EF. Loomis-— Observations of the U. S. Signal Service. 15 
successively over the same station, causing a change of tem- 
perature of 50° in a single hour. 
he other cases of sudden change above enumerated admit 
of similar explanation. During January 7th there was a great 
rise of the thermometer at Denver, accompanied by a dry wind 
from the southwest. The next morning the thermometer fell 
suddenly with a wind from the northeast, which brought back 
the cold air which steadily prevailed at stations in the north. 
This change also accompanied the progress of an area of low 
pressure, which was apparently central near Virginia City on 
the morning of. the 7th, and was central at St. Paul on the 
morning of the 8th. This storm was accompanied by similar 
changes of temperature at Virginia City, Cheyenne, and the 
other stations within the area of low pressure, but at none of 
them were the changes as great or as sudden as at Denver, 
for the reason that Denver is most favorably situated for feel- 
ing the influence of the mountains. The stations of the Signal 
Service which are nearest to the dividing ridge of the Rocky 
Mountains are Denver, Cheyenne and Virginia City. On the 
West of Denver, at the distance of only 40 miles, rises a con- 
tinuous mountain range of 12,000 feet elevation, while at Chey- 
enne the mountains are more distant and of less height, and 
near Virginia City the height of the mountains is still less. 
n the morning of January 3d the wind at Denver blew from 
the West with a velocity of 12 miles per hour. The air was 
warm and very dry. Between 3 and 5 p.m. the thermometer 
fell suddenly with a north wind. Similar changes were expe- 
rienced at Virginia City and Cheyenne, accompanied by an area 
of high pressure pursuing an area of moderately low pressure. 
The case of January 5th was more remarkable for the sudden- 
ness of the change of temperature than for the magnitude of 
the change, and resulted from the passage of an area of slight 
barometric depression. 
Barometric minima cross the Rocky Mountains. 
In former papers, particularly Nos. 8 and 9, I have shown 
the United States. In order to investigate this subject more 
fully, I selected from the published observations of the Signal 
Service (Sept., 1872, to Jan., 1875, and Jan. te May, 1877,) 
all those cases in which the barometer at Corinne or Salt Lake 
City was at least 0-4 inch below its mean height for that month. 
(‘The observations at Corinne ceased March 19, 1874, and those 
at Salt Lake City commenced the next day.] These cases are 
shown in the following table, in which column Ist gives the 
number of the storm; column 2d the date of the observation ; 
