Meteorological Sketches. 263 
deemed, by late European writers, to be as yet unknown. These 
fluctuations of the barometer appear to differ in their character _ 
origin, and may be classed under the following heads. 
I. The regular semi-diurnal oscillation, which’ in the srSpieal hats 
itudes is gt its maximum from 9 to 10 A. M., and at its minimum 
about 3 P. M. In the temperate latitudes, the effect appears to be 
nearly the same ; but Professor Forbes has shown that in very high 
latitudes the effect is reversed, the minimum being at 10 and the 
maximum at 3 o’clock. This oscillation appears to indicate a sys- 
tem of atmospheric tides, resulting from the rotation of the pics 
and its relations to the solar system. 
Il. The more striking and irregular variations which witond the 
presence and passage of storms of wind, especially in the higher 
latitudes. This class of fluctuations is believed to admit of an easy 
and satisfactory explanation. 
t appears from a careful examination of the phenomena of hur- 
ricanes and storms, as they occur in various regions, that the great- 
est depression of the barometer is found within the body of the 
storm,—that this depression constantly accompanies the storm during 
its progress from one region to another, notwithstanding the tenden- 
ey of the air to move from all sides towards the point of least pres- 
sure,—and that the wind in these storms is found to blow in a lat- 
eral or circuitous direction, around the point of greatest depression, 
which is near the geographical center of the storm. Now when 
these facts are considered, it becomes evident that the centrifugal 
action of the air in this powerful rotative movement, effectually op- 
poses the gravitating tendency towards the point of least pressure, 
and thus maintains, mechanically, the constant rarefaction which 
causes the depression of the mercury under the storm. Were it 
possible to produce a movement of the wind from all sides of a 
storm, in the direction of its center, the depression of the barom- 
eter would at once be terminated, and an accumulated pressure 
would immediately take place. 
A demonstrative proof of our position is also found in the barome- 
trical depression which is so constantly exhibited in the permanent 
atmospheric eddy at Cape Horn and the Strait of Magalhaens, which 
is caused by the westerly winds that press upon the Cape and are 
disgorged into the Southern ocean around the southern termination 
of the Andes. Capt. P. P. King, who surveyed this region and 
who was furnished with the best instruments, adjusted to the standard 
