EF, Loomis— Observations of the U. S. Signal Service. 107 
velocity not exceeding five miles per hour; two feathers indi- . 
cate a velocity from six to ten miles; three feathers from eleven 
to fifteen miles; four feathers from sixteen to twenty miles, and 
so on for higher velocities. 
The center of low pressure advanced eastward along the dot- 
ted line represented on Plate IV. Some have claimed that this 
advance of storms is simple drzft, the entire mass of air within the 
low area being carried bodily eastward, that being the average 
direction in which the atmosphere moves in the middle lati- 
tudes. This explanation will pot stand examination. If while 
the winds are circulating around a low center, the entire atmos- 
phere within the low area is carried bodily eastward, the effect 
of this movement should be different upon the northern and 
southern portions of the storm. In my former papers I have 
shown that on the north side of low areas in the United States 
the average direction of the wind is nearly northeast, an 
the south side it is from the southwest, and the average velocity 
of the winds on both sides is nearly the same, viz: eight miles 
per hour, while the average progress of the low center is twen- 
ty-six miles per hour. Suppose now the velocity of progress 
to be increased to fifty miles per hour, if the entire mass of air 
within the low area is carried bodily eastward, the velocity of 
the wind relative to the earth’s surface should be greatly in- 
creased on the south side of the low area, while that on the 
Storm, the average velocity of the winds is 14°9 miles per hour, 
and on the south side it is 85 miles; that is, on the north side 
the average velocity of the winds is seventy-five per cent greater 
than it is on the south side. The air within this low area did 
'g bodily from place to place at the rate of twenty-five miles 
per hour, the wind at the center should blow at the rate of 
