266 ADVENTURES AMONG BIRDS 



foliage, has a singular attraction. In some instances 

 the effect on the hill itself of its crown of trees is to 

 give it the appearance of a vast mound artificially raised 

 by man on which to build or plant his temple. This is 

 most striking when, as at Badbury Rings, in Dorset, 

 the hill is round and low, with a grove of old, very large 

 trees. In this case the effect is heightened by the huge 

 prehistoric earthworks, ring within ring, enclosing the 

 grove on the space inside. Indeed, the sublimest of 

 these temple-groves are not those which stand on the 

 highest hills ; in many cases they stand but a little above 

 the surrounding level, as in the case of Badbury Rings 

 and of Hollywater Ckunp in Wolmer Forest, where the 

 soil is sand. 



To my mind the best appearance presented by the 

 higher hill-top groves is on a hot, windless summer 

 day, during the phenomenon of "visible air," or "heat," 

 when the atmosphere near the surface appears as a 

 silvery mist, or as thinnest white and crystalline flames, 

 asceading, wavering, dancing, and producing an illusion 

 of motion in all distant solid objects, such as houses, 

 fences, trees, and cattle. If the sun had greater power, 

 this silvery flame-like appearance would become more 

 visible still and take the appearance of water of a 

 marvellous brilliancy, as of molten silver, flowing over 

 the earth, with cattle standing knee-deep in it, and dis- 

 tant buildings and groves rising like islands out of it. 

 This effect of mirage is occasionally visible In England 

 in hot, dry summers, but is very rare. It is on these 

 burning silvery days, when air and sunlight have a new 



