p 
CLIMATE—-PURITY OF THE AIR—MIRAGE. 251 
The remarkably high range of the thermometer, exhibited by this table, cannot fail to be 
noticed. It is very common to find it standing at 110? during the day, but after the sun 
descends the temperature rapidly decreases, and the nights are refreshingly cool. "The mean 
temperatures of the seasons, deduced from the observations of the last five years, are as 
follows: Spring, 72.10; Summer, 89.95 ; Autumn, 75.69; Winter, 56.80. The mean annual 
temperature being 73.62.' The sudden diminution of temperature at night is facilitated by 
the absence of clouds, or watery vapors, in the air, which allows free and rapid radiation of 
heat from the surface, and permits the sun's rays to penetrate with remarkable heating power 
and chemical effect during the day. The nights and mornings, during our stay upon the Desert, 
and at Camp Yuma, were always cool and comfortable, and sometimes cold, but with the rising 
sun the heat returned, lasting, without intermission, until the unclouded sun was hid behind 
the western mountains. Similar conditions were observed by Bayard Taylor in the Nubian 
Desert. The temperature varied from 509—559 at 6 a. m. to 809—859? at 2 p. m. The 
extremes being 47? and 1009.? The heat, therefore, is not as great as on the Colorado Desert. 
The remarkable purity of the air of the Desert has been noticed in the Itinerary. It is so clear 
that it permits small objects to be distinctly seen at great distances; and the mountain ridges, 
when ten or twelve miles distant, appear close at hand. Small objects on the broad plain are 
often imagined to be of immense size, and at a great distance, there being nothing with which 
to compare their magnitude. "The air also appears to produce a wonderful effect in coloring the 
ranges of distant mountains. The tints which they assume are beautifully clear and uniform, 
but vary with the distance. A deep, beautiful blue is most common, but all shades of brown, 
purple, violet, and even rose, are seen. The transparency of the air becomes very evident at 
night, when the stars shine out with all the splendor which they have in the clear nights of 
mid-winter in New England. | 
All the phenomena of mirage, as described by travellers of the Desert regions of Africa and 
Asia, are exhibited on a grand scale upon the Colorado Desert. Mountain ranges, so far distant 
as to be below the horizon, are made to rise into view in distorted and changing outlines. 
Inverted images of smaller objects, and apparent lakes of clear water, are often seen, and invite 
the traveller to turn aside for refreshment. The first exhibition of mirage that was seen was 
from the margin of the plain at Carrizo Creek, looking towards the Gila, about ninety miles 
distant, It was early in the morning, aud the eastern sky had that golden hue which precedes 
the rising sun. Tall, blue columns, towers, and the spires of churches, and overhanging preci- 
pices seemed to stand upon the verge of the plain. Their outlines were changing gradually, 
and as the sun rose higher they were entirely dissipated. Figures of these outlines are given 
in the Itinerary.—(See also View XIL) After reaching Fort Yuma, and witnessing the strangely 
precipitous and pinnacled outline of the mountains beyond, it was at once apparent that the 
mirage consisted of their distorted images. When we were upon the northern part of the 
Desert, the peak of Signal Mountain was often distorted and raised above the horizon. The 
points of distant ranges also seemed, at times, to be elevated above the surface, precisely as 
headlands of a coast sometimes appear to rise above the water at sea. Many of the phenomena 
called mirage are not due to refraction, but are believed to be the result of reflection from the sand 
or smooth surface of clay, or the polished pebbles. Thesmooth clay forms an excellent reflector 
for all the rays which are incident at a slight angle, and is most frequently the cause of the 
! Army Meteorological Register, from 1843-1854, p. 641. 
? Journey to Central Africa, p. 
