 

 on the similarly coloured foreground, no distinguishable image. It will be 

 seen, without farther explanation, that, as the observer raises himself more 

 and more out of the rarefied air, more and direct rays from the base of 

 the hill will be enabled to reach him, and the refracted rays from the sky 



* The rays will be bent in a curve line, not at an angle, as represented in 

 the sketch, in order to show, with less confusion, the direction in which 

 they reach the eye. 



