MIRAGES—GORDON 331 
The commonest mirage in our desert country today is a seeming 
strip of water on the highway which appears when we are at some dis- 
tance and vanishes as we come near. The frequency of this type is a 
modern development, paralleling that of the automobile and paved 
highways; it was very rarely seen on the old dirt roads. Frequently it 
may show the seeming reflection of an approaching car or, after sun- 
down, its headlights in the seeming water. An asphalt surface is its 
favorite location. On warm afternoons, the air temperature next to 
this surface may be 10° higher than that an inch above, 20° higher 
than 6 inches above, supplying the foundation for an air lens. 
Perhaps you have wondered why these little mirages appear only 
on scattered parts of the road surface. Their location is definite; 
they do not move with you. If you had looked back you probably 
would have seen the first one pop into sight again after you had 
gone a hundred yards or so. But there would be long stretches with 
none at all, in spite of the sun’s beating down on the surface. ‘These 
mirageless areas have long been a puzzle. Looking for an explana- 
tion, I took a series of temperature readings about 3:30 on warm 
June afternoons in a mirage-forming area and in a nonforming one 
about 10 feet away. The bulb was held at the surface, barely above 
it, up one-third inch, 1, 2, 3, 6, and 12 inches. There was appreciable 
wind movement at all times. To make sure the temperature shown 
by the bulb was not affected by direct radiation from the asphalt, 
a sheet of paper was interposed between the two, resulting in negli- 
gible change. Readings in the two locations were practically identi- 
cal. The mirage-forming area seemed to be surrounded by other air 
with just as favorable conditions. The only hint of an explanation 
seemed to be found in the fact that there was certainly a general 
tendency for mirage-forming areas to be in depressions. They might 
be quite appreciable or so slight as to be hardly more than a token, 
such as the converging cracks in the surface or depressed lines left 
by street repair. 
It was said for years that the reason mirages were so much more 
common over asphalt than over dirt was that the black surface became 
so much hotter under the sunshine. To test this, comparative read- 
ings were taken in the same manner as described earlier, over two 
small mirage-forming areas of the asphalt about a hundred feet apart, 
and in several locations over the dry, loose soil of an adjacent vacant 
lot. Results were so unexpected that 12 sets of readings were taken. 
Surface-temperature readings over the dirt ran from 6° to 10° higher 
than over the asphalt, this difference gradually growing smaller with 
elevation. Actually, the difference was logical enough, and any bare- 
foot boy could have told you that the dirt was hotter. Asphalt is a 
much better conductor of heat than dry dirt. Heat absorbed by the 
pavement was carried down and distributed throughout the mass, 
