Observations of the United States Signal Service. 7 
generally there was some rain reported whenever the barometer 
fell below 29°5 inches. I have found no storm of great violence 
which was not accompanied by a considerable fall of rain. 
Rainfall is not, however, generally the cause of that first move- 
ment of the wind which results in a great barometric depres- 
sion. This appears from column 8th of the table on page 2, 
which shows that over a circle of 600 miles in diameter sur- 
rounding the locality where the storm originated, in thirty-one 
cases no rain for the preceding eight hours was reported from 
any station, and in only one instance did the total rain-fall 
within this circle exceed one-tenth of an inch. It may be said 
that in the neighborhood of these localities the stations were 
few in number, and that rain-fall may have occurred at inter- 
mediate points from which we have no reports. But in at least 
a quarter of all the cases, this circle of 600 miles in diam- 
eter included as many as four or five stations, so that we 
seem fully justified in concluding that generally the inward 
movement of the air toward a central area begins before there 
is any considerable precipitation of vapor. 
ter an area of low barometer has been formed, it soon 
begins to change its position. This movement appears to be 
mainly determined by the same causes which control the general 
circulation of the atmosphere. Throughout the United States 
(with the exception of the extreme southern margin) the aver- 
age annual progress of the wind is from west to east, and this 
movement is determined by causes which are general in their 
operation, and cannot be permanently changed by the influence 
of local storms. When an area of low barometer is formed, 
the wind sets in both from the east and west sides to restore 
storm, and generally extends much beyond these limits, and at 
ll points is exerted nearly in parallel lines. Moreover, the 
disturbance of the atmosphere by storms is mainly confined to 
the lower half of the atmosphere, while the regular movement 
of the upper half is much less disturbed. The force of this 
pr current from the west, combined with that of the lower 
f of the atmosphere, pressing upon the west side of an = 
ha 
