64 TRANSACTIONS OF THE TEXAS ACADEMY OE SCIENCE. 
tures, to which any one will vouch who has driven through a river val- 
ley or an irrigated bottom in the arid region. The sensible difference 
between the temperature of Phoenix and other points of southern Ari- 
zona is decidedly marked; Phoenix being sultry in the hot season and 
chilly during the cold. The actual temperature shows greater extremes 
for reasons which depend on the fundamental principles of natural phi- 
losophy. Moist air is heavier than dry air, and is a poorer conductor of 
heat and cold. Consequently during the intense heat of summer radia- 
tion does not take place with sufficient rapidity during the night to lower 
the temperature to that of other localities outside the irrigated district. 
For the same reason in winter the heat of the sun during the day is not 
sufficient to raise the temperature to that of the other stations. It is 
not the presence of the river which causes the increase of the relative 
humidity, for Yuma, which is situated at the junction of the Gila and 
Colorado rivers, has the lowest relative humidity of any region of the 
territory where the records are kept. But at Phoenix, for several miles 
above and below the city, the country is watered by irrigation and the 
soil is so thoroughly saturated that the level of the water beneath the 
surface has been raised from a depth of seventy to from twenty-five to 
thirty feet. This condition is no more pronounced at Phoenix than at 
any other point where irrigation is practiced. As for example, in north- 
ern Colorado, where prior to the introduction of irrigation an extremely 
low percentage of moisture existed. Since all that region has been 
irrigated the relative moisture has been so increased that they now have 
dews where formerly dews were unknown. The emanations from the 
growing vegetation also increase the relative humidity, although suffi- 
cient areas have not been irrigated to make any perceptible difference in 
the amount of precipitation by rainfall. What these artificial changes 
may bring about in time, no one can predict; but it is well known that 
bodies of water which are surrounded by irrigated regions, such as the 
Great Salt Lake in TJtah, are gradually increasing in volume. The sur- 
face of the Great Salt Lake has risen several inches in the last few years. 
“The report of the United States Weather Observer at Phoenix, makes 
the relative humidity at that point, for the six months October to 
March, 1895-6, inclusive, 52.82 per cent. Records for Tucson during 
the same period are not obtainable. But compared with other years, 
when the precipitation was about the same as for this season, we find 
the relative humidity in the irrigated districts about twelve or fifteen 
per cent higher than at the other points in southern Arizona.” 
These same conditions mentioned by Dr. Rodgers for the Salt River 
Valley in Arizona, obtain in the Rio Grande Valley in New Mexico. It 
is both warmer in winter, and cooler in summer, on the San Augustine 
plains, twenty to twenty-five miles east of Las Cruces, on the other (east) 
