108 On the Law of Storms in the 
This force creates a cone-shaped depression on the outer 
surface of the atmosphere, rarefies the air in the neighbour- 
hood of the axis, and by accumulation condenses that towards 
the circumference of the storm. The atmospheric pressure on 
the surface of the sea around the centre of the storm is less, 
and at the circumference is greater than the mean pressure ; 
therefore at the centre the sea is raised above, and at its cir- 
cumference is depressed below, the mean level, about a foot 
for each inch of range of mercury in the barometer. 
For the same reason the mercury in the barometer will rise 
above the mean height at the circumference, and fall below it 
at the centre ; so that when such a storm approaches a ship, 
the mercury rises: as it passes over her the mercury falls, at 
first gradually, but afterwards rapidly until the centre reaches 
her, when the mercury reaches its minimum height. 
During the passage of the second half of the storm, the 
mercury rises rapidly, afterwards more gradually, and finally 
sinks to its mean height. It is probable, however, that the 
fluctuations of level in an ordinary barometer occur some 
three or four hours after the atmospheric changes by which 
they are caused; for, at page 207, vol. 2, of Daniell’s 
Meteorology, it is stated that the water barometer of the 
Royal Society preceded by one hour the mercurial barometer 
of 4 inch bore; and the latter, the mountain barometer of 
‘15 inch bore, by the same interval in their indications. 
The knowledge that barometric fluctuations are not 
simultaneous with, but often considerably later than, the 
corresponding atmospheric changes, may remove the 
apparent anomalies which often arise from the sluggish 
indications of an instrument of narrow bore, and perhaps 
otherwise imperfect. 
Each wind of definite direction raises a swell parallel to 
that radius of the circle which is perpendicular to the wind. 
