EF. Loomis— Observations of the U. S. Signal Service, 19 
cated. One is by curves representing from day to day the fluc- 
tuations of the barometer at the various stations, as shown in 
Plates I and I, accompanying my ninth paper. These curves 
indicate distinctly the eastward progress of low areas, but they 
do not show the center of a low area, and therefore do not 
indicate the exact path of the low center. There are two 
methods by which the position of the center of a low area can 
be traced from day to day, viz: by isobaric lines or isabnormal 
lines. If we employ isobaric lines, it is requisite that the ob- 
servations at all the stations should be carefully reduced to 
sea level, and, in the case of the mountain stations, this in- 
noticeable in very violent storms. Plate II exhibits the isobars 
for April 11, 1874, at 4:35 p.m. This is a storm which was 
moved from Lake Superior to Norfolk, Va.; that is, from 
northwest to southeast, maintaining during the whole time 
nearly the same pressure. 
The direction of the winds, April 11th at 4:35 P. M., west of 
the Mississippi river, corresponded with what is usually ob- 
served about a low center, with the exception of Portland, 
r., and Virginia City. The latter observation seems to indi- 
eate that the center of the low area was further north than is 
represented upon the map; but we find frequent anomalies in 
the direction of the winds~at the mountain stations, as will be 
