Observations of the United States Signal Service. 17 
Throughout an extensive region east of the Rocky Mountains 
the annual rain-fall is very small. The winds of this region are 
therefore dry winds, while the air of the Mississippi valley con- 
tains an. abundance of vapor. The difference in the specific 
gravity of the air over the two localities is sufficient to accel- 
erate considerably the west and northwest winds which sweep 
over the dry prairies west of the Mississippi river. _ 
same causes which affect the average velocity of the 
winds affect also the frequency of occurrence of the most vio- 
lent winds. The region where violent winds are of most fre- 
uent occurrence is near the Gulf of St. Lawrence. It has been 
mentioned that the observations at Cape Rosier and Father 
‘oint embrace a period of only seven months, and should 
therefore be multiplied by three in order to render them com- 
parable with observations at the other stations. An important 
reason for the greater violence of the winds in this region, is 
the greater magnitude of the barometric fluctuations and the 
unusual contrast which exists between the warm and moist 
