260 ADVENTURES AMONG BIRDS 



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 Clump 

 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, ascending, 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 appear- 

 ance of water of a marvellous brilliancy, as of molten 

 silver, flowing over the earth, with cattle standing 

 knee-deep in it, and distant 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 magic, that I like 

 best to see the hill-top grove; when at a distance of 

 a mile or two the tall columnar trunks of the pines, 

 showing the light between, seem to have a wavering 

 motion, and, with the high dense roof of branches, 

 look absolutely black against the brilliant whiteness 

 of the air and the pale hot sky beyond. 



The downland groves are, however, less to me in 



