108 EE. Loomis— Observations of the U. 8. Signal Service. 
twenty-five miles per hour. We must then conclude that the 
progressive movement of areas of high barometer is like that 
wave, and its apparent motion results from a subtraction 
of air from one side and an addition of air to the opposite side. 
The progress of areas of low barometer must be due to sim- 
ilar causes. e pressure is diminished on the east side of the 
low area and increased on the west side, in consequence of 
which the iow center suffers a displacement with reference to 
the earth’s surface, and the rate of progress of the low center 
will depend upon the rate at which the pressure is reduced on 
the east side, and restored on the west side. 
The advance of storm centers across the United States may 
be affected by atmospheric conditions prevailing much beyond 
the limits of the signal service maps, so that we cannot be sure 
that we know all the circumstances which influence the case 
we are considering, but from Plate IV we can see a reason why 
the pressure on the west side of the low area should be rapidly 
restored. The air from the north and northwest rushed in with 
great velocity. This air had a very low temperature, the ther- 
mometer at 4.35 pP. M. (Washington time) being below zero of 
Boston on the east. The moist and warmer air on the east side 
of the low center rises from the earth’s surface and is supplan 
by the cold air which presses in upon the west side. The great 
extension of the rain area on the east side causes an unusually 
rapid fall of the barometer on that side, and a corresponding 
advance of the storm’s center. 
e have seen from the table on page 99 that in the middle 
latitades, storms generally travel eastward, even though the 
age rain-fall should be on the west side of the low center; 
but when the principal rain-fall is on the east side of the low 
center, this causes a diversion of the winds in that direction, 
and the low center travels eastward with increased rapidity. 
