in the daytime, sending up a vast column of hot air, which cannot
escape eastward, for Arizona manufactures a like column. It flows high
above the mountains westward till it strikes the Pacific and parts with
its heat, creating an immense vacuum which is filled by the air from
the coast flowing up the slope and over the range, and plunging down
6000 feet into the desert. "It is easy to understand," says Mr. Van
Dyke, making his observations from the summit of the Cuyamaca, in San
Diego County, 6500 feet above the sea-level, "how land thus rising a
mile or more in fifty or sixty miles, rising away from the coast, and
falling off abruptly a mile deep into the driest and hottest of American
deserts, could have a great variety of climates.... Only ten miles away
on the east the summers are the hottest, and only sixty miles on the
west the coolest known in the United States (except on this coast), and
between them is every combination that mountains and valleys can
produce. And it is easy to see whence comes the sea-breeze, the glory of
the California summer. It is passing us here, a gentle breeze of six or
eight miles an hour. It is flowing over this great ridge directly into
the basin of the Colorado Desert, 6000 feet deep, where the temperature
is probably 120 deg., and perhaps higher. For many leagues each side of us
this current is thus flowing at the same speed, and is probably half a
mile or more in depth. About sundown, when the air on the desert cools
and descends, the current will change and come the other way, and flood
these western slopes with an air as pure as that of the Sahara and
nearly as dry.

[Illustration: BIRD'S-EYE VIEW OF RIVERSIDE.]

"The air, heated on the western slopes by the sea, would by rising
produce considerable suction, which could be filled only from the sea,
but that alone would not make the sea-breeze as dry as it is. The
principal suction is caused by the rising of heated air from the great
desert.... On the top of old Grayback (in San Bernardino) one can feel
it [this breeze] setting westward, while in the canons, 6000 feet below,
it is blowing eastward.... All over Southern California the conditions
of this breeze are about the same, the great Mojave Desert and the
valley of the San Joaquin above operating in the same way, assisted by
interior plains and slopes. Hence these deserts, that at first seem to
be a disadvantage to the land, are the great conditions of its climate,
and are