38 ; REPORT—1840. 
towards the west. However, as the air rolls off above, towards the 
north, it will be constantly passing over portions of the earth’s surface, 
which have a less diurnal velocity than the part from which it set out; 
and as from the nature of inertia it still inclines to retain the diurnal 
velocity towards the east, which it originally possessed, when it reaches 
the latitude of about 20 or 25 degrees, it will then probably be moving 
nearly towards the north, and beyond that latitude its motion will be 
north-easterly. 
“Tf violent storm clouds, which necessarily rise to a great height into 
the upper current, are driven forward in the direction of the upper 
current, it is probable that the barometer will rise higher in that part 
of the annulus which is in front of the storm, than in the rear, and if 
so, a sudden rise of the barometer, in particular localities, may become, 
when properly understood, one of the first symptoms of an approach- 
ing storm. In consequence of the high barometer in front of the storm 
in a semiannulus, the air will be forced downwards there, and cause, in 
some cases, a more violent action of the air or wind backwards, meet- 
ing the approaching storm, than will be experienced in the rear of the 
storm. As the barometer will probably be highest in the centre of the 
semiannulus, north-east of the storm, in middle latitudes, the tendency 
of the wind to blow outwards on all sides from the centre may cause 
the wind in the beginning of the storm to blow so as to appear to whirl 
from left to right, on the east side of the storm, and from right to left 
on the west side. 
“As the air comes downwards in the semiannulus in front of the 
storm, it will come under greater pressure, and any clouds which it 
may contain will probably be dissolved by the heat of greater pressure, 
and therefore on the passage of the annulus it will probably be fair 
weather. Also, as the air above always contains more caloric to the 
pound than the air below, there will be an increase of temperature on 
the passage of the annulus, partly from the increased pressure, but 
chiefly by the descent of the air. In very hot climates, this increase of 
temperature in front of the storm will be very sensibly felt. The in- 
creased pressure in the annulus round a volcano, when it suddenly 
bursts out, will sometimes, under favourable circumstances, be very 
great, and of course the air will be depressed from a great height ; so 
that some portion of the very air which has gone up in the central 
parts of the ascending column, and formed cloud by the cold of di- 
minished pressure, will be forced down to the surface of the earth, 
bringing with it the caloric of elasticity which it received from the 
condensing vapour; if so, the heat experienced at the time of this 
descent will be very great. 
“These hot blasts of air will alternate with cold blasts; for the air 
which is forced down from great heights in the annulus will not only 
be very hot, but very dry, having condensed its vapour in its previous 
ascent. Now, when this hot dry air flows inwards again towards the 
voleano, and ascends, it will not form cloud, because of its want of 
vapour, and therefore the process of cloud-forming will cease, and con- 
sequently hail and rain will cease too, until more air from a greater 
