352 Experiments on Artificial Mirages and Tornadoes. 



sheet of thin writing-paper with an arc-lamp behind it works 

 very well, but a large mirror set in a window and reflecting 

 the sky is better, when daylight is to be had. 



Between the sky and the desert a small range of mountains, 

 cut out of pasteboard, should be set up. The individual peaks 

 should be from 1 to 2 cm. high, and the valleys between them 

 should be only a trifle above the level of the desert. The general 

 arrangement is shown in fig. 5. 



The plates are now heated by means of a row of burners, 

 which should be moved about from time to time in order to 

 prevent overheating any one place. 



If now we look along the desert, holding the eye only a 

 trifle above the level of the sand, we shall see the mountains 

 sharply outlined against the sky: as the temperature rises a 

 lake begins to form in front of the mountain-chain, and in a 

 few moments the inverted images of the peaks appear as if 

 reflected in the water. If the eye be depressed a trifle, the 

 base of the mountain-chain vanishes completely in the illusory 

 lake, which now appears as an inundation. These appear- 

 ances are shown in fig. 6, the photographs having been taken 

 of the actual mirage on the artificial desert. The first of the 

 three shows the appearance when the plates are cold, the 

 second the apparent lake with the images of the peaks in the 

 water, and the third the vanishing of the lower portions of 

 the range. Two or three palm-trees, cut out of paper, were 

 stuck up to add to the effect. Vertical magnification can also 

 be shown on the hot desert: if the mountains are removed 

 and a small marble be laid on the sand at the farther end of 

 the desert, it will be found that if the eye be brought into 

 the right position, the circular outline will change into an 

 ellipse, and as the eye is lowered the image will contract to a 

 point and eventually disappear. The magnification in this 

 case is of course due to the running together of the direct 

 and refracted images. I have observed similar cases in 

 looking across our lake, when the water was warm and the 

 air cold, patches of snow on the opposite shore, too small to 

 be visible to an eye several metres above the level of the lake, 

 coming out very distinctly when one walked down a bank to 

 the water's edge. 



The atmospheric conditions existing when mirages of this 

 description are observed are such as give rise to the dust- 

 whirls, so often seen on the American desert, and when 

 existing on a larger scale, to tornadoes. There seemed no 

 reason why these whirlwinds should not be produced on a 

 small scale as well as the mirages. One of the metal plates 

 was sprinkled with precipitated silica and heated with a few 

 burners : in a few minutes most beautiful little whirlwinds 



