14 E. Loomis— United States Weather Maps. 
from this region. the average course of storms is almost exactly 
east 
Where do the storms which seem to come from the far west 
originate ? 
place of origin, I have taken the volume of signal service obser- 
vations for Sept., 1872, and projected all the barometric obser- 
vations in curves, arranging the stations in the order of longi- 
tude. From a comparison of these curves it appears that for 
the month of September, 1872, all the principal oscillations of 
the barometer at Portland, Oregon, can be traced at Fort Ben- 
ton, Virginia City, Fort Sully, and at each of the other stations 
extending eastward to Lake Superior and Lake Michigan. In 
was reported on that day from any of the signal service stations, 
but the weather was reported “threatening” at Fort Benton and 
Corinne, and “cloudy” at Virginia City, Cheyenne, etc. The 
first curve on the left shows the isobar 29°8 for 74 a. M., Sept. 
19th. The second oval shows the isobar 29°8 for 7} a. M., Sept. 
20th. On this day slight rain fell at Cheyenne; the weather was 
reported “threatening” at Denver, Omaha and Leavenworth; 
and “cloudy” at Breckenridge, Fort Sully, St. Paul, ete. The 
oval on the right shows the isobar 29°8 for 74 a. M., Sept. 21st. 
On this day, half an inch of rain was reported at Duluth, 2 inch 
at Marquette, and slight rain at a few other stations near Lake 
Michigan. In two days the center of these isobars traveled 
eastward about 980 miles, or at an average rate of 20 miles per 
hour. The next day the storm advanced toward the northeast, 
and passed beyond the range of our stations of observation. 
About three days later, another and more considerable de- 
pression of the barometer followed nearly in the track of the 
preceding. Plate 1 exhibits the isobar 29-7 for four successive 
fea On the 22d of Sept. a considerable fall of the barometer 
was observed from Oregon to Dacota. At 74 4. M. the isobar 
of 29°7 was an oval alge: westward nearly to the Pacific 
Ocean, and eastward to the Missouri River, a distance of ubout 
1100 miles. On the same day “heavy rain” was re 
Portland, 
ported at 
Oregon ; at Fort Benton over a foot of snow fell on 
