20 HK. Loomis—Observations of the U. S. Signal Service. 
4. In North America, south of latitude 35°, areas of low 
pressure are less frequent and generally exhibit a less depres- 
sion than near latitude 45°, because the area over which a 
cyclonic movement of the winds prevails is small; and this 
area is small because if a cyclonic area could be formed having 
a radius of 1,000 miles with its center in latitude 30°, its cir- 
cumference must extend southward to latitude 16°, where the 
trade winds are steady and seldom interrupted. Such a diver- 
sion of the winds toward the north, even if it could be produced, 
could not be long maintained; so that a large cyclonic area 
with its center in latitude 30° is well nigh impossible ; and it 
is impossible that there should be a great depression of the 
barometer in latitude 80°, except with a wind having a hurricane 
velocity. This is believed to be the reason why in North 
merica the centers of great storms are generally found north 
of latitude 40°. 
erally the original cause of the barometric depression, but 
rather an incident of the cycloidal movement of the atmos- 
here. The fall of the barometer during a rain storm cannot 
ye ascribed to the simple condensation of the vapor of the 
atmosp as some have supposed, since a rain-fall of one or 
two inches prevailing over an area 300 miles in diameter near 
. latitude 30° produces scarcely an appreciable effect upon the 
barometer. e tables on pages 2 and 3. 
7. The progress of areas of low barometer in all latitudes is 
determined mainly by the same causes which determine the 
general system of circulation of the atmosphere; and their 
normal direction is changed by whatever causes may change. 
the direction of the winds. 
