38 W. Ferrel on motions of Fluids and Solids 
depression there, and a greater part of the atmosphere to be 
thrown into the northern hemisphere. 
This also accounts for the mean position of the equatorial 
calm belt being, in general, a little north of the equator. But 
in the Pacific Ocean, where there is nearly as much water north 
of the equator as south, its position nearly coincides with the 
equator. 
For the same reason the tropical calm belt of the northern 
hemisphere is farther from the equator than that of the southern 
hemisphere ; and, on account of the irregular distribution of the 
land and water of the two hemispheres in different longitudes, it 
does not coincide with any parallel of latitude. In the longitude 
of Asia, where there is all land in the northern hemisphere an 
the Indian Ocean in the southern, this belt, which is also the 
dividing line which separates the winds which blow east from 
those which blow west, is farther from the equator than at any 
other place, as shown by Professor Coffin’s c 
6. 
In winter, the difference of temperature between the equa- _ 
tor and the poles, upon which the disturbance of the atmosphere 
depends, is much greater than in summer; this causes the east- 
ward motion of the atmosphere in either hemisphere during its _ 
winter to be greater, while in the other hemisphere it is less. 
Hence a portion of the volume of the atmosphere in winter is 
thrown into the other hemisphere; but, although the volume or 
height of the atmosphere is then less, yet, being more dense, the 
barometric pressure remains nearly the same. The difference — 
at Paris, and in the middle latitudes generally, between wint 
and summer, is only about J of an inch. 
On accoun 
t of this alternate change with the seasons of the — 
velocity of the eastward motion of the atmosphere in the two _ 
hemispheres, the equatorial and tropical calm belts change their 
ena ag little, moving north during our spring, and south in 
III. The Motions of the Atmosphere arising from local disturbances. 
27. Besides the general disturbance of equilibrium arising 
from a difference of specific gravity between the equator and the 
poles, which causes the general motions of the atmosphere, treat- 
ed in the last section, there are also more local disturb: 
arising from a greater rarefacti 
portions of the earth’s surface, which 
storms, tornadoes, and water-spouts, en, on account of greater 
heat, or a greater amount of aqueous vapor, the atmosphere at 
any place becomes more rare than the surrounding portions, it 
ascends, and the surrounding heavier atmosphere flows in below, 
to supply its place, while a counter current is consequently pro- 
a ' a 
tion of the atmosphere over limited a 
Spite oc : , give rise to the various 
irregularities in its motions, includir cyclones or revolving — 
