6 EF. Loomis— Observations of the U. S. Signal Service. 
the absolute height of the barometer. Plate I exhibits the 
oscillations of the barometer for Nos. 2, 8 and 9 at San Francisco 
and several other stations, extending eastward to the Valley of 
the Mississippi, and a change of pressure of one-tenth of an rial 
is represented by one-tenth of an inch in the diagram. Plate 
II represents three other cases in which the minimum of press- 
ure is pretty sharply defined, and the progress of the barometric 
wave is very distinctly indicated. These examples show conclu- 
sively that barometric waves sometimes travel from the Pacific 
coast across the Rocky Mountains into the Valley of the Missis- 
sippi, with so little change as to leave no doubt of their identity. 
It will be noticed, however, that the barometric oscillation gen- 
erally increases quite rapidly as soon as the wave reaches the 
Mississippi Valley; and in several of the diagrams palpable 
changes will be usin in the form of the curves from one 
station to another. In several of the’ cases, not here represented, 
these changes are still more considerable. 
minimum at Salt Lake City usually occurs about sixteen 
howl later than at San Francisco, and at Cheyenne about one 
day later than at San Francisco. This indicates a velocity of 
forty miles per hour, which is greater than the velocity usually 
found for barometric Waves ; but it is probable that the motion 
of the center of lo w pressure was not parallel to the line joining 
San ds fo Cheyenne, so that the velocity of the center 
of low pressure was less than forty miles per hour. It sere 
then, to be ‘italy ucabtiobead that barometric waves frequen 
travel from the Pacific coast across the Rocky Mountains ra 
reach the Mississippi Valley with but little a The 
Rocky Mountains form an uninterrupted barrier 6,000 feet in 
height from British America southward to vitals 32°, and the 
Sierra Nevadas present a barrier of the same height extending 
from British America southward to latitude 36°, with but three 
interruptions amounting in the aggregate to less than one hun- 
miles. The Roc ky Mountains form a barrier of 10,000 feet 
in height, which eit nearly half the — from latitude 
49° to latitude 32°, and which is continuous for about 350 miles 
in the neighborhood of Colorado. The Sierra Ne evadas also 
present short ranges of equal altitude, but the longest of them 
is less than 150 miles 
Thus we see that an unbroken mountain range of 6,000 feet 
in height cannot stop the progress of atmospheric waves; neither 
do omae of more than 10, 000 feet i in height, broken as in North 
le obstacle. A great t barometric 
