6 EK Loomis— Observations of the U. 8S. Signal Service. 
Stations. on: Stations. | Stations. L4 
Pioche, Nev. 40°3 ||Dodge City, Kan. | 62°3|\Vicksburg, Miss. | 69:0 
Santa Fe, N. Mex. | 42°4||St. Louis, Mo. 63°9 |\Shreveport, La. 69°3 
Winnemucca, Nev.| 4274 ||Corsicana, Tex. 65°7 ||Davenport, Iowa | 69°5 
Denver, Col. 44:0 || Bi k, Dak. 66°4 ||Omaha, Neb 69°6 
Salt Lake City, Ut.| 45-1 ||/Denison, Tex. 6°7 || Yankton, Dak ‘sf 
( ne, Wyo Ft. Gibson, Ind. T. | 67-5 ||Keokuk, Iowa 70°3 
Boise City, Idaho | 54:1 ||Dubuque, Iowa 67°9 ||Pembina, Dak 72°] 
Virginia City, Mont.| 54°5 ||New Orleans, La. | 68°5 ||/Duluth, Minn 73°0 
Red Bluff, Cal 54:7 ||Leavenworth, Kan.| 68°7 ||Indianola, Tex 13°3 
Umatilla, Oregon 5 mphis, Tenn. 68°8 ||Galveston, _ TAT 
North Platte, Neb.} 61-6 |/St. Paul, Minn. 68 8 ||Breckenridge, Minn.| 76°0 
Pike’s Peak, Col. | 62°0 ||LaCrosse, Wisc. 68°9 
Plate I exhibits the curves of equal relative humidity for the 
stations east of the Rocky Mountains, showing that on the 
east side of these Mountains there is a narrow belt of territory 
where the mean relative humidity is less than one-half; an 
there is a belt at least 400 miles wide where the mean humidity 
is less than two-thirds; and in ae serio we find the 
humidity to increase still further. What is the cause of this 
dry atmosphere? Only one Bomiatca 6 seems possible. The 
westerly winds from the Pacific Ocean have their moisture 
mostly condensed in passing over the Sierra Nevadas, so that 
between these Mountains and the Rocky Mountains, the air is 
generally extrerhely dry. By passing over the Rocky Moun- 
tains there is a further condensation of vapor, so that when the 
air descends on the eastern side of these Mountains it is almost 
destitute of moisture. The vapor which comes up from the 
Gulf of Mexico is diffused over the Mississippi Valley, and 
mingles with the dry air which comes from beyond the Moun 
tains, so that the dryness of the air rapidly diminishes as we 
advance eastward from the Rocky Mountains. 
n order to determine whether the sudden changes of tempe- 
rature, sometimes experienced near the level of ne sea, ever re- 
sult from the sudden descent of cold air from a great height, I 
ave made an extensive comparison of the o pcrvations at 
h 
Peak is 30°8° below that at Denver, and the difference of eleva- 
tion is 8882 feet, showing a fall of temperature of 1° for an ele- 
vation of 288 feet, This represents nearly the Senliees of 
equilibrium of a vertical column of the atmosphere. If the 
air at Pike’s Peak should be 40° colder than at Denver it would 
tend to sink, and the air at Denver would ténd to rise. 
accordingly selected from the volumes of published _observa- 
tions (Nov. 1873 to Jan. 1875, and from me n. 1877 to © Peak 
1877) all the cases in which the. temperature at Pike’s P 
was 40° lower than at Denver. The number of these cases in 
twenty months of observations was 843. Only thirty-nine of these 
ale Pea 
